Brain Research, 119 (1977) 345-356

345

© Elsevier/North-HollandBiomedicalPress, Amsterdam - Printed in The Netherlands

CENTRAL VISION OF MAN A N D MACAQUE: CONE A N D ROD SENSITIVITY

M. L. J. CRAWFORD Sensory Sciences Center, The University of Texas, Graduate School of Biomedical Sciences, Houston, Texas 77030 (U.S.A.)

(Accepted May 10th, 1976)

SUMMARY The distribution of photopic and scotopic sensitivity of the rhesus monkey has been described for central vision and compared to sensitivities of human observers. For small, brief, green and red test flashes the monkey's sensitivity was comparable to man, but was considerably more sensitive than man's to small, blue (450 nm) test flashes. This superior photopic sensitivity to blue was correlated with a low density of macular pigment. The scotopic sensitivities of man and monkey were comparable, with the distribution of central sensitivity of the monkey being demonstrated to be related to the density of rods around the center fovea.

INTRODUCTION The distribution over the retina of visual sensitivity is not known for the infrahuman primate eye. The acquisition of such information and relating the sensitivity of the eye to the distribution of receptors within the retina would be of considerable value in establishing the adequacy ot the particular primate as a research surrogate visual system for man. Additionally, this information is prerequisite to research efforts to describe retinotopic color and form processing by the unanesthetized monkey. Recently Crawford z described the behavioral techniques for gaining control over the placement of discrete test flashes upon the macaque retina and measuring the detection response of the monkey to such stimulation. This report describes the distribution of sensitivity of the macaque eye for small monochromatic test flashes presented throughout the central 12° of vision, for both photopic and scotopic levels of adaptation. Comparison data collected on the same apparatus and by the same procedures are also presented for human subjects.

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Subjects While three macaque monkeys (Macaca mulatta) have been trained by these procedures, the psychophysical data reported here have been in the main from one macaque and one human subject.

Procedures The training procedures have been described in detail elsewhere z and are presented here in abbreviated form. The monkey was seated before the final eyepiece of the optical system as illustrated in Fig. 1 where the visual display was presented to the eye in Maxwellian view. Within the 18° background field (emanating from a tungsten lamp indicated as B in Fig. 1 at the origin of the middle optical path) the relationship between a test flash (formed along the upper optical path) and a fixation point could be changed so as to present the test flash to any point within the field. Therefore, in order to map the sensitivity at discrete locations on the retina, the fixation point and the test flash were separated by some desired visual angle and the increment-threshold sensitivity of the subjects' eye determined by the method of limits where the intensity of the test

347 flash was reduced on 0.1 log10 unit steps until the animal failed to report the test flash. Threshold was defined as logx0 of the average number of quanta contained in the test flash where the subject failed to report that the flash had occurred on two successive flash presentations. The observing and reporting task of the subject was initiated by depressing and holding down the lever located to the right of the primate chair, whereupon the fixation target, labeled (S1) began to flash repetitively. S1 was a 7' × 15' (of visual angle) bar of light of very low contrast which could be located with peripheral vision but had to be foveally fixated by human subjects in order to resolve its orientation. S1 was flashed with a horizontal orientation as the lever was held down. After a variable interval, S1 changed momentarily to a vertical orientation (labelled S1A). The program required that the monkey release the lever within 500 msec of this change in order to continue in the program. A successful detection of SI A was followed by the opportunity of the monkey to press one of two buttons located to his left. The correct button to be pressed was signalled by a second visual stimulus of 50 msec duration (labeled $2) which preceded the occurrence of S 1A randomly and on half of the trials. If the test flash had occurred, pressing the right button was followed by orange juice delivered through the mouth piece. If the test flash $2 did not occur, pressing of the left button was similarly reinforced. Therefore, by this procedure the monkey had to hold fixation in order to continue in the sequence, remember whether or not a test flash had occurred, and then report correctly if he were to obtain juice. By these procedures, we first measured the photopic threshold sensitivity at intervals along the optical horizontal meridian, on both temporal and nasal sides o f the center fovea. Test flashes of 15' diameter, and 450, 550 and 620 nm were presented for 50 msec upon a photopic (0.472 lumens per steradian; 3000 trolands) white background. Additionally, spectral sensitivity for each of 29 wavelengths was determined for two human subjects who viewed a 2° diameter test flash presented to the central fovea. RESULTS AND DISCUSSION

