STABILIZED IMAGES: PROBE ANALYSIS PATTERN AND COLOR ANALYTIC MECHANISMS’
OF
U.K.BROWS,MARKP.COSGROVE,GEORGE
A. KOHL,DAN D. FULGHAH’ and MARTY J. SCHMIDT~
Department of Psychological Sciences. Purdue University, West Lafayette. Indiana 47907. U.S.A. (Rrcrirrd 14 March
1974; itr rvoisedfbrrn 15 May 1974)
Abstrac+Four experiments investigated the sensitivity of receptive fields when achromatic and monochromatic dots and lines were inserted into a circular field from which a stabilized test line had faded. The data support the conclusion that the human visual cortex processes pattern. in part, through the activities of spatial analytic mechanisms which receive input through independent color channels.
This set of studies continues a program of research designed to help clarify the nature of the pattern analytic mechanisms of human vision through the application of stabilized image techniques. Earlier studies in the series have supported the general conclusion that the characteristic structured fading of stabilized retinal images reflects the functioning of populations of supraretinal pattern analyzers (Brown et al., 1972, 1973; Cosgrove et al., 1971,1972, 1974; Schmidt et al., 1971, 1972). They have demonstrated the nature of selected characteristics of receptive fields for achromatic stimuli (Brown et al., 1973), and they have shown that fading is sensitive to color, as well as spatial. pattern attributes (Cosgrove et al., t974). The specific purpose of these particular studies is to psychophysically examine the interactions which occur in the processing of “colored things”. Although highly suggestive. neither the psychophysical nor the neurophysiological literature have made it clear whether color information is distributed among common brightness mechanisms which code information about achromatic brightness patterns, or whether individual colors are associated with separate brightness channels. The absence ofenduring color perception in the subjective and physical absence of brightness contours (Krauskopf. 1963; Yarbus, 1967; Cohen, 19%: Weintraub, 1964; Leibman, 1927) and the chromatic adaptation of pattern-specific mechanisms (McCollough. 1965; Lovegrove and Over, 1972), suggest that the perception of “things” and the perception of the i This research was supported by Research Grant HD00909 from the National Institute of Child Health and Human Development, D. R. Brown, Principal Investigator. ’ Present Address: WilIiams Air Force Base, Arizona, U.S.A. ’ Present Address: Department of Psychology. University of New Hampshire. U.S.A.. 03824. “27. 15,2--o
209
“color of things” are inti~tely related. Chromaticspecific differences in human contrast thresholds (Green. 1968), in the visibility of stabilized contours (Clowes. 1962; West. 1967). and in the adaptation of orientation and size-specific mechanisms {Slay, 1972). suggest that wavelength information is Transmitted through chromatic-specific pattern sensitive channels. The majority of cells in monkey visual cortex have been shown to be highly sensitive to line stimuli in a specific orientation. but insensitive to their wavelength (Hubel and Wiesel, 1968), suggesting a separate population of specialized color cells at the cortical level to receive color information from lower levels of processing in the visual system. Dow and Gouras (1973) found a large proportion of cells with orientation but not color specificity (spatial-opponent cells). fewer cells with color but not pattern signaling capability (coloropponent cells). and a small proportion of cells with both color and spatial-opponent properties. They conclude that spatial information is transmitted within both the red and green channels (but rarely- within the blue channel), and that color-opponent processes arise from interactions between color channels. AS Dow and Gouras (1973) point out. their interpretation of the neurophysiological data are consistent with some psychophysical results for humans. For example. they suggest that the color-opponent cells may account for the persistence and spread of color when color-filled contours fade under stabilized viewing (\Vaiis, 1953: Yarbus. 1938; Gerrits and Vendrik. 1970) and that the failure of Julesz patterns composed solely of color differences to create depth illusions implies some independence of color-processing and luminance-processing systems (Lu and Fender. 1971). The studies reported herein rely heavily upon previous studies in the series for their methodology. Brown er al. (1973) utilized a receptive field probe technique in conjunction with stabilized images which essentially allowed the isolation and description of pat-
D.R. BROW
210
tern-sensitive receptive fields for achromatic patterns. Working from the assumption that faded stabilized images reflect the temporary adaptation or inhibition of the units responsible for their visibility. the retinal area surrounding the faded images was probed with probe dots and lines in the interest of describing the receptive field characteristics of the assumed visual mechanisms. The results reflected the operation of length and orientation specific analytic mechanisms which are similar to the known characteristics of cortical receptive fields (e.g. Hubel and Diesel. 1968). A subsequent set of studies (Cosgrove. Kohl. Schmidt and Brown. 1974) investigated the adaptation produced by borders which were described by both chromatic and luminance differences in an effort to provide initial data concerning the processing of chromatic information. The data strongly supported the conclusion that adaptation to stabilized images is both wavelength and pattern specific. Moreover. interocular transfer of the effects argues for &he involvement of supraretinal mechanisms. These studies seek to extend the existing data for both chromatic and achromatic processing mechanisms. GESERAL METHODS The probe procedure utilized is shown schematically in Fig. I. The method utilizes an optical system which can present either of two pairs of optical channels in stabilized Maxwellian view. A test lint and background is presented in stabilized view through two test channels. When the line has completely faded from view. the observer triggers a shutter system which substitutes a probe channel pair for the test channel pair. The probe channels do not contain the test stimulus. but rather a probe stimulus and background. The probe can be either a small colored dot of variable wavelength or a line of variable orientation. When the probe stimulus is inserted into the held in an area formerly occupied by the test stimulus which has completely faded from view. the detection of the probe stimulus is typically delayed
Target
~
rtui.
