Detecting and remembering pictures with and without visual noise

Objects in a scene are often partially occluded without causing the viewer any problem: the occluded parts are apparently represented via amodal completion. To evaluate human ability to perceive and remember partially occluded pictures, we showed sequences of pictures using rapid serial visual presentation (RSVP) for durations of 53 ms, 107 ms, 213 ms, or 426 ms/picture. Participants either attempted to detect a named target (e.g., "businessmen at table") or were given a yes-no recognition memory test of one item. In Experiment 1, with as much as 30% of the picture area covered, detection and recognition were both well above chance. More interestingly, occlusion significantly affected recognition memory but not target detection. In Experiment 2, when pictures were inverted, occlusion impaired detection as severely as recognition. For target detection, the interaction between occlusion and inversion was significant. By contrast, taking away color information did not significantly reduce detection's tolerance of occlusion (Experiment 3). Finally, Experiment 4 showed that with 40% of the picture area occluded, detection performance was impaired. These results support the hypothesis that contextual gist information facilitates visual processes that tolerate occluding noise. Although inversion and color were tested in particular, the presented paradigm can also be used to investigate the role of other factors in gist representation.     

The mechanisms of collinear integration

Low-contrast visual contour fragments are easier to detect when presented in the context of nearby collinear contour elements (U. Polat & D. Sagi, 1993). The spatial and temporal determinants of this collinear facilitation have been studied extensively (J. R. Cass & B. Spehar, 2005; Y. Tanaka & D. Sagi, 1998; C. B. Williams & R. F. Hess, 1998), although considerable debate surrounds the neural mechanisms underlying it. Our study examines this question using a novel stimulus, whereby the flanking "contour" elements are rotated around their own axis. By measuring contrast detection thresholds to a brief foveal target presented at various phases of flanker rotation, we find peak facilitation after flankers have rotated beyond their collinear phase. This optimal facilitative delay increases monotonically as a function of target-flanker separation, yielding estimates of cortical propagation of 0.1 m/s, a value highly consistent with the dynamics of long-range horizontal interactions observed within primary visual cortex (V1). A curious new finding is also observed: Facilitative peaks also occur when the target flash precedes flanker collinearity by 20-80 ms, a range consistent with contrast-dependent cortical onset latencies. Together, these data suggest that collinear facilitation involves two separate mechanisms, each possessing distinct dynamics: (i) slowly propagating horizontal interactions within V1 and (ii) a faster integrative mechanism, possibly driven by synchronous collinear cortical onset.     

Aging effects on collinear facilitation

Normal aging has been shown to alter performance on several suprathreshold spatial tasks such as contour integration and perceptual measures of center-surround interactions. The purpose of this study was to investigate the effects of aging on collinear facilitation. Despite all related lateral interactions that are presumed to involve neural architecture within primary visual cortex, collinear facilitation differs from contour integration and surround suppression tasks in that it is a purely foveal, threshold phenomenon. Collinear facilitation was measured for 20 younger (19-31 years) and 15 older (59-71 years) adults with measures repeated over two identical test sessions. Contrast thresholds were measured for a central Gabor patch in the presence of flankers of varying interelement distances and orientations. A reduced magnitude of facilitation was found for the older observers. Our results demonstrate abnormalities of spatial interactions in older adults.     

Temporal asymmetry of collinear lateral interactions

The visibility of a target improves when the target is presented simultaneously or with a delay between two collinear flankers. Here we tested the temporal properties of lateral facilitation by manipulating the timing of both onsets and offsets of the target and flankers. The results show that lateral facilitation critically depends on the order of presentation of the target and flankers. A typical pattern of lateral interactions was observed when the flankers preceded the target, or were presented simultaneously with it, but not when the target preceded the flankers. This result is incompatible with a feedforward account of lateral interactions, according to which the two temporal effects are linearly summed within a higher level receptive field. We propose that both facilitation and masking are the result of excitatory and inhibitory interactions within neuronal networks. The temporal asymmetry can be accounted for by assuming different time courses for the excitatory and the inhibitory interactions. Although the excitation is slow to develop, lagging behind the stimulus both in onset and offset, inhibition is rapid and follows the onset and offset of the stimulus more closely. Such a network exhibits fast transitions between slow processes.     

