Contrast Sensitivity in Amblyopia: Masking Effects of Noise

Transcription

1 Investigative Ophthalmology & Visual Science. Vol. 33. No. 10, September 1992 Copyright Association for Research in Vision and Ophthalmology Contrast Sensitivity in Amblyopia: asking Effects of Noise J. P. Nordmann,* R. D. reeman, and C. Cosonovof Contrast sensitivities were determined for sinusoidal gratings of varying spatial frequencies with and without the presence of a random noise pattern superimposed on the gratings. Control subjects with normal binocular vision and observers with amblyopia were tested to determine the relative effects of noise on contrast sensitivity. or both amblyopes and normal subjects, contrast sensitivities are reduced by the presence of noise. Effects are maximal at 4 cycles/degree and are minimal at low and high spatial frequencies. Dichoptic presentation of noise and gratings to opposite eyes is equivalent to monoptic results for both amblyopes and normal subjects. asking effects are eliminated if gratings are drifted while noise patterns are static. The contrast sensitivity of amblyopes is reduced by relatively similar amounts to that of normal subjects when noise is added to the stimulus. Overall, masking effects are virtually identical for amblyopes and for subjects with normal binocular vision. Invest Ophthalmol Vis Sci 33: , 1992 The resolution of visual targets is impaired by adaptation to similar stimuli. or example, contrast sensitivity for gratings is reduced at the spatial frequency of prior adaptation. 1 " 3 Reductions in resolution are caused also by masking effects of adjacent visual stimuli or stimuli that are superimposed on the target to be resolved. or example, visual interference by lateral masking has been observed for letters which form words 4 or with spaced arrays of various configurations. 5 Adjacent contours positioned near letter targets also decrease resolution. 6 " 9 These latter effects are reported to be equivalent for monoptic and dichoptic presentations, ie. for targets and contours viewed by the same eye or by opposite eyes. 10 Therefore, the physiological basis of the contour interactions is presumably cortical. (Note, however, that for aftereffects, interocular transfer does not by itself establish a central origin.") asking has also been studied for sinewave gratings, with respect to both monocular 12 and binocular 1314 effects. Results vary with spatial frequency and rom the Group in Ncurobiology. School of Optometry. University of California. Berkeley. California. Supported by grant EY01175 and CO grant EY03I76 from the National Eye Institute (Bethesda. D). ellowships from the RC of Canada and Bourse Lavoisier of rance provided partial support forcc and JPN. respectively. Submitted for publication: ebruary : accepted April * Current address: Departemcnt d'ophthalmologic. Hopital SaintAntoine. 184 faubourg. SaintAntoine Paris. rance. t Current address: Department d'ophthalmologic. acultcde edecinc. Univcrsitc dc Shcrbrookc. Sherbrooke(Quebec)Canada J III 5N4. Reprint requests: R. D. reeman. 360 inor Hall. University of California. Bcrkclcv. CA contrast levels. In these latter studies, gratings were used for both masking and tests. Other investigations have explored the influence of noise on grating detection. or example, effects of varying the noise spectrum have been determined for different spatiotemporal combinations of grating parameters. 15 The performance of subjects with faulty binocular vision has also been investigated with masking stimuli. Dichoptic presentation has been employed in which a test and a masking grating are presented to opposite eyes. The main result is that sensitivity is lowered by the masking grating for a range of orientations and spatial frequencies. This effect is similar for normal subjects and for those with abnormal binocular vision Results of other studies also indicate that spatial tuning characteristics can be similar for amblyopic and nonamblyopic eyes of strabismic amblyopes A somewhat different result has been obtained by the use of dynamic visual noise presented to one or both eyes. or observers with normal binocular vision, there is a large increase in masking effect with dichoptic as compared to monoptic noise. A similar summation of masking does not occur in abnormal binocular vision. 20 In other work, as part of a control procedure, masking effects were determined for different noise levels for subjects with normal and abnormal binocular vision. 21 In this case, values appear similar for both groups but specific data are not given. onocular and binocular reaction times have also been measured in response to presentation of gratings. Observers with normal binocular vision exhibit strong binocular summation, while those with deficient stereopsis perform at the level of probability summation. 22 or normal subjects, random noise 2975

