Standard definitions of chromatic induction fail to describe induction with S-cone patterned backgrounds

Inducing patterns that selectively stimulate the S cones can induce large shifts in color appearance. For example, a “peach” test-ring presented within contiguous purple and non-contiguous lime inducing rings appears pink while the physically identical peach test-ring appears orange when presented within contiguous lime and non-contiguous purple inducing rings (Fig. 1c). These shifts have been accounted for by a neural substrate which predicts that chromatic assimilation and simultaneous contrast can operate synergistically to produce large shifts with these patterns [Monnier, P., & Shevell, S. K. (2004). Chromatic induction from S-cone patterns. Vision Research, 44, 849-856].Here, induction was measured for test-rings that stimulated the S cones either more or less than did the inducing rings. According to standard definitions of induction, color shifts for test s-chromaticities either lower or higher than both inducing chromaticities should be attenuated compared to test-rings of intermediate S-cone stimulation. On the other hand, a previously proposed model of induction predicted independence of the color shifts with test-ring s-chromaticity. Consistent with standard definitions of induction, a reduction in the magnitude of the color shifts for test-ring chromaticities either lower or higher in S-cone excitation than the inducing chromaticities was observed. Additional measurements with patterns that have been shown to isolate assimilation and simultaneous contrast were conducted. For these patterns, expectations based on standard definitions of induction suggested that the magnitude of the color shifts should be monotonic with the S-cone stimulation of the test-ring, and the direction of the color shift should reverse for test-ring chromaticities either lower or higher than both inducing chromaticities compared to test-rings of intermediate chromaticity. In contrast, the previously proposed model of induction based on a receptive-field with S-cone spatial antagonism predicted the color shifts should be independent of the test-ring chromaticity (Monnier & Shevell, 2004). Color shifts were generally independent of the level of the test-ring chromaticity, supporting the S-cone antagonistic model of induction.     

Brightness contrast and assimilation from patterned inducing backgrounds

Theories of induction propose that the brightness of a test patch within a complex surround is explained by local contrast or by integrating contrasts from various regions within the surround, weighted inversely with the distance from the test. Results here corroborate that brightness induction from a patterned background depends on both contiguous and non-contiguous surrounding light, but the measurements were inconsistent with any linear integration of contrast at edges within the scene. In some conditions, assimilation rather than contrast to contiguous surrounding light was observed, depending on the luminance of the light in non-contiguous regions. This finding implies that brightness induction from patterned backgrounds depends on neural processes that can cause contrast and/or assimilation, depending on the luminance relation between contiguous and non-contiguous regions. A model in the literature postulating that the influence of a non-contiguous edge is regulated by the amount of contrast at the contiguous edge can accommodate brightness induced by these patterned backgrounds.     

Chromatic induction from S-cone patterns

Chromatic induction from patterned backgrounds depends on the spatial as well as the chromatic aspects of the background light. Color appearance with patterned and uniform backgrounds was compared using chromaticities distinguished by only the S cones; all backgrounds were equivalent to equal-energy white in terms of L-cone and M-cone stimulation. The measurements showed larger shifts in color appearance with a patterned chromatic background than with a uniform background at any chromaticity within the pattern. The measurements also showed that inducing light within different spatial regions could cause opposite shifts in color appearance: inducing light near a test field shifted appearance toward the inducing chromaticity (assimilation), while the same light some distance from the test shifted appearance away from the inducing chromaticity (simultaneous contrast). The shifts in color appearance were accounted for by a neural receptive field with S-cone spatial antagonism.     

Color perception under chromatic adaptation: "supersensitivity" with dim backgrounds

The amounts of 540 nm light and 660 nm light that when mixed result in a yellow (neither reddish nor greenish) percept remain in constant ratio, regardless of the total retinal illumance level of the mixture. A similar result holds if the mixture is superimposed upon a very dim 660 nm adapting field, except that a larger proportion of 540 nm light is required than in the dark (contrary to an effect of receptor desensitization, which would result in a smaller proportion of 540 nm light). Mixtures of 460 and 579 nm lights that give yellow/blue equilibrium percepts show the same effect: under very dim 579 nm adaptation, a larger proportion of 460 nm light is required than in the dark. These results cannot be accounted for by cone sensitivity reduction nor by a postreceptoral restoring effect; they reveal an additional mechanism of adaptation.     

