Vision RPS. Vol. 17. pp. 1071 to 1074. Pergamon
Press
1977. Printed
in Great
Britain.
THE ROLE OF AFTERIMAGES IN VISUAL SPATIAL AFTEREFFECTS JOHN UHLARIK and MITCHELL BRIGELL Department of Psychology, Kansas State University, Mandate, (Received 8 April 1976; in revised form 3 back
KS 66506, U.S.A. 1977)
Abstract-Displacement aftereffects were obtained with alternate presentations of an inducing figure and its complement which is a finding contrary to an explanation based on retinal afterimages. Both the strength and decay rate of the aftereffects obtained for this condition were comparable to those found when a single inducing figure was presented for the entire adaptation period. For aI1 conditions a stronger and longer aftereffect resulted for binocular stimulus presentation than was the case for dichoptic presentation. The results suggest that central processes, sensitive to relative figureground contrast renardless of polarity, are responsible for both the temporal protraction of the inducing figure and the diiplacement of the iest figure.
It is well known that visual stimuli presented in ciose spatial proximity can induce shifts in the apparent locations of the stimuli. The term “figural aftere%cV’ (Kohler and Wallach, 1944) has been used to describe the apparent displacement of the spatial position of a test stimulus due to the prior presentation of an inducing stimulus in close proximity on the retina. The present study was designed to examine the relation between the luminance characteristics of the ipducing figure and the manner in which it displaces the test figure. If a retinal afterimage is the process responsible for the temporal protraction of the inducing figure, then the net luminance distribution resulting from viewing this figure should determine both the magnitude and decay characteristics of displacement aftereffect. In other words, a weak afterimage of the inducing figure should produce a weak and rapidly decaying influence (displacement) on the test figure. Temporal alternation of an inducing figure with its complement (i.e. photo~aphi~ negative) would tend to produce a homogeneously bleached retina due to the relatively slow rate of regeneration of photopigments. The presence of a strong fig& aftereffect in such a condition would imply that the residual neural effect of an inducing figure is based on relative contrast (i.e. contour) information per se, and not simply the net luminance distribution of the inducing figure. In this context relative contrast is defined as:
is important to note that two luminance distributions of opposite polarity have equivalent relative contrast as long as the differences between the maximum and minimum luminances are equal. Thus, there would be high relative contrast at all times during an a~p~tion period ~vol~ng alternate exposure of a luminance distribution and its complement, even though the net sum, over time, of these distributions would be zero. If a process relying upon relative contrast is responsible for the temporal protraction of the inducing figure, then both a condition involving alternation of an inducing figure with its complement It
and a condition involving constant exposure of a single luminance distribution should yield similar inducing effects, resulting in aftereffects with similar decay rates. In addition, comparisons between binocular and dichoptic presentation of the inducing and test figures were also investigated in order to specify further the nature and locus of the interaction between the components of spatial aftereffects. METHOD Subjects
The 12 subjects were undergraduate students enrolled in an introductory psychology course at Kansas State University.
The inducing and test figures are shown in Fig. 1. The inducing figure consisted of either a white square on a black background or black square on a white background. A fixation point was situated to the left and below the lower left-hand cbrner of the inducing square. The whiteon-black and black-on-white inducing figures were presented in separate chmnels of an Iconix 6131 tachistoscope. A pre-exposure fixation point was displayed in a third channel. The tachistoscopic field of view subtended a visual angle of 5” vertically and 8” horizontally. The luminance of the white areas of the inducing stimuli was 31.4 mL and was l.OmL for the black areas. The test figure consisted of two vertical lines (Fig. lb) displayed on the CRT of a 561a Tektronix oscilloscooe . co&a&g a fast decay phosphor (p-11). The luminance of the test lines was estimated at 0.6mL and that of the background was 0.3 mL. The length of each test line was the same as a side of the inducing square, and the fixation point was situated halfway between, and to the left of, the vertical lines. The high-contrast inducing figures, lowcontrast test figure, and the interfigural distances between the upper test line and the inducing figure (0.13”) were chosen to produce a maximal displa~ment aftereffect (cf. Poliack, 1958). The observer’s task was to make a vernier aligmnent of the two vertical lines by adjusting the horizontal position of the lower line. The experimenter monitored the voltage applied to this signal on a digital voltage meter which provided adequate resolution to measure the aftereffects. Viewing distance was controlled by having the observers use a viewing hood mounted on the osciIloscope.