2 2976 INVESTIGATIVE OPHTHALOLOGY 6 VISUAL SCIENCE / Seprember 1992 Vol. 33 used as a mask for one eye caused an elevation in reaction time to gratings presented to the other eye. Dichoptic masking was not found for a stereoblind subject. 22 Other measures of reaction times to gratings show that those for amblyopic eyes are relatively prolonged and that masking normal subjects with noise produces similar results These studies of masking effects provide information concerning differences between normal and abnormal binocular vision. However, it appears that no systematic study has been made of the relative masking effects of noise on the contrast sensitivities of gratings for subjects with deficient binocular vision. The expected outcome of this study depends on which of two plausible physiological processes is applicable in amblyopia. irst, receptive field structure of cells in pathways from the amblyopic eye could be disorganized. If, during normal early development, excitatory and inhibitory regions of receptivefieldsare regularly arranged, abnormal visual input could disrupt this order. In this case, random noise might not degrade an image in an amblyopic eye as much as that in a normal eye, since transmission would be limited by the irregular receptive field structure. Therefore, a noise mask might have relatively less effect on the contrast sensitivity of a grating in the case of an amblyope. The second physiological possibility is that the organization of receptive field structure is normal in amblyopia but afferent connections are weak or partially disconnected so as to produce a scaled down sensitivity especially for high spatial frequency stimuli. In this case, a noise mask would be likely to cause a relatively similar reduction in contrast sensitivity for amblyopes as that for normal observers. Of course, both physiological processes could apply and in this case, the noise masking effect for an amblyope might be relatively less than that for a normal observer. In the study reported here, we have determined contrast sensitivities for a range of spatial frequencies for gratings with superimposed noise masks. Our results show that masking effects are identical for amblyopic and normal subjects. aterials and ethods Apparatus Gratings are generated on a large, bright (200 cd/ m 2 ) monitor (Joyce Electronics) with a P4 phosphor. The screen of this monitor is divided into two areas (4 X 7 each) placed sidebyside, separated by a black mask (1.3 X 7 ). Noise (texture) is produced via a graphics board (Imagraph) on a second monitor. A beam splitter is used to superimpose the two patterns. Generation of visual stimuli, data acquisition, and analysis are under computer control. Stimuli Horizontal gratings with sinusoidal luminance distribution are used to test contrast sensitivity at five spatial frequencies separated in octave steps between 0.5 and 8 cycles/degree. A sixth frequency, 14 c/deg, is also used for the test. The noise or texture pattern is produced on a 22 X 28 monitor with a resolution of 512 by 512 pixels. Each pixel is rectangular (0.043 X or 2.6 X 3.3 min of arc). Rectangular texture elements have a density of 1/256 or an average of 1.6 elements/deg. 2 This density remains constant so that the number of noise elements per cycle of grating is inversely proportional to the spatial frequency of the grating. The use of this noise element density is based on preliminary tests in which we varied density to determine values that caused measurable reductions in contrast sensitivity for a range of spatial frequencies. To determine the spectrum of the noise, a twodimensional discrete ourier analysis was performed. This analysis shows that there are no extraneous peaks in the spectrum, which might cause nonuniform effects at different frequencies. Therefore, if the noise pattern were composed of infinitesimally small dots, the spectrum would be flat. However, since the noise consists of elements of finite dimension, the spectrum must be analyzed as follows. The spectrum of the noise is identical to that of a single rectangular shape and is given by sin (2irfa) 2 (1) where /is the spatial frequency and a is one half of the height of the rectangular shape. or our stimulus, a = (22 /[512 X 2]). The power of the noise decreases monotonically as a function of spatial frequency for the range concerned and declines to one quarter of the peak at 14 c/deg. The expected masking effect, therefore, is smaller at higher spatial frequencies. Procedure We use a twoalternative spatial forcedchoice procedure. A grating is presented for 3 sec in one of the two laterally adjacent windows on the monitor. The subject activates one of two buttons to indicate which window contained the grating. eedback is given by two different auditory signals to indicate correct or incorrect responses, respectively. A staircase procedure is used in which three consecutive correct responses result in a reduction of contrast of the grating and one incorrect response results in an increase in contrast. After 10 reversals, a threshold is reached which corresponds to 79% correct. Around 80 presen

