Alcohol does not affect visual contrast gain mechanisms.

It has been suggested that acetylcholine plays a role in contrast discrimination performance and the regulation of visual contrast gain (Smith, 1996). Since alcohol has been shown to reduce levels of acetylcholine and contrast sensitivity, the present study measured the effects of alcohol on contrast discrimination and explored whether the deficits could be explained as a consequence of reduction in contrast gain. Detection thresholds and contrast increment thresholds under placebo and alcohol (0.06% BAC) conditions were measured in six volunteers. Alcohol was found to impair both detection and discrimination of only high spatial frequencies. However, when the base contrasts used in the increment threshold task were equal multiples of detection threshold, no alcohol-induced changes in increment thresholds were obtained at any spatial frequency. We conclude that alcohol impairs contrast discrimination performance but that no change in contrast gain mechanisms need be postulated to account for the data.     

Orientation discrimination depends on spatial frequency

Thresholds were measured for discriminating the orientation of sinusoidal gratings of varying spatial frequency, and found to decrease monotonically with increasing spatial frequency. For discrimination of high-contrast (10 times threshold) near-vertical gratings, thresholds ranged from about 1 deg at 0.04 c/deg to 0.5 deg at 0.2 c/deg, after which there was little improvement. At lower contrasts and for discriminations around a mean of 45 deg, thresholds varied more so, and continued to improve until 1 c/deg. The variation of orientation discrimination thresholds with spatial frequency follows a similar trend to the variation in orientation bandwidth of visual units over the same range of spatial frequencies. Thus the present results are consistent with recent "opponent-process" models of orientation discrimination, that predict that thresholds to be limited (at least in part) by the maximum slope of orientation selectivity of visual detectors. That thresholds for high contrast vertical gratings did not improve for frequencies higher than 0.2 c/deg implies that orientation bandwidth and noisiness of oriented detectors may not be the sole factor limiting orientation discrimination, and suggests the existence of more central noise sources.     

Suppressive effect of sustained low-contrast adaptation followed by transient high-contrast on peripheral target detection

We observed that presenting a low-contrast Gabor patch (2 cpd, 5 degrees eccentricity, contrast=4%) for 8 s and then flashing a 20-30 ms high-contrast patch over it could elicit the perceptual disappearance of a subsequent low-contrast stimulus, whereas neither low-contrast adaptation nor high-contrast flash alone had any considerable effect (p<0.00001). In other experiments we found (a) suppressive components are phase-insensitive, (b) the effect transfers between eyes, (c) suppression is selective for orientation, and (d) the induction by the transient high-contrast Gabor patch could be transferred to another previously adapted location up to a few degrees. Results indicate synergy between contrast and adaptation through a non-linear interaction between rapid gain adjustment to transient change and adaptation to sustained spatial patterns. Findings are compatible with non-local mechanisms presumably at the cortical level.     

Orientation selectivity in luminance and color vision assessed using 2-d band-pass filtered spatial noise

We evaluated orientation discrimination in color and luminance vision using an external noise paradigm. Stimuli were spatiotemporal patches of 2D orientation noise isolating the achromatic, red-green and blue-yellow mechanisms, and matched in multiples of contrast detection threshold. We found a monotonic increase of orientation discrimination thresholds with the stimuli orientation bandwidths that is similar for both color and luminance contrasts. This dependence was fitted with two suitable models. A variance summation model suggests that internal orientation noise is significantly greater for the chromatic than for the achromatic mechanisms, while the efficiencies are similar. A gain control model of orientation tuning suggests that both chromatic and achromatic mechanisms are characterized by broadly tuned orientation detectors and that the relative chromatic deficit in orientation discrimination may only result from a slightly broader orientation tuning for the chromatic mechanisms. The moderate deficiency in chromatic orientation discrimination may account for the small differences found in shape perception between color and luminance vision.     

