Binocular contrast discrimination needs monocular multiplicative noise.

The effects of signal and noise on contrast discrimination are difficult to separate because of a singularity in the signal-detection-theory model of two-alternative forced-choice contrast discrimination (Katkov, Tsodyks, & Sagi, 2006). In this article, we show that it is possible to eliminate the singularity by combining that model with a binocular combination model to fit monocular, dichoptic, and binocular contrast discrimination. We performed three experiments using identical stimuli to measure the perceived phase, perceived contrast, and contrast discrimination of a cyclopean sine wave. In the absence of a fixation point, we found a binocular advantage in contrast discrimination both at low contrasts (<4%), consistent with previous studies, and at high contrasts (≥34%), which has not been previously reported. However, control experiments showed no binocular advantage at high contrasts in the presence of a fixation point or for observers without accommodation. We evaluated two putative contrast-discrimination mechanisms: a nonlinear contrast transducer and multiplicative noise (MN). A binocular combination model (the DSKL model; Ding, Klein, & Levi, 2013b) was first fitted to both the perceived-phase and the perceived-contrast data sets, then combined with either the nonlinear contrast transducer or the MN mechanism to fit the contrast-discrimination data. We found that the best model combined the DSKL model with early MN. Model simulations showed that, after going through interocular suppression, the uncorrelated noise in the two eyes became anticorrelated, resulting in less binocular noise and therefore a binocular advantage in the discrimination task. Combining a nonlinear contrast transducer or MN with a binocular combination model (DSKL) provides a powerful method for evaluating the two putative contrast-discrimination mechanisms

Saccadic latency in amblyopia.

We measured saccadic latencies in a large sample (total n = 459) of individuals with amblyopia or risk factors for amblyopia, e.g., strabismus or anisometropia, and normal control subjects. We presented an easily visible target randomly to the left or right, 3.5° from fixation. The interocular difference in saccadic latency is highly correlated with the interocular difference in LogMAR (Snellen) acuity-as the acuity difference increases, so does the latency difference. Strabismic and strabismic-anisometropic amblyopes have, on average, a larger difference between their eyes in LogMAR acuity than anisometropic amblyopes and thus their interocular latency difference is, on average, significantly larger than anisometropic amblyopes. Despite its relation to LogMAR acuity, the longer latency in strabismic amblyopes cannot be attributed either to poor resolution or to reduced contrast sensitivity, because their interocular differences in grating acuity and in contrast sensitivity are roughly the same as for anisometropic amblyopes. The correlation between LogMAR acuity and saccadic latency arises because of the confluence of two separable effects in the strabismic amblyopic eye-poor letter recognition impairs LogMAR acuity while an intrinsic sluggishness delays reaction time. We speculate that the frequent microsaccades and the accompanying attentional shifts, made while strabismic amblyopes struggle to maintain fixation with their amblyopic eyes, result in all types of reactions being irreducibly delayed

Crowding in Peripheral Vision: Why Bigger Is Better

SummaryWe enjoy the illusion that visual resolution is high across the entire field of vision. However, this illusion can be easily dispelled by trying to identify objects in a cluttered environment out of the corner of your eye. This reflects, in part, the well-known decline in visual resolution in peripheral vision; however, the main bottleneck for reading or object recognition in peripheral vision is crowding. Objects that can be easily identified in isolation seem indistinct and jumbled in clutter. Crowding is thought to reflect inappropriate integration of the target and flankers in peripheral vision [1, 2]. Here, we uncover and explain a paradox in peripheral crowding: under certain conditions, increasing the size or number of flanking rings results in a paradoxical decrease in the magnitude of crowding—i.e., the bigger or more numerous the flanks, the smaller the crowding. These surprising results are predicted by a model in which crowding is determined by the centroids of ≈4–8 independent features within ≈0.5× the target eccentricity. These features are then integrated into a texture beyond the stage of feature analysis. We speculate that this process may contribute to the illusion of high resolution across the field of vision

“Phase capture” in amblyopia: The influence function for sampled shape

AbstractThis study was concerned with what stimulus information humans with amblyopia use to judge the shape of simple objects. We used a string of four Gabor patches to define a contour. A fifth, center patch served as the test pattern. The observers’ task was to judge the location of the test pattern relative to the contour. The contour was either a straight line, or an arc with positive or negative curvature. We asked whether phase shifts in the inner or outer pairs of patches distributed along the contour influence the perceived shape. That is, we measured the phase shift influence function. Our results, consistent with previous studies, show that amblyopes are imprecise in shape discrimination, showing elevated thresholds for both lines and curves. We found that amblyopes often make much larger perceptual errors (biases) than do normal observers in the absence of phase shifts. These errors tend to be largest for curved shapes and at large separations. In normal observers, shifting the phase of inner patches of the string by 0.25 cycle results in almost complete phase capture (attraction) at the smallest separation (2λ), and the capture effect falls off rapidly with separation. A 0.25 cycle shift of the outer pair of patches has a much smaller effect, in the opposite direction (repulsion). While several amblyopic observers showed reduced capture by the phase of the inner patches, to our surprise, several of the amblyopes were sensitive to the phase of the outer patches. We used linear multiple regression to determine the weights of all cues to the task: the carrier phase of the inner patches, carrier phase of the outer patches and the envelope of the outer patches. Compared to normal observers, some amblyopes show a weaker influence of the phase of the inner patches, and a stronger influence of both the phase and envelope of the outer patches. We speculate that this may be a consequence of abnormal “crowding” of the inner patches by the outer ones