3 No. 10 ASKING, CONTRAST SENSITIVITY, AND ADLYOPIA / Nordmann er ol 2977 tations are necessary to obtain 10 staircase reversals. Three testing sessions are used for each subject. During each session, the order of the two conditions, gratings with and without noise, is randomized. or monocular tests, the nontested eye is occluded. or dichoptic tests, the beam splitter is positioned so that gratings and noise patterns are overlapped. If binocular rivalry results, the grating and noise patterns are presented to the opposite eyes. usion is attained for the subjects tested dichoptically. Subjects Thirty one subjects were tested. Of these, 14 were used for control data and 17 were amblyopic. Details concerning the amblyopic subjects are given in Table 1. In general, seven subjects had anisometropia with no obvious sign of strabismus, although we used only clinical observations to determine this. An additional 10 subjects had strabismus or strabismus plus anisometropia. In nearly all cases, the clinical histories indicate longstanding conditions so that a reasonable assumption can be made that the disorders were present in childhood. All subjects, including those with normal or abnormal binocular vision, were checked clinically with respect to refractive status. In each case, optimal correcting lenses were used during the tests. Results Normal Subjects Typical results for monocular tests of normal subjects are shown in igure 1 (open symbols). Contrast sensitivity values are standard for these conditions and the functions show typical high and low frequency falloffs with peak sensitivities around 24 c/ deg. When noise is added to the grating patterns, a relatively small but consistent decrement is observed across a wide range of spatial frequencies (filled symbols). However, for the lowest spatial frequency (0.5 Table 1. Refractive errors and clinical data for amblyopic subjects Name Sex. ('He Refractive error (D) Visual. acuity Strabismus Strabismic and strabismic anisometropic amblyopcs BR S TK CB HD W V J DK R Anisometropic amblyopes (.50 x X x 60 ).50x x x x X X X X x /100 20/40 20/80 20/70 20/70 20/40 20/50 20/15 20/100 20/50 20/ A ; csotropia 5A ; csotropia 12A ; csotropia I0A ; csotropia 5A ; csotropia 6A ; csotropia 8A ; esotropia 4A ; exotropia 9A ; exotropia 8A ; exotropia H K PC WD KC HJ D x x x x 90 I.OOx x x x 80 I.OOx x /15 20/ /50 20/40 20/40 20/50 20/25

4 2978 INVESTIGATIVE OPHTHALOLOGY & VISUAL SCIENCE / September 1992 Vol. 33 D " \ 10 ig. I. Contrast sensitivity functions for sinusoidal gratings arc presented for normal binocular subjects (A). or each subject, the two sets of data represent results with (filled symbols) and without (open symbols) noise superimposed on the gratings. Standard errors are approximately equal to the size of the symbols in this and in subsequent figures. \ I c/deg), the effect is absent, and for the higher frequencies (ie, 8 and 14 c/deg), effects are minimal. Although there are small differences between subjects, the overall patterns are consistent, and this holds for the other normal subjects for whom data are not shown. The variation of the effect of noise on contrast sensitivity, as a function of spatial frequency, does not appear to be due solely to the spectral composition of the noise pattern. In particular, the masking effect is minimal at the lowest spatial frequency. sensiitiv Comtrast Amblyopic Subjects Representative contrast sensitivity functions for amblyopic subjects are shown in igure 2. These results are for monocular tests through the amblyopic eye. or comparison, the mean for the normal subjects is shown, too (triangle symbols, thick curve). The increased reductions relative to the curve for normal subjects indicate a progression of degrees of amblyopia from miid to extensive. The contrast sensitivity ig. 2. Contrast sensitivity functions are shown for four amblyopic subjects to illustrate the range of deficits. Open squares, open circles, filled squares, and filled circles represent data for S. CB, BR. and GL. respectively (sec Table 1). or comparison, a composite function (bold curve) is shown which represents the mean values for the 14 normal control subjects. Vertical bars each represent 1 standard error of the mean.