by an interval on the order of 1-2 set following channel change. By recording the elapsed time between insertion and detection of the probe of variable wavelength and orientation a measure is gained oi the chromatic and spatial characteristics of the adapted mechanisms. Ohsniurs Two males. MC and GK. each 16 years of age. served as observers (OS). Vision was corrected to 20~20 in the left viewing eyes with indiv~duaily fitted scleraf contact lenses worn during all experimental sessions. Both OS had normal color vision as measured by the Ishihara Color Blindness Test. and were well practiced in stabilized image viewing. An optical system. similar in principle to that described by Clowes and Ditchburn (1959) and Brown rf a[. (19733was used to oresent a stabilized image of a target slide in Maxwellian ;ieu f through any of 1 channels whose axes are congruent after passing through beamsplitters (BS,. BSZ. BS,). This system is shown schematically in Fig. 2. The final lens in the system was an achromatizing lens (AL) to correct for chromatic aberration of the eye (see Bedford and Wyzecki. 1957). Stabilization was achieved by means of an optical lever rrgected from a small mirror fixed to a contact lens worn on the viewing eye. The system was designed so that movements of the eye (and. therefore. movements of the contact lens mirror) produced the appropriate movements of the stimulus image to keep it motionless with respect to the retina. A bite board with XYZ motion allowed the observer to fix his head in the appropriate position. Contact lenses were scleral lenses individually molded to tit each observer. A small mirror (5 mm dia) was mounted on the stage. normal to the optical axis. Before each experimental session the observer received one drop of Oph-
Probe
(7-J I. Wavelength
0 ~ I
2. Orientation
Fig. I. Ilhtstration of the probe detection paradigm used in these experiments.
Fig. 1. Latency of probe dot detection on control (dotted line) and test (solid lines) trials for OS MC (0) and GK (0). Each data point represents the mean of 10 trials. The arrow indicates the wavelength of the test line.
Stabilized images thainc anesthetic (Squibb) in the viewing eye. The non-viewing eye was occluded with an eyepatch. Siimrrli
Stimulus patterns were prepared as 35 mm slides for insertion into the optical system. Each target slide was mounted on a microscope slide positioning device which allowed the target stimuli to be accurately located in the field of view. Field luminance was adjusted by means of neutral density wedges and maintained by continuous monitoring of filament current at each source lamp. Schott (Veril S60) interference wedges were used to insure narrowband chromatic stimulation. Half band width throughout the spectrum was between I I and 16nm. Heterochromatic luminances were equated among colored lines by matching each colored line with a common white standard luminance using the criterion of a minimally distinct border between two precisely juxtaposed fields. To obtain these matches, a bipartite field with precisely juxtaposed borders was constructed. The field was a vertically divided circle subtending 2’ of visualangle. The Us adjusted the radiance of the chromatic side of the bipartite field until a minimally distinct border was obtained with respect to the white reference field. The match was taken as the mean of 10 adjustments for each observer. The method and rationale underlying this approach are outlined in Boynton and Kaiser (1968). Chromatic and achromatic stimulus patterns with and without white surrounding fields were used throughout. Test patterns were always a black. luminous, white or colored 2 x 45’ vertical line centered in a light-filled circular field. 1’ diam. Test wavelengths were 475. 525 and 650 nm. Probe stimuli were either a 3’ dia colored dot. or a 2 x 45’ colored or white line. Probe stimulus wavelengths were 475. 500. 525. 550. 575. 600. 625. 650 or 675nm. Probe line orientations were 0. 5, 10. 15 or 20” from the vertical. Field luminance. measured with a Gamma Scientific photomultiplier and photometer. was a measured 1.08 mL for the line and 0.5-t mL for the background. Proceriurr
Each trial began with the 0 viewing a stabilized colored or white line in the test channels while the probe channels were occluded by a shutter. Prior to each trial the experimenter adjusted the wavelength and luminance. and/or the orientation of the probe stimulus. The 0 was. therefore. naive with regard to probe conditions during test trials. When the test line had completely faded from view. the 0 depressed a hand-held pushbutton, activating a stepping motor system which simultaneously occluded the test channels and opened the probe channels. Substitution of channels was nearly undetectable except, of course. for the difference between the stimulus parameters in each channel. and was accomplished within a measured 2.5 msec with a I msec discontinuity between channels. The 0 kept the push-button depressed until he detected the presence of any part of the probe stimulus. whereupon he released the button. stopping a msec clock. The clock reading indicated the elapsed time between channel change and the detection of the probe. No attempt was made to record the accuracy of probe detection. Each test trial was followed by a control trial of the same color and pattern conditions. In these trials. however, the 0 initiated the channel change while the test line was still clearly visible. In these control trials the probe stimulus was immediately visible for all probe conditions.
211 EXPERIMENT
I
Earlier studies in this laboratory (Brown er al., 1973) have demonstrated that with achromatic line patterns. there exists an area of adaptation surrounding a faded image which varies as a function of proximity to the faded pattern and which is highly orientation sensitive. These data are easily understood if it is assumed that they reflect the operation of either spatial-opponent or color- and spatial-opponent cells. Experiment 1 was designed to replicate the earlier achromatic data and to partially examine the degree to which the data are independent of color properties of the stimuli. In order to examine orientation sensitivity. a white test line without a background tieId was presented and an identical probe line rotated 0, 5. 10. 15 or 20” from the vertical was presented on probe trials. Probe lines were randomly rotated in either direction from the vertical and the degree of rotation was randomly assigned for each probe. For each 0. the target line was probed a total of 100 times, 20 times for each of the five orientations. An equal number of control trials was completed. To examine wavelength specificity of previous results, a vertical white test line on a white background was probed by dots of variable wavelength on an identical background. The probe dot was always centered on the test line. For each 0, the target was probed a total of 180 times. 20 times for each of the nine wavelengths. Wavelengths were varied randomly over probe trials. An equal number of control trials was conducted.
Results Mean latencies to detect the probes are shown for both OS and for all conditions of viewing in Fig. 3. In Fig. 3, the dotted line portrays mean latency for all control trials. As seen in Fig. 3, detection of the probe dot did not vary as a function of the wavelength of the probe. The pattern of results is highly similar for both OS and to previous data which they were intended to replicate (Brown et al., 1973).
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Fig. 3. Mean latency of probe dot detection (left panel) and probe line detection (right panel) on control (dotted line) and test (solid line) trials for MC (e) and GK (0). Each data point represents the mean of 10 trials. The test line was in all cases white.