Perceptual learning in parafoveal vision

AbstractThe present study tests the effects of practice on parafoveal vernier and resolution acuity. By measuring task specificity, transfer of training to other retinal locations in the trained eye and transfer of training to the untrained eye, we directly address whether improvement on these tasks is the result of changes in the underlying physiological processes or simply the development of new cognitive strategies. We found that: (1) significant learning can occur for both vernier and resolution acuity in many (but not all) individuals; (2) there were significant individual differences in the degree and time-course of learning: (3) learning transfers to the untrained task; and (4) learning transfers to the other eye particularly when the visual pathway leads to the trained hemisphere. These results suggest that both physiological and cognitive processes contribute to the improvement seen after repetitive practice on these visual tasks

Dynamic random noise shrinks the twinkling aftereffect induced by artificial scotomas

AbstractPhysiological alterations in cortical neurons are induced during adaptation to an artificial scotoma, a small homogeneous patch within a dynamic random noise or patterned background. When the dynamic noise is replaced by an equiluminant gray background, a twinkling aftereffect can be seen in the location of the artificial scotoma. Following binocular adaptation, we discovered that the perceived size of the twinkling aftereffect was dramatically smaller than the inducing artificial scotoma. Dichoptic adaptation induced shrinkage in the twinkling aftereffect that was similar to that found after binocular adaptation, suggesting that the twinkling aftereffect and its shrinkage both have cortical origins. We speculate that this perceptual shrinkage may reflect the interaction between two cortical mechanisms: a twinkling aftereffect mechanism that spreads throughout the artificial scotoma, and a filling-in mechanism that has a greater influence at the edges of the artificial scotoma and spreads inwards

Learning to identify contrast-defined letters in peripheral vision

AbstractPerformance for identifying luminance-defined letters in peripheral vision improves with training. The purpose of the present study was to examine whether performance for identifying contrast-defined letters also improves with training in peripheral vision, and whether any improvement transfers to luminance-defined letters. Eight observers were trained to identify contrast-defined letters presented singly at 10° eccentricity in the inferior visual field. Before and after training, we measured observers’ thresholds for identifying luminance-defined and contrast-defined letters, embedded within a field of white luminance noise (maximum luminance contrast=0, 0.25, and 0.5), at the same eccentric location. Each training session consisted of 10 blocks (100 trials per block) of identifying contrast-defined letters at a background noise contrast of 0.5. Letters (x-height=4.2°) were the 26 lowercase letters of the Times-Roman alphabet. Luminance-defined letters were generated by introducing a luminance difference between the stimulus letter and its mid-gray background. The background noise covered both the letter and its background. Contrast-defined letters were generated by introducing a differential noise contrast between the group of pixels that made up the stimulus letter and the group of pixels that made up the background. Following training, observers showed a significant reduction in threshold for identifying contrast-defined letters (p<0.0001). Averaged across observers and background noise contrasts, the reduction was 25.8%, with the greatest reduction (32%) occurring at the trained background noise contrast. There was virtually no transfer of improvement to luminance-defined letters, or to an untrained letter size (2× original), or an untrained retinal location (10° superior field). In contrast, learning transferred completely to the untrained contralateral eye. Our results show that training improves performance for identifying contrast-defined letters in peripheral vision. This perceptual learning effect seems to be stimulus-specific, as it shows no transfer to the identification of luminance-defined letters. The complete interocular transfer, and the retinotopic (retinal location) and size specificity of the learning effect are consistent with the properties of neurons in early visual area V2

Visual deficits in anisometropia

AbstractAmblyopia is usually associated with the presence of anisometropia, strabismus or both early in life. We set out to explore quantitative relationships between the degree of anisometropia and the loss of visual function, and to examine how the presence of strabismus affects visual function in observers with anisometropia. We measured optotype acuity, Pelli-Robson contrast sensitivity and stereoacuity in 84 persons with anisometropia and compared their results with those of 27 persons with high bilateral refractive error (isoametropia) and 101 persons with both strabismus and anisometropia. All subjects participated in a large-scale study of amblyopia (McKee et al., 2003). We found no consistent visual abnormalities in the strong eye, and therefore report only on vision in the weaker, defined as the eye with lower acuity. LogMAR acuity falls off markedly with increasing anisometropia in non-strabismic anisometropes, while contrast sensitivity is much less affected. Acuity degrades rapidly with increases in both hyperopic and myopic anisometropia, but the risk of amblyopia is about twice as great in hyperopic than myopic anisometropes of comparable refractive imbalance. For a given degree of refractive imbalance, strabismic anisometropes perform considerably worse than anisometropes without strabismus – visual acuity for strabismics was on average 2.5 times worse than for non-strabismics with similar anisometropia. For observers with equal refractive error in the two eyes there is very little change in acuity or sensitivity with increasing (bilateral) refractive error except for one extreme individual (bilaterally refractive error of –15D). Most pure anisometropes with interocular differences less than 4D retain some stereopsis, and the degree is correlated with the acuity of the weak eye. We conclude that even modest interocular differences in refractive error can influence visual function