Results for the light-adapted eye Fig. 2 summarizes the change in threshold sensitivity as a function of eccentricity along the horizontal meridian. The left set of curves are for the monkey, those to the right are for the human subject. For the monkey, it appears that photopic sensitivity for all three of the small colored test flashes decreases by about only 0.4 of a log unit between the central fovea and 6 °, and the decrease appears to be somewhat greater for the nasal side of the fovea. Comparable data for a human observer are shown by the set of curves to the right where a change in sensitivity with eccentricity is greater than for ,*he monkey, being about 0.8 log unit. It is seen that the human possesses higher sensitivity than the monkey to small red and green test flashes only within the central fovea; man and macaque are about equal in sensitivity at about

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Fig. 2. Threshold photopic sensitivity for monkey (left curves) and the human subject (right curves) for points along the central 12° of the horizontal meridian. Test conditions: 18° diameter white background (0.472 lumens/steradian; 3000 trolands); 0.25 ° diameter test flash; 50 msec duration; 450 nm, • - - - • ; 550 nm, © - - - © ; 620 nm, • .. 0 . 2 ° eccentricity, which c o r r e s p o n d s to the o u t e r edge o f the foveal depression 4. M o r e p e r i p h e r a l t h a n this 2 ° location, m a c a q u e a n d m a n have a b o u t the same sensitivity. T h e r e is one n o t a b l e difference between m a n a n d m a c a q u e , which is related to the wavelength o f the test flash. N o t e t h a t the m a c a q u e has greater sensitivity to 450 n m test flashes over the entire central 12 ° field, a l t h o u g h the decrease o f p h o t o p i c sensitivity for the blue test flash with increasing eccentricity does n o t a p p e a r to be p a r t i c ularly different f r o m the curves for the r e d or green test flashes. A s h u m a n p h o t o p i c sensitivity for small blue test flashes varies with b o t h age a n d retinal position, sensitivity at two retinal p o i n t s for the small 450 n m test flash was c o m p a r e d on a y o u n g e r h u m a n subject a n d is shown as small squares at the center a n d 2 ° eccentric locations. M o r e o v e r , when the c o m p l e t e spectral sensitivity curves are c o m p a r e d for the h u m a n subjects (Fig. 3) they are virtually identical t h r o u g h out. Therefore it a p p e a r s t h a t the h u m a n sensitivity for blue shown here is n o t a n o malous, a n d t h a t indeed the m a c a q u e possesses a superiority for detecting small blue test flashes t h r o u g h o u t the central 12° range.

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The obvious difference in blue sensitivity between the macaque and human subjects required further consideration. Inasmuch as the human subject does not appear to have anomalous color sensitivity, another explanation must be sought. The most likely mechanism for modifying the blue sensitivity of the primate eye is by the density of the macular pigment 1,5, which in the human eye has been described as having a peak absorption near 450 nm (the wavelength of our blue test flash). Therefore, if the macaque had less of the pigment in his retina as compared to the human subject, the difference in sensitivity for blue light might be accounted for. Upon the death of the monkey, the eyes were removed and placed in cold Ringer's solution. Within a few minutes the eyes were opened and radial cuts were made through the posterior eye cup to flatten the retina with the ganglion cell side up. Using an 8 mm diameter trephine, a disk of retina, centered upon the fovea, was cut and gently pulled away from the pigment epithelium and then mounted on a microscope slide. A ring of silicone grease on the slide surrounding the disk of retina served to retain the bathing solution and allowed controlled compression of a coverslip so as to flatten and hold in place, yet not distort, the retina. The slide was mounted on a Leitz microscope and viewed through a 6.3 × objective. Using a grating monochromator (Ferrand, Inc.) with a coupled photomultiplier tube (RCA-1P21) and

