Human Orientation Discrimination: Changes With Eccentricity in Normal and Amblyopic Vision

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1 Human Orientation Discrimination: Changes With Eccentricity in Normal and Amblyopic Vision Erik Vandenbussche, Rufin Vogels, and Guy A. Orban The authors measured orientation discrimination of a single line as a function of eccentricity, line length, and standard orientation. Orientation discrimination improved with increasing line length at all eccentricities. The shortest length at which orientation discrimination was optimal increased with eccentricity. When a line length was used for which discrimination was optimal at all eccentricities, it was found that orientation discrimination performance changed little with increasing eccentricity. Under the same conditions, the oblique effect in orientation discrimination decreased with increasing eccentricity. Similar results were also obtained in both eyes of strabismic amblyopes. The difference between the just noticeable differences in orientation of the amblyopic and nonamblyopic eye decreased with increasing line length for central vision. This interocular difference also decreased with increasing eccentricity. The absence of the oblique effect in orientation discrimination at large eccentricities support the hypothesis that the area 17 S-cell orientation anisotropy underlies the oblique effect in orientation discrimination of long lines since this sensorial anisotropy is limited to the central visual field. Invest Ophthalmol Vis Sci 27: , 1986 Human line orientation discrimination shows meridional variations: just noticeable differences (JNDs) in orientation are smaller when measured at horizontal or vertical standard orientations than at oblique standard orientations.' This type of oblique effect is extremely robust 2 and has been linked to the anisotropy in preferred orientation of a particular class of area 17 cells: the S-cells. These S-cells have narrow receptive fields with almost completely nonoverlapping ON and OFF subregions and show considerable length summation. 3 Both in cat and monkey, this class of cells has the narrowest orientation tuning and is the only class to have a meridional anisotropy in preferred orientation. 3 " 5 In order to gain further support for this analogy type 6 of linking hypothesis, we have derived predictions from the physiological properties of S-cells for line orientation discrimination. In humans 1 as well as in cats, 7 meridional variations in line orientation discrimination are larger for long than for short lines, as predicted from the length summation of S-cells. The present report deals with another prediction derived from the properties of S-cells. In cat as well as in monkey, the bias in preferred orientation of S-cells towards horizontal and vertical orientations vanishes with increasing eccentricity. 3 ' 48 " The first aim of the pres- From the Laboratorium voor Neuro-en Psychofysiologie, KU- Leuven, Campus Gasthuisberg, Belgium. Submitted for publication: April 16, Reprint requests: G. A. Orban, MD, PhD, Laboratorium voor Neuro-en Psychofysiologie, KULeuven, Campus Gasthuisberg, B Leuven, Belgium. ent experiments was to test the prediction that the oblique effect in orientation discrimination in humans decreases with increasing eccentricity. The second aim of this study was to compare the steepness of changes in JNDs in orientation and in acuity with increasing eccentricity. A large difference in the dependence of orientation discrimination and of acuity on distance from the fixation point would give further support to our view that different neuronal mechanisms underlie acuity and contrast sensitivity on one hand and orientation discrimination on the other.'' The third aim of the present study was to compare the changes in orientation discrimination with eccentricity in normal and amblyopic subjects. In a previous study 2 we showed that orientation discrimination of long lines is little affected in amblyopia. Only orientation discrimination around the principal orientations is significantly affected in the amblyopic eye. In that study, eye position was not precisely controlled and, although unlikely, eccentric fixation could have accounted for the results. The present experiments allow us to rule out this explanation. They rather show that, at least qualitatively, central vision of strabismic amblyopes is similar to the peripheral visual system of normal subjects. Stimuli and Apparatus Materials and Methods A single 0.25 wide light bar was backprojected on a circular screen positioned at 114 cm from the ob- 207