Probe uot wavelength,
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Fig. 4. &an latency of probe dot dskxtion a~ 3 function of waveleygth (left panel) and of line probe varying in rotation irght paneit for a test line of 525 nm For OS MC (*I and GK (3). Each data point represents the mean of IO trials. The arrow indicates the wavekngth of the test line.
The involvement of color-opponent or color- and sp~t~~~-o~pon~nt cells in the processing of colared parterns is suggested by a variety of psychophysical data mentioned earlier. Our observation (Cosgrove rt al.. 197-Qthat visibilitv ofstabilized lines varies as a function of the chromaiic aspects of stimulation is also supportive of this same notion. Experiment ?.was designed to estcnd these previous data by mapping the chromatic specificity of adapted receptive fields and assessing orientation specificit)- for chromatic line patterns. In these experimental sessions a monochromatic test line of 375. 53 or 65Onm wx probed by dots of various rvavelsngtbs. and by lines of the same wavelength and size as the test line, but varying in rotation. For each 0 each colored target line was probed 180 rimes. 20 times with dots of each of nine waveIengths. One-half of the trials conducted were control trials, in which the test line was probed when it was not faded from view. ~~~lvelen~th probe conditions were presented in a random sequence. In the same manner each monochromatic test linz was probed by an identical mono~hro~dti~ line a total of 100 times. 20 times with iines of each of fiw or~entat~ons. One-half of the triafs were control trials. orientation conditions were presented in a random sequence white colored test tine conditions were blocked into separate: daily sessions. Background luminances were not used in the orientation probe trials. Rr.%rir.s Figures 4. 5 and 6 show the results of the wavelength and orientation probe trials for the 525. 650 and 175 nm test lines respectively. The dotted line in each graph presents mean latency for control trials. In general. for wavelength test trials the latency to probe dut detection decreased with increasing differences between test and probe waveleng&s. thus forming narrow-band functions centered around the test Iine wavelength in Mach case. There \vas also an increase in
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Fig. 5. Mean latency of probe dot detection as a function of wavelength Ileft panel\ and of line probe varving in rotation (right panel) For a test Iine of 5x3 nm for Ds MC ie,) and GK (0). Each data point reprcsems the mean of ID trials. The arrow indicates the uavekngth of the test line.
latency for probe wavelengths which wre markedly different from the test pattern wavekngths. Both OS showed highly similar results. The right panels in Figs. 3. 5 and 6 show the results of the orientation probes of these same three test line wavelengths. The results for the 53 and 630 nm test lines are similar to the results for the achromatic test line shov.n in Fig. 3. In each case there is a decrease in detection latency with increasing diffxences b~ttvs~n test and probe line ori~nt~~tjons~The same detection latencv vaIue that the white test iine trials showed by 15’ ret&on {Fig. 31was not reached during the 53 and $50 nm test line trials until the probe iine had been rotated by 33’. For the 475 nm test fine there was no substan& change in probe detection latency 8s probe orientation was varied. The probe line was delay-ed by 7W9QO msec for all orientations. The results are. again. highly similar for both OS. EXPEKINENT 3
For reasons which Ml be discussed in detail later. the data from the second experiment are extremely
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of the Dow and Gouras data for monkey cortex. These data. however. only show that there is wavelength selectivity (based upon the dot probe summarized in the left panel of Figs. 4. 5 and 6) and for the test wavelengths. there is orientation sensitivity (the right panel in each figure). They do nor show that a common mechanism which has both wavelength and orientation sensitivity is involved. Taken together, however. the two panels of each figure predict a family of wavelength and orientation functions if such mechanisms are involved. The third study tests these predictions. Test lines of 525 and 650 nm were each probed by lines of various orientations and various wavelengths. Each test line was probed with 0’. 5’. LO’.15’ and 20‘ orientation lines. The 525 nm test line was probed with 5X.575 and 650 nm lines: and the 650 nm test line was probed with 650,600, 550 and 500 nm probe lines. For each 0 each test line was probed 20 times for each wavelength and orientation condition. Orientation conditions were presented in a random sequence. Wavelength conditions for both test and probe stimuli were blocked into separate experimental sessions. Test line and probe line luminances were I.08 mL. No background fields were used.
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The results of Experiment 3 are shown in Figs 7 and S. The results for both OS show that probe detection latencies vary with differences in both the probe and test line orientation and wavelength, in a manner predicted by the wavelength functions shown in Figs. J and 5. From Figs. 4 and 5, one would predict both the height and slope of the functions in Figs. 8 and 9. Given the relatively small number of observations for each point, the stability of these data for the two OS is extremely encouraging. EXPERIMEST
4
The fourth study was carried out to provide data in support of the argument that the data of the first three 525 nm tat
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experiments are due to chromatic. not luminance. aspects of the colored stimuli. The study is based upon the argument that the adapted receptive field ofa faded image should show differential sensitivity- as a function of the varying spectral purity of a probe for any given level of luminance and that different luminances should generate a family of parallel purity functions if these two aspects of color processing are operating independently to produce the phenomena under stud\. This hy-pothesis was tested for a single wavelength. SOI nm. A vertical 501 nm line stimulus of three possible luminance values (0.1. 1.0s and 1OS ftL) vvas probed by a 1.08 ftL dot of variable spectral purity. Probe purity values were located along a line connecting 501 and 610nm and passing through white for a tungsten source in the CIE standard diagram. 501 and 610 nm were chosen as wavelengths because they provide a good estimate of human dichromacy. Spectral purity of the probe dot was varied by adding 25. 50. 75 and 100 per cent white light to the probe dot and subtracting an equal percentage of the colored light. The test line in each luminance condition was presented through Channel 1 of the optical system. No background field was used. The probe dot was presented through Channels 3 and 4 of the optical system (white and color). The per cent of white or colored light in the dot was controlled by neutral density wedges located in each channel. The 501 nm test line was probed IO times for each 0 and for each of nine spectral purity conditions. Probes of 0.24 50. 75 and 100 per cent purity for each of the two wavelength values the probe dot could assume were presented in a random sequence during testing. The three luminance conditions for the test line were blocked into separate experimental sessions for each 0. Heterochromatic luminances were equated among the probe dots by matching each chromatic luminance with a white standard luminance using the
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Fig. 9. Latency of probe dot detection for MC (left panel) and GK (right panelj for a test line of 501 nm and luminance of0.l ftt (e), 1ftL (0). or 10 ftL (a) and probe dots of varying purity. Each data point represents the mean of 10 trials.