1071 V.R.17/g--
P
I072
J~FIN UHLARIK and MITcvfm. BRIGI-LL
Fig. 1. Schematic representation of inducing (A) and test (B) figures. (Field size is not drawn to scale.)
Design and procedure
A mixed factorial design consisting of one between-subjects and three within-subjects variables was used. Ocular mode of presentation was the between-subjects independent variable. For half of the subjects, presentation was ~jc~opti~. In this condition, the inducing figure was pres” ented to one eye and the test figure to the other eye. The remaining subjects received binocular presentation of both the test and inducing figures. Temporal mode of presentation of the inducing figure was a within-subjects independent variable. This variable involved temporal variation of the figure-ground luminance relationships of the inducing figure. In the constant condition, a single inducing figure was presented for 60 see and was always either white-on-black or black-onwhite. In the alternation condition the white-on-black and biack-on-white inducing figures were alternated every 500 msec for 60 repetitions so that the total &xposure time was equivalent to that of the constant condition. Intermittent presentation of either the black-on-white or the whiteon-black inducing figure was included to control for the differences in the number of onsets and offsets between the alternation and constant conditions. The intermittent condition consisted of ~ternatety presenting one of the inducing stimuli for 460msec and a homogeneous neutral gray “off” field for 4Omsec. Thus the intermittent condition consisted of 65 transient repetitions, similar to the alternation condition, and 59.8 set of exposure to a single inducing figure luminance dis~~bution which was comparable to the constant condition. For both the intermittent and constant conditions, each subject was exposed to separate trials using the bIack”on-white and the white-onblack inducing figures. In addition, the order or presentation of the alternating inducing stimuli was counterbalanced within each subject, so that on one alternation trial the black-on-white inducing figure was presented first, and on the other trial the white-on-black fignre was the first in the alternation sequence. Thus each observer was run in each of six dSerent conditions. A different random order of these six trials was used for each observer.
Each trial consisted of five premeasures of vernier acuity, an inter~lat~ exposure period to one of the inducing figure conditions, and five postmeasures (decay intervals) taken at 15-set intervals following termination of exposure to the inducing figure. The adjustable (lower) comparison line was randomly placed at one of three starting positions [O.P, 1.0 and 1.1” to the left of the fixed (upper) test line] prior to each measure. Each observer was dark adapted for 10 min prior to each trial, and only two trials were run in an experimental session. Sessions were separated by a minimum of 24 hr for any given subject.
RESULTS
A dispIa~ment aftereffect was repre~nted as a shift in the apparent location of the test line away from the position of the previously presented inducing figure. The difference between each postmeasure and the mean of the five premeasures of vernier acuity (for a given trial) was obtained for each decay interval. These difference scores were signed such that positive entries indicated leftward shifts of the adjustable lower line, and hence the presence of a displacement aftereffect. Figure 2 illustrates the mean displacement aftereffect as a function of the five decay intervals for aiternation, constant, and intermittent presentation of the inducing figure. Table 1 presents these same mean aftereffects and their 95% cont3dence intervals. Tests based on the confidence intervals indicated that the displacement aftereffects were reliably different from zero for the 0- and lS-set decay intervals of the constant and alternation conditions, and for the O-set decay interval of the intermittent condition. The mean aftereffects for ail other decay intervals were not reliably unrent from zero. Decay of the strength of the
1073
Afterimages and displacement aftereffects
W
CONSTANT
H
ALTERNATlON
O---O
INTERMITTENT
2.5
2.5
2.0
-
1.5
-
: z
l \
tj,
8lNOCULAR
M
DICHQPTIC
2.0
z ‘\ ‘.