5 No. 10 ASKING, CONTRAST SENSITIVITY, AND ADLYOPIA / Nordmonn er ol 2979 profiles are typical of amblyopes in that they generally show more pronounced deficits as spatial frequency is increased. 24 or reasons described earlier, the contrast sensitivity of an amblyopic eye might be relatively less affected by noise than that of a nonamblyopic eye. On the other hand, previous studies of masking, as described above, suggest that effects are similar for amblyopic and nonamblyopic eyes, although interocular masking has been reported to be normal in one study 16 and abnormal in another. 22 Effects of superimposed noise on contrast sensitivity in amblyopia are illustrated in igure 3, which has the same format as igure 1. The three subjects whose results are given here are typical and they represent varying degrees of amblyopia. What is striking here is that there is a close similarity to the data from normal subjects. Both aspects of the effects of noise are seen again. irst, the reduction is modest but consistent, and second, effects are maximal in the middle frequency range and are reduced or absent at both ends of the spatial frequency scale. To visualize these results more clearly, the data have been converted as follows. At each spatial frequency, the ratio of mean contrast sensitivity with the addition of noise to that without the noise was computed. This gives the relative decrease at each spatial frequency, which is expressed as a percent reduction in igure 4. In igure 4A, the two sets of data represent the nonamblyopic eyes of amblyopic subjects (filled squares) and the data from control subjects with normal binocular vision who were tested monocujarjy (open circles). igure 4B also contains two sets of data, one for the amblyopic eyes (open squares) and the other for the nonamblyopic eyes of amblyopic subjects (filled squares). There is a remarkable similarity in all fours sets of data. The relative reductions are nearly identical so that all four curves may be nearly superimposed. The major effects of the noise are found for gratings of 4 c/deg, where sensitivity is reduced to around 60% of that without noise. Note that noise masking is less pronounced for the highest spatial frequencies where sensitivities are reduced by around 20%. There are i ig. 3. Data are presented for threc.amblyopic subjects tested through their nonamblyopic (AC) or amblyopic (D) eyes. Once again, a range of deficits is shown from mild (A, D) to pronounced (C, ). In each case, tests were conducted with (filled symbols) and without (open symbols) noise. Subjects PC, CB. and R were used to provide data for (AD), (BE), and (C), respectively. c o U I

6 2980 INVESTIGATIVE OPHTHALOLOGY & VISUAL SCIENCE / Seprember 1992 Vol x three separate sessions. Both contrast sensitivities and the effects of noise were consistent in all cases. ean differences between sessions for a given variable were around 10%. The patterns of sensitivity were consistent for all subjects and varied little between sessions. c o a 8 1 o U B ig. 4. The supprcssivc effects of the noise arc illustrated here. or each spatial frequency, ratios have been computed of mean contrast sensitivities with noise to those without noise. Values are then expressed as percentages. In each case, vertical bars represent I standard error of the mean. In (A), the data represent ratios for the nonamblyopic eyes of 17 amblyopcs (filled squares) and 14 normal binocular control subjects tested through one eye (open circles). In (B), the values arc for nonamblyopic (filled squares) and amblyopic eyes (open squares) of the amblyopic subjects. three major implications here. irst, the normal control data are not distinguishable from those obtained from the nonamblyopic eyes of amblyopic subjects, with respect to the effects of noise on contrast sensitivity. Second, effects of the noise are also virtually identical in amblyopic and nonamblyopic eyes of amblyopic subjects. Third, the influence of texture or noise patterns of the type we use here appears indistinguishable in amblyopic and in normal vision. In particular, there are no reduced masking effects for amblyopes in the extraction of visual clues needed to see a grating, when noise is added to the pattern. Repeated Tests Practice effects may be important and for other masking studies, they have been shown to influence the results To see if practice effects are a factor in our results, we tested six subjects (three amblyopes and three normal control subjects) completely during Site of Interaction Having found that the influence of noise is constant and closely similar in amblyopic and normal vision, we next considered the site of interaction. In addition to monoptic presentations of gratings and noise, we conducted some tests dichoptically so that gratings and noise were presented to separate eyes. If contrast sensitivities for these two procedures are