Backward masking suppresses collinear facilitation in the visual cortex

Perceptual facilitation in detecting low-contrast Gabor patches (GPs) is induced by collinearly oriented high-contrast flankers. Our recent Visual Evoked Potentials (VEPs) study provided new physiological evidence for these collinear interactions, reflected by nonlinear modulation of multiple waveform components and frequencies [Sterkin, A., Yehezkel, O., Bonneh, Y. S., Norcia, A., & Polat, U. (2008). Multi-component correlate for lateral collinear interactions in the human visual cortex. Vision Research, 48(15), 1641-1647]. Here we used VEPs to study the temporal structure of this process. Low-contrast, foveal target GP (T) was simultaneously flanked by two collinear high-contrast GPs with a spatial separation that induces facilitation of T (lateral masking, LM). Another mask, identical to LM, was presented at different time-intervals (ISIs) after LM (backward masking, BM-on-LM). The responses were compared to separate waveforms evoked by T-alone and mask-alone at different ISIs. BM canceled the physiological markers of facilitation at an ISI of 50 ms, in agreement with earlier psychophysical findings, whereas no BM effect on T-alone was observed. This ISI coincides with the active time-window of lateral interactions, confirming our working model. The waveform amplitude of the negative N1 peak of LM was modulated toward the linear prediction of no interactions and the spectrum was shifted toward suppression, with no evidence of facilitation. Moreover, the P1 peak amplitude of BM was decreased at the same ISI, indicating that there is a mutual interference in cortical representation of both events. Waveform subtraction between BM-on-LM and LM suggests a mechanism of extended persistence of the target representation underlying facilitation in LM. We suggest an explanation for the role of improved detection of collinear stimuli in grouping of contours.     

Multi-component correlate for lateral collinear interactions in the human visual cortex

Perceptual facilitation, a decrease in detection threshold for low-contrast Gabor patches (GPs) occurs when the GP is flanked by collinearly oriented high-contrast patches. There is earlier evidence suggesting a spatial architecture of excitatory and inhibitory interactions. Here we used Visual Evoked Potentials (VEPs) to study the temporal structure of this process. We measured VEPs elicited by a foveal near-threshold target GP presented in isolation (T), T in the presence of two flanking collinear high-contrast GPs (lateral masking, LM), or the flankers alone (F). Stimuli were presented for 50 ms every 1000 ms. The choice of the set parameters elicited behavioral facilitation of T detection. Significant modulation of peak amplitudes in LM compared with linearly summed waveforms elicited by T and F was found for five alternating polarity components, ranging from 65 to 290 ms after stimulus onset. In the frequency domain, suppression at lower frequencies (up to 0.8 log units) was followed by facilitation at higher frequencies (4-6 Hz, up to 0.8 log units). Although no differences in the latencies were found, lateral interactions were reflected by non-linear waveform modulation of multiple components and frequencies, including components as early as 65-75 ms. Spectrum analysis suggests that both suppression and facilitation may be found for the same configuration of stimuli, simultaneously, distributed at different temporal frequencies and/or sources. The physiological correlates of lateral interactions may thus originate at multiple sources, only some of which are explicitly facilitatory. The final perceptual outcome of this complex spatio-temporal representation is determined by combining sensory and cognitive factors.     

