Patch Stimuli Research Articles

Attentional mechanisms in visual signal detection: The effects of simultaneous and delayed noise and pattern masks

The attentional cuing effects in detection and some discrimination tasks depend on the use of backward masks and on the presence of external noise in the display. These effects have been attributed to an interruption masking mechanism, which terminates stimulus processing prematurely, and an external noise exclusion mechanism, which minimizes the perceptual effects of noise. To test whether the dependencies on masking and external noise are expressions of a single mechanism, observers detected grating patch stimuli, masked with noise masks or pattern masks, presented either simultaneously or after a delay of 60-90 msec. Contrary to an external noise exclusion account, but consistent with an interruption masking account, cuing effects were largest when the masks were delayed. However, weaker cuing effects were obtained with simultaneous masks, contrary to an interruption masking account. These results suggest that attentional effects in simple visual judgments are affected by mechanisms of both kinds.

Localizing Sites of Activation in Primary Visual Cortex Using Visual-Evoked Potentials and Functional Magnetic Resonance Imaging

This study compared retinotopic map identification in primary visual cortex (V1) using: (i) functional magnetic resonance imaging (fMRI) and (ii) visual evoked potentials (VEPs) coupled with dipole source localization (DSL). A multielectrode array was used to record VEPs while subjects viewed a flickering dartboard pattern modulated by a 16-bit m-sequence. The stimulus preferentially activates V1. Using a common time function DSL algorithm, the primary source of each stimulus patch was found independent of the fMRI. The VEP/DSL and fMRI localization data for each subject were aligned by a rigid translation and rotation. The average distance between VEP and corresponding fMRI sources was 10.8 mm +/- 3.8 mm. To assess the significance of the results, fMRI and DSL solutions were scrambled so the comparisons were no longer for corresponding patches. The average distance between the noncorresponding data sets was 17.2 mm for 50 million scrambles. The probability of the scrambled data yielding a better fit than the real data was p < 10(-7). The combination of multielectrode recording, multiinput visual stimulation and common time function DSL analysis can provide a detailed retinotopic map of visual cortex that has high correspondence with independent fMRI localization analysis on the same subject.

Electrophysiological estimate of human cortical magnification

Objective: The cortical magnification factor characterizes the area of human primary visual cortex activated by a stimulus as a function of angular distance from an observer's line of sight. This study estimates human cortical magnification using an electrophysiological method with excellent temporal resolution: visual evoked potential (VEP) dipole source localization.Methods: For each of 60 independently modulated checkerboard patches within the central 18deg of the visual field, location, orientation, magnitude, and time-course of the dipole current source that best described the VEP distribution across a multi-electrode array was obtained. At numerous eccentricities, cortical magnification was determined using two different techniques: (1) the distance between each pair of adjacent stimulus patches was matched to the corresponding distance between adjacent cortical sources; and (2) the area of each stimulus patch was matched to the magnitude of the corresponding cortical source (which was assumed to be proportional to cortical area).Results: The estimates of human cortical magnification using our electrophysiological method were similar to previous estimates from psychophysics, cortical stimulation, and functional magnetic resonance imaging.Conclusions: The concordance of results provided by these disparate technologies, with differing spatial and temporal limitations, supports their combination in studying the spatio-temporal dynamics of human brain function.

Attention and luminance detection: Effects of cues, masks, and pedestals.

Three experiments investigated the effects of attention on the detection of luminance targets in a spatial-cuing paradigm. Gabor patch stimuli of 30, 60, or 90 ms duration, equated for detectability, were presented (a) against a uniform field with a checkerboard backward mask, (b) atop a 15% luminance pedestal with a backward mask, or (c) atop a luminance pedestal with no backward mask. Signal detection analysis showed that detection sensitivity was significantly enhanced at attended locations for all observers when backward masks were used, both when targets were presented against a uniform field and when a pedestal was used. However, when no masks were used there was no cuing advantage of any kind. The results show that the cuing effect in simple detection depends on the use of backward masks, a finding that resolves the inconsistencies previously associated with studies of this type.

Attention and luminance detection: effects of cues, masks, and pedestals.

Three experiments investigated the effects of attention on the detection of luminance targets in a spatial-cuing paradigm. Gabor patch stimuli of 30, 60, or 90 ms duration, equated for detectability, were presented (a) against a uniform field with a checkerboard backward mask, (b) atop a 15% luminance pedestal with a backward mask, or (c) atop a luminance pedestal with no backward mask. Signal detection analysis showed that detection sensitivity was significantly enhanced at attended locations for all observers when backward masks were used, both when targets were presented against a uniform field and when a pedestal was used. However, when no masks were used there was no cuing advantage of any kind. The results show that the cuing effect in simple detection depends on the use of backward masks, a finding that resolves the inconsistencies previously associated with studies of this type.