Orientation discrimination across the visual field: matching perceived contrast near threshold

Performance can often be made equal across the visual field by scaling peripherally presented stimuli according to F=1+E/E2 where E2 is the eccentricity at which stimulus size must double to maintain foveal performance levels. Previous studies suggest that E2 for orientation discrimination is in the range of 1.5 degrees -2 degrees when stimuli are presented at contrasts well above detection threshold. Recent psychophysical and physiological evidence suggests spatial reorganization of receptive fields at near-threshold contrasts. Such contrast-dependent changes in receptive field structure might alter the amount of size scaling necessary to equate task performance across the visual field. To examine this question we measured orientation discrimination thresholds for a range of stimulus sizes and eccentricities (0 degrees -15 degrees ). We used the same procedure previously employed except that stimuli were presented at near-threshold contrasts. We controlled for the effects of perceptual contrast on thresholds through a matching procedure. A standard line of 3 degrees in length presented at fixation was set to 2 just noticeable differences above detection threshold. The perceived contrast of all other stimuli was adjusted by the subject to match this one. Orientation discrimination thresholds were then obtained at these matching contrasts for all stimulus sizes and eccentricities. E2 values of 3.42 degrees and 3.50 degrees were recovered for two subjects; these values were about a factor of two larger than E2 values previously found for this task when stimuli were presented at higher physical contrasts.     

Spatial frequency discrimination and detection characteristics for gratings defined by orientation texture

We describe evidence consistent with the proposal that the visual system contains a parallel array of size-tuned mechanisms sensitive to orientation texture-defined (OTD) form, and propose that the relative activity of these mechanisms determines spatial frequency discrimination threshold for OTD gratings. Using a pattern of short lines we measured spatial frequency discrimination thresholds for OTD gratings and luminance-defined (LD) gratings. For OTD gratings, the orientation of texture lines varied sinusoidally across the bars of the gratings, but line luminance was constant. For LD gratings, line orientation was constant, but line luminance varied sinusoidally across the bars of the grating. When the number of texture lines (i.e. spatial samples) per grating cycle was below about six, spatial sampling strongly affected both the spatial frequency discrimination and grating detection thresholds for OTD and LD gratings. However, when the number of spatial samples per grating cycle exceeded about six, plots of both discrimination threshold and detection threshold were different for OTD and LD gratings. For an OTD grating of any given spatial frequency, spatial frequency discrimination threshold fell as the number of samples per grating cycle was increased while holding texture line length constant: the lower limit was reached at six to ten samples per cycle. When we progressively increased the viewing distance (keeping the cycles per degree (cpd) constant), spatial frequency discrimination threshold reached a lower limit and increased thereafter. We propose that this minimum threshold represents a balance between opposing effects of the number of samples per grating cycle and the length of texture lines, and approaches the absolute physiological lower limit for OTD gratings. Spatial frequency discrimination was possible up to at least 7 cpd. Grating acuity for an OTD grating was considerably lower than the physiological limit for LD gratings, presumably because detectors of OTD form include a spatial integration stage following the processing of individual lines. For an LD grating, discrimination threshold fell as the number of samples per grating cycle was increased and asymptoted at six to ten samples per cycle. Spatial frequency discrimination thresholds for OTD and LD gratings were similar at low spatial frequencies (up to 3-4 cpd), but increased more steeply for OTD gratings at high spatial frequencies. For both OTD and LD gratings, discrimination threshold fell steeply as the number of grating cycles was increased from 0.5 to ca. 2.5 cycles, and thereafter decreased more slowly or not at all suggesting that, for both OTD and LD gratings, spatial frequency discrimination can be regarded as a special case of line interval or bar width discrimination. As orientation contrast was progressively increased, discrimination threshold for an OTD grating fell steeply up to about four to five times grating detection threshold, then saturated. This parallels the effect of luminance contrast on discrimination threshold for an LD grating.     

Position-based motion perception for color and texture stimuli: effects of contrast and speed

Motion can be perceived either through low-level, motion-energy detection or through tracking the change in position of features. Previously we have shown that, while luminance-based motion likely is detected with velocity-sensitive motion-energy units, patterns defined by texture or binocular disparity ('second-order' stimuli) were tracked by a position-sensitive mechanism (Seiffert & Cavanagh (1998) Vision Research, 38, 3569-3582). Here, we use the same technique, measuring motion amplitude thresholds of oscillating gratings over a range of temporal frequencies and find that the motion of low-contrast equiluminant red/green gratings is also detected with position tracking. In addition, we find that as contrast or speed increases these results change: high-contrast or high-speed equiluminant color or texture-based motion is detected by velocity-sensitive mechanisms. These results help resolve the dispute over the processes which detect the motion of non-luminance based stimuli. Both systems are available, but their relative efficiency changes as a function of contrast and speed. A position-tracking process is more sensitive at low contrasts and low speeds whereas a motion-energy system is more sensitive at high contrasts and high speeds.     

