Apparent contrast of suprathreshold gratings varies with stimulus orientation

Human contrast sensitivity is known to vary with both stimulus spatial frequency and orientation. Previous research has indicated, however, that the apparent contrast of a sinusoidal grating is independent of spatial frequency at suprathreshold levels. We have found that the apparent contrast of suprathreshold gratings is not independent of its orientation. Observers adjusted the apparent contrasts of vertical and oblique gratings to match a 40% standard. Contrast match deviations were found at a number of spatial frequencies, indicating a lack of contrast constancy with stimulus orientation. The apparent contrast of gratings in different orientations could not be predicted in any simple fashion from either contrast threshold differences or ratios.     

Gender differences in response to spatial frequency and stimulus orientation

Male and female subjects with normal or corrected-to-normal visual acuity and less than .25 diopter of corrected astigmatism were asked to make contrast threshold judgments in response to both stationary and drifting grating displays. Results indicate a sex difference in contrast sensitivity as a function of spatial frequency for vertical and oblique orientations.     

Recovery from adaptation as a function of stimulus orientation

These experiments examined the oblique effect in an adaptation paradigm. Reaction times (RT) to the presence of a grating test stimulus were obtained following adaptation to either a blank field or a grating of the same orientation as the test stimulus. Horizontal, vertical, and oblique test and adaptation orientations were employed. Test gratings were presented at several interstimulus intervals following offset of the adaptation stimulus. RTs following grating adaptation were elevated to a greater extent (relative to blank adaptation) for oblique then for horizontal or vertical stimuli, for two grating spatial frequencies. Differences in RT can be related to differences in sensitivity among channels responsible for detection of the various orientations.

     

Adaptive filtering in spatial vision: evidence from feature marking in plaids

Much evidence shows that early vision employs an array of spatial filters tuned for different spatial frequencies and orientations. We suggest that for moderately low spatial frequencies these preliminary filters are not treated independently, but are used to perform grouping and segmentation in the patchwise Fourier domain. For example, consider a stationary plaid made from two superimposed sinusoidal gratings of the same contrast and spatial frequency oriented +/- 45 degrees from vertical. Most of the energy in a wavelet-like (e.g. simple-cell) transform of this stimulus is in the oblique orientations, but typically it looks like a compound structure containing blurred vertical and horizontal edges. This checkerboard structure corresponds with the locations of zero crossings in the output of an isotropic (circular) filter, synthesised from the linear sum of a set of oriented basis-filters (Georgeson, 1992 Proceedings of the Royal Society of London, Series B 249 235-245). However, the addition of a third harmonic in square-wave phase causes almost complete perceptual segmentation of the plaid into two overlapping oblique gratings. Here we confirm this result psychophysically using a feature-marking technique, and argue that this perceptual segmentation cannot be understood in terms of the zero crossings marked in the output of any static linear filter that is sensitive to all of the plaid's components. If it is assumed that zero crossings or similar are an appropriate feature-primitive in human vision, our results require a flexible process that combines and segments early basis-filters according to prevailing image conditions. Thus, we suggest that combination and segmentation of spatial filters in the patchwise Fourier domain underpins the perceptual segmentation observed in our experiments. Under this kind of image-processing scheme, registration across spatial scales occurs at the level of spatial filters, before features are extracted. This contrasts with many previous schemes where feature correspondence is required between spatial edge-maps at different spatial scales.     

Spatial frequency comparisons of rotated gratings

Reaction time for determining if two square-wave gratings are the same or different is a bimodal function of the relative angular separation (RAS) between them. Response time peaks at 60° and 120° separation and drops siguificantly at approximately 90°, provided that the gratings being compared are of the same spatial frequency. This pattern of results is not in line with those previously reported for more complex figures, nor may it be attributed to any “oblique effect,” since the same function is obtained with the comparison gratings placed at different retinal orientations. One possible explanation is that orthogonally tuned orientation channels interact in a facilitatory fashion in the human visual system.     

The effects of adaptation to square-wave gratings as a function of grating orientation

An adaptation technique was used to measure the selectivity or tuning for grating orientation in the visual system for different orientations of the inspection stimulus. Duration thresholds for grating patterns of constant luminance were determined for 13 test gratings oriented from ±5 to 90 deg away from each of five adaptation gratings: 0, 22, 45, 67, and 90 deg. Threshold data obtained for test gratings without prior adaptation indicated higher sensitivity for gratings oriented along the horizontal and vertical axis than along the oblique axis. After adaptation, thresholds increased (sensitivity was reduced) for gratings having similar orientations as the test gratings. However, the functions relating sensitivity reduction to degree of angular disparity between test and adaptation grating did not vary across the five inpsection orientations, i.e., selectivity or tuning for grating orientation appeared to be independent of the orientation of the adapting stimulus.     

Monkeys show an oblique effect.

Monkeys aligned a cursor bar with high-contrast square-wave gratings presented in a variety of orientations. Alignment time increased with increasing spatial frequency from 6 to 24 cycles deg-1 regardless of the orientation of the grating. At higher spatial frequencies, alignment tasks took longer for obliquely oriented gratings than for horizontal and vertical ones. Reducing grating contrast by blurring the image of the 24 cycle deg-1 grating also produced longer alignment times for the obliques. These data indicate that monkeys have an oblique effect similar to that found in humans, implying that the monkey is a useful animal model for investigating the development of meridional anisotropies.     