Using multi-stimulus VEP source localization to obtain a retinotopic map of human primary visual cortex

Objective: The goal of this study was to acquire a detailed spatial and temporal map of primary visual cortex using a novel VEP stimulus and analysis technique.Methods: A multi-stimulus array spanning the central 18 degrees of the visual field was used where each of 60 checkerboard stimulus ‘patches’ was simultaneously modulated with an independent binary m-sequence (Sutter, 1992). VEPs corresponding to each patch were recorded from 3 subjects using a dense posterior electrode array. For each stimulus patch, single dipole source localization was conducted to determine the location, magnitude, and time-function of the underlying neural activation. To reduce ambiguity in the solution, a common time-function was assumed for stimulus patches at the same visual eccentricity (defining an annulus). The analysis was conducted independently for each annulus composed of 4–12 patches.Results: The loci of the dipole solutions followed a smooth retinotopic pattern across annuli consistent with the classical organization of primary visual cortex. Specifically, each dipole was found contralateral to the corresponding stimulus patch and field inversion was observed for all subjects.Conclusions: Using this technique, the most detailed spatial and temporal retinotopic map of primary visual cortex to date has been obtained.

The effect of target size and eccentricity on reflex disparity vergence.

This study examined the effects of stimulus size and eccentricity on reflex disparity vergence: the small, involuntary corrections of eye alignment which serve to minimize the binocular disparity of fixated targets. Subjects were instructed to fixate steadily on a small, stationary mark superimposed on the center of a dynamic random dot stereogram. The stereogram was binocularly uncorrelated except for a fully correlated patch whose size and eccentricity were varied systematically across trials. The disparity of the patch was varied sinusoidally over time to stimulate vergence following movements. The overall purpose was to determine the relative contributions of various field loci in controlling binocular fixation by finding the smallest patch which would reliably drive vergence against the effort to fixate steadily. Psychophysical thresholds for detection of the correlated patch stimuli were also measured for comparison to the oculomotor results. Results showed that the smallest effective patch increased with eccentricity similarly for both vergence responses and psychophysical detection, suggesting they depend on a common, presumably cortical matching process. The dependence of response on eccentricity is roughly consistent with changes in the cortical magnification factor, suggesting that the area of cortex stimulated may be the determining factor in vergence responses to this class of stimulus.

Motion perimetry in anisometropic amblyopia: Elevated size thresholds extend into the midperiphery

Purpose: Our purpose was to determine whether motion detection abnormalities in patients with anisometropic amblyopia exist and to determine the extent of these abnormalities in the central and midperipheral visual field. Methods: We used of motion perimetry to evaluated 10 anisometropic subjects with no manifest strabismus. Each of 44 locations in the visual field corresponding to the test sites of the Humphrey 24-2 program was tested with circular patches of motion (random dot cinematograms) displayed on a computer screen. Stimulus patch size was reduced in a 2 1 staircase manner to determine the smallest patch of motion detectable at each test location (threshold). Data from 15 age-matched normal subjects were used as controls. Results: Vision in the amblyopic eye ranged from 20 25 to count fingers. The overall mean size threshold for amblyopic eyes was elevated (61% ± 73%) compared with fellow eyes and age-matched normal eyes ( p < 0.03) (i.e., the moving patch of dots in the field had to be larger for it to be detected when viewing with the amblyopic eye). The increase in size threshold was consistent across the visualfield and was not greater for central locations. Conclusion: The amblyopia caused by anisometropia is associated with an abnormality in motion detection that extends into the midpheriphery of the visual field.

In search of a contrast metric: matching the perceived contrast of Gabor patches at different phases and bandwidths.

The definition of contrast in a complex scene is a long-standing problem. The local contrast in an image may be approximated by the contrast of a Gabor patch of varying phase and bandwidth. Observers' perceived (apparent) contrast, as indicated by matching of such patterns, were compared here to the physical contrast calculated by a number of definitions. The 2 c/deg 1-octave Gabor patch stimuli of different phases were presented side by side, separated by 4 deg. During each session the subjects (n = 5) were adapted to the average luminance, and four different contrast levels were randomly interleaved. The subject's task was to indicate which of the two patterns was lower in contrast. Equal apparent contrast was determined by fitting a psychometric function to the data. The results of the matching rejected the hypothesis that either the Michelson formula or the King-Smith and Kulikowski contrast metric (CKK = (Lmax-Lbackground)/Lbackground) was used by the subjects to set the matching. The use of the Nominal contrast (the Michelson contrast of the underlying sinusoid) as an estimate of apparent contrast could not be rejected. In a second experiment the apparent contrast of a 1-octave Gabor patch was matched to the apparent contrast of a 2-octave Gabor patch (of Nominal contrast of 0.1, 0.3, 0.6, 0.8) using the method of adjustment. The results of this experiment disagree with the prediction of the Nominal contrast definition as well. The local band-limited contrast measure (Peli, 1990), when used with the modifications suggested by Lubin (1995) as an estimate of apparent contrast, could not be rejected by the results of either experiment. These results suggest that a computational contrast metric based on multi-scale bandpass filtering is a better estimate of apparent perceived contrast than any of the other metrics tested.