The role of local grouping and global orientation contrast in perception of orientation-modulated textures

We explored the contribution to perception of orientation-modulated textures of visual processes selective either for orientation contrast or orientation grouping. To distinguish between these two processes we manipulated the axis of local grouping of texture elements independently of the direction of global orientation modulation. The general question posed was whether visibility of texture structure (measured as threshold for discriminating spatial-frequency of texture structure) is dependent on the magnitude of orientation contrast, strength and direction of local grouping, or some combination of the two. We demonstrated that the factor of primary importance is the amplitude of global orientation contrast rather than the presence of local grouping content. Using orientation-interleaved textures (containing two superimposed textures modulated around orthogonal orientations), we further showed that orientation single-opponent processes are a more likely candidate for detecting orientation contrast than double-opponent processes.     

Segregation of mesh-derived textures evaluated by resistance to added disorder.

In the "figure detection task" the strength of segregation for a particular texture pair was estimated by the threshold amount of added disorder that prevented segregation of a textured figure from a textured ground. Disorder was either jitter in the orientation of the texture elements, or jitter in their xy positions, or a mixture of the two. Other procedures included lowpass filtering, and a task requiring discrimination between textured figures of different shapes. Orientation cues are weakly or inconsistently used for segregating mesh textures. The low spatial harmonics are very important. A new finding is that orientation and position jitter thresholds for a set of figure/ground texture patterns are often proportional. In a mixture the one disorder can be exchanged for the other.     

A facilitation effect in orientation discrimination

The minimal stimulus for orientation discrimination consists of two spots of light which define the orientation of an imaginary line. Luminance thresholds for discrimination of orientation were measured with two 5′ test spots, separated by 10′. 20′. 30′ or 40′. located approximately 2° from the fovea. Test flashes were of 2-msec duration and varied in temporal relation from simultaneity to nearly 0.5 sec asynchrony. When measurements were made by an ascending method of limits with both test flashes increasing together, luminance thresholds for orientation discrimination were close to light detection thresholds and were uninfluenced by the temporal relation. When one of the flashes was presented at a constant luminance 0.6 log units above detection threshold and the luminance of the other was the dependent variable, the luminance threshold for discrimination of orientation of the two spots varied with their temporal and spatial relations. For 30′ separation it was approximately 0.4 log units below light detection threshold when the variable luminance spot preceded the fixed luminance spot by about 140 msec for each of two observers. Results with haploscopic presentation suggest that the effect may represent facilitation at the cortex.     

The three dimensions of human visual sensitivity to first-order contrast statistics

This work studies the preattentive discrimination of achromatic textures composed of mixtures of different (Weber) contrasts. These textures differ not at all in local spatial structure, but only in the relative proportions of different contrasts they comprise. It is shown that, like chromatic discrimination, preattentive discrimination of such textures is three-dimensional. The current results do not uniquely determine the characteristics of the three texture filters mediating human discrimination of these textures; they do, however, define the space of textures with 4th-order polynomial histograms to which human vision is sensitive. Three real-valued functions of contrast that collectively capture human sensitivity to the textures in this space are presented.     

Asymmetric dynamics of adaptation after onset and offset of flicker

We measured human psychophysical detection thresholds for test pulses which are superimposed on spatially homogeneous backgrounds that have abrupt onsets and offsets of high-contrast 25 Hz flicker. After the onset of the background flicker, test thresholds reach their steady-state levels within 20-60 ms. After the offset of the background flicker, test thresholds remain elevated above their steady-state level for much longer durations. Adaptation after onsets and offsets of background flicker is modeled with a divisive gain control that is activated by temporal contrast. We show that a feedback structure for the gain control can explain the asymmetric dynamics observed after onsets and offsets of the background contrast. Finally, we measure detection thresholds for tests presented on steadily flickering backgrounds as a function of the contrast of the background flicker. We show that the divisive feedback model for contrast gain control can describe these results as well.     