similar, then the masking effect is likely to occur centrally at a physiological level of binocular interaction. Procedurally, this test is difficult because of binocular rivalry. or normal subjects, we determined the eye which could be tested with gratings while noise was presented to the other eye. or the amblyopes, noise was presented to the nonamblyopic eye. In general, the reverse combination resulted in binocular rivalry that was too severe to allow measurements to be made. Results of measurements for dichoptic presentation of gratings and noise are shown in igure 5 for three normal (AC) and three amblyopic (D) observers. Three sets of data are shown for each subject. Contrast sensitivities alone and with noise, determined monoptically, are denoted by open and closed squares, respectively. The third set of data (open circles) represents dichoptic presentation of noise to one eye and gratings to the other eye. Once again, there is a remarkable similarity in the results such that monoptic and dichoptic data are indistinguishable. We conclude that the interaction between noise and grating stimuli occurs at or after binocular convergence of signals from both eyes. Effects of ovement With noise, contrast sensitivity is reduced by about 40% in the middle spatial frequency range and around 20% at high spatial frequencies. These masking effects are for static conditions of grating and noise presentation. or moving gratings, the masking influence of the noise should be less severe at low and medium spatial frequencies since contours in motion in this range are easier to detect compared to the same stimuli presented in a stationary mode. 27 However, the expected difference may not be equivalent for amblyopic vision. To investigate this factor, we determined contrast sensitivities for drifting gratings of the same spatial frequencies used in the static tests. We used a temporal frequency of 13 Hz, which is in the range of

7 No. 10 ASKING, CONTRAST SENSITIVITY, AND ADLYOPIA / Nordmonn er ol 2981 Normal Amblyopic D 100 " ij». 5. Three different tests arc depicted here. irst, contrast sensitivities arc determined for gratings alone (open squares). Second, effects of superimposed texture arc shown (tilled squares). Third, gratings are seen by one eye while texture is viewed by the other (open circles). Data shown in (AC) arc from normal subjects. or the amblyopic subjects (D), gratings arc viewed by the amblyopic eye and noise by the nonamblyopic eye. Amblyopic subjects in (D). (K). and () are D. S. and DK, respectively. O u oo \ 10 ' peak sensitivity for both normal and amblyopic subjects. 28 Results for the normal control subjects, shown in igures 6A, B, and C, are typical in that they show peak sensitivities at relatively low spatial frequencies and steep fallofts at the high frequency end. The data for detection of gratings alone and gratings in the presence of noise are virtually superimposed. There is no masking effect of the noise in this condition. The contrast sensitivity functions for amblyopic subjects, shown in igures 6E and, are of the same general form as those of the normal control subjects. Once again, there is no masking effect of the noise in this condition. The amblyopic subject of igure 6D exhibits a different function in that no lowfrequency falloff is seen. But as with the other subjects, there is no masking effect of the noise. These results show that static noise does not mask the contrast sensitivities for moving gratings. Discussion or both amblyopes and normal control subjects, contrast sensitivities are reduced over a broad range of spatial frequencies by the addition of noise texture. However, effects vary with spatial frequency. They are minimal at 0.5 c/deg and are maximal at 4 c/deg. where sensitivites are reduced by around 40%. or the condition in which noise is presented to one eye and the grating to the other, sensitivity is reduced to an extent which is nearly identical to that of monocular presentation. There is also a close similarity between effects of noise when the nonamblyopic eye is compared to one eye of a normal binocular subject. asking effects are virtually eliminated when gratings are drifted across a static pattern of noise. This indicates that noise interference is offset by movement detection sensitivity for the pattern. The most striking finding is that the masking effects for normal control sub