The dynamics of collinear facilitation: fast but sustained

The mechanism by which flanking Gabors facilitate the detection of a central test Gabor is not well understood. Since a knowledge of the dynamics of this effect will help constrain the class of possible model, we conducted three different but interrelated experiments designed to assess different aspects of the dynamics associated with this facilitation. In experiment 1, collinear facilitation was measured at different onset times of a test target for flanks whose contrast was sinusoidally-modulated at 1 Hz. In experiment 2, the order between test target and flanks was investigated by varying the SOA, both stimuli being presented for 50 ms. Experiment 3 assessed temporal summation with and without the flanks. The results obtained do not support either a single channel masking explanation which predicts transient dynamics or an explanation-based solely on the conduction of facilitatory impulses from flanks to target via long-range horizontal connections which predicts transient but delayed dynamics. The results suggest that the dynamics of facilitation are fast but sustained. We propose two underlying mechanisms, a rapid signal to initiate facilitation across large retinal distances, based on feedback from higher centers and a sustained facilitative response based on the temporal integration of locally-responsive, lower-level mechanisms.     

Facilitation from collinear flanks is cancelled by non-collinear flanks

Detection of a central Gabor pattern is facilitated by the presence of collinear flanking patterns. We find that this facilitation is greatly reduced when the collinear flanks are combined with non-collinear flanks to form a coherent surround. These results are unlikely to be explained by mechanisms that merely transduce local contrast in a nonlinear fashion. A model wherein the outputs of such mechanisms are combined anisotropically provides a better account for these results.     

Detecting disorder in spatial vision

In normal foveal vision, visual space is accurately mapped from retina to cortex. However, the normal periphery, and the central field of strabismic amblyopes have elevated position discrimination thresholds, which have often been ascribed to increased 'intrinsic' spatial disorder. In the present study we evaluated the sensitivity of the human visual system (both normal and amblyopic) to spatial disorder, and asked whether there is increased 'intrinsic' topographical disorder in the amblyopic visual system. Specifically, we measured thresholds for detecting disorder (two-dimensional Gaussian position perturbations) either in a horizontal string of N equally spaced samples (Gabor patches), or in a ring of equally spaced samples over a wide range of feature separations. We also estimated both the 'equivalent intrinsic spatial disorder' and sampling efficiency using an equivalent noise approach. Our results suggest that both thresholds for detecting disorder, and equivalent intrinsic disorder depend strongly on separation, and are modestly increased in strabismic amblyopes. Strabismic amblyopes also show markedly reduced sampling efficiency. However, neither amblyopic nor peripheral vision performs like ideal or human observers with added separation-independent positional noise. Rather, the strong separation dependence suggests that the 'equivalent intrinsic disorder' may not reflect topographic disorder at all, but rather may reflect an abnormality in the amblyopes' Weber relationship.     

Detecting disparity in two-dimensional patterns

One can measure the disparities between two retinal images in several different ways. Experiments were conducted to identify the measure that is invariant at the threshold for detecting the disparity of two-dimensional patterns. The patterns used were stereo plaids, which permit a partial dissociation between the disparity of the pattern and the disparities of its one-dimensional components. For plaids with near-horizontal disparities, thresholds are limited by a disparity phase shift equal to the threshold phase shift for single gratings. For non-horizontal disparities, thresholds are elevated, yet are still phase-limited. In no disparity direction are thresholds for detecting disparity determined by the spatial extent of the plaids' disparity. Effects of the number and the orientation of components with task-relevant disparities indicate that plaid thresholds are limited by the disparity of the plaid's one-dimensional components. No evidence was found that these components form any higher-order pattern that can be used in detecting disparity. Oblique and near-vertical disparities generate elevated thresholds at a stage beyond component disparity detection. This second stage combines component disparities, which are ambiguous about depth, into pattern disparities capable of supporting veridical depth perception.     