Apparent spatial frequency and contrast of gratings: separate effects of contrast and duration

Variations in apparent spatial frequency of sinusoidal gratings were assessed using a matching method. Perceived spatial frequency increased as contrast decreased and as duration of presentation decreased (from 320 to 20 msec). The effect of duration was not produced via changes in apparent contrast (a) because the changes in perceived spatial frequency were much the same when subjective contrasts were matched as when physical contrasts were matched and (b) because reducing duration to 20 msec produced very little change in perceived contrast, despite a factor of four reduction in threshold contrast sensitivity. The effects of duration and contrast on apparent spatial frequency are therefore dissociable, but both might exert their effects via changes in the balance of centre-surround antagonism within receptive fields; the "selectivity shift" theory of these size perception phenomena is discussed.     

基于空间频率通道的弱视眼与正常眼对比敏感度的差异分析

背景 对比敏感度(CS)作为弱视诊疗手段之一在临床中已广泛应用,但其与空间频率通道之间的相互影响还有待研究. 目的 比较儿童弱视眼与正常眼的CS,通过空间频率通道探讨弱视眼CS缺损的原因.方法 采用OPTEC 6500型视功能测试仪对166眼正常眼和143眼弱视眼进行CS测量,通过主成分分析法和非正交旋转法推导空间频率通道的调谐曲线,由半高宽法计算通道带宽,比较通道数目和通道带宽,对弱视眼和正常眼的CS进行差异分析.空间频率通道的有效性采用43眼弱视眼CS数据进行交叉验证.结果 在空间频率为1.5、3.0、6.0、12.0、18.0cpd时,弱视眼CS分别为:36.35±21.40、50.33±33.46、46.88±41.72、16.24±17.26、4.67±5.79;正常眼CS分别为:49.49±24.69、87.23±40.87、93.18±51.99、36.63±24.72、15.70±13.87(H=27.83、66.61、68.34、78.23、89.88,P＜0.05).弱视眼和正常眼的CS存在三个空间频率通道;在空间频率峰值为3.0、6.0、12.0 cpd时,正常眼的通道带宽分别为1.03、1.02、0.99 octaves,弱视眼的通道带宽分别为1.04、1.01、0.73 octaves. 结论 弱视眼CS的缺损可能由对应空间频率通道带宽的缩小引起.     

A mechanistic inter-species comparison of spatial contrast sensitivity

The validity of the Rovamo-Barten modulation transfer function model for describing spatial contrast sensitivity in vertebrates was examined using published data for the human, macaque, cat, goldfish, pigeon and rat. Under photopic conditions, the model adequately described overall contrast sensitivity for changes in both stimulus luminance and stimulus size for each member of this diverse range of species. From this examination, optical, retinal and post-retinal neural processes subserving contrast sensitivity were quantified. An important retinal process is lateral inhibition and values of its associated point spread function (PSF) were obtained for each species. Some auxiliary contrast sensitivity data obtained from the owl monkey were included for these calculations. Modeled values of the lateral inhibition PSF were found to correlate well with ganglion cell receptive field surround size measurements obtained directly from electrophysiology. The range of vertebrates studied was then further extended to include the squirrel monkey, tree shrew, rabbit, chicken and eagle. To a first approximation, modeled estimates of lateral inhibition PSF width were found to be inversely proportional to the square root of ganglion cell density. This finding is consistent with a receptive field surround diameter that changes in direct proportion to the distance between ganglion cells for central vision. For the main species examined, contrast sensitivity is considerably less than that for the human. Although this is due in part to a reduction in the performance of both optical and retinal mechanisms, the model indicates that poor cortical detection efficiency plays a significant role.     