The spatial distribution of the antagonistic surround of MT/V5 neurons.

The majority (217/325, 66%) of the neurons in the middle temporal (MT) area/V5 show strong antagonistic surrounds, defined here by a decrease of at least 50% in the summation curve. We mapped the antagonistic surround in 145 such cells, using eight circularly distributed surround stimulus patches (Surround Asymmetry Test, SAT) and also mapped the surround in 51 of these 145 cells using a grid consisting of 25 square patches (Surround Mapping Test, SMT). Both tests showed that the angular surround distribution was non-uniform in the majority of these neurons. In half the neurons, the antagonistic surround was asymmetric, and arose from a single region on one side of the excitatory receptive field (ERF). In another quarter of the sample the surround was bilaterally symmetric, and arose from a pair of regions on opposite sides of the ERF. Only the remaining 20% showed a circularly symmetric surround distribution. These three groups differed in their laminar distribution. The SMT showed that, radially, the surround antagonism reached a maximum, on average, at 1.5 times the ERF radius. Detailed comparisons of the spatial relationships of excitatory and inhibitory regions of the RF components shows that non-homogeneity of the surround influence appears to be an intrinsic property of the surround. Such a property may underly the extraction of the surface orientation and curvature from speed patterns.

The effect of prey movement on attack behavior and patch residence decision rules of wolf spiders (Araneae: Lycosidae)

We used a video imaging technique to test the effects of prey movement on attack behavior and foraging patch residence time decision rules of wolf spiders. TwelveSchizocosa ocreata (Hentz) (Lycosidae) were tested in an artificial foraging patch stimulus chamber consisting of a microscreen television displaying a computer digitized, animated image of a cricket. Four prey movement treatments were used: (1) a blank screen, (2) a stationary cricket control, (3) a cricket moving for 1 min, and (4) a cricket moving for 10 min. Spiders stayed significantly longer in treatments with higher cricket activity. Spiders also stayed longer when they attacked the stimulus than when they did not. The distribution of patch residence times of spiders indicates a decision rule based on a fixed probability of leaving.

Spatial layout affects speed discrimination.

We address a surprising result in a previous study of speed discrimination with multiple moving gratings: discrimination thresholds decreased when the number of stimuli was increased, but remained unchanged when the area of a single stimulus was increased [Verghese & Stone (1995). Vision Research, 35, 2811-2823]. In this study, we manipulated the spatial- and phase relationship between multiple grating patches to determine their effect on speed discrimination thresholds. In a fusion experiment, we merged multiple stimulus patches, in stages, into a single patch. Thresholds increased as the patches were brought closer and their phase relationship was adjusted to be consistent with a single patch. Thresholds increased further still as these patches were fused into a single patch. In a fission experiment, we divided a single large patch into multiple patches by superimposing a cross with luminance equal to that of the background. Thresholds decreased as the large patch was divided into quadrants and decreased further as the quadrants were maximally separated. However, when the cross luminance was darker than the background, it was perceived as an occluder and thresholds, on average, were unchanged from that for the single large patch. A control experiment shows that the observed trend in discrimination thresholds is not due to the differences in perceived speed of the stimuli. These results suggest that the parsing of the visual image into entities affects the combination of speed information across space, and that each discrete entity effectively provides a single independent estimate of speed.

Orientation Discrimination at Isoluminance

Colour and form are important attributes of the objects in our visual environment. We tested the hypothesis that colour and orientation are processed independently in the visual system. Orientation perception for stimuli defined by luminance is characterised by a decrease in thresholds with increasing stimulus contrast, and by a strong oblique effect: stimuli along the horizontal or vertical axes are discriminated more easily. We determined orientation discrimination thresholds for stationary, slow (1 Hz) or fast (8 Hz) moving sine-wave gratings (1 cycle deg−1) defined by luminance or isoluminant (red — green) contrast. In a 4AFC paradigm, three of four stimulus patches were identical and the fourth differed in orientation, contrast, or in both. When we measured orientation discrimination thresholds as a function of stimulus contrast, thresholds decreased for all stimuli with increasing contrast. At all temporal frequencies the functions relating orientation thresholds to stimulus contrast had similar shapes for luminance and isoluminant gratings. On a cone-contrast metric, thresholds for stationary and slowly moving stimuli were consistently lower for isoluminant compared to luminance stimuli. For fast-moving stimuli orientation thresholds were similar for both kinds of gratings. For both types of stimuli a marked oblique effect was observed. To characterise processing of contrast and orientation completely, we measured simultaneous thresholds for contrast and orientation. The shapes of the resulting two-dimensional threshold contours were similar for luminance and isoluminant gratings, indicating similar rules for combining differences in contrast and orientation. We conclude that processing of isoluminant and luminance stimuli undergoes the same neural processing at least for the low spatial frequencies used here.