Beyond fourth-order texture discrimination: generation of extreme-order and statistically-balanced textures

Julesz introduced the concept of statistically defined textures and their perceptual discrimination. Julesz showed that discrimination was possible with statistics equated to third-order, specifying fourth-order textures. Klein and Tyler offered a variety of paradigms suggesting that fourth order might be the limit on human texture processing. To go beyond this limit, new texture paradigms are now introduced to avoid contamination by luminance extrema, to control local and long-range texture properties, and to provide textures without global statistical structure. Local luminance contamination is avoided by novel orientation plaids, in which higher-order rules govern the orientation of local elements rather than their coloring. These textures allow evaluation of texture discrimination up to thirty-second order by cortical pattern elements. Long-range processing is studied by random strip rotation and by interlacing of independent textures. Each substantially degrades the visibility of the fourth-order textures, revealing that the fourth-order information is conveyed largely by local rather than long-range perturbations from random statistics. Finally, textures equated at all orders can be defined in terms of their global statistics, but may nevertheless readily be discriminated in human vision. The discrimination on the basis of local perturbations implies that human vision assesses textures through a local sampling window, and is largely insensitive to longer-range statistical properties.     

Higher order texture statistics impair contrast boundary segmentation

Texture boundary segmentation is conventionally thought to be mediated by global differences in Fourier energy, i.e., low-order texture statistics. Here, we have examined the importance of higher order statistical structure of textures in a simple second-order segmentation task. We measured modulation depth thresholds for contrast boundaries imposed on texture samples extracted from natural scene photographs, using forced-choice judgments of boundary orientation (left vs. right oblique). We compared segmentation thresholds for contrast boundaries whose constituent textures were either intact or phase scrambled. In the intact condition, all the texture statistics were preserved, while in the phase-scrambled condition the higher order statistics of the same texture were randomized, but the lower order statistics were unchanged. We found that (1) contrast boundary segmentation is impaired by the presence of higher order statistics; (2) every texture shows impairment but some substantially more than others; and (3) our findings are not related to scrambling-induced changes in detectability. The magnitude of phase-scrambling effect for individual textures was uncorrelated with variations in their amplitude spectra, but instead we suggest that it might be related to differences in local edge structure or sparseness.     

The contributions of figure and ground textures to segmentation.

Several models of texture segmentation use spatial gradients in the activity of early filters to locate texture boundaries. The models assume that these filters are identical to those involved in the detection and discrimination of near threshold patterns. The models differ in how activity gradients from different types of filters are combined. We examined this question by measuring the respective contributions of a figure and a ground texture to segmentation. Vertical and horizontal line segments were used to construct two perfectly discriminable textures and these textures were used to construct four types of displays. Each display contained an obliquely oriented figure, but the displays differed in the way this figure was defined. Displays consisted of either (1) a horizontally textured figure on a blank background, (2) a blank figure on a vertically textured background, (3) a horizontally textured figure on a vertically textured background or (4) a figure with a mixed texture (50% vertical lines, 50% horizontal lines) on a blank background. In a two-alternative forced-choice experiment, observers were asked to judge the figure's orientation (right or left oblique), and the contrast of the textures was varied across trials. The resulting psychometric functions for segmentation were very similar for the four types of displays, suggesting ways in which a simple model of segmentation should be modified.     

Sensitivity and configuration-specificity of orientation-defined texture processing in infants and adults

Here we use textures made up of widely spaced Gabor patches to compare infant and adult sensitivity to the global organization of the elements comprising the textures. Visual Evoked Potentials (VEPs) were recorded to alterations between random images and images containing varying proportions of patches that were of the same orientation. The patches were placed on rectangular, hexagonal or random lattices. Texture-specific responses were robust in adults and their VEP threshold was reached when 1-17% of the patches had the same orientation in the structured image. Infant thresholds were approximately 20-60%. While infants are capable of detecting the global structure of our textures, their sensitivity is low. In adults we found, unexpectedly, that sensitivity and response gain were higher for horizontal compared to vertical global orientations. Infant sensitivity was the same for the two orientations. Comparable orientation anisotropies have not been previously reported for gratings, suggesting that the Gabor-defined textures are tapping different mechanisms. There were small, but measurable effects of the lattice type in adults, with the rectangular lattice producing the largest responses.     

Dual nonlinearities regulate contrast sensitivity in pattern discrimination tasks

Many current psychophysical models propose that visual processing in cortex is hierarchical, with nonlinearities sandwiched between linear stages of processing. In earlier publications, we proposed a model of this type to account for masking effects found with spatial frequency and orientation discriminations. Our model includes two nonlinear mechanisms that regulate contrast sensitivity in early cortical mechanisms. The first is a local within-pathway nonlinearity that accelerates at low contrasts but is compressive at high. The second is a pooled nonlinear gain control process that operates over a broad range of neurons with different tuning characteristics. Here, we test predictions of the model for spatial frequency discriminations. The model predicts that at low contrasts, adding a grating mask oriented parallel to test gratings will improve discrimination performance via operation of the within-pathway nonlinearity, analogous to the "dipper effect" found with contrast discriminations. Adding an orthogonally oriented mask is predicted to have no effect at low contrasts, where pooled gain control processes contribute little to performance. At high contrasts, the model predicts that performance will asymptote and become independent of contrast with either parallel or orthogonal masks. The results confirm model predictions.     