Spatial summation of S-cone ON and OFF signals: effects of retinal eccentricity

We studied spatial summation for S-cone ON and OFF signals as a function of retinal eccentricity in human subjects. S-cone isolation was obtained by the two-colour threshold method of Stiles, modified by adding blue light to the yellow background. Test stimuli were blue light increments or decrements within a circular area of variable size. These were presented for 100 ms at 0 to 20 deg along the horizontal temporal retinal meridian. Ricco's area of complete spatial summation was measured from the threshold vs. area curves. This was nearly constant and approximately the same for both types of stimuli within the 0-5 deg range and increased beyond this range. The decremental area increased faster, suggesting that separate mechanisms, presumably ON and OFF, integrate S-cone increments and decrements. The results appear to provide new evidence for the existence of separate S-cone ON and OFF pathways. We compare the data with known morphology of primate retina and assume that, if S-cone decrements are detected via separate OFF cells, these should differ in density and dendritic field size from the S-cone ON cells, but only in the retinal periphery.     

Cone weights for the two cone-opponent systems in peripheral vision and asymmetries of cone contrast sensitivity

In understanding the basis of the changes in human color vision across eccentricity, one key piece of information remains unknown, whether the relative cone weights of the two cone opponent mechanisms vary. Here we measure detection threshold contours within three planes in a 3-dimensional cone contrast space to reveal the L, M and S-cone weights to the two cone opponent mechanisms, L/M and S/(L+M). We find these remain constant across eccentricity suggesting the underlying structures of the cone opponent mechanisms are invariant. The contrast sensitivities of two poles of the S-cone opponent mechanism also remain symmetrical, whereas small asymmetries develop in L/M opponency from about 15 degrees.     

屈光参差性弱视治疗的空间频率评价

目的 观察弱视治疗的空间频率变化特点.方法 对屈光参差性弱视52只眼,随机分为治疗组(26只眼)和未治疗组(26只眼).治疗组采用SZ-C综合治疗仪治疗,观察治疗前后视力变化情况、不同空间频率图形翻转VEP的P100波振幅和峰潜时改变以及眩光对比敏感度不同空间频率改变.结果 治疗后视力变化差异具有统计学意义(p＜0.01),两组间P100波振幅和峰潜时差异具有统计学意义(p＜0.01),但治疗前后高空间频率P100波未见统计学差异.眩光对比敏感主要表现为高空间频率异常.结论 视力不是评价弱视疗效的惟一标准,应重视进一步的视觉功能训练.     

The effect of spatial frequency adaptation on the latency of spatial contrast detection

The effect of spatial frequency adaptation on detection response time was studied using 2-D Gabor functions as stimuli. On the basis of pilot studies, it was expected that reaction time to a given spatial frequency at a low contrast would increase following adaptation to that spatial frequency at a high contrast. Subjects were tested using 2-D Gabor functions that ranged in frequency from 25 to 24 cpd. Subjects' reaction times to the Gabor functions were measured prior to adaptation and after adaptation to a particular spatial frequency. The adapting spatial frequency was either 1, 2, 4, 8, 10, or 16 cpd. The test stimuli were 0.3, 0.5, or 0.7 log units above the unadapted threshold contrast. The subjects adapted to the high contrast test grating for 3 min (80% contrast) and reaction times were again measured in an adapt-test-readapt paradigm. The results showed the greatest increase in reaction time after adaptation when adapting and test spatial frequencies within an octave of the adapting spatial frequency also showed an increase in reaction time but to a lesser extent. Reaction times to gratings with spatial frequencies more distant from the adapting spatial frequency were not significantly affected by the adaptation. The results obtained resemble the tuning curves found for threshold data. Reaction times for stimuli at 0.5 and 0.7 log units above the unadapted threshold were affected less by adaptation than those at 0.3 log units above the unadapted threshold. These results were evaluated in terms of a shifting contrast gain mechanism which may account for both the spatial frequency specific effects of adaptation and the differences found for the different contrast test levels.     