Spatial heterogeneity of inhibitory surrounds in the middle temporal visual area.

A recurrent theme in the organization of vertebrate visual cortex is that of receptive fields with an associated "silent" opponency component. In the middle temporal area (area MT), a cortical visual area involved in the analysis of retinal motion in primates, this opponency appears in the form of a region outside the classical receptive field (CRF) that in itself gives no response but suppresses responses to motion evoked within the CRF. This antagonistic motion surround has been described as very large and symmetrically arrayed around the CRF. On the basis of this view, the primary function of the surround has long been thought to consist of simple figure-ground segregation based on movement. We have made use of small stimulus patches to map the form and extent of the surround and find evidence that the surround inhibition of many MT cells is in fact confined to restricted regions on one side or on opposite sides of the CRF. Such regions endow MT cells with the ability to make local-to-local motion comparisons, capable of extracting more complex features from the visual environment, and as such, may be better viewed as intrinsic parts of the receptive field, rather than as separate entities responsible for local-to-global comparisons.

Maximizing the dynamic range of the humphrey field analyzer for blue-on-yellow perimetry.

The conditions employed for blue-on-yellow perimetry (i.e. a blue stimulus filter and a high luminance yellow background) are in conflict with those that allow a maximum dynamic range. A modification is described for increasing the dynamic range of the Humphrey Field Analyzer (HFA) for blue-on-yellow perimetry. The position of the HFA stimulus bulb was lowered by reducing the length of both terminals. A concave mirror with a 28 mm radius of curvature was positioned behind the lowered stimulus bulb to give a filament image partly above and partly overlapping the original filament profile within the confines of the size V stimulus patch diameter. The modification achieved a 60% increase in the light output on the default 0 dB value of 10,000 apostilbs and, therefore, a useable dynamic range for blue-on-yellow perimetry of 2.94 log units. The effective increase in voltage supply to the stimulus bulb permitted a filament life of 260 operating hours. Furthermore, the modification ensured the correct calibration of the instrument.

Contrast sensitivity to patch stimuli: Effects of spatial bandwidth and temporal presentation

Models of the spatial response of human vision are important for applied work, but the available contrast sensitivity function (CSF) data vary widely due to the diverse spatiotemporal stimuli used over the years. To assist selection, this paper: (1) reports measurements of the effects on the CSF of varying the spatial and temporal windows of grating patches; (2) demonstrates that the widely discrepant CSFs from previous studies can be accounted for by using these results; and (3) discusses simple criteria for choosing CSFs for practical applications. CSFs were measured for several combinations of spatial and temporal waveforms, using the same subjects under otherwise identical conditions. The CSF was measured over the range of 0.5-10 c/deg using Gabor-type patches of 1.0-, 0.5-, 0.25-, and 0.125-octave spatial bandwidths using both abrupt and gradual temporal presentations. The results were compared with the CSF obtained with a fixed aperture (4 deg x 4 deg) grating pattern. Increasing the number of cycles resulted in increased sensitivity at intermediate frequencies, changing the CSF to a narrower bandpass shape. For each patch bandwidth, the gradual presentation CSF had a narrower spatial pass band than with the abrupt presentation. The relevance of the large differences in the CSFs obtained with different stimuli to our understanding of visual performance is discussed.

Spectral sensitivity in the pigeon.

Measurements of light- and dark-adapted absolute thresholds were obtained from three pigeons at 15 wavelengths ranging from 380 mμ to 700 mμ. Pecking responses caused a stimulus patch to fluctuate in intensity up and down across the pigeon’s threshold, and a record of the intensity provided the sensitivity data. The sensitivity of the birds was followed throughout a period of 80 min following a standard pre-exposure to white light. Four complete dark adaptation curves were obtained from each bird at each wavelength. Spectral sensitivity functions derived from these curves place the photopic maximum at 560–580 mμ, and the scotopic maximum at about 500 mμ. The scotopic function is fitted closely by aphakic human data. The photopic function shows inflections that may be related to similar inflections in corresponding human curves. The functions are quite similar to those found in electrophysiological studies of the pigeon eye. They also correspond rather well to the absorption spectra of chicken rhodopsin and iodopsin.