Discrimination of moving gratings at and above detection threshold

Two experiments examined the discriminability of moving gratings. Experiment 1 measured the difference between detection and discrimination thresholds for gratings of equal spatial frequency drifting in the same direction at different rates. It was found that, as Watson and Robson [Vision Res.21, 1115–1122 (1981)] had found with counterphase modulated gratings, only very coarse discriminations could be made. The results suggest that just two labelled channels can account for the velocity discrimination of gratings at detection threshold. The second experiment investigated the discriminability of suprathreshold moving gratings. These results also support the idea that rate of movement is mediated by two broadly tuned channels.     

Contrast matching across spatial frequencies for isoluminant chromatic gratings

Contrast matching was performed with isoluminant red-green and s-cone gratings at spatial frequencies ranging from 0.5 to 8 c/deg. Contrast threshold curves were low-pass in shape, in agreement with previous findings. Contrast matching functions resembled threshold curves at low contrast levels, but became flat and independent of spatial frequency at high contrasts. Thus, isoluminant chromatic gratings exhibited contrast constancy at suprathreshold contrast levels in a similar manner as has been demonstrated for achromatic gratings.     

Summation and discrimination of gratings moving in opposite directions

We have measured the amount of summation occurring at threshold between gratings which move in opposite directions. The small amount of summation observed at low spatial and high temporal frequencies is approximately consistent with the action of direction-selective mechanisms, as proposed by Levinson and Sekuler (1975), provided that probability summation between such mechanisms is taken into account. However, at high spatial and low temporal frequencies much more summation is found, an amount approximately consistent with detection by directionally non-selective mechanisms.We have also measured thresholds for identifying the direction of a moving grating. For those gratings which show little summation, direction of motion is judged correctly at the detection threshold, while for those gratings which show the most summation, the identification threshold is considerably above the detection threshold.     

Visual evoked potentials for red-green gratings reversing at different temporal frequencies: asymmetries with respect to isoluminance

Human visual evoked potentials (VEPs) were recorded for abrupt reversals of 2 cycles/deg (c/deg) square-wave gratings combining high red-green contrast with different levels of luminance contrast. Response characteristics--2nd harmonic amplitudes and peak latencies as a function of luminance contrast--were compared for four different reversal rates ranging from 6.25 Hz to 12.5 Hz. At every reversal frequency, the VEP amplitude and latency plots were nonsymmetrical with respect to isoluminance. The amplitude dropped to a minimum within a region of rapid phase change, always at a red-green luminance contrast for which the green color had the higher luminance, at about 40% or 50% Michelson luminance contrast. The rapid phase shift around this contrast suggested a sudden change in the relative impact of VEP generators with different latencies, possibly dominated by parvocellular or magnocellular input. The most prominent VEP waveform through most of the luminance contrast range, P110, is interpreted in terms of a parvo-mediated response that is attenuated with increasing reversal frequency. Contrast-dependent changes in the P110 amplitude appear to be responsible for the VEP asymmetries reported here.     

Contrast detection and direction discrimination of drifting gratings

Observers performed simple detection and left right discrimination of drifting sinusoidal gratings. Ratio of detection to discrimination sensitivities was measured under variations in several experimental parameters. In the first experiment, it was found that combinations of spatial and temporal frequency which resulted in the same velocity produced similar detection discrimination ratios. At an exposure duration of 800 msec, the relationship between the ratio and velocity described a power function with the intercept at 0.6 sec^?1. Decreasing duration shifted the curve to higher velocities. I examined the effect of grating orientation in a second experiment. Visual sensitivity was poorer for oblique than for vertical gratings with detection and discrimination exhibiting similar size anisotropies. In a third experiment, observers viewed gratings presented to different retinal loci, Visual performance in both detection and discrimination fell with greater eccentricity. However, motion discrimination fell more steeply resulting in an increase in the ratio. The results demonstrate that form and motion analyzing mechanisms cannot be distinguished by their response to changes of spatial frequency, orientation or retinal locus.     