2 238 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE'/ February 1986 Vol. 27 Table 1. Ophthalmological data of the amblyopic subjects Refraction Snellen acuity Strabism Fixation Subject C.G. Nonamblyopic eye Amblyopic eye +0.5D +2D / 3/ esotropic* foveal, stable foveal, stable Subject I.G. Nonamblyopic eye Amblyopic eye +3D +3D 90 +6D +3.25D 90 12/ 2/ exotropic* foveal, stable parafoveal, unstable * Chirurgically corrected. server. The luminance of the bar was 0.14 cd/m 2 while the background luminance equalled cd/m 2 (contrast log AI/I = 1.35). Due to this extremely low background luminance, no visual reference could influence the discrimination of the subjects. The orientation of the light bar was changed by turning a dove prism with a stepping motor. Exposure time and intertrial interval were 600 and 5000 msec respectively. A light spot was used as fixation point. When testing most normal subjects, the fixation spot was turned off during stimulus presentation. For one normal subject and the amblyopic subjects, the fixation spot was only turned off for 0- and 5-deg eccentricity. Stimulus presentation, recording of the responses (depressing keys), and auditory feedback (correct or incorrect) were controlled by a microprocessor (Rockwell, AIM 65). Subjects The eight subjects of this study ranged in age from 20 to 25 yr. Four of the normal subjects were emmetropes; two were corrected myopes. The ophthalmological data of the two strabismic amblyopes are given in Table 1. The refraction errors were corrected during the discrimination tests. Informed consent was obtained from the subjects. All subjects were well trained before the final data collection. All subjects were tested monocularly: in normals only the right eye was tested; in the amblyopes both eyes were tested. Procedure Two psychophysical procedures were used. Both involved the sequential presentation of a single line. Simultaneous presentation of the two lines to be discriminated in orientation was avoided because of possible configurational interactions. Studies in our laboratory 12 have shown that decision process factors and, in particular, memory noise are isotropic in these orientation discrimination tasks. In most subjects with normal vision we used a method of single stimuli with five stimulus and two response alternatives described previously. 1 ' 2 The five equally spaced orientations spanned an interval symmetrical around the standard orientation. The subject had to judge the line as tilted anticlockwise or clockwise from the standard orientation. After probit analysis, the standard deviation was taken as the JND (84% correct level) in orientation. In one normal subject (C.C.) and in both amblyopes, a transformed up-down staircase procedure 13 was run using a temporal two-alternative, forced choice design. The subject had to discriminate between the sequences, standard orientation-orientation tilted As deg clockwise from standard, and the sequence, orientation tilted As deg clockwise from standard orientation-standard orientation. The orientation difference As between the two stimulus presentations of a trial was decreased by 20% after 4 successive correct responses and increased by the same factor after one incorrect response. The geometric mean of the orientation differences of the last 5 midrun estimates 14 in a staircase of 20 runs was used to compute a 84% correct JND in orientation. The data were subjected to an analysis of variance with subjects treated as blocks. 15 These analyses of variances were performed on log transformed JNDs. The error term derived from the analysis of variance was used to test differences between means with a priori t-tests. 15 Results Experiment 1. Normal Vision: Homogeneity of the Visual Field with Respect to Orientation Discrimination In a preliminary experiment, we measured the JNDs in orientation along the horizontal and vertical retinal meridians at 30 from the fixation point. The line length was 15 and two different widths were used: 0.25 and All measurements were made for three standard orientations (vertical, horizontal, and right oblique) in one subject (Fig. 1). Comparison of the JNDs obtained with the 2 slit widths shows that the level of performance was fairly similar in both instances: JNDs hovered around 2 for the different retinal loci and standard orientations for the 0.25 width as well as for the 1.75 width. And indeed an analysis of variance with standard orientation, retinal locus, and slit width as factors revealed no significant effect of slit