Dynamics of collinear contrast facilitation are consistent with long-range horizontal striate transmission

It is well established that activity of striate neurons may be either facilitated or suppressed by visual stimuli presented outside of their classical receptive field (CRF) limits. Whilst two general mechanisms have been identified as candidates for these contextual effects; those based on extra-striate feedback and long-range horizontal striate connections; the physiological data supporting these models is both ambiguous and inconsistent. Here we investigate psychophysically the phenomenon of collinear facilitation, in which contrast detection thresholds for foveally presented Gabor stimuli are reduced via concurrent presentation of remote collinear flankers. Using backward noise masking, we demonstrate that the minimum exposure duration required to induce facilitation increases monotonically with greater target-flanker separation. The inferred cortical propagation velocities of this process (0.10-0.23 ms(-1)) closely correspond with depolarising activity observed to travel across striate cortex of several species. These dynamics strongly suggest that contrast facilitation is mediated via long-range horizontal striate connections. This conclusion complements a recent suggestion that collinear induced long-range suppressive dynamics depend on extra-striate feedback.     

Induced internal noise in perceptual artificial scotomas created by surrounding dynamic noise

Research has shown that exposure to a homogeneous gray patch surrounded by a dynamic noise background causes filling-in of the artificial scotoma by the twinkling noise from the surround. When the background is switched off, observers report perception of a prolonged patch of twinkling noise in the unstimulated area. We studied the effects of exposure to a centrally presented artificial scotoma and the twinkling aftereffect on the threshold for detecting a foveal Gabor patch embedded in external scotoma noise. The detection thresholds were mainly elevated in the absence of scotoma noise and less affected at higher levels of scotoma noise. The analysis of the experimental data using the equivalent input noise approach revealed that the reduced contrast sensitivity is due to induced internal noise whose variance is proportional to the strength of the surrounding noise. We did not find significant effects on the internal noise in a control experiment using flickering Gaussian noise samples of 1.6 Hz which did not cause filling-in and dynamic afterimage. These findings suggest that the perceptual phenomena caused by artificial scotomas may reflect increased variability of neural activity due to long-range interactions between the surrounding noise and unstimulated region of the artificial scotoma.     

Contrast thresholds in additive luminance noise: Effect of noise temporal characteristics

This study investigated the way in which the temporal properties of additive luminance noise influence threshold contrast and affect estimates of equivalent noise and sampling efficiency. Threshold contrast was obtained from four visually normal observers for a 2-cycle-per-degree Gabor patch across a range of target durations in the absence and presence of additive luminance noise that was either static or dynamic. In addition, the temporal relationship between target and noise was either synchronous (simultaneous presentation of both) or asynchronous (noise duration longer than target duration). For both synchronous and asynchronous presentation modes, the extent of temporal integration differed for targets presented in dynamic vs. static noise. Furthermore, for a fixed-duration target, increasing the degree of temporal asynchrony between target and noise monotonically increased threshold contrast in dynamic noise, but had a non-monotonic effect on threshold contrast in static noise. For both dynamic and static noise, estimates of equivalent noise and sampling efficiency were dependent on the degree of temporal asynchrony between target and noise. The observed differences between the effects of dynamic and static noise are consistent with a previous proposal that detection of targets of intermediate spatial frequency in the presence of these two noise types is governed by sustained-like and transient-like visual mechanisms, respectively.     

Detecting color vision in a malingerer

A patient describing himself as totally color blind was ordered by the judicial system to have his color vision investigated in order to establish his suitability for military service. Basic clinical (Farnsworth Panel D-15, Moreland and Rayleigh anomaloscope equations), electroretinographic (ERG) and psychophysical techniques (spectral sensitivities) were applied to determine the extent of his color discrimination performance and cone function. These standard procedures were complemented by a test for cone interaction (transient tritanopia) and by newly developed cone-isolating flicker large-field ERG recordings. The patient's data consistently indicate the function as well as the functional interaction of the middle-wavelength-sensitive (M-) and the short-wavelength-sensitive (S-) cones. But the function of the long-wavelength-sensitive (L-) cones was completely absent. Hence the patient was correctly demonstrated to be a protanope. This study establishes that standard classical procedures, in combination with newly developed and easy to apply psychophysical and ERG ones, which can be reliably used to assess true color discrimination performance, in difficult cases of malingering.