Monocular and binocular detection of moving sinusoidal gratings

We compared observers' contrast sensitivities for monocularly presented drifting gratings with their sensitivities to various pair-combinations of these gratings. If the two gratings are presented one to each eye, are of low spatial frequency, and move in same direction, contrast sensitivity is nearly twice the monocular value; if the gratings move in opposite directions, there is little or no sensitivity difference between the monocular and dichoplic conditions. As spatial frequency increases, the difference between the same- and opposite-direction conditions becomes less marked, and both pair-combinations are about 1.4 times as detectable as their monocular components. A monocular combination of gratings drifting in opposite directions (a counterphase modulated grating) gives results much like the dichoptic opposite-direction combination at all spatial frequencies. If spatial frequency is fixed at a moderate value, and drift rate is varied, then the difference between same and opposite conditions increases with drift rate; the natural conclusion that stimulus velocity determines this difference is not. however, completely borne out by experiments in which spatial and temporal frequency are varied to hold velocity constant. Thus under conditions where velocity and direction information are known to be available at detection threshold, our results show that binocular combination depends on the direction of movement: directional information must thus be extracted at or prior to the confluence of binocular signals.     

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.     

S‐cone excitation ratios for reaction times to blue‐yellow suprathreshold changes at isoluminance

We examined different contrast metrics to scale visual latencies for suprathreshold stimuli modulated along tritan confusion lines. S‐cone increments (‘blue’) and decrements (‘yellow’) were isolated along two different tritan confusion lines, each one having a different luminance value. Reaction times (RT) were evaluated as a function of the Weber contrast and the S‐cone excitation ratio between the test stimulus and the background. RTs were described using a model that generalizes Piéron’s law and incorporates the notion of threshold units and power law scaling. Our results show that RTs for S‐cone increments and decrements equate better when using the S‐cone excitation ratio. However, a single function did not describe all RT data. S‐cone RTs are better described by separate functions. We conclude that S‐cone increments and decrements do not scale in the same manner. Both Weber contrast and the S‐cone excitation ratio are plausible metrics at isoluminance. The implications for the S‐cone pathways are discussed.     

Visual evoked potentials for reversals of red-green gratings with different chromatic contrasts: asymmetries with respect to isoluminance

Human visual evoked potentials (VEPs) were recorded for abrupt 6.25-Hz reversals of 2 c/deg square-wave gratings combining red-green contrast with different levels of luminance contrast. Response characteristics-- amplitudes and peak latencies as a function of luminance contrast--were compared for four different pairs of red-green colors and an isochromatic yellow grating. For each of the red-green color pairs, the plots of VEP amplitudes and latencies were nonsymmetrical with respect to isoluminance. The amplitude dropped to a minimum within a region of rapid phase change, at a different contrast for each color pair but always at a luminance contrast for which the greener color had the higher luminance. When the contrast-response curve for each of the four red-green pairs was modeled by a simple |CL - CM| opponency of L- and M-cone contrast using a fixed CL/CM weighting ratio of about two, there was a close correspondence between the contrast giving a null in the modeled response and that giving a minimum in the VEP amplitude. So for the stimulus parameters applied here, the reversal VEP appeared to be dominated by L/M-opponent response contributions for which the signed CL/CM-cone weighting ratio was close to a value of minus two rather than to a value of minus one, which is characteristic of the psychophysical red-green detection mechanism and representative of CL/CM weighting ratios for precortical cells in the parvocellular pathway.     

Motion contrast: a new metric for direction discrimination

The Adelson-Bergen energy model (Adelson, E. H., & Bergen, J. R. (1985). Spatiotemporal energy models for the perception of motion. Journal of the Optical Society of America A, 2, 284-299) is a standard framework for understanding first-order motion processing. The opponent energy for a given input is calculated by subtracting one directional energy measure (EL) from its opposite (ER), and its sign indicates the direction of motion of the input. Our observers viewed a dynamic sequence of gratings (1 c/deg) equivalent to the sum of two gratings moving in opposite directions with different contrasts. The ratio of contrasts was varied across trials. We found that opponent energy was a very poor predictor of direction discrimination performance. Heeger (1992). Normalization of cell responses in cat striate cortex. Visual Neuroscience, 9, 181-197) has suggested that divisive inhibition amongst striate cells requires a contrast gain control in the energy model. A new metric can be formulated in the spirit of Heeger's model by normalising the opponent energy (EL - ER) with flicker energy, the sum of the directional motion energies (EL + ER). This new measure, motion contrast (EL - ER)/(EL + ER), was found to be a good predictor of direction discrimination performance over a wide range of contrast levels, but opponent energy was not. Discrimination thresholds expressed as motion contrast were around 0.5 +/- 0.1 for the sampled drifting gratings used in our experiments. We show that the dependence on motion contrast, and the threshold of about 0.5, can be predicted by a modified opponent energy model based on current knowledge of the response functions and response variance of cortical cells.     