8 2982 INVESTIGATIVE OPHTHALOLOGY & VISUAL SCIENCE / September 1992 Vol. 33 a Normal a Amblyopic D 100: 10 10!S iood 3 ig. 6. Results arc shown here for contrast sensitivities determined for drifting gratings with (filled squares) and without (open squares) stationary noise. Amblyopic subjects in (D)? (E), and () are BR. DK, and KC. respectively. As before, all tests arc monocular. 100 jects and those for amblyopes are virtually indistinguishable for all conditions tested. This result applies to a wide range of amblyopia, from mild to severe cases (Table 1). Therefore, amblyopes are able to extract, in a relative way, the same information required to detect the gratings as normal binocular subjects. This conclusion is limited, of course, to the conditions we have used. or example, we have not attempted to vary the parameters of the noise. In previous studies in which the amount of noise has been varied, low levels have been found to have little or no effect. As the noise levels increase, sensitivity decreases in a proportional way, ie, noise versus sensitivity functions have a slope of around one The speculation based on the current results is that this relationship would also hold for amblyopes. The masking effect of noise varies with spatial frequency. At the lowest frequency (0.5 c/deg), there is almost no effect, while reductions at other frequencies range from around 2040%. Clearly, the masking effects of noise are maximal for spatial frequencies at which contrast sensitivities are highest. This masking pattern does not conform to what is predicted from spectral analysis. As noted above, the ourier spectrum for our noise pattern shows that the power of the noise decreases monotonically as a function of spatial frequency. Analysis of paireddot noise patterns and their power spectra has been reported previously. 31 Their spatial frequency analysis model is put forward to account for the processing of visual information. It incorporates ideal ourier channels that relate the degree of contrast sensitivity reduction at a particular spatial frequency to the power at the frequency in the ourier domain of the pattern used for adaptation. This kind of analysis is clearly an approximation to an actual ourier analysis because of the conditions for this process which are not fulfilled in the visual system. With this in mind, and to provide comparisons between theoretical and experimental findings, De Valois and Switkes 31 suggest the use of a transform by which power density per cycle may be converted to power density per octave. This gives the amount of

9 No. 10 ASKING, CONTRAST SENSITIVITY, AND ADLYOPIA / Nordmonn er ol 2983 spectral input within a single channel's bandwidth. 32 We haven't performed this kind of analysis, and it is not clear that our results can be accounted for on the basis of spectral content of the noise. To pursue this further, one could convolve the noise spectrum with the contrast sensitivity function. The results of igure 4 show major effects only for the middle frequencies. or these frequencies, gratings whose contrast is a fraction of a percent can be detected. Since contrast sensitivity is highest here, noise masking, relatively, has the greatest effect. So far. results have been considered by grouping all amblyopes together. However, amblyopes are generally classed into two separate main types, strabismic and anisometropic, and differences have been reported for these categories. Contrast sensitivity for sinusoidal gratings is reduced over a broad range of spatial frequencies in both types 2433 " 35 and in meridional amblyopia as well, in which deficiencies are found for specific grating orientations However, spatial localization, which is markedly reduced in amblyopia, 38 appears to be differentially affected in strabismic and anisometropic amblyopia Another apparent difference between the major types of amblyopia is that strabismic but not anisometropic varieties exhibit distortions of positional information at low spatial frequencies. 41 In the present study, we find the effects of noise to be indistinguishable between anisometropic and strabismic amblyopes. One of the difficulties with this type of comparison is that strabismus and anisometropia often occur together. In our sample, for example, four of the 10 strabismic subjects have substantial anisometropia. It is also possible that some of the anisometropes have small amounts of strabismus that are not readily detected by standard clinical tests. These factors suggest that strabismic and anisometropic types of amblyopia are difficult to clearly differentiate. onoptic and dichoptic effects of noise and grating presentations are equivalent. In the dichoptic case, the masking effect is presumably mediated by intracortical connections. It is important to point out that a dichoptic masking effect does not allow a firm conclusion concerning location of the process. In the present case, noise could mask the grating at peripheral levels and also at a cortical level in the dichoptic case. The dichoptic masking could be caused in part by binocular rivalry. It is therefore difficult to differentiate the source or sources of the interaction. However, the fact that dichoptic results are the same for normal and amblyopic subjects suggests that the presumably few binocular neurons that function in amblyopes are sufficient to provide a complete