criterion of a minimally distinct border between two precisely juxtaposed fields. Rrsrtirs
The results of experiment 3 are shown for each 0 in Fig. 9. which shows that probe detection latencies decrease in a regular manner as the percentage of purity of the probe dot decreases. This is true for all three test line luminance 1eveIs. Detection of a 501 nm probe dot of 100 per cent purity is delayed by about 7OOmsec whereas an equally luminous probe of 0 per cent purity (white) is delayed little beyond reaction time values. This suggests that the adaptation caused by the faded 50 I nm stimulus line is indeed largely chromatic. However, the fact that detection of the 610 nm probe dot is also somewhat delayed by the faded 501 nm test line and that as test line luminance increased the probe detection latency values also increased slightly, suggests that at least some of the adaptation produced by a faded 501 nm line can be attributed to the luminance of the Iine. However, these luminance effects are small when compared to the purity effects.
The original intent of this program of studies was two-fold, to demonstrate that the visibility of stabilized images reflects the operation of central visual processing mechanisms and to utilize this methodology to provide more detailed information concerning the nature of central human pattern and color vision than has heretofore been available. The data presented here are important, we believe, in both respects. fn attempting to understand how we see the color of things. it is wet1 to remember that the n~urophysioIogical data which classify types of central processing units are. undoubtedly. quote incomplete. Dow and Gouras (1973) have shown the existence ofcolor opponent processing units without sensitivity to patterned input. a large proportion of spatial-opponent cells which receive input independently through red and green. but
perhaps not biut. cone channels. and cells which are poorip characterized. but which signal spatial information through the interaction of cone channels. It is not necessary, and. indeed, probably not accurate. to account for all ofour data in terms of the operation of a sinele class of cells. However. these data are strikingly similar to the predictions one would make from the assumption that they reflect the operation of ceils which receive input from cone channels each with its own independent spatial inhibition. In this regard. it is worth noting that the orientation “tuning” was highly similar for achromatic stimuli. red and green lines. but inconclusive for blue lines. Earlier data (Cosgrove et ai.. 1973) have shown that blue lines fade at higher rates than do red or green lines. Dow and Gouras were unabie to find spatial-opponent cells which received input through blue channels. To handle the data from the first experiment. the fact that faded achromatic lines lead to insensitivity to probes of all wavelengths, requires the assumption that achromatic patterns adapt all cone mechanisms equally. It is also possible that we “see” achromatic Imes with mechanisms which receive inputs from all cone channels. but are only sensitive to the contour information carried by these channels. Stabilized image data hav-e rarely bscn utilized as a foundation for the systematic study of central visual processes and have been repeatedly criticized on methodological grounds. Given that it is becoming possible to create stabilized images without the inconvenience ofcontact lenses (Cornsweet and Crane. 1970). we suggest that it may be a fruitful approach to the study of a much broader range of pattern stimulus parameters. as viewed by one or both eyes.
REFEREYCES
Bedford R. E. and Wyzecki G. (1957) Axial chromatic aberration of the human eye. 1. Opt. Sot. .+ner. 47, 56C 165. Bovnton R. M. and Kaiser P. K. (1968) Vision. The additiLity law made to work for heterochromatic photometry with bipartite fields. Scimcr .L’..Yl 161, 366-368. Brown D. R.. Schmidt hf. J.. Cosgrove &l. P. and Zuber J. J. (1972) Stabilized images: Further evidence for central pattern processing, Ps)cho~to~nic Sci. 29. 106-105. Brown D. R.. Schmidt M. J.. Fulgham D. D. and Cosgrove XI. P. (1973) Human receptiv-e field characteristics: Probe analysis of stabilized images. l’ision Rrs. 13, 231-2-U. Clowes M. B. (I 962) A note on colour discrimination under conditions of retinal image constraint. Opcica .A%, 9, 65 65. Clowes M. B. and D~tchburn R. W. (1959) An improved apparatus for producing a stabilized retinal image. Optica .kta. 6, 252-965, Cohen W. (19%) Color-perception in the chromatic Ganzfeld. Atn. J. Ps.~hoI. 71, 39@-394. Cornsweet T. N. and Crane H. D. (1970) Servo-controlled infra-red ontometer. J. Ont. Sot. Atnrr. 60, X&j%.
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Stabilized images Cosgrove M. P.. Schmidt M. J.. Fulgham D. D. and Brown D. R. (1971) The dependent variable in stabilized retinal imaee studies. C’ision Res. 1I. I 183-i 187. Cosgrove M. P.. Schmidt M. J.. Fulgham D. D. and Brown D. R. (1971) Stabilized images: Dependent variable specificity of pattern-specific effects with prolonged viewing. Percept. Ps~hophys. 11, 398-402. Cosgrove M.-P.. Kohl G. A., Schmidt M. J. and Brown D. R. (1974) Chromatic substitution with stabilized images: Evidence for chromatic-specific pattern processing in the human visual system. Y&ion Res. 14, 23-19. Dow B. M. and Gouras P. (1973) Color and spatial specificity of single units in Rhesus monkey fovea1 striate cortex. J. Newophysiol. 36, 79100. Gerrits H. J. M. and Vendrik A. J. H. (1970) Simultaneous contrast. filling-in process and information processing in man’s visual system. Esp. Brain Rrs. 11. II i-430. Green D. G. (1968) The contrast sensitivity of the colour mechanisms of the human eye. J. Ph_vsiol. 1%. 415-429. Hubel D. H. and Wiesel T. N. (1968) Receptive fields and functional architecture of monkey striate cortex. J. PhySol. 195, 215-213. Krauskopf J. Effect of retinal image stabilization on the appearance of heterochromatic targets. J. Opt. Sot. rlmrr. 53.741-744. Liebman S. (1927) tiber das Verhalten farbiger Formen bei Hclligketsgleichneit von Figur and Grund. Psychol. Forsch. 9.3%353.