Luminance mechanisms mediate the motion of red-green isoluminant gratings: the role of "temporal chromatic aberration"

In this paper we use a dynamic noise-masking paradigm to explore the nature of the mechanisms mediating the motion perception of drifting isoluminant red-green gratings. We compare contrast thresholds for the detection and direction discrimination of drifting gratings (1.5 cpd), over a range of temporal frequencies (0.5-9 Hz) in the presence of variable luminance or chromatic noise. In the first experiment, we used dynamic luminance noise to show that direction thresholds for red-green grating motion are masked by luminance noise over the entire temporal range tested, whereas detection thresholds are unaffected. This result indicates that the motion of nominally isoluminant red-green gratings is mediated by luminance signals. We suggest that stimulus-based luminance artifacts are not responsible for this effect because there is no masking of the detection thresholds. Instead we propose that chromatic motion thresholds for red-green isoluminant gratings are mediated by dynamic luminance artifacts that have an internal, physiological origin. We have termed these "temporal chromatic aberration". In the second experiment, we used dynamic chromatic noise masking to test for a chromatic contribution to red-green grating motion. We were unable to find conclusive evidence for a contribution of chromatic mechanisms to the chromatic grating motion, although a contribution at very high chromatic contrasts cannot be ruled out. Our results add to a growing body of evidence indicating the presence of dynamic, internal luminance artifacts in the motion of chromatic stimuli and we show that these occur even at very low temporal rates. Our results are compatible with our previous work indicating the absence of a chromatic mechanism for first order (quasi-linear) apparent motion [Vision Res. 40 (2000) 1993]. We conclude that previous conclusions based on the motion of chromatic red-green gratings should be reassessed to determine the contribution of dynamic luminance artifacts.     

Meridional anisotropies of orientation discrimination for sine wave gratings

The difference limen for perceived stimulus orientation was measured for thin lines, and for sine wave gratings between 2.5 and 20.0 c/deg. All observers exhibited a marked meridional anisotropy on this task with both the thin line and grating test targets. For the sine wave gratings orientation discrimination was not found to depend on their spatial frequency. Contrast threshold measurements with the same set of stimuli confirmed earlier reports that the meridional anisotropy for contrast detection increases with test spatial frequency. The data are consistent with published hypotheses (Regan and Beverley, 1985) that detection and discrimination of spatial patterns may be processed differently by orientation selective elements of the visual system and that there are fewer of these mechanisms subserving oblique orientations than either vertical or horizontal orientations.     

Motion of contrast-modulated gratings is analysed by different mechanisms at low and at high contrasts

We used a pedestal test [Lu & Sperling (1995a). Vision Research, 35, 2697-2722] to determine whether motion discrimination of contrast-modulated gratings has different properties at low contrast (4.5%) and at high contrast (45%). The amplitude-modulated gratings consisted of a 5 c/deg static carrier modulated by a moving 1 c/deg contrast envelope. We found that when contrast is low direction discrimination for contrast-modulated gratings is vulnerable to pedestals and becomes impossible at about 4 Hz. At high contrast contrast-modulated gratings are unaffected by pedestals and modulation sensitivity in a motion direction-discrimination task remains high up to 12 Hz. These results are consistent with the hypothesis that separate mechanisms analyse motion of contrast-modulated gratings at low and at high contrast; at low contrast motion analysis is based on feature tracking, whereas at high contrast, contrast-modulated gratings are analysed by spatio-temporal filters.     