Flicker contrast sensitivity in normal and specifically disabled readers

Temporal contrast sensitivity for counterphase flicker was determined for specifically disabled and normal readers to investigate whether the two groups differ in the functioning of their transient systems. In experiment 1, temporal contrast sensitivity was measured over a range of temporal frequencies with a spatial frequency of 2 cycles deg-1. Disabled readers were less sensitive than the control subjects at all temporal frequencies. In experiment 2, temporal contrast sensitivity was measured at a temporal frequency of 20 Hz over a range of spatial frequencies. Disabled readers were less sensitive than the controls at all spatial frequencies, with the differences between the groups increasing as spatial frequency increased. Both these findings are interpreted as supporting the hypothesis of a transient-system deficit in the visual systems of disabled readers.     

Acuity for orientation measured with a sequential recognition task and signal detection methods

An orientation recognition task with two stimuli was employed as the basis of a psychophysical method for measuring sensitivity to orientation. Essentially, this method depends on a subject’s ability to discriminate between two stimuli differing in orientation and presented to the same retinal area in random succession from trial to trial. Advantages of this sequential method over traditional simultaneous matching methods are discussed. Its feasibility is demonstrated in a signal detection analysis of acuity for the orientation of short luminous slits presented foveally on a dark field at seven reference orientations varying between 0° and 90°. In both subjects employed, sensitivity was greater for horizontal and vertical orientations than for other slopes (the oblique effect).     

Variations in apparent spatial frequency with stimulus orientation: IL Matching data collected under normal and interferometric viewing conditions

Using an adjustment procedure, human observers matched the apparent spatial frequencies of sinusoidal gratings presented in different orientations (0, 45, 90, and 135 deg). Matches were made between all possible pairwise orientation combinations. Significant match deviations indicated that the apparent frequency of a grating depends on its orientation. The most consistent deviations were found between horizontal and vertical gratings, with horizontal gratings appearing to be of a lower spatial frequency than vertical gratings. These effects were relatively independent of stimulus contrast and persisted when the optics of the eye were bypassed with laser interferometry. A neurophysiological explanation of these effects is proposed.     

An oblique effect of chromatic gratings measured by color-mixture thresholds

Contrast sensitivity is lower for obliquely oriented achromatic gratings than for vertical or horizontal gratings at high spatial and low temporal frequencies. Although this response is suggestive of mediation by P-pathway cortical correlates, no clear sensory (i.e. class 1) oblique effect has been demonstrated with isoluminant chromatic stimuli. In the present experiment, a two-alternative forced-choice detection task was used to measure observers' sensitivity to spatiotemporal sinusoids varying in orientation and color contrast. A maximum-likelihood method fit ellipses to the thresholds, with the length of each ellipse taken as a measure of chromatic contrast sensitivity at isoluminance, and the width as luminance contrast threshold. A chromatic oblique effect was observed at about 3 cycles deg-1 suggesting an orientation bias within the cortical stream conveying P-cell activity.     

Eliminating the haptic oblique effect: influence of scanning incongruity and prior knowledge of the standards

Although the oblique effect has been conceptualized as a purely visual phenomenon, recent studies report its occurrence in a haptic matching task and present the hypothesis that differences in haptic orientational sensitivity might be responsible for the results. The possibility that procedural variables could be responsible was investigated. Specifically, the effect of prior knowledge of the stimulus orientation standards and of use of bilateral haptic exploration of standard and comparison orientations was examined. The results indicate that the reported oblique effect is reduced either when subjects are not informed which orientations will be tested, or when a unilateral matching procedure instead of a bilateral one is used. When both conditions are combined, the haptic oblique effect is eliminated. It is concluded that this particular manifestation of the oblique effect is not related to haptic sensitivity, but stems from the use of well-established imagery as referent for a match (imagery for oblique stimulus orientations is inferior) and the inherently different scanning patterns required in bilateral exploration of obliques (percepts of standard and comparison obliques will be necessarily different).     

Genetic and environmental factors in orientation anisotropy: a field study in the British Isles

Visual acuity for the detection of gratings at four orientations was measured for groups of ten boys and ten girls aged five to seven years, from the following four populations: Scots in Glasgow, Pakistanis in Glasgow, Gaels in Stornoway (Outer Hebrides) and East Anglians in Littleport (Cambridgeshire fenlands). The Glaswegians, both Scottish and Pakistani, showed the normal pattern of anisotropy, with poorest acuity for oblique orientations; the East Anglians showed no significant anisotropy; while the Gaels were unusual in showing poorest horizontal acuity. A group of fourteen Pakistani children in Stornoway differed slightly from a matched group of Gaels. The group differences bore little relation to the visual environments, and were probably due to genetic or cultural factors. The relatively poor horizontal acuity of the Gaels was not correlated with astigmatism. Sex differences were also found, with the boys showing higher mean acuity and a higher ratio between vertical and oblique acuity.     