3 No. 2 PERIPHERAL ORIENTATION DISCRIMINATION / Vandenbussche er al. 239 width nor an interaction between slit width and standard orientation. The analysis of variance also revealed no interaction between standard orientation and retinal locus as has been described for grating acuity. 16 A more careful inspection of the data reveals that for the 1.75 width there is an effect of retinal locus on the JNDs for different standard orientations. In this case the JND for the standard orientation parallel to the meridian on which the slit center was positioned was the smallest. Indeed, for the upper and lower retinal field position (ie, on the vertical meridian), the JNDs for vertical were lower than those for horizontal. The converse was true for the temporal and nasal fields (ie, along the horizontal meridian). The slit width used in our previous experiments 1 ' 2 was Since performance for a peripherally presented slit did not improve with increase in slit width and since the visualfieldwas more homogeneous for the small slit width, all further experiments were performed with a 0.25 wide slit positioned along the horizontal meridian of the temporal retina. Experiment 2. Normal Vision: Changes in Orientation Discrimination with Line Length For central vision, orientation discrimination depends on line length, and longer lines are needed to reach optimal orientation sensitivity at larger eccentricities. 19 Therefore, we investigated the influence of line length on orientation discrimination. In 2 subjects, we measured JNDs in orientation for 8 different line lengths ranging between 0.5 and 15 at 0, 15 and 30 eccentricity for different standard orientations: horizontal, vertical, and right oblique in subject B.G. and vertical and left oblique in subject B.D.B. The results of subject B.G. for two eccentricites are shown in Figure 2A and B. For central vision (Fig. 2A), the JNDs in orientation for right oblique decreased between 0.5- and 4-deg length by a factor 1.5 or 2. The JNDs in orientation for the principal orientation decreased between 0.5 and 4 length by a factor 4 or 5. Hence, the oblique effect in orientation discrimination increased with line length. Comparing the mean JNDs of both subjects for principal and oblique orientations yields 1.06 and 2.24 respectively for a line length of 15 and 6.44 and 4.58 for a line length of 0.5. This confirms our previous observations. 1 At an eccentricity of 30, subject B.G. could not discriminate orientation for lines shorter than 4. Between 4 and 8 length, the JNDs in orientation dropped steeply for all 3 standard orientations and remained constant for lines longer than 8 (Fig. 2B). It is noteworthy that at this eccentricity the JND in orientation-line length relationships were identical for all three standard orientations. SUBJECT :B.G. 30 eccentricity width 0.25 width 1.75 Fig. 1. JND in orientation as a function of position in the visual field at 30 eccentricity (subject B.G.). The magnitude of the JND in orientation is indicated by the distance between the datapoint and the center of the circles. JNDs in orientation at the horizontal, vertical, and right oblique standard orientation are indicated by open circles, filled triangles, and filled circles respectively. The position of the datapoints corresponds to the position in the visual field of the right eye (eg left corresponds to nasal). The data for the two stimulus widths are indicated separately. Since we were primarily interested in the changes of the JNDs in orientation-line length relationship with eccentricity, we pooled the thresholds across subjects and standard orientations. Figure 2C shows the resulting mean JNDs plotted as a function of line length

4 240 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / February 1986 Vol C "o c 2 Q 1 : 05 SUBJECT : B.G. 0 eccentricity line length (degrees) 0.5 B SUBJECT: B.G. 30 eccentricity K U 8 16 line length (degrees) X. Fig. 2. The influence of line length on orientation discrimination measured at o.5l 20 N=2 different eccentricities. A-B, JNDs in orientation of subject B.G. for 3 different standard orientations at 0 (A) and 30 (B) eccentricity. Same conventions as in Figure 1. C, Mean JNDs in ori- \ "*"* entation as a function of line 0 o"v length measured at three different eccentricities: 0 (full lines), 15 (stippled lines), and 30 (dotted lines). The lines were fitted to the dataos i 2 t 8 16 points with linear regression. line length (degrees) The JNDs f or a u standard orientations of two subjects were pooled. on a log-log scale. Three aspects of the JND in orientation-line length relationship change with eccentricity. First, the optimal line length (inflexion point of the curve) increases with increasing eccentricity. The increase, however, is modest compared to the change in minimal angle of resolution. Compared to 0 eccentricity, the optimal lengths at 15 and 30 eccentricity are a factor 1.6 and 2.2 larger. This contrast with minimum angle of resolution for which these factors are 20 and 25 respectively. Second, the length summation becomes steeper with increasing eccentricity. The exponent of the negative power law describing the relationship for suboptimal lengths increases from 0.54 at 0 to 1.09 and 1.64 at 15 and 30 