&
‘. ‘0
ii! 5
5
*\
a
1.0
-
‘\\ ‘\
F 4 f w =
‘\ 0.5
‘\
_\
‘\\ ‘7,
0.0
*. ‘x - 0.5
Q-
-
1
I
0
15 DECAY
1
I
30
15
INTERVAL
-___o
L 60
’
I
,
I
I
0
15
30
45
60
DECAY
(SEC)
INTERVAL
ISECI
Fig. 2. The effect of the temporal mode of presentation of the inducing tigure (i.e. constant, alternation and intermittent) on mean displacement of the test tlgure as a function of the decay interval.
Fig. 3. The effect of ocular mode of presentation of the inducing figure (i.e. binocular and dichoptic) on mean displacement of the test figure as a function of the decay interval.
aftereffect is also indicated in an analysis of variance which showed a reliable overall main effect for decay interval, F(4,40) = 9.62, P -C 0.01. Neither the main effect nor any of the interactions involving the temporal mode of presentation of the inducing figure were significant. It should be noted that differential decay rates for the alternation, constant and/or intermittent conditions would have been indicated by a signiticant decay interval by temporal mode of presentation interaction which, OFcourse, did not obtain. Rather, Fig. 2 shows that the decay functions are essentially parallel for these three conditions. Table 1 also supports the notion that there were no reliable differences between the three temporal modes of presentation of the inducing figure. For any decay interval, the 95% confidence intervals associated with the mean aftereffects for the constant, alternation, and intermittent conditions overlapped. Figure 3 shows the mean displacement aftereffect as a function of the five decay intervals for binocular and dichoptic presentation (occur mode) of the inducing and test figures. Overah, the mean strength of me aftereffect was greater for binocular presentation (X = l.~~n} than was the case for dichoptic presentation (X = 0.1 mm). This main effect for ocular mode of presentation was statistically significant, F (1,lO) = 12.10, P < 0.01, but none of the higher order
interactions involving this variable were significant. In addition, the mean aftereffects for the binocular condition were reliably different from zero for the O-, 15, and 30-set decay intervals (based on 95% con% dence intervals); whereas the mean aftereffects For the dichoptic condition were reliably different from zero only for the 0-set decay interval. DISCUSSlON The results of the present experiment suggest that a retinal afterimage of the inducing figure is not the process that controls the temporal decay of spatial aftereffects. Strong and prolonged aftereffects were found in the alternation condition. The decay rate of the aftereffect in this condition was equivalent to that of the constant condition despite the relatively homogeneous pattern of retinal bleaching produced in the former condition. Although not statisticaily significant, the displacement aftereffects reported for the constant condition were consistently larger than those reported for either the alternation or intermittent conditions. However, the ma~itude of these differences was much smaller than an aFterimage process would suggest. Furthermore, the aftereffects in the intermittent condition were not systematically stronger than those in the alternation condition.
Table 1. Mean displacement aftereffects(min) and 95% confidence intervals as a function of decay interval for the constant, atternation and inte~ittent
conditions
Decay interval (set)
Constant
Temporal mode Alternation
Intermittent
0 15 30 45 60
2.2 (k 1.2) 2.3 (k 1.3) 1.1 (k1.5) l.O($-1.2) 0.3 ( f 0.9)
1.9(&1.2) 1.4(+1.2) 0.8 (+ 1.2) -0.2(&1.4) -1.1(*1.3)
1.8(+1.1) 1.2(&1.3) 0.1 (&LO) -0.5(+1.4) -0.5 (kO.7)
1073
JOH\ UttL.mtK and MITUICLL
Ganz (1966a,b) has proposed that spatial displacement aftereffects are due to a simultaneous lateral interaction between the neural representation of the test figure and the inhibitory effects of the afterimage of the inducing figure. The essential difference between this theory and those preceding it (Kohler and Wallath. 1944: Osgood and Heyer. 1952; Day. 1962; Deutch. 