Attentional effects on contrast discrimination in humans: evidence for both contrast gain and response gain

In order to understand how attention affects visual processing, we investigated the degree to which attention effects can be accounted for by increases in the contrast gain of the contrast response function, CRF (represented by an increase in effective contrast) vs. increases in the response gain (represented by an overall amplification of response). To this end, we used a dual-task paradigm to compare psychophysical "threshold vs. pedestal contrast" (TvC) curves obtained under conditions of full- vs. poor-attention. The attention effect, defined as the ratio of thresholds for poor- vs. full-attention conditions, was roughly four-fold at a pedestal contrast of 0% (i.e., at detection threshold) and there was a significant decrease in attention effect with increasing pedestal contrast, from approximately ten-fold at the lowest non-zero pedestal contrast tested (0.25%) to three-fold at the highest pedestal contrast tested (64%). These findings are consistent with the existence of both contrast gain effects of attention (needed to account for the substantial attention effect at detection threshold and the decrease in attention effect with increasing pedestal contrast) as well as response gain effects of attention (needed to account for the fact that attention was beneficial across all pedestal contrasts-rather than harmful at some contrasts, as a pure contrast gain model would predict). The results of a model fitting Naka-Rushton CRF equations to the TvC data also support this conclusion. Here we found a two-fold increase in contrast gain and a five-fold increase in response gain in the CRF for the full-attention, as compared to the poor-attention, condition. Because pure contrast gain effects, on the order of two-fold, have been observed at early stages of visual processing (for example in areas V4 and MT), our psychophysical results suggest a hybrid model of attention; contrast gain control at an early stage of visual processing, followed by response gain control at a later stage.     

Effects on grating detection of vertically displaced peripheral gratings

Contrast threshold for a sinusoidal target in the simultaneous presence of the vertically displaced peripheral gratings was measured as a function of the peripheral contrast and the separation. When the signal and peripheral gratings were in phase, the low-contrast peripheral gratings produced the improvement in signal threshold, while the high-contrast gratings produced threshold elevation. The facilitation effect was extended to the separation equating to 10 grating cycles, whereas the inhibition effect was restricted to the region near the border. When the two gratings were out of phase, the low-contrast peripheral gratings produced threshold elevation, while the high-contrast gratings exerted little effect on signal threshold. The results were explained in terms of spatial summation and lateral inhibition.     

Two-dimensional Aspect Ratio Discrimination for Shape Defined by Orientation Texture

A pattern of 12,860 short (0.15 × 0.05 deg) texture lines contained an orientation texture-defined (OTD) rectangle of aspect ratio a/b (a was the rectangle's height and b was its width). All the lines within the rectangle had the same orientation and all the lines outside the rectangle had the same orientation. These two orientations were θ deg symmetrically clockwise and anticlockwise of the vertical. The rectangle's visibility was created entirely by the orientation difference 2θ. Aspect ratio discrimination threshold for the texture-defined rectangle was a U-shaped function of θ that was approximately symmetrical about θ = 45 deg. The lowest values of aspect ratio discrimination threshold were 2.8% (SE = 0.1%), 2.7% (SE = 0.1%) and 5.1% (SE = 0.3%) for three observers. A luminance-defined (LD) rectangle with the same spatial sampling as the OTD rectangle was created by removing all texture lines outside the rectangle. Aspect ratio discrimination threshold for the LD rectangle was 1.1% (SE = 0.1%), 1.7% (SE = 0.1%) and 2.2% (SE = 0.1%)) for the same three observers. Although these discrimination thresholds were not greatly lower than discrimination thresholds for the OTD rectangle, they were significantly lower at the P < 0.001 level. Discrimination thresholds for the OTD rectangle are comparable with the lowest aspect ratio discrimination thresholds for motion-defined (MD) rectangles (2 and 3% for two observers), and for disparity-defined (DD) rectangles (3.1, 3.4, 4.0 and 7.4% for four observers), even though the MD and DD rectangles were much smaller than the 185 deg² OTD rectangle used in the present study. Copyright © 1996 Elsevier Science Ltd.