Lovegrove W. J. and Over R. (1972) Color adaptation of spatial frequency detectors in the human visual system. Science. 1v.Y.176. 541-343. Lu C. and Fender D. ( 1971) Interaction of color and luminance in stereoscopic vision. 1. Opt. Sot Amer. 61, 1567-1568. May J. G. (1972) Chromatic adaptation of orientation and size-soecific visual processes in man. Vision Rrs. 12. 150% ijl8.’ McCullough C. (19653Color adaptation ofedge-detectors in the human visual svstem. Scirrxr. S. Y 149, I I I I- I I 16. Schmidt M. J.. Fulgham D. D. and Brown D. R. (1971) Stabilized images: the search for pattern elements. Prrcrpt. Psvchophys. 10,29>299. Schmidt M. J.. Cosgrove M. P. and Brown D. R. (1972) Stabilized images: Functional relationships among populations of orientation-specific mechanisms in the human visual system. Prrcrpt. Psychoph_vs. 11, 389-392. Walls G. L. (1954) The filling-in process. rlrn. J. Optometry. 31, 329-340. Weintraub D. J. (1964) Successive contrast involving luminance and purity alterations of the Ganzfield. J. Exp. Psyhol.
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West D. C. (1967) Brightness discrimination with a stabilized retinal image. C’isionRex 7, 949-974. Yarbus A. L. (1969) Epr hfovenrrnts ami Visiotl. Plenum Press. New York.
OF
U.K.BROWS,MARKP.COSGROVE,GEORGE
A. KOHL,DAN D. FULGHAH’ and MARTY J. SCHMIDT~
Department of Psychological Sciences. Purdue University, West Lafayette. Indiana 47907. U.S.A. (Rrcrirrd 14 March
1974; itr rvoisedfbrrn 15 May 1974)
Abstrac+Four experiments investigated the sensitivity of receptive fields when achromatic and monochromatic dots and lines were inserted into a circular field from which a stabilized test line had faded. The data support the conclusion that the human visual cortex processes pattern. in part, through the activities of spatial analytic mechanisms which receive input through independent color channels.
This set of studies continues a program of research designed to help clarify the nature of the pattern analytic mechanisms of human vision through the application of stabilized image techniques. Earlier studies in the series have supported the general conclusion that the characteristic structured fading of stabilized retinal images reflects the functioning of populations of supraretinal pattern analyzers (Brown et al., 1972, 1973; Cosgrove et al., 1971,1972, 1974; Schmidt et al., 1971, 1972). They have demonstrated the nature of selected characteristics of receptive fields for achromatic stimuli (Brown et al., 1973), and they have shown that fading is sensitive to color, as well as spatial. pattern attributes (Cosgrove et al., t974). The specific purpose of these particular studies is to psychophysically examine the interactions which occur in the processing of “colored things”. Although highly suggestive. neither the psychophysical nor the neurophysiological literature have made it clear whether color information is distributed among common brightness mechanisms which code information about achromatic brightness patterns, or whether individual colors are associated with separate brightness channels. The absence ofenduring color perception in the subjective and physical absence of brightness contours (Krauskopf. 1963; Yarbus, 1967; Cohen, 19%: Weintraub, 1964; Leibman, 1927) and the chromatic adaptation of pattern-specific mechanisms (McCollough. 1965; Lovegrove and Over, 1972), suggest that the perception of “things” and the perception of the i This research was supported by Research Grant HD00909 from the National Institute of Child Health and Human Development, D. R. Brown, Principal Investigator. ’ Present Address: WilIiams Air Force Base, Arizona, U.S.A. ’ Present Address: Department of Psychology. University of New Hampshire. U.S.A.. 03824. “27. 15,2--o
209
“color of things” are inti~tely related. Chromaticspecific differences in human contrast thresholds (Green. 1968), in the visibility of stabilized contours (Clowes. 1962; West. 1967). and in the adaptation of orientation and size-specific mechanisms {Slay, 1972). suggest that wavelength information is Transmitted through chromatic-specific pattern sensitive channels. The majority of cells in monkey visual cortex have been shown to be highly sensitive to line stimuli in a specific orientation. but insensitive to their wavelength (Hubel and Wiesel, 1968), suggesting a separate population of specialized color cells at the cortical level to receive color information from lower levels of processing in the visual system. Dow and Gouras (1973) found a large proportion of cells with orientation but not color specificity (spatial-opponent cells). fewer cells with color but not pattern signaling capability (coloropponent cells). and a small proportion of cells with both color and spatial-opponent properties. They conclude that spatial information is transmitted within both the red and green channels (but rarely- within the blue channel), and that color-opponent processes arise from interactions between color channels. AS Dow and Gouras (1973) point out. their interpretation of the neurophysiological data are consistent with some psychophysical results for humans. For example. they suggest that the color-opponent cells may account for the persistence and spread of color when color-filled contours fade under stabilized viewing (\Vaiis, 1953: Yarbus. 1938; Gerrits and Vendrik. 1970) and that the failure of Julesz patterns composed solely of color differences to create depth illusions implies some independence of color-processing and luminance-processing systems (Lu and Fender. 1971). The studies reported herein rely heavily upon previous studies in the series for their methodology. Brown er al. (1973) utilized a receptive field probe technique in conjunction with stabilized images which essentially allowed the isolation and description of pat-
D.R. BROW
210
tern-sensitive receptive fields for achromatic patterns. Working from the assumption that faded stabilized images reflect the temporary adaptation or inhibition of the units responsible for their visibility. the retinal area surrounding the faded images was probed with probe dots and lines in the interest of describing the receptive field characteristics of the assumed visual mechanisms. The results reflected the operation of length and orientation specific analytic mechanisms which are similar to the known characteristics of cortical receptive fields (e.g. Hubel and Diesel. 1968). A subsequent set of studies (Cosgrove. Kohl. Schmidt and Brown. 1974) investigated the adaptation produced by borders which were described by both chromatic and luminance differences in an effort to provide initial data concerning the processing of chromatic information. The data strongly supported the conclusion that adaptation to stabilized images is both wavelength and pattern specific. Moreover. interocular transfer of the effects argues for &he involvement of supraretinal mechanisms. These studies seek to extend the existing data for both chromatic and achromatic processing mechanisms. GESERAL METHODS The probe procedure utilized is shown schematically in Fig. I. The method utilizes an optical system which can present either of two pairs of optical channels in stabilized Maxwellian view. A test lint and background is presented in stabilized view through two test channels. When the line has completely faded from view. the observer triggers a shutter system which substitutes a probe channel pair for the test channel pair. The probe channels do not contain the test stimulus. but rather a probe stimulus and background. The probe can be either a small colored dot of variable wavelength or a line of variable orientation. When the probe stimulus is inserted into the held in an area formerly occupied by the test stimulus which has completely faded from view. the detection of the probe stimulus is typically delayed
Target
~
rtui.