1964) is that Ganz proposed that protraction of the inducing figure and displacement of the test figure are accounted for by two different processes. Specifically, Ganz’s theory employs the afterimage of the inducing figure as the process which bridges the time gap and lateral inhibition as the process which produces the displacement. According to Ganz. “The inspection of a figure, even momentarily, induces rrtirml procrssrs which decay exponentially in time.. As the eye recovers, the afterimage decays exponentially. indicating that the residual process-the neural processes responsible for light adaptationis decaying through time. As the afterimage weakens. its power as an inducing figure weakens and the displacement magnitude decays” (1966b, p. 163. italics added). The results of the present study indicate that local heterogeneities in retinal light and dark adaptation are not involved in temporally protracting the inducing figure, and any process, at any level in the visual system, that attempts to account for the residual effects of the inducing figure cannot depend upon either the state of retinal adaptation or the coding of luminance information per sr. Rather, the present findings suggest that both the initial magnitude and decay rate of displacement aftereffects rely only on the presence of relative contrast in the inducing stimulus. For Ganz’s (1966a,b) theory to remain viable, it would have to be modified so that it is not the net luminance, but rather the relative contrast (independent of polarity) of the inducing stimulus that is coded at some level in the visual svstem. The results of the present study also showed that dichoptic presentation of the inducing and test figure resulted in a weaker displacement aftereffect than was the case when both figures were presented binocularly. Specifically. both the initial displacement was smaller and the decay rate faster in the dichoptic condition This pattern of results has generally led researchers to conclude that peripheral processing is involved. However, Andrews (1972) has suggested that a reduced dichoptic aftereffect does not necessarily imply that precortical processing is involved. Rather, such results could indicate the absence of participation of cortical cells with monocular receptive fields. Specifically. the involvement of cortical cells. some having monocular and others having binocular excitatory inputs. could account for the decrement in the strength of the aftereffect that obtained for the dichoptic mode of presentation in the present study. Our results indicate that the residual neural activity arising from the inducing figure could be due to a process that is sensitive only to relative contrast and not to the polarity of the luminance distribution. Insofar as this distribution of neural activity could interact directly, via lateral inhibition, with the neural activity produced by the test figure. Ganz’s (1966a) postulation of separate neural loci for temporal pro-
BKtot.LL
traction and displacement functions does not seem warranted. Rather. it seems reasonable that the two effects could be caused by the interactions of two distrihutions of neural activity at a single neural locus. ~.(.l\/lOWledy~int~rlts-This research was supported by the Bureau of General Research, Kansas State University, and was carried out while Mitchell Brigell was receiving support from NIMH Experimental Training Grant STOI-MH 08359. The authors would like to thank Paul Goldhorn for his assistance in conducting the experiment. REFERENCES
Andrews D. P. (1973) The site of the adautation shown in figural aftereffects. c’isio~ Rrs. 12, 3065-1067. Day R. H. (1962) Excitatory and inhibitory processes as the basis of contour shift and negative aftereffect. PSJBcltologia 5. 185-193. Deutsch J. A. (1964) Neurophysiological contrast phenomena and figural aftereffects. Psychol. Rro. 71. 19-26. Ganz L. (1966a) The mechanism of the figural aftereffect. Psycho/. Reo. 73, 12X-150. Ganz L. (lY66b) Is the figural aftereffect an aftereffect? A review of its intensity onset, decay. and transfer characteristics. Psychol. Bull. 66. ISI 165. Kohler W. and Wallach H. (1944) Figural aftereffects: an investigation of visual processes. Ani. pliil. Sot. Proc. 88, 2699357. Osgood C. E. and Heyer A. W. (1952) A new interpretation of the fieural-aftereffect. Psrchol. Rev. 59. 9X-l 18 Pollack Ry H. (1958) Figural aftereffects. Quantitative studies of displacement. .4ust. J. PsychoI. 10. 269-277.
APPENDIX COMPLETE ANALYSIS OF VARIANCE OF MEAN DISPLACEMENT AFTEREFFECTS
Source Between subjects Ocular mode Between-subjects error Within subjects Temporal mode OM x TM Error Figure-ground contrast OM x FGC Error Decay interval OM x Dl Error TM x FGC OM x TM x FGC Error TM x DI OM x TM x DI Error FGC x DI OM x FGC x DI Error TM x FGC x DI OM x TM x FGC x DI Error * P < 0.01. t P < 0.05.
d.f.