masking effect. Alternatively, effects could be caused by inhibition between monocular neurons. It would have been instructive to compare dichoptic effects for noise presented first to the amblyopic and then to the nonamblyopic eyes. In this case, the prediction would be that intracortical inhibitory input to the amblyopic eye is relatively stronger than that in the other direction. However, as noted above, we found it possible procedurally to present noise only to an eye that seemed dominant for the conditions of our tests. It may be possible to overcome this limit to some extent by using brief presentations. The fact that effects of noise are identical in amblyopic and normal subjects should be noted in conjunction with the finding that noise masking is independent of the degree of amblyopia. The implication here is that the site of amblyopia may be at a stage of visual processing beyond that of the noise masking. In addition, since monoptic and dichoptic effects are the same, the amblyopic defect may occur mainly upstream of the site of binocular interaction, ie, beyond striate cortex. On the other hand, these effects could be explained by a change of gain at a given site of convergence or by inhibitory interactions that are not affected by amblyopia. It is also important to note that our noise mask affected mainly middle and low spatial frequencies and the most marked deficits in amblyopia occur at high spatial frequencies. Therefore, a different noise mask, that affects primarily high spatial frequencies, might elicit effects which depend on the degree of amblyopia. Since response properties of single neurons in the LGN and visual cortex have been studied with noise as the stimulus, it is of interest to examine other possible physiological implications of our results. The most strikingfindingof previous physiological tests in the visual cortex is that complex but not simple cells are reported to be responsive to noise. 42 However, recent studies suggest that these two cell types respond similarly to noise or texture Each result has different implications. or example, if simple cells are suppressed and only complex cells are activated by noise, the suppression is likely to be due to intracortical inhibitory input from complex cells. However, if both simple and complex cells are activated, alternative mechanisms of suppression are possible. In either case, dichoptic sequences in which gratings are presented to one eye and noise to the other would determine if the suppression of cortical cells by texture is mediated by intracortical inhibition. If simple and complex cells are differentially activated by texture, inferences about these cell types may be made from psychophysical tests. However, since it currently appears that this is not the case, psychophysical and physiological masking effects of texture must be compared without respect to cell type. In summary, the main finding of the present experi

10 2984 INVESTIGATIVE OPHTHALOLOGY 6 VISUAL SCIENCE / Seprember 1992 Vol. 33 merits is that the masking effects of noise are indistinguishable in amblyopes and in subjects with normal binocular vision. This result is in accord with those of Levi et al, 17 who describe strong dichoptic masking effects in stereoblind subjects. However, Blake 20 reports that for normal observers, binocularly viewed noise has a much greater effect than that seen monocularly (by around 70%). This result is attributed to binocular summation. A stereoblind subject tested in the same way did not exhibit this summation, ie, there was no dichoptic masking. As Blake points out, his study and that of Levi et al used methods that were considerably different. In the former case, a standard binocular summation task was used and in the latter study, an interpretation was made in terms of binocular inhibition. Our present results and those of Levi et al suggest that the binocular neurons which remain in subjects with defective binocular vision are capable of transmitting information from masking stimuli in a robust manner. In other work, we have found that cells which appear to be monocular when tested through one eye at a time are often clearly binocular when tested dichoptically. 