by an interval on the order of 1-2 set following channel change. By recording the elapsed time between insertion and detection of the probe of variable wavelength and orientation a measure is gained oi the chromatic and spatial characteristics of the adapted mechanisms. Ohsniurs Two males. MC and GK. each 16 years of age. served as observers (OS). Vision was corrected to 20~20 in the left viewing eyes with indiv~duaily fitted scleraf contact lenses worn during all experimental sessions. Both OS had normal color vision as measured by the Ishihara Color Blindness Test. and were well practiced in stabilized image viewing. An optical system. similar in principle to that described by Clowes and Ditchburn (1959) and Brown rf a[. (19733was used to oresent a stabilized image of a target slide in Maxwellian ;ieu f through any of 1 channels whose axes are congruent after passing through beamsplitters (BS,. BSZ. BS,). This system is shown schematically in Fig. 2. The final lens in the system was an achromatizing lens (AL) to correct for chromatic aberration of the eye (see Bedford and Wyzecki. 1957). Stabilization was achieved by means of an optical lever rrgected from a small mirror fixed to a contact lens worn on the viewing eye. The system was designed so that movements of the eye (and. therefore. movements of the contact lens mirror) produced the appropriate movements of the stimulus image to keep it motionless with respect to the retina. A bite board with XYZ motion allowed the observer to fix his head in the appropriate position. Contact lenses were scleral lenses individually molded to tit each observer. A small mirror (5 mm dia) was mounted on the stage. normal to the optical axis. Before each experimental session the observer received one drop of Oph-
Probe
(7-J I. Wavelength
0 ~ I
2. Orientation
Fig. I. Ilhtstration of the probe detection paradigm used in these experiments.
Fig. 1. Latency of probe dot detection on control (dotted line) and test (solid lines) trials for OS MC (0) and GK (0). Each data point represents the mean of 10 trials. The arrow indicates the wavelength of the test line.
Stabilized images thainc anesthetic (Squibb) in the viewing eye. The non-viewing eye was occluded with an eyepatch. Siimrrli
Stimulus patterns were prepared as 35 mm slides for insertion into the optical system. Each target slide was mounted on a microscope slide positioning device which allowed the target stimuli to be accurately located in the field of view. Field luminance was adjusted by means of neutral density wedges and maintained by continuous monitoring of filament current at each source lamp. Schott (Veril S60) interference wedges were used to insure narrowband chromatic stimulation. Half band width throughout the spectrum was between I I and 16nm. Heterochromatic luminances were equated among colored lines by matching each colored line with a common white standard luminance using the criterion of a minimally distinct border between two precisely juxtaposed fields. To obtain these matches, a bipartite field with precisely juxtaposed borders was constructed. The field was a vertically divided circle subtending 2’ of visualangle. The Us adjusted the radiance of the chromatic side of the bipartite field until a minimally distinct border was obtained with respect to the white reference field. The match was taken as the mean of 10 adjustments for each observer. The method and rationale underlying this approach are outlined in Boynton and Kaiser (1968). Chromatic and achromatic stimulus patterns with and without white surrounding fields were used throughout. Test patterns were always a black. luminous, white or colored 2 x 45’ vertical line centered in a light-filled circular field. 1’ diam. Test wavelengths were 475. 525 and 650 nm. Probe stimuli were either a 3’ dia colored dot. or a 2 x 45’ colored or white line. Probe stimulus wavelengths were 475. 500. 525. 550. 575. 600. 625. 650 or 675nm. Probe line orientations were 0. 5, 10. 15 or 20” from the vertical. Field luminance. measured with a Gamma Scientific photomultiplier and photometer. was a measured 1.08 mL for the line and 0.5-t mL for the background. Proceriurr
Each trial began with the 0 viewing a stabilized colored or white line in the test channels while the probe channels were occluded by a shutter. Prior to each trial the experimenter adjusted the wavelength and luminance. and/or the orientation of the probe stimulus. The 0 was. therefore. naive with regard to probe conditions during test trials. When the test line had completely faded from view. the 0 depressed a hand-held pushbutton, activating a stepping motor system which simultaneously occluded the test channels and opened the probe channels. Substitution of channels was nearly undetectable except, of course. for the difference between the stimulus parameters in each channel. and was accomplished within a measured 2.5 msec with a I msec discontinuity between channels. The 0 kept the push-button depressed until he detected the presence of any part of the probe stimulus. whereupon he released the button. stopping a msec clock. The clock reading indicated the elapsed time between channel change and the detection of the probe. No attempt was made to record the accuracy of probe detection. Each test trial was followed by a control trial of the same color and pattern conditions. In these trials. however, the 0 initiated the channel change while the test line was still clearly visible. In these control trials the probe stimulus was immediately visible for all probe conditions.