eccentricity respectively. Hence orientation discrimination improves more rapidly with line length in the peripheral visual field than in central vision. Finally, the optimal level of performance reached at large line lengths is only a factor 2 larger at 30 eccentricity compared to 0 eccentricity. Hence, the change in orientation discrimination with eccentricity is much smaller than the decrease in acuity with eccentricity. Experiment 3. Normal Vision: The Influence of Eccentricity on Orientation Discrimination at Four Standard Orientations We measured JNDs in orientation at eccentricities ranging from 0 to 30 and 4 standard orientations (vertical, horizontal, left and right oblique) in 3 subjects. For each subject we used the line length as short as optimal at 30 eccentricity, eg, 8 for subject B.G. (see Fig. 2B). Figure 3A shows the results for this subject. The orientation discrimination at oblique standard orientations did not change at all with eccentricity. The JNDs in orientation at principal orientations did not change up to from the fixation point, increased steeply between and 15 eccentricity and then remained at a level close to that of JNDs for oblique standard orientations. As a consequence, the oblique effect in orientation discrimination vanished in this subject between 12 and 15 eccentricity. In the two other subjects it vanished between 15 and 20 (subject R.V.) and between 20 and 30 (subject C.C.). The results pooled across subjects are shown in Figure 3B. Again the oblique JNDs are hardly affected by eccentricity, while principal JNDs increase more or less abruptly at 15 eccentricity. For eccentricites over 20, there was no difference between JNDs for oblique and principal orientation. The analysis of variance (eccentricity and standard orientation as factors) confirmed that the interaction between eccentricity and standard orientation was significant (F(15, 46) = 2.96; P < 0.05). It is noteworthy that in the pooled results the changes in JND for the vertical standard orientation are slightly larger than for the horizontal standard orientation. Three additional subjects were tested at three eccentricities: 0, 15, and 30 and two standard orientations (vertical and left oblique). Pooling those results with those of the three subjects tested at all four standard orientations confirms the previous results (Fig. 3C). At 0 eccentricity, the oblique JNDs were on average a factor 2.3 larger than the vertical JNDs. At 30 eccentricity, this difference had completely disappeared. Analysis of variance of the results again confirmed that the interaction between standard orientation and eccentricity was significant (F(2, 25) = 3.9, P < 0.05). Experiment 4. Amblyopic Vision: Effect of Line Length on Orientation Discrimination at Different Eccentricities We measured the influence of line length on orientation discrimination in both eyes of the two strabismic amblyopic subjects at two eccentricities (0 and 20

5 No. 2 PERIPHERAL ORIENTATION DISCRIMINATION / Vandenbussche er ol. 241 deg) and at two standard orientations (vertical and right oblique). Figure 4A shows the results pooled across subjects and standard orientations. The results show the same trend as those of normal subjects (compare with Fig. 2C). At both eccentricities, for the nonamblyopic as well as for the amblyopic eye, orientation discrimination improves with line length and reaches a stable level over a given optimal length. As in normals, this optimal length increases with eccentricity, in the nonamblyopic eye as well as in the amblyopic eye. The main difference between the amblyopic and nonamblyopic eyes concerns the level of performance for short lines. At 0 eccentricity, the JNDs in orientation are higher for the amblyopic eye than for the 20 3_ 0.5 B 8 15 SUBJECT : B.G B 0.7l OL A 8 line length (degrees) Fig. 4. The effect on line length on orientation discrimination in amblyopic subjects. A, The mean (two subjects) JND in orientation as a function of line length; filled symbols: 0 eccentricity, open symbols: 20 eccentricity, circles: nonamblyopic eye, triangles: amblyopic eye. One subject could not discriminate the orientation of the 2 long line at 20 eccentricity with her amblyopic eye. The corresponding datapoint will be larger than (indicated by arrow). B, The interocular ratio as a function of line length for the principal (filled squares) and the oblique (open squares) standard orientations calculated for the 0 eccentricity data L eccentricity (degrees) Fig. 3. The effect of eccentricity on orientation discrimination of long lines for different standard orientations. A, JNDs in orientation of subject B.G. as a function of eccentricity for the horizontal (open circles), vertical (triangles), left oblique (crosses), and right oblique (filled circles) standard