45 It is possible that these types of cells mediate noise masking in subjects with abnormal binocular vision. Key words: amblyopia, contrast sensitivity, noise masking Acknowledgments The authors thank R. Blake, 1. Ohzawa, and J. G. Robson for their helpful comments on a draft of this manuscript. References 1. Gilinsky AS: Oricniationspccitic effects of patterns of adapting light on visual acuity. J Opt Soc Am [A] 58:13, Pantlc A and Sekuler R: Sizedetecting mechanisms in human vision. Science 162:1146, Blakcmorc C and Campbell W: On the existence of neurones in the human visual system selectively sensitive to the orientation and size of retinal images. J Physiol (Lond) 203: Bouma 11: Visual interference in the paralbveal recognition of initial and final letters of words. Vision Res 13: Anstis S: A chart demonstrating variations in acuity with retinal position. Vision Res 14: Rom C. Wcymouth W. and Kahneman D: Visual resolution and contour interaction. J Opt Soc Am [A] 53: b. 7. Loomis J: Lateral masking in foveal and eccentric vision. Vision Res 18: Jacobs RJ: Visual resolution and contour interaction in the fovea and periphery. Vision Res 19: Skottun BC and reeman RD: Perceived size of letters depends on intcrlcttcr spacing: A new visual illusion. Vision Res 23: lom C. Heath GG. and Takahashi II: Contour interaction and visual resolution: Contralateral effects. Science 142: a. 11. Anderson P. itchell DE, and Timney B: Residual binocular interaction in stereoblind humans. Vision Res 20: Lcgge GE and oley J: Contrast masking in human vision. J Opt Soc Am [A] 70:1458, Abadi RV: Induction masking: A study of some inhibitory interactions during dichoptic viewing. Vision Res 16:269, LcggcGE: Spatial frequency masking in human vision: Binocular interactions. J Opt Soc Am [A] 69:838, Pelli DG: Effects of Visual Noise. PhD Thesis. University of Cambridge, Cambridge. UK, Levi D. Harwerth RS, and Smith EL III: Humans deprived of normal binocular vision have binocular interactions tuned to size and orientation. Science 206:852, Levi D, Harwerth RS, and Smith EL III: Binocular interactions in normal and anomalous binocular vision. Doc Ophthalmol 49:303, Rentschler 1. Hilz R, and Brcttel H: Spatial tuning properties in human amblyopia cannot explain the loss of optotype acuity. Behav Brain Res 1: Hess R: A preliminary investigation of neural function and dysfunction in amblyopia: I. Sizeselective channels. Vision Res 20: Blake R: Binocular vision in normal and stereoblind subjects. Optom Vis Sci 59: Holopigian K and Blake R: Abnormal spatial frequency channels in esotropic cats. Vision Res 24: Blake R. artens W, and Di Gianfilippo A: Reaction time as a measure of binocular interaction in human vision. Invest Ophthalmol Visual Sci 19:930, Loshin DS and Levi D: Suprathreshold contrast perception in functional amblyopia. Doc Ophthalmol 55: Bradley A and reeman RD: Contrast sensitivity in anisomctropic amblyopia. Invest Ophthalmol Visual Sci 21: Wolford G and Chambers L: Lateral masking as a function of spacing. Perception and Psychophysics 33: anny, ern KN. Loshin DS. and artinez AT: The effects of practice on contour interaction. Clinical Visual Sci 3: Kelly DH: Retinal inhomogencity: I. Spatiotcmporal contrast sensitivity. J Opt Soc Am [A] 1: Bradley A and reeman RD: Temporal sensitivity in amblyopia: An explanation of conflicting reports. Vision Res 25: Stromcycr C and Julcsz B: Spatial frequency masking in vision: Critical bands and spread of masking. J Opt Soc Am [A] 62: Anderson PA and ovshon JA: Binocular combination of contrast signals. Vision Res 29: Dc Valois KK and Switkes E: Spatial frequency specific interaction of dot patterns and gratings. Proc Natl Acad Sci USA 77: Lcgge GE: Adaptation to a spatial impulse: Implications for ourier transform models of visual processing. Vision Res 16: Gstalder RJ and Green DG: Laser interferomctric acuity in amblyopia. J Pediatr Ophthalmol Strabismus 8: Hess R and Howcll ER: The threshold contrast sensitivity function in strabismic amblyopia: Evidence for a two type classification. Vision Res 17: Levi D and Harwerlh RS: Spatiotemporal interactions in anisometropic and strabismic amblyopia. Invest Ophthalmol 16: itchell DC and Wilkinson : The effect of early astigmatism on the visual resolution of gratings. J Physiol 243:739, reeman RD and Thibos LN: Contrast sensitivity in humans with abnormal visual experiences. J Physiol 247:

11 No. 10 ASKING, CONTRAST SENSITIVITY, AND ADLYOPIA / Nordmonn er ol reeman RD and Bradley A: onocularly deprived humans: Nondcpri ved eye has supernormal vernier acuity. J Neurophysiol 43:1645, Lcvi D and Klein S: Differences in vernier discrimination for gratings between strabismic and anisometropic amblyopes. Invest Ophthalmol VisSci 23:398, Levi D and Klein S: Spatial localization in normal and amblyopic vision. Vision Res 23: Bedell HE and lom C: onocular spatial distortion in strabismic amblyopia. Invest Ophthalmol Vis Sci 20: Hammond P and ackay D: Differential responsiveness of simple and complex cells in cat striate cortex to visual texture. Exp Brain Res 30:275, Gulyas B, OrbahGA, Duysens J, and acs H: Thcsuppressivc influence of moving textured backgrounds on responses of cat striate neurons to moving bars. J Neurophysiol 57:1767, Skottun BC, Grosof D, and DeValois R: Responses of simple and complex cells to random dot patterns: A quantitative comparison. J Ncurophysiol 59:1719, reeman RDand Ohzawa I: onocularly deprived cats: Binocular tests of cortical cells reveal functional connections from the deprived eye. J Ncurosci 8:2491, 1988.