211 EXPERIMENT
I
Earlier studies in this laboratory (Brown er al., 1973) have demonstrated that with achromatic line patterns. there exists an area of adaptation surrounding a faded image which varies as a function of proximity to the faded pattern and which is highly orientation sensitive. These data are easily understood if it is assumed that they reflect the operation of either spatial-opponent or color- and spatial-opponent cells. Experiment 1 was designed to replicate the earlier achromatic data and to partially examine the degree to which the data are independent of color properties of the stimuli. In order to examine orientation sensitivity. a white test line without a background tieId was presented and an identical probe line rotated 0, 5. 10. 15 or 20” from the vertical was presented on probe trials. Probe lines were randomly rotated in either direction from the vertical and the degree of rotation was randomly assigned for each probe. For each 0. the target line was probed a total of 100 times, 20 times for each of the five orientations. An equal number of control trials was completed. To examine wavelength specificity of previous results, a vertical white test line on a white background was probed by dots of variable wavelength on an identical background. The probe dot was always centered on the test line. For each 0, the target was probed a total of 180 times. 20 times for each of the nine wavelengths. Wavelengths were varied randomly over probe trials. An equal number of control trials was conducted.
Results Mean latencies to detect the probes are shown for both OS and for all conditions of viewing in Fig. 3. In Fig. 3, the dotted line portrays mean latency for all control trials. As seen in Fig. 3, detection of the probe dot did not vary as a function of the wavelength of the probe. The pattern of results is highly similar for both OS and to previous data which they were intended to replicate (Brown et al., 1973).
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Fig. 3. Mean latency of probe dot detection (left panel) and probe line detection (right panel) on control (dotted line) and test (solid line) trials for MC (e) and GK (0). Each data point represents the mean of 10 trials. The test line was in all cases white.
Probe uot wavelength,
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Fig. 4. &an latency of probe dot dskxtion a~ 3 function of waveleygth (left panel) and of line probe varying in rotation irght paneit for a test line of 525 nm For OS MC (*I and GK (3). Each data point represents the mean of IO trials. The arrow indicates the wavekngth of the test line.
The involvement of color-opponent or color- and sp~t~~~-o~pon~nt cells in the processing of colared parterns is suggested by a variety of psychophysical data mentioned earlier. Our observation (Cosgrove rt al.. 197-Qthat visibilitv ofstabilized lines varies as a function of the chromaiic aspects of stimulation is also supportive of this same notion. Experiment ?.was designed to estcnd these previous data by mapping the chromatic specificity of adapted receptive fields and assessing orientation specificit)- for chromatic line patterns. In these experimental sessions a monochromatic test line of 375. 53 or 65Onm wx probed by dots of various rvavelsngtbs. and by lines of the same wavelength and size as the test line, but varying in rotation. For each 0 each colored target line was probed 180 rimes. 20 times with dots of each of nine waveIengths. One-half of the trials conducted were control trials, in which the test line was probed when it was not faded from view. ~~~lvelen~th probe conditions were presented in a random sequence. In the same manner each monochromatic test linz was probed by an identical mono~hro~dti~ line a total of 100 times. 20 times with iines of each of fiw or~entat~ons. One-half of the triafs were control trials. orientation conditions were presented in a random sequence white colored test tine conditions were blocked into separate: daily sessions. Background luminances were not used in the orientation probe trials. Rr.%rir.s Figures 4. 5 and 6 show the results of the wavelength and orientation probe trials for the 525. 650 and 175 nm test lines respectively. The dotted line in each graph presents mean latency for control trials. In general. for wavelength test trials the latency to probe dut detection decreased with increasing differences between test and probe waveleng&s. thus forming narrow-band functions centered around the test Iine wavelength in Mach case. There \vas also an increase in
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Fig. 5. Mean latency of probe dot detection as a function of wavelength Ileft panel\ and of line probe varving in rotation (right panel) For a test Iine of 5x3 nm for Ds MC ie,) and GK (0). Each data point reprcsems the mean of ID trials. The arrow indicates the uavekngth of the test line.
latency for probe wavelengths which wre markedly different from the test pattern wavekngths. Both OS showed highly similar results. The right panels in Figs. 3. 5 and 6 show the results of the orientation probes of these same three test line wavelengths. The results for the 53 and 630 nm test lines are similar to the results for the achromatic test line shov.n in Fig. 3. In each case there is a decrease in detection latency with increasing diffxences b~ttvs~n test and probe line ori~nt~~tjons~The same detection latencv vaIue that the white test iine trials showed by 15’ ret&on {Fig. 31was not reached during the 53 and $50 nm test line trials until the probe iine had been rotated by 33’. For the 475 nm test fine there was no substan& change in probe detection latency 8s probe orientation was varied. The probe line was delay-ed by 7W9QO msec for all orientations. The results are. again. highly similar for both OS. EXPEKINENT 3
For reasons which Ml be discussed in detail later. the data from the second experiment are extremely
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of the Dow and Gouras data for monkey cortex. These data. however. only show that there is wavelength selectivity (based upon the dot probe summarized in the left panel of Figs. 4. 5 and 6) and for the test wavelengths. there is orientation sensitivity (the right panel in each figure). They do nor show that a common mechanism which has both wavelength and orientation sensitivity is involved. Taken together, however. the two panels of each figure predict a family of wavelength and orientation functions if such mechanisms are involved. The third study tests these predictions. Test lines of 525 and 650 nm were each probed by lines of various orientations and various wavelengths. Each test line was probed with 0’. 5’. LO’.15’ and 20‘ orientation lines. The 525 nm test line was probed with 5X.575 and 650 nm lines: and the 650 nm test line was probed with 650,600, 550 and 500 nm probe lines. For each 0 each test line was probed 20 times for each wavelength and orientation condition. Orientation conditions were presented in a random sequence. Wavelength conditions for both test and probe stimuli were blocked into separate experimental sessions. Test line and probe line luminances were I.08 mL. No background fields were used.