350 amplifier, the amount of 450 nm light transmitted through a 50 #m diameter section of macula was measured. In primate maculas, the density varies greatly as a function of the distance from the center fovea, in the rhesus monkey being most dense in a narrow ring of about 0.25 mm radius, centered upon the fovea 3. The maximum pigment density in the retina of the monkey used in the present experiment was only 0.05 absorbance units: 1 450 nm transmitted A : log10 ~-; T = 450 nm incident which amounts to only about 10 ~o attenuation of the 450 nm test flash energy. This value is quite low when one considers that in man as much as 90 ~ (A = 1.0) of the 450 nm light is absorbed by the pigment ~. Therefore, the psychophysical differences in sensitivity to the small 450 nm test flash reported here could be related to the low density of macular pigment in the macaque eye,

Photopic sensitivity and cone density A comparison of the change in macaque threshold photopic sensitivity with the eccentric distribution of cones is shown in Fig. 4. The eyes of two adult monkeys were prepared as described later, and the number of cone nuclei were counted in 100/~m intervals beginning at the very center of the fovea and extending for 14

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successive intervals (1.4 mm) along the nasal horizontal meridian. As the cone densities for the two eyes did not differ significantly the counts were pooled and the means and standard deviations of cone density for 12, 2/~m thick, serial sections are shown. Plotted on the same graph and normalized to the central fovea cone counts is the average threshold sensitivity of the monkey for the 15' diameter test flashes. It is seen that over the nasal 5 ° o f the horizontal meridian the decline in photopic sensitivity of the m o n k e y for small test flashes approximates the eccentric decrease in the density of cones.

352

Dark-adapted sensitivity Next we asked the questions : what is the eccentric sensitivity for the rod system ? H o w does it compare to man? We blocked the background adapting field, substituted a dim red, 700 nm fixation bar and tested with a 500 nm 0.5 ° diameter test flash. The monkey or the human subject was dark-adapted for 1 h before data were collected. The 700 nm fixation bar was of such an intensity as to allow foveal fixation whilst it could not be seen with rods. The opposite was the case for the 500 nm test flash, being just detectable by rods, but not by cones. Threshold determinations for vertical scans are shown in Fig. 5A. Here, thresholds are shown at intervals along the vertical meridian from 2° above to 2 ° below the horizontal meridian. Parallel scans were made at intervals both into the nasal and temporal retina (0.5 °, 0.1 ° and 2°). It is seen that the relative loss in sensitivity is greatest along the vertical meridian where sensitivity is decreased by 1.6 log units at the intersection of the vertical and horizontal meridian, i.e. the center of the fovea. The adjacent threshold scans suggest a contour of decreasing sensitivity as the center of the fovea is approached. However, as is seen for the vertical meridian scan, sensitivity remains low at 2 ° below the horizontal meridian, which suggested that the full contour of sensitivity had not been described. We therefore replicated the determinations of dark-adapted sensitivity, only this time we used scans along and parallel to the horizontal meridian, with the area being tested extended an additional 2° into the lower retina. The results are shown on Fig. 5B along with comparable data taken on the experimenter's eye. First, note the threshold scans along the horizontal meridian made symmetrically about the vertical meridian (Fig. 5B). Beginning at 2 ° on the temporal retina, dark-adapted sensitivity falls by 1.6 log units at the center of the fovea, is approached at 0 °, and appears to recover in a symmetrical manner to 2 ° into the nasal retina. Parallel scans into the upper retina suggest a recovery of sensitivity as a function of vertical distance from the fovea. Now note that the change in sensitivity of the horizontal scans into the lower retina is comparable to the observation on the vertical scan estimates where dark-adapted sensitivity remained low (1.6 log units) for 2° below the horizontal meridian; it is seen here that these scans replicate the earlier result. Scans 3° and 4 ° below the horizontal meridian show that the sensitivity has recovered to levels comparable to the upper retina. The greatest loss in sensitivity, 2 log units, is seen at the 0.5 ° lower retinal location. Overall, these scans of threshold darkadapted sensitivity define a contour of decreasing sensitivity which is about 2° in vertical extent, and is centered upon the vertical meridian. In the comparable curves taken on a human subject, the extent of the depression in sensitivity is about the same as that seen for the macaque with the maximum loss in sensitivity centered upon the horizontal meridian. For the human eye, the sensitivity contours appear to describe a circular depression rather than the symmetrical groove shape seen for the macaque.