MS
F
I
5328.20 440.34
12.10*
10 2 2 20 1 1 IO
1180.41 35.52 400.84
2.94
Press
1977. Printed
in Great
Britain.
THE ROLE OF AFTERIMAGES IN VISUAL SPATIAL AFTEREFFECTS JOHN UHLARIK and MITCHELL BRIGELL Department of Psychology, Kansas State University, Mandate, (Received 8 April 1976; in revised form 3 back
KS 66506, U.S.A. 1977)
Abstract-Displacement aftereffects were obtained with alternate presentations of an inducing figure and its complement which is a finding contrary to an explanation based on retinal afterimages. Both the strength and decay rate of the aftereffects obtained for this condition were comparable to those found when a single inducing figure was presented for the entire adaptation period. For aI1 conditions a stronger and longer aftereffect resulted for binocular stimulus presentation than was the case for dichoptic presentation. The results suggest that central processes, sensitive to relative figureground contrast renardless of polarity, are responsible for both the temporal protraction of the inducing figure and the diiplacement of the iest figure.
It is well known that visual stimuli presented in ciose spatial proximity can induce shifts in the apparent locations of the stimuli. The term “figural aftere%cV’ (Kohler and Wallach, 1944) has been used to describe the apparent displacement of the spatial position of a test stimulus due to the prior presentation of an inducing stimulus in close proximity on the retina. The present study was designed to examine the relation between the luminance characteristics of the ipducing figure and the manner in which it displaces the test figure. If a retinal afterimage is the process responsible for the temporal protraction of the inducing figure, then the net luminance distribution resulting from viewing this figure should determine both the magnitude and decay characteristics of displacement aftereffect. In other words, a weak afterimage of the inducing figure should produce a weak and rapidly decaying influence (displacement) on the test figure. Temporal alternation of an inducing figure with its complement (i.e. photo~aphi~ negative) would tend to produce a homogeneously bleached retina due to the relatively slow rate of regeneration of photopigments. The presence of a strong fig& aftereffect in such a condition would imply that the residual neural effect of an inducing figure is based on relative contrast (i.e. contour) information per se, and not simply the net luminance distribution of the inducing figure. In this context relative contrast is defined as:
is important to note that two luminance distributions of opposite polarity have equivalent relative contrast as long as the differences between the maximum and minimum luminances are equal. Thus, there would be high relative contrast at all times during an a~p~tion period ~vol~ng alternate exposure of a luminance distribution and its complement, even though the net sum, over time, of these distributions would be zero. If a process relying upon relative contrast is responsible for the temporal protraction of the inducing figure, then both a condition involving alternation of an inducing figure with its complement It
and a condition involving constant exposure of a single luminance distribution should yield similar inducing effects, resulting in aftereffects with similar decay rates. In addition, comparisons between binocular and dichoptic presentation of the inducing and test figures were also investigated in order to specify further the nature and locus of the interaction between the components of spatial aftereffects. METHOD Subjects
The 12 subjects were undergraduate students enrolled in an introductory psychology course at Kansas State University.
The inducing and test figures are shown in Fig. 1. The inducing figure consisted of either a white square on a black background or black square on a white background. A fixation point was situated to the left and below the lower left-hand cbrner of the inducing square. The whiteon-black and black-on-white inducing figures were presented in separate chmnels of an Iconix 6131 tachistoscope. A pre-exposure fixation point was displayed in a third channel. The tachistoscopic field of view subtended a visual angle of 5” vertically and 8” horizontally. The luminance of the white areas of the inducing stimuli was 31.4 mL and was l.OmL for the black areas. The test figure consisted of two vertical lines (Fig. lb) displayed on the CRT of a 561a Tektronix oscilloscooe . co&a&g a fast decay phosphor (p-11). The luminance of the test lines was estimated at 0.6mL and that of the background was 0.3 mL. The length of each test line was the same as a side of the inducing square, and the fixation point was situated halfway between, and to the left of, the vertical lines. The high-contrast inducing figures, lowcontrast test figure, and the interfigural distances between the upper test line and the inducing figure (0.13”) were chosen to produce a maximal displa~ment aftereffect (cf. Poliack, 1958). The observer’s task was to make a vernier aligmnent of the two vertical lines by adjusting the horizontal position of the lower line. The experimenter monitored the voltage applied to this signal on a digital voltage meter which provided adequate resolution to measure the aftereffects. Viewing distance was controlled by having the observers use a viewing hood mounted on the osciIloscope.