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The results of Experiment 3 are shown in Figs 7 and S. The results for both OS show that probe detection latencies vary with differences in both the probe and test line orientation and wavelength, in a manner predicted by the wavelength functions shown in Figs. J and 5. From Figs. 4 and 5, one would predict both the height and slope of the functions in Figs. 8 and 9. Given the relatively small number of observations for each point, the stability of these data for the two OS is extremely encouraging. EXPERIMEST
4
The fourth study was carried out to provide data in support of the argument that the data of the first three 525 nm tat
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experiments are due to chromatic. not luminance. aspects of the colored stimuli. The study is based upon the argument that the adapted receptive field ofa faded image should show differential sensitivity- as a function of the varying spectral purity of a probe for any given level of luminance and that different luminances should generate a family of parallel purity functions if these two aspects of color processing are operating independently to produce the phenomena under stud\. This hy-pothesis was tested for a single wavelength. SOI nm. A vertical 501 nm line stimulus of three possible luminance values (0.1. 1.0s and 1OS ftL) vvas probed by a 1.08 ftL dot of variable spectral purity. Probe purity values were located along a line connecting 501 and 610nm and passing through white for a tungsten source in the CIE standard diagram. 501 and 610 nm were chosen as wavelengths because they provide a good estimate of human dichromacy. Spectral purity of the probe dot was varied by adding 25. 50. 75 and 100 per cent white light to the probe dot and subtracting an equal percentage of the colored light. The test line in each luminance condition was presented through Channel 1 of the optical system. No background field was used. The probe dot was presented through Channels 3 and 4 of the optical system (white and color). The per cent of white or colored light in the dot was controlled by neutral density wedges located in each channel. The 501 nm test line was probed IO times for each 0 and for each of nine spectral purity conditions. Probes of 0.24 50. 75 and 100 per cent purity for each of the two wavelength values the probe dot could assume were presented in a random sequence during testing. The three luminance conditions for the test line were blocked into separate experimental sessions for each 0. Heterochromatic luminances were equated among the probe dots by matching each chromatic luminance with a white standard luminance using the
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Fig. 9. Latency of probe dot detection for MC (left panel) and GK (right panelj for a test line of 501 nm and luminance of0.l ftt (e), 1ftL (0). or 10 ftL (a) and probe dots of varying purity. Each data point represents the mean of 10 trials.
criterion of a minimally distinct border between two precisely juxtaposed fields. Rrsrtirs
The results of experiment 3 are shown for each 0 in Fig. 9. which shows that probe detection latencies decrease in a regular manner as the percentage of purity of the probe dot decreases. This is true for all three test line luminance 1eveIs. Detection of a 501 nm probe dot of 100 per cent purity is delayed by about 7OOmsec whereas an equally luminous probe of 0 per cent purity (white) is delayed little beyond reaction time values. This suggests that the adaptation caused by the faded 50 I nm stimulus line is indeed largely chromatic. However, the fact that detection of the 610 nm probe dot is also somewhat delayed by the faded 501 nm test line and that as test line luminance increased the probe detection latency values also increased slightly, suggests that at least some of the adaptation produced by a faded 501 nm line can be attributed to the luminance of the Iine. However, these luminance effects are small when compared to the purity effects.
The original intent of this program of studies was two-fold, to demonstrate that the visibility of stabilized images reflects the operation of central visual processing mechanisms and to utilize this methodology to provide more detailed information concerning the nature of central human pattern and color vision than has heretofore been available. The data presented here are important, we believe, in both respects. fn attempting to understand how we see the color of things. it is wet1 to remember that the n~urophysioIogical data which classify types of central processing units are. undoubtedly. quote incomplete. Dow and Gouras (1973) have shown the existence ofcolor opponent processing units without sensitivity to patterned input. a large proportion of spatial-opponent cells which receive input independently through red and green. but
perhaps not biut. cone channels. and cells which are poorip characterized. but which signal spatial information through the interaction of cone channels. It is not necessary, and. indeed, probably not accurate. to account for all ofour data in terms of the operation of a sinele class of cells. However. these data are strikingly similar to the predictions one would make from the assumption that they reflect the operation of ceils which receive input from cone channels each with its own independent spatial inhibition. In this regard. it is worth noting that the orientation “tuning” was highly similar for achromatic stimuli. red and green lines. but inconclusive for blue lines. Earlier data (Cosgrove et ai.. 1973) have shown that blue lines fade at higher rates than do red or green lines. Dow and Gouras were unabie to find spatial-opponent cells which received input through blue channels. To handle the data from the first experiment. the fact that faded achromatic lines lead to insensitivity to probes of all wavelengths, requires the assumption that achromatic patterns adapt all cone mechanisms equally. It is also possible that we “see” achromatic Imes with mechanisms which receive inputs from all cone channels. but are only sensitive to the contour information carried by these channels. Stabilized image data hav-e rarely bscn utilized as a foundation for the systematic study of central visual processes and have been repeatedly criticized on methodological grounds. Given that it is becoming possible to create stabilized images without the inconvenience ofcontact lenses (Cornsweet and Crane. 1970). we suggest that it may be a fruitful approach to the study of a much broader range of pattern stimulus parameters. as viewed by one or both eyes.
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Bedford R. E. and Wyzecki G. (1957) Axial chromatic aberration of the human eye. 1. Opt. Sot. .+ner. 47, 56C 165. Bovnton R. M. and Kaiser P. K. (1968) Vision. The additiLity law made to work for heterochromatic photometry with bipartite fields. Scimcr .L’..Yl 161, 366-368. Brown D. R.. Schmidt hf. J.. Cosgrove &l. P. and Zuber J. J. (1972) Stabilized images: Further evidence for central pattern processing, Ps)cho~to~nic Sci. 29. 106-105. Brown D. R.. Schmidt M. J.. Fulgham D. D. and Cosgrove XI. P. (1973) Human receptiv-e field characteristics: Probe analysis of stabilized images. l’ision Rrs. 13, 231-2-U. Clowes M. B. (I 962) A note on colour discrimination under conditions of retinal image constraint. Opcica .A%, 9, 65 65. Clowes M. B. and D~tchburn R. W. (1959) An improved apparatus for producing a stabilized retinal image. Optica .kta. 6, 252-965, Cohen W. (19%) Color-perception in the chromatic Ganzfeld. Atn. J. Ps.~hoI. 71, 39@-394. Cornsweet T. N. and Crane H. D. (1970) Servo-controlled infra-red ontometer. J. Ont. Sot. Atnrr. 60, X&j%.
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