Spatial selectivity of contrast adaptation: models and data

Contrast threshold elevation was measured in human observers at a single spatial frequency (4 or 8 c/deg) after adaptation to gratings at different contrasts and spatial frequencies. When plotted against adapting contrast, the threshold elevation functions for different adapting frequencies were neither straight nor parallel, which poses difficulties for the "equivalent contrast transformation". The different functions appear to form a single family, differing by a scaling factor that depends on spatial frequency but is independent of contrast. This is similar to expressing the aftereffect as a proportion of its peak value. A multiple channel, "fatigue" model of adaptation is shown to be consistent with the results, provided the channel shape is appropriate and channels overlap considerably in the frequency domain. Channel bandwidth is estimated at 1.4 octaves, similar to that of cortical cells.     

The effect of aging on the spatial frequency selectivity of the human visual system

Changes in the physiological properties of senescent V1 neurons suggest that the mechanisms encoding spatial frequency in primate cortex may become more broadly tuned in old age (Zhang et al., European Journal of Neuroscience, 2008, 28, 201-207). We examined this possibility in two psychophysical experiments that used masking to estimate the bandwidth of spatial frequency-selective mechanisms in younger (age ≈22 years) and older (age ≈65 years) human adults. Contrary to predictions from physiological studies, in both experiments, the spatial frequency selectivity of masking was essentially identical in younger and older subjects.     

The effect of spatial frequency and contrast on the latency in the visual evoked potential

The latency in the visual evoked potential was measured at spatial frequencies of 2–12 c/deg in 10 subjects. The contrast levels of the sinuosoidal grating patterns were set at 1.5, 1.75, 2.0, 2.25, 2.5, 2.75 and 3.0 log units above each subject's contrast sensitivity threshold. Two factors were shown to influence the latency: suprathreshold contrast and, to a lesser extent, spatial frequency. The visual evoked potential latencies at contrast sensitivity threshold were extrapolated. These threshold latencies showed considerable variation with spatial frequency and between subjects. Therefore, the visual evoked potential latency cannot be considered a useful tool for estimating the contrast sensitivity function.     

Emergence of crowding: The role of contrast and orientation salience

Crowding causes difficulties in judging attributes of an object surrounded by other objects. We investigated crowding for stimuli that isolated either S-cone or luminance mechanisms or combined them. By targeting different retinogeniculate mechanisms with contrast-matched stimuli, we aim to determine the earliest site at which crowding emerges. Discrimination was measured in an orientation judgment task where Gabor targets were presented parafoveally among flankers. In the first experiment, we assessed flanked and unflanked orientation discrimination thresholds for pure S-cone and achromatic stimuli and their combinations. In the second experiment, to capture individual differences, we measured unflanked detection and orientation sensitivity, along with performance under flanker interference for stimuli containing luminance only or combined with S-cone contrast. We confirmed that orientation sensitivity was lower for unflanked S-cone stimuli. When flanked, the pattern of results for S-cone stimuli was the same as for achromatic stimuli with comparable (i.e. low) contrast levels. We also found that flanker interference exhibited a genuine signature of crowding only when orientation discrimination threshold was reliably surpassed. Crowding, therefore, emerges at a stage that operates on signals representing task-relevant featural (here, orientation) information. Because luminance and S-cone mechanisms have very different spatial tuning properties, it is most parsimonious to conclude that crowding takes place at a neural processing stage after they have been combined.     