1071 V.R.17/g--
P
I072
J~FIN UHLARIK and MITcvfm. BRIGI-LL
Fig. 1. Schematic representation of inducing (A) and test (B) figures. (Field size is not drawn to scale.)
Design and procedure
A mixed factorial design consisting of one between-subjects and three within-subjects variables was used. Ocular mode of presentation was the between-subjects independent variable. For half of the subjects, presentation was ~jc~opti~. In this condition, the inducing figure was pres” ented to one eye and the test figure to the other eye. The remaining subjects received binocular presentation of both the test and inducing figures. Temporal mode of presentation of the inducing figure was a within-subjects independent variable. This variable involved temporal variation of the figure-ground luminance relationships of the inducing figure. In the constant condition, a single inducing figure was presented for 60 see and was always either white-on-black or black-onwhite. In the alternation condition the white-on-black and biack-on-white inducing figures were alternated every 500 msec for 60 repetitions so that the total &xposure time was equivalent to that of the constant condition. Intermittent presentation of either the black-on-white or the whiteon-black inducing figure was included to control for the differences in the number of onsets and offsets between the alternation and constant conditions. The intermittent condition consisted of ~ternatety presenting one of the inducing stimuli for 460msec and a homogeneous neutral gray “off” field for 4Omsec. Thus the intermittent condition consisted of 65 transient repetitions, similar to the alternation condition, and 59.8 set of exposure to a single inducing figure luminance dis~~bution which was comparable to the constant condition. For both the intermittent and constant conditions, each subject was exposed to separate trials using the bIack”on-white and the white-onblack inducing figures. In addition, the order or presentation of the alternating inducing stimuli was counterbalanced within each subject, so that on one alternation trial the black-on-white inducing figure was presented first, and on the other trial the white-on-black fignre was the first in the alternation sequence. Thus each observer was run in each of six dSerent conditions. A different random order of these six trials was used for each observer.
Each trial consisted of five premeasures of vernier acuity, an inter~lat~ exposure period to one of the inducing figure conditions, and five postmeasures (decay intervals) taken at 15-set intervals following termination of exposure to the inducing figure. The adjustable (lower) comparison line was randomly placed at one of three starting positions [O.P, 1.0 and 1.1” to the left of the fixed (upper) test line] prior to each measure. Each observer was dark adapted for 10 min prior to each trial, and only two trials were run in an experimental session. Sessions were separated by a minimum of 24 hr for any given subject.
RESULTS
A dispIa~ment aftereffect was repre~nted as a shift in the apparent location of the test line away from the position of the previously presented inducing figure. The difference between each postmeasure and the mean of the five premeasures of vernier acuity (for a given trial) was obtained for each decay interval. These difference scores were signed such that positive entries indicated leftward shifts of the adjustable lower line, and hence the presence of a displacement aftereffect. Figure 2 illustrates the mean displacement aftereffect as a function of the five decay intervals for aiternation, constant, and intermittent presentation of the inducing figure. Table 1 presents these same mean aftereffects and their 95% cont3dence intervals. Tests based on the confidence intervals indicated that the displacement aftereffects were reliably different from zero for the 0- and lS-set decay intervals of the constant and alternation conditions, and for the O-set decay interval of the intermittent condition. The mean aftereffects for ail other decay intervals were not reliably unrent from zero. Decay of the strength of the
1073
Afterimages and displacement aftereffects
W
CONSTANT
H
ALTERNATlON
O---O
INTERMITTENT
2.5
2.5
2.0
-
1.5
-
: z
l \
tj,
8lNOCULAR
M
DICHQPTIC
2.0
z ‘\ ‘.