orientation. B, Mean (three subjects) JNDs in orientation as a function of eccentricity and standard orientation. Same conventions as in A. C, Mean (three subjects) JNDs in orientation as a function of eccentricity for a vertical (triangles) and left oblique (crosses) standard orientations. Standard errors are indicated by vertical bars. nonamblyopic for lines shorter than 4. The same is true, to a lesser degree, at 20 eccentricity. The ratio of JNDs in amblyopic and nonamblyopic eyes at 0 eccentricity averaged for the 2 subjects is plotted as a function of line length for the principal and oblique standard orientation in Figure 4B. This shows that the deficit in the amblyopic eye for short lines affects both the oblique and principal standard orientations, contrary to the deficit at long line lengths which affects only the principal standard orientations (see below). Experiment 5. Amblyopic Vision: Influence of Eccentricity on Orientation Discrimination at Four Standard Orientations We measured JNDs in orientation at different eccentricities ranging from 0 to 30 and 4 standard ori-

6 242 INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / February 1986 Vol NONAMBLYOPIC EYE AMBLYOPIC EYE eyes confirms our previous observation that for long lines, only JNDs at principal orientations are affected in the amblyopic eye. The ratio of JNDs in the amblyopic eye and in the nonamblyopic eye averaged over the two subjects is plotted as a function of eccentricity for principal and oblique standard orientations (Fig. 5C). When tested with long lines, the deficit in orientation discrimination of the amblyopic eye is limited to the principal standard orientation and to small eccentricities (below 20 deg). For central vision (0 eccentricity) the difference between JNDs in both eyes is significant for principal standard orientations (t(47) = 1.82, P < 0.05) but not for oblique standard orientations (t(47) = 0.36, n.s.). At 30 eccentricity, the differences in JNDs between both eyes are significant neither for the principal (t(47) = -0.36, n.s.) nor for the oblique standard orientations (t(47) = 0.53, n.s.). Discussion S 2 0.5L eccentricity (degrees) Fig. 5, A-B, JNDs in orientation of long lines as a function of eccentricity in the nonamblyopic (5A) and amblyopic (5B) eye. The conventions are the same as in Figure 3. C, The interocular ratio as a function of eccentricity for the principal (filled squares) and oblique (open squares) standard orientations. entations in both eyes of the two same strabismic amblyopes. As in normals, we used for each subject the line length as short as optimal at 30 eccentricity. The results pooled across subjects are shown in Figure 5A for the nonamblyopic eye and in Figure 5B for the amblyopic eye. For both eyes the results are qualitatively similar to those of normals (see Fig. 3B). The JNDs for oblique standard orientations hardly change with eccentricity, while those for the principal standard orientations rise steeply around 20 eccentricity. At 30 eccentricity, the difference (averaged over the 2 eyes) in JNDs in orientation between oblique and principal standard orientations has vanished: t(47) = 1.51, n.s. Two further observations are worth noting. First, in both eyes the JNDs for the vertical standard orientation change more steeply with eccentricity than the JNDs for the horizontal standard orientation. Second, the increase in JNDs for principal orientations occurs in both eyes at the same eccentricity. This implies that the difference between both eyes is not due to eccentric fixation. Comparison of the results in both The two main results of the present study are (1) orientation discrimination changes little with increasing eccentricity, and (2) the oblique effect in orientation discrimination is limited to small eccentricities below deg. For normal subjects as well as for both eyes of strabismic amblyopes, JNDs in orientation for oblique standard orientations hardly change with eccentricity, while the JNDs for principal standard orientations steeply rise at deg eccentricity to the level of the oblique JNDs. Our finding that orientation discrimination for optimal line length changes little with eccentricity is in good agreement with those of Andriessen and Bouma 20 and Scobey. 19 These authors showed that, provided line length was optimized, orientation discrimination at a vertical standard orientation did not change between 0 and eccentricity. Although our findings are qualitatively similar, the optimal lengths for central vision found in the present study (over 4 ) are much larger than those of Andriessen and Bouma (70 minarc) and of Scobey (21 minarc). There are, however, a number of differences in the stimuli used: (1) our stimuli as well as the background had a much lower luminance, and (2) our stimuli were less sharp and wider than those used in the other studies. We performed a control experiment to investigate the influence of the sharpness and width of lines. It could be that with a sharp line subjects use the orientation of the edge of the line rather than that of the body of the line and the line length dependence for both cues could be different. Furthermore, when the orientation of the body of the line is used (unsharp stimulus), linear theory (2-D Fourier) predicts a dependence of orientation bandwidth on the length/width ratio. 21 Therefore, we measured in one subject the JND