Intermediate spatial frequency letter contrast sensitivity: its relation to visual resolution before and during amblyopia treatment

We examined the loss of letter contrast sensitivity (LCS) measured using the Pelli-Robson chart, and the extent to which any such loss was modulated by spectacle wear and occlusion therapy in children participating in an amblyopia treatment trial. Their initial mean interocular difference in logMAR acuity was approximately three times that of their LCS (0.45 vs 0.14 log units). Log LCS was weakly though significantly correlated with logMAR visual acuity (VA) for all VAs better than 0.90 (r = -0.19, 95% CI: -0.28 to -0.10) whereas for all VAs of 0.90 or poorer, log LCS was markedly and significantly correlated with VA (r = -0.72, 95% CI: -0.83 to -0.53). LCS in those children with a > or =0.1 log unit interocular difference on this test improved commensurately with VA during treatment. We conjecture that the spatial visual loss in all but the most severe amblyopes occurs in an area of resolution and contrast space that lies beyond that sampled by the Pelli-Robson chart.     

The significance of retinal image contrast and spatial frequency composition for eye growth modulation in young chicks

PURPOSE: This study sought further insight into the stimulus dependence of form deprivation myopia, a common response to retinal image degradation in young animals. METHODS: Each of 4 Bangerter diffusing filters (0.6, 0.1, <0.1, and LP (light perception only)) combined with clear plano lenses, as well as plano lenses alone, were fitted monocularly to 4-day-old chicks. Axial ocular dimensions and refractive errors were monitored over a 14-day treatment period, using high frequency A-scan ultrasonography and an autorefractor, respectively. RESULTS: Only the <0.1 and LP filters induced significant form deprivation myopia; these filters induced similarly large myopic shifts in refractive error (mean interocular differences+/-SEM: -9.92+/-1.99, -7.26+/-1.60 D, respectively), coupled to significant increases in both vitreous chamber depths and optical axial lengths (p<0.001). The other 3 groups showed comparable, small changes in their ocular dimensions (p>0.05), and only small myopic shifts in refraction (<3.00 D). The myopia-inducing filters eliminated mid-and-high spatial frequency information. CONCLUSIONS: Our results are consistent with emmetropization being tuned to mid-spatial frequencies. They also imply that form deprivation is not a graded phenomenon.     

Correlation between spatial frequency and orientation selectivity in V1 cortex: Implications of a network model

We addressed how spatial frequency and orientation selectivity coexist and co-vary in Macaque primary visual cortex (V1) by simulating cortical layer 4Cα of V1 with a large-scale network model and then comparing the model’s behavior with a population of cells we recorded in layer 4Cα. We compared the distributions of orientation and spatial frequency selectivity, as well as the correlation between the two, in the model with what we observed in the 4Cα population. We found that (1) in the model, both spatial frequency and orientation selectivity of neuronal firing are greater and more diverse than the LGN inputs to model neurons; (2) orientation and spatial frequency selectivity co-vary in the model in a way very similar to what we observed in layer 4Cα neurons; (3) in the model, orientation and spatial frequency selectivity co-vary because of intra-cortical inhibition. The results suggest that cortical inhibition provides a common mechanism for selectivity in multiple dimensions.     

Measuring and modelling the spatial contrast sensitivity of the chicken (Gallus g. domesticus)

The spatial contrast sensitivity (CSF) of the chicken has been measured using a behavioural technique. The results obtained show that spatial vision in this species is relatively poor compared with the human observer. For a visual stimulus luminance of 16 cd m⁻², the upper frequency limit of spatial vision in the chicken (acuity) was found to be about 7.0 c deg⁻¹, with peak spatial vision occurring at around 1.0 c deg⁻¹. Under equivalent stimulus conditions, the acuity of the human is around 50 c deg⁻¹ with a peak in spatial vision at about 3.0 c deg⁻¹. Peak spatial contrast sensitivity in the chicken was also found to be only about 2% that for the human. At a lower stimulus luminance of 0.1 cd m⁻², the chicken CSF reduced in overall magnitude and indicated an acuity level of about 5.0 c deg⁻¹. These experimental results were successfully modelled using modulation transfer (MTF) theory. This theoretical treatment enabled important neural mechanisms underlying spatial vision in the chicken to be revealed. The role played by spatial vision in the chicken’s ability to recognise detailed shapes in its visual environment was also examined by deploying the CSF as a visual weighting function with the Fourier series of a chicken comb.