The relationship between the subjective and objective aspects of visual filling-in

AbstractWe explored the relationship between filling-in processes and the known increase in detection sensitivity observed for targets presented between collinear flankers. Filling-in was probed using a Yes/No detection task by measuring the false-positive reports (false-alarm, FA) and hit rate (Hit) for a low-contrast Gabor target with different target-flankers distances. Observers increased the number of reports on the presence of a target (FA and Hit) when the flankers’ distance was within the known range of facilitatory lateral interactions. This bias in reporting was reduced with blocked stimulation, when the target-flanker distance was kept fixed across trials. When different distances were mixed by trials the bias followed the pattern of lateral interactions across distance. The effect was maximal when flankers and targets were aligned. These false perceptions are most likely the result of a filling-in process by lateral excitation that produces illusory contours

Elongated physiological summation pools in the human visual cortex1Preliminary results were reported at the 1996 Annual Meeting of the Society for Neuroscience: [50].1

AbstractThe visibility of gratings improves with increasing stimulus area. This effect is usually interpreted as being due to probability summation between the outputs of linear, independent spatial filters, although non-linear spatial summation can have similar effects [1]. In order to distinguish between probabilistic and physiological summation models, we measured contrast thresholds using the Visual Evoked Potential (VEP). Our previous work [2]suggests that spatial summation in the VEP is nonlinear and that it occurs preferentially for collinear configurations. Traditional probability summation models predict that areal summation will improve threshold independent of stimulus configuration. Contrast thresholds were derived from VEP contrast response functions for either circular or elongated Gabor patches with aspect ratios up to 6:1. The carrier orientation was either the same as the patch envelope orientation (collinear) or orthogonal to it. Response amplitudes were larger and contrast sensitivity was higher for collinear configurations. The results are consistent with nonlinear, configuration dependent summation that is more extensive along the axis of orientation

Abnormal Long-range Spatial Interactions in Amblyopia

AbstractNeural interactions between widely separated stimuli were explored with psychophysical and visual evoked potential (VEP) measures in normal and amblyopic observers. Contrast detection thresholds were measured psychophysically for small foveally viewed Gabor patches presented in isolation and in the presence of similar, but laterally displaced flanks. The amplitude and phase of VEPs elicited by similar targets were also measured. The presence of neural interaction between the target and flank responses was assessed by comparing the unflanked threshold to the flanked threshold in the psychophysical experiments and by comparing the response predicted by the algebraic sum of test and flank responses to that measured when test and flanks were presented simultaneously. In normal observers simultaneous presentation of test and flank targets produces a VEP response that is up to a factor of two larger than the linear prediction (facilitation). Psychophysical threshold is also facilitated by a comparable factor. Facilitation was found mainly for configurations in which local (carrier) and global (patch) orientations resulted in collinearity, independent of global orientation (meridian). Amblyopic observers showed several deviations from the normal pattern. The facilitation for the collinear configurations was either markedly lower than normal or was replaced by inhibition. The normal pattern of spatial interaction may facilitate the grouping of collinear line segments into smooth curves. In contrast, abnormal long-range spatial interactions may underlie the grouping disorders and perceptual distortions found in amblyopia. © 1997 Elsevier Science Ltd. All rights reserved

Response similarity as a basis for perceptual binding

Detection of low-contrast Gabor patches (GPs) is improved when flanked by collinear GPs, whereas suppression is observed for high-contrast GPs. The facilitation resembles the principles of Gestalt theory of perceptual organization. We propose a model for contour integration in the context of noise that incorporates a temporal element into this spatial architecture. The basic principles are (1) the response increases with increasing contrast, whereas the latency decreases; (2) activity-dependent interactions: facilitation for low and suppression for high activity; (3) the variance increases with contrast for responses, rates, and latency; and (4) inhibition has a shorter time constant than excitation. When a texture of randomly oriented GPs is presented, the response to every element decreases due to fast inhibition between the neighboring elements, shifting the activity toward the range of collinear facilitation. Next, the slower excitation induces selective facilitation along the contour elements. Consequently, the response to the contour increases, whereas the variance of the rate and latency decreases, providing better temporal correlation between the contour elements. Thus, collinear facilitation increases the saliency of contours. Our model may suggest a solution to the binding problem by bridging between the temporal and spatial aspects of lateral interactions that determine the encoding of perceptual grouping

Training improves visual processing speed and generalizes to untrained functions

Studies show that manipulating certain training features in perceptual learning determines the specificity of the improvement. The improvement in abnormal visual processing following training and its generalization to visual acuity, as measured on static clinical charts, can be explained by improved sensitivity or processing speed. Crowding, the inability to recognize objects in a clutter, fundamentally limits conscious visual perception. Although it was largely considered absent in the fovea, earlier studies report foveal crowding upon very brief exposures or following spatial manipulations. Here we used GlassesOff's application for iDevices to train foveal vision of young participants. The training was performed at reading distance based on contrast detection tasks under different spatial and temporal constraints using Gabor patches aimed at testing improvement of processing speed. We found several significant improvements in spatio-temporal visual functions including near and also non-trained far distances. A remarkable transfer to visual acuity measured under crowded conditions resulted in reduced processing time of 81 ms, in order to achieve 6/6 acuity. Despite a subtle change in contrast sensitivity, a robust increase in processing speed was found. Thus, enhanced processing speed may lead to overcoming foveal crowding and might be the enabling factor for generalization to other visual functions