7 No. 2 PERIPHERAL ORIENTATION DISCRIMINATION / Vondenbussche er ol. 243 in orientation-line length relationship for different line widths and different degrees of blur. Only the vertical standard orientation and central vision were tested. With a sharp stimulus, one can use the orientation of edges; this is demonstrated by the fact that one can judge the orientation of a square. For each line width, we looked for the minimum blur required to prevent subjects from judging the orientation of a square of the same dimension. For a 0.1 wide line, a 1-D blur was enough; for a 0.25 wide line, 2 diopters were required. Figure 6 shows the JND in orientation line length relationship for three widths (0.1, 0.25, and 2 ) and blurring with a 1-D lens (Fig. 6A) and a 2-D lens (Fig. 6B). There is no difference in those relationships whether edge information is present or not (compare 0.1 and 0.25 width with 1-D blur or 0.25 and 2 widths with 2-D blur). Nor is there any dependence of the relationship on width when edge information is removed (compare 0.1 and 0.25 widths with 2-D blur). Hence, it is very likely that the longer optimal lengths observed in this study are due to the lower luminances of the line and particularly of the background. The improvement of JNDs in orientation with line length up to optimal length indicates that the orientation sensitive mechanisms improve their operation with increasing line length. Indeed, physiological studies have shown that orientation tuning of cortical cells becomes sharper with increasing slit length both in endfree cells 22 and in endstopped cells. 23 The implication of the present results is that this sharpening of orientation tuning occurs for shorter lengths in photopic vision than in scotopic vision. Bisti et al 24 have shown that the width of orientation tuning does not depend on background luminance, but unfortunately these authors only used long slits. The increase of orientation tuning width of oriented mechanisms with decreasing line length is likely to depend on the spatial grain of the sampling grid. Although no physiological evidence is available since the above mentioned reports only dealt with central vision, this increase will be larger for a coarser grain. 18 This could explain the severe effects of eccentricity, amblyopia, and blur on the orientation discrimination of short lines. Our observation that orientation discrimination changes little with eccentricity is in marked contrast with the steep changes in acuity with eccentricity. This adds further evidence to our view that, despite the fact that both tasks show oblique effects, contrast sensitivity and orientation discrimination depend on different neuronal mechanisms." While grating acuity and contrast sensitivity depend heavily on eccentricity, blurring, and amblyopia, orientation discrimination for long lines depends little on these variables. Moreover, stimulus-length hardly affects grating acuity, while ori- s V SUBJECT : K.C A line length (degrees) Fig. 6. The effect of line length on orientation discrimination of lines of different width and with different amount of blur. A, -MD blur. B, +2D blur. Triangles: 0.1 width, squares: 0.25 width; crosses: 2 width. entation discrimination depends strongly on line length. Finally, selective practice abolishes the oblique effect in contrast sensitivity but not in orientation discrimination. 25 While acuity and orientation discrimination of long lines depend on different neuronal mechanisms, both show an oblique effect limited to small eccentricities in the visual field. Comparison of the present results with those of Berkley et al 26 suggests that the oblique effect in grating acuity fades at smaller eccentricities than that in orientation discrimination. The modest changes in orientation discrimination with eccentricity involve both the level of performance (JNDs) and the optimal length. Our observation that JNDs in orientation at principal standard orientations are more affected by eccentricity than JNDs in orientation at oblique standard orientations fits with our earlier observations that principal JNDs are more affected by blurring and in amblyopia. It seems that within orientation discrimination, discrimination at a principal orientation is more susceptible to deterioration of the spatial grain of the visual system. Our observation that optimal length for orientation discrimination changes less steeply with eccentricity than minimal angle in resolution is not surprising in the view that the former probably corresponds to the min- 15