&
‘. ‘0
ii! 5
5
*\
a
1.0
-
‘\\ ‘\
F 4 f w =
‘\ 0.5
‘\
_\
‘\\ ‘7,
0.0
*. ‘x - 0.5
Q-
-
1
I
0
15 DECAY
1
I
30
15
INTERVAL
-___o
L 60
’
I
,
I
I
0
15
30
45
60
DECAY
(SEC)
INTERVAL
ISECI
Fig. 2. The effect of the temporal mode of presentation of the inducing tigure (i.e. constant, alternation and intermittent) on mean displacement of the test tlgure as a function of the decay interval.
Fig. 3. The effect of ocular mode of presentation of the inducing figure (i.e. binocular and dichoptic) on mean displacement of the test figure as a function of the decay interval.
aftereffect is also indicated in an analysis of variance which showed a reliable overall main effect for decay interval, F(4,40) = 9.62, P -C 0.01. Neither the main effect nor any of the interactions involving the temporal mode of presentation of the inducing figure were significant. It should be noted that differential decay rates for the alternation, constant and/or intermittent conditions would have been indicated by a signiticant decay interval by temporal mode of presentation interaction which, OFcourse, did not obtain. Rather, Fig. 2 shows that the decay functions are essentially parallel for these three conditions. Table 1 also supports the notion that there were no reliable differences between the three temporal modes of presentation of the inducing figure. For any decay interval, the 95% confidence intervals associated with the mean aftereffects for the constant, alternation, and intermittent conditions overlapped. Figure 3 shows the mean displacement aftereffect as a function of the five decay intervals for binocular and dichoptic presentation (occur mode) of the inducing and test figures. Overah, the mean strength of me aftereffect was greater for binocular presentation (X = l.~~n} than was the case for dichoptic presentation (X = 0.1 mm). This main effect for ocular mode of presentation was statistically significant, F (1,lO) = 12.10, P < 0.01, but none of the higher order
interactions involving this variable were significant. In addition, the mean aftereffects for the binocular condition were reliably different from zero for the O-, 15, and 30-set decay intervals (based on 95% con% dence intervals); whereas the mean aftereffects For the dichoptic condition were reliably different from zero only for the 0-set decay interval. DISCUSSlON The results of the present experiment suggest that a retinal afterimage of the inducing figure is not the process that controls the temporal decay of spatial aftereffects. Strong and prolonged aftereffects were found in the alternation condition. The decay rate of the aftereffect in this condition was equivalent to that of the constant condition despite the relatively homogeneous pattern of retinal bleaching produced in the former condition. Although not statisticaily significant, the displacement aftereffects reported for the constant condition were consistently larger than those reported for either the alternation or intermittent conditions. However, the ma~itude of these differences was much smaller than an aFterimage process would suggest. Furthermore, the aftereffects in the intermittent condition were not systematically stronger than those in the alternation condition.
Table 1. Mean displacement aftereffects(min) and 95% confidence intervals as a function of decay interval for the constant, atternation and inte~ittent
conditions
Decay interval (set)
Constant
Temporal mode Alternation
Intermittent
0 15 30 45 60
2.2 (k 1.2) 2.3 (k 1.3) 1.1 (k1.5) l.O($-1.2) 0.3 ( f 0.9)
1.9(&1.2) 1.4(+1.2) 0.8 (+ 1.2) -0.2(&1.4) -1.1(*1.3)
1.8(+1.1) 1.2(&1.3) 0.1 (&LO) -0.5(+1.4) -0.5 (kO.7)
1073
JOH\ UttL.mtK and MITUICLL
Ganz (1966a,b) has proposed that spatial displacement aftereffects are due to a simultaneous lateral interaction between the neural representation of the test figure and the inhibitory effects of the afterimage of the inducing figure. The essential difference between this theory and those preceding it (Kohler and Wallath. 1944: Osgood and Heyer. 1952; Day. 1962; Deutch. 