Contrast detection and detection of contrast modulation for noise gratings

Threshold contrast was measured for noise gratings of various spatial frequency bandwidths and central frequencies. The results indicate that the contrast reaches threshold whenever the spatial power of the stimulus, filtered according to the contrast sensitivity function, exceeds a fixed level, irrespective of the width of the spatial frequency range over which the power is distributed. No “critical band effect” is found. For suprathreshold noise gratings of various bandwidths we measured thresholds for the detection of sinusoidal (amplitude) modulation of the contrast. The results indicate that the visual system uses the spatial power always with about the same efficiency, irrespective of the bandwidth over which it is distributed. The thresholds depend on the spatial frequency of the modulation: the visual system appears to be unable to use information from more than one cycle of the modulation frequency.     

Velocity discrimination thresholds for flowfield motions with moving observers

The visual flow field, produced by forward locomotion, contains useful information about many aspects of visually guided behavior. But locomotion itself also contributes to possible distortions by adding head bobbing motions. Here we examine whether vertical head bobbing affects velocity discrimination thresholds and how the system may compensate for the distortions. Vertical head and eye movements while fixating were recorded during standing, walking or running on a treadmill. Bobbing noise was found to be larger during locomotion. The same observers were equally good at discriminating velocity increases in large accelerating flow fields when standing or walking or running. Simulated head bobbing was compensated when produced by pursuit eye movements, but not when it was part of the flow field. The results showed that these two contributions are additive and dealt with independently before they are combined. Distortions produced by body/head oscillations may also be compensated. Visual performance during running was at least as good as during walking, suggesting more efficient compensation mechanisms for running.     

Visual search at isoluminance: Evidence for enhanced color weighting in standard sub-set and preview-based visual search

Isoluminant displays depend on responses from the parvocellular visual stream, known to code color information. We examined the influence of isoluminance on attentional guidance by color using two procedures: (i) color sub-set search (Egeth, Virzi, & Garbart, 1984) and (ii) preview search (Watson & Humphreys, 1997). We used displays that do not generate a sub-set search advantage with luminant stimuli. Despite this, a sub-set search advantage was present for small color groups with isoluminant displays. Under preview-search conditions, presenting items at isoluminance amplified the effects of a negative color carry-over from a preview display to a new target, but only when there was an extended preview duration. Both findings demonstrate that presenting items at isoluminance increases the influence of color on visual search. Collectively, the data are consistent with the notion of a flexible inhibitory mechanism that can change the weighting applied to visual features in search.     

Perceiving illusory contours: figure detection and shape discrimination

We investigate the relationship between illusory figure detection and discrimination of its shape, asking whether these depend on a single, two separate, or two sequential processes. In a simultaneous detection-discrimination experiment, we presented subjects with brief, backward-masked Kanizsa-type patterns consisting of four "pacmen," arranged as if at the corners of a 60-degree parallelogram. Pacman openings were oriented in a quarter of the trials so as to induce an illusory parallelogram. In another quarter, three of the pacmen induced an equilateral triangle. In the remaining half, pacmen were rotated so as not to induce a complete figure. For each trial, subjects reported whether they perceived an illusory figure (detection) and which shape they saw (discrimination), "guessing" the shape even when it was not explicitly perceived. Average detection and discrimination psychometric curves were similar with significantly better-than-chance detection and discrimination beginning at approximately 100 ms. Nevertheless, we found three patterns of performance, representing different detection-discrimination relationships, suggesting these may be separate processes. Detection was not always followed by correct discrimination, especially for poorer performers. Interestingly there were also cases where discrimination was accurate, even without detection, especially in mid-level performers. One detection-discrimination interaction was that only with explicit detection did shape discrimination use local features (such as the orientation of the fourth pacman in the case of an illusory triangle). We suggest that illusory figure detection and shape discrimination are separate tasks, with their relationship being determined individually.     

Failure of motion discrimination at high contrasts: Evidence for saturation

The ability of human observers to discriminate the direction of motion of a briefly-presented, slowly moving, 1 c/deg sinusoidal grating varies non-monotonically with the contrast of the grating. At low contrasts, performance improves with increasing contrast, but it reaches a peak between 95% and 100% correct at a contrast of 0.02-0.05. With further increases in contrast performance declines, reaching chance levels at a contrast of about 0.4. Detection of the same stimulus improves with increasing contrast to 100% correct and stays there. This behaviour would be expected if the visual signal which determines direction-of-motion is given by the difference between the responses of paired direction-selective filters tuned to opposite directions of motion and if the responses of these paired filters saturate at modest contrasts.