353

Fig. 6. A horizontal section through the retina of a macaque monkey to illustrate the clear distinction between the dark nuclei for rods (arrows) and the larger and lighter-stained nuclei of cones. Araldite; 2/tm thickness; toludine blue stain. The section is through the side of the foveal depression.

Comparison of the distribution of foveal rods to scotopic sensitivity To determine the agreement between threshold scotopic sensitivity for the macaque eye and the eccentric distribution o f receptors, an eye f r o m a y o u n g m a c a q u e m o n k e y was perfused with a 2 ~ glutaraldehyde, 2 ~ paraformaldehyde solution, embedded in Araldite resin and cut horizontally in 2 / t m thick sections. These sections

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were then stained with toluidine blue. By these procedures, the nuclei for the rods stain darkly and are easily distinguished from cone nuclei throughout the retina as seen in Fig. 6, which illustrates the easily recognizable rod nuclei in a horizontal section taken through the side of the foveal depression. By scaling the distances on the retina according to the report of Rolls and Cowey 4 where 0.246 m m is equal to 1° visual angle, sections were selected which were comparable in retinal location to the locus of measured scotopic sensitivity. These sections were projected and traced, then superimposed to scale upon a set of contours of the scotopic sensitivity for the macaque eye.

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Fig. 7 shows the relationship between rod density and scotopic sensitivity around the center of the fovea. It is seen that the loss in sensitivity converges upon the area centered upon the vertical meridian and just below the horizontal retinal meridian. Clearly there is a good agreement between scotopic sensitivity and rod distribution for the most part throughout center vision. That is, the loss in scotopic sensitivity near the center of the fovea matches the decrease in density or the absence of rods. Since foveation and fixation in the dark-adapted eye are difficult to maintain, even by a human observer, variations in fixation within the fovea could obliterate boundaries around the fovea where rods terminate. However, examination of the rod distribution in serial section around the fovea showed that the rod boundary is neither smooth in radial curvature or abrupt and only in the most central sections (approx. 1° × 1°) are there no rods to be found. More peripheral to this central zone the density of rods increases gradually in the manner shown in Fig. 8 where the average number of rod nuclei 0.1 mm along the nasal horizontal meridian is plotted. In summary, the data presented in this report demonstrate the high correlation between the sensitivity and receptor distribution of the macaque eye, and further support the contention that the macaque visual system is a good research surrogate for visual processing in man. ACKNOWLEDGEMENTS

I am indebted to Sister Clement Johnson for the sectioning and staining; to David Garrett for fabricating the optical system and offering constructive suggestions; to Richard Kelly for collecting most of the psychophysical data; and to a Grant from NIH, EY-00381-07AI, for support.

356 REFERENCES 1 Boettner, E. A. and Wolter, J. R., Transmission of the Human Eye, USAF Aerospace Medicine, Brooks AFB, Texas, 1966, No. 05096-1-F. 2 Crawford, M. L. J., Behavioral control of visual fixation of the rhesus monkey, J. exp. Analyt. Behav., I (1976). 3 Crawford, M. L. J. and Marc, R. E., Radial density of the macular pigment in primates, in preparation. 4 Rolls, E.T. and Cowey, A., Topography of the retina and striate cortex and its relationship to visual acuity in rhesus monkeys and squirrel monkeys, Exp. Brain Res., 10 (1970) 298-310. 5 Ruddock, K. H., Light transmission through the ocular media and in macular pigment and its significance for psychophysical investigation. In D. Jameson and L. M. Hurvich (Eds,), Handbook of Sensory Physiology, VIII4, Springer, New York, 1972, Ch. 17. 6 Wald, G., Human vision and the spectrum, Science, 101 (1945) 653.