1964) is that Ganz proposed that protraction of the inducing figure and displacement of the test figure are accounted for by two different processes. Specifically, Ganz’s theory employs the afterimage of the inducing figure as the process which bridges the time gap and lateral inhibition as the process which produces the displacement. According to Ganz. “The inspection of a figure, even momentarily, induces rrtirml procrssrs which decay exponentially in time.. As the eye recovers, the afterimage decays exponentially. indicating that the residual process-the neural processes responsible for light adaptationis decaying through time. As the afterimage weakens. its power as an inducing figure weakens and the displacement magnitude decays” (1966b, p. 163. italics added). The results of the present study indicate that local heterogeneities in retinal light and dark adaptation are not involved in temporally protracting the inducing figure, and any process, at any level in the visual system, that attempts to account for the residual effects of the inducing figure cannot depend upon either the state of retinal adaptation or the coding of luminance information per sr. Rather, the present findings suggest that both the initial magnitude and decay rate of displacement aftereffects rely only on the presence of relative contrast in the inducing stimulus. For Ganz’s (1966a,b) theory to remain viable, it would have to be modified so that it is not the net luminance, but rather the relative contrast (independent of polarity) of the inducing stimulus that is coded at some level in the visual svstem. The results of the present study also showed that dichoptic presentation of the inducing and test figure resulted in a weaker displacement aftereffect than was the case when both figures were presented binocularly. Specifically. both the initial displacement was smaller and the decay rate faster in the dichoptic condition This pattern of results has generally led researchers to conclude that peripheral processing is involved. However, Andrews (1972) has suggested that a reduced dichoptic aftereffect does not necessarily imply that precortical processing is involved. Rather, such results could indicate the absence of participation of cortical cells with monocular receptive fields. Specifically. the involvement of cortical cells. some having monocular and others having binocular excitatory inputs. could account for the decrement in the strength of the aftereffect that obtained for the dichoptic mode of presentation in the present study. Our results indicate that the residual neural activity arising from the inducing figure could be due to a process that is sensitive only to relative contrast and not to the polarity of the luminance distribution. Insofar as this distribution of neural activity could interact directly, via lateral inhibition, with the neural activity produced by the test figure. Ganz’s (1966a) postulation of separate neural loci for temporal pro-
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traction and displacement functions does not seem warranted. Rather. it seems reasonable that the two effects could be caused by the interactions of two distrihutions of neural activity at a single neural locus. ~.(.l\/lOWledy~int~rlts-This research was supported by the Bureau of General Research, Kansas State University, and was carried out while Mitchell Brigell was receiving support from NIMH Experimental Training Grant STOI-MH 08359. The authors would like to thank Paul Goldhorn for his assistance in conducting the experiment. REFERENCES
Andrews D. P. (1973) The site of the adautation shown in figural aftereffects. c’isio~ Rrs. 12, 3065-1067. Day R. H. (1962) Excitatory and inhibitory processes as the basis of contour shift and negative aftereffect. PSJBcltologia 5. 185-193. Deutsch J. A. (1964) Neurophysiological contrast phenomena and figural aftereffects. Psychol. Rro. 71. 19-26. Ganz L. (1966a) The mechanism of the figural aftereffect. Psycho/. Reo. 73, 12X-150. Ganz L. (lY66b) Is the figural aftereffect an aftereffect? A review of its intensity onset, decay. and transfer characteristics. Psychol. Bull. 66. ISI 165. Kohler W. and Wallach H. (1944) Figural aftereffects: an investigation of visual processes. Ani. pliil. Sot. Proc. 88, 2699357. Osgood C. E. and Heyer A. W. (1952) A new interpretation of the fieural-aftereffect. Psrchol. Rev. 59. 9X-l 18 Pollack Ry H. (1958) Figural aftereffects. Quantitative studies of displacement. .4ust. J. PsychoI. 10. 269-277.
APPENDIX COMPLETE ANALYSIS OF VARIANCE OF MEAN DISPLACEMENT AFTEREFFECTS
Source Between subjects Ocular mode Between-subjects error Within subjects Temporal mode OM x TM Error Figure-ground contrast OM x FGC Error Decay interval OM x Dl Error TM x FGC OM x TM x FGC Error TM x DI OM x TM x DI Error FGC x DI OM x FGC x DI Error TM x FGC x DI OM x TM x FGC x DI Error * P < 0.01. t P < 0.05.
d.f.
MS
F
I
5328.20 440.34
12.10*
10 2 2 20 1 1 IO
1180.41 35.52 400.84
2.94