8 244 INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / February 1986 Vol. 27 imal length, producing a sharp orientation tuning of cortical cells. It is likely that this length does change more gradually than cortical magnification or even retinal density. Our observation that the oblique effect in orientation discrimination of long lines fades with increasing eccentricity bears out the prediction derived from our hypothesis that the S-cell anisotropy underlies the oblique effect in orientation discrimination. Recently our results on the effect of selective practice on line orientation discrimination have led to the recognition of two sensorial sources of the oblique effect: one due to S-cells in area 17 and one due to the further processing of S-cell signals. The fact that the predictions derived from S-cell properties are borne out so well suggests that the latter step is relatively close to the S- cells and perhaps resides in the combination of the S- cell output. We have suggested that the level of performance in orientation discrimination depends on the square root of the number of S-cells activated by the line. 1 Orientation discrimination performance changes by a factor 2 between 0 and 30 eccentricity (Fig. 2C); this then implies that the number of S-cells is reduced by a factor 4 between 0 and 30 eccentricity. This could be achieved, despite the large changes in cortical magnification (by a factor 30 or so 27 ) if the S-cells represent a larger proportion of VI cortical cells at 30 eccentricity than at 0 eccentricity. And indeed, the proportion of oriented cells (and hence of S-cells) increases with eccentricity in monkey V1, 28 as does the proportion of endfree cells (and hence of S-cells) in VI of the cat. 4 It has been suggested that in peripheral vision, cortical cells and especially S-cells prefer orientations parallel to the line linking the RF center to the fovea. 9 One would therefore expect orientation discrimination to be better for the standard orientations parallel to retinal meridians on which the line center was positioned than for the orthogonal standard orientations. Such a tendency was present for wide lines (see Fig. 1), but much weaker for narrow widths, although Figure 3 and Figure 5 suggest that a small tendency in this direction was also present in our JNDs measured with narrow lines. Our present observations on amblyopic subjects confirm our earlier observation 2 that only JNDs in orientation at principal orientations are affected in amblyopia. The present results extend this observation and show (1) that the deficit is limited to central and near peripheral vision, and (2) that it is not due to eccentric fixation. The present study reveals a second deficit in orientation discrimination of amblyopes: the orientation discrimination of short lines is strongly impaired at all standard orientations. Our results also give further support to the frequently held view that, at least in strabismic amblyopes, central vision of amblyopes is similar to peripheral vision in normals. 29 " 32 In fact; the orientation discrimination of our two amblyopic subjects for central vision corresponds to the performance of our normal subjects at an eccentricity of to 15. If this was to be verified further, this would imply that the magnification factor-eccentricity relationship for the afferents of the two eyes would be different, the one for the afferents from the amblyopic eye being flatter than that for the normal eye afferents. Key words: orientation discrimination, oblique effect, eccentricity, amblyopia Acknowledgments The technical assistance of P. Kayenbergh, G. Vanparrijs, and J. Warmoeskerken as well as the computerized typing of Y. Celis are gratefully acknowledged. The refraction and acuity measurements were done by M. Van Lammeren, from the Department of Ophthalmology, KUL Medical School. References 1. Orban GA, Vandenbussche E, and Vogels R: Human orientation discrimination tested with long stimuli. Vision Res 24:121, Vogels R, Orban GA, and Vandenbussche E: Meridional variations in orientation discrimination in normal and amblyopic vision. Invest Ophthalmol Vis Sci 25:720, Orban GA and Kennedy H: The influence of eccentricity on receptive field types and orientation selectivity in areas 17 and 18 of the cat. Brain Res 208:203, Orban GA: Neuronal Operations in the Visual Cortex. Berlin, Springer Verlag, 1984, 367 p. 5. 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