Decision processes in visual discrimination of line orientation

The contribution of decision factors to the meridional variations in line orientation discrimination was determined for two-alternative forced-choice experimental designs. Using Johnson's (1980) formalization of decision processes in discrimination tasks, we identified three decision factors: the decision rule, memory variance, and criterial noise. In a first experiment, we showed the effect of experimental design on orientation discrimination to be similar at horizontal and oblique standard orientations, indicating that the meridional variations in orientation discrimination were not due to a decision rule anisotropy. In a second experiment, the effect of the interstimulus interval was also found to be similar at both standard orientations, suggesting that the memory variance is isotropic in the orientation domain. The results of two other experiments supported the hypothesis that the meridional variations in orientation discrimination are not due to a criterial noise anisotropy. These different results strongly suggested that the oblique effect in line orientation discrimination is due to sensorial factors rather than to decision factors. Therefore, they further support the hypothesis linking the anisotropy of the preferred orientation distribution of Area 17-S cells (a single physiologically defined class of cells in the primary visual cortex) and the meridional variations in line orientation discrimination.     

Temporal integration and contrast sensitivity in foveal and peripheral vision

Spatial contrast sensitivity functions and temporal integration functions for gratings with dark surrounds were measured at various eccentricities in photopic vision. Contrast sensitivity decreased with increasing eccentricity at all exposure durations and spatial frequencies tested. The decrease was faster at high than at low spatial frequencies, but similar at different exposure durations. When cortically similar stimulus conditions were produced at different eccentricities by M-scaling, contrast sensitivity became independent of visual field location at all exposure durations tested. The results support the view that in photopic vision spatiotemporal information processing is qualitatively similar across the visual field, and that quantitative differences result from retino-topical differences in ganglion cell sampling. For gratings of constant retinal area temporal integration (improvement of contrast sensitivity with increasing exposure duration) was more extensive at high than at low retinal spatial frequencies but independent of cortical spatial frequency and eccentricity. For M-scaled gratings temporal integration was more extensive at high than at low cortical spatial frequencies but independent of retinal spatial frequency and eccentricity. The results suggest that the primary determinant of temporal integration is not spatial frequency but grating value that is calculated as AF2 square cycles (cycle2), where A is grating area and F spatial frequency.     

Contrast and spatial frequency components in visual preferences of newborns

A preferential-looking procedure was used to investigate newborns' responses to square-wave gratings varying in spatial frequency and contrast. A preliminary study confirmed that the gratings used in the experiment were suprathreshold. In the experiment newborns' preference for a grating of 0.1 cycle deg-1 within the peak contrast sensitivity range was examined. Reduction in the contrast of this grating led to a transfer of the preference to a high-contrast grating of the same space-averaged luminance with a spatial frequency outside this range (0.42 cycle deg-1). The findings are discussed with reference to the role of the contrast sensitivity function in pattern preferences of newborns: it is suggested that contrast and spatial frequency interact in determining pattern preferences.     

The effect of contrast on the completeness of binocular rivalry suppression

Binocular rivalry between orthogonal sine wave gratings was studied by asking subjects to indicate when they saw just one or the other grating, as opposed to a composite. Using this measure, it was found that the completeness of rivalry (1) usually increased with grating contrast; (2) increased as a trial progressed, up to about 40 sec; and (3) depended on spatial frequency, with the optimal spatial frequency being lower than that at which contrast sensitivity was maximal. These findings supplement what has been learned about rivalry by other methods.     

Direct evidence for the existence of energy-based texture mechanisms

Two classes of models have been proposed to explain how the visual system processes texture modulations. In 'feature models', abstract representations of the featural properties of local texture regions (eg orientation, spatial frequency, contrast) are first generated, after which differences in individual feature properties across space are detected. In 'energy models', on the other hand, differences across space in the response energies of linear simple-cell-like filters are detected. This model thus processes the existing differences between texture regions directly without generating a full representation of the individual texture regions. We provide here direct evidence for the existence of the second, energy model, using an adaptation paradigm in conjunction with textures simultaneously modulated in two dimensions--orientation and spatial frequency. We found that the mechanism that processed the conjoint modulation was tuned to orientations and spatial frequencies that could not be predicted by any feature model, but which were precisely predicted by the energy model.     

Effects of spatial attention on detection and identification of oriented lines

In this paper, we study the effects of spatial attention on detection and identification of oriented lines presented at near-threshold luminance. In the first experiment, we found that the cuing effect was greater when observers had to discriminate between two close orientations than when they had to discriminate between two far-apart orientations. In the second experiment, we examined the effects of peripheral cues on summation of low-contrast oriented lines. We found that the range of orientations within which summation occurred was greater when the cues were invalid than when they were valid. Our results suggest that attention to a specific location in the visual field modulates the neural channels which code orientation in at least two different ways: (a) spatial attention increases the responsivity of these channels to stimuli presented at this location, and (b) it reduces the bandwidths or ranges of orientations to which these channels are sensitive. These results suggest that the properties of the orientation-tuned channels, including those that seem to exert their effects at early stages of orientation processing (e.g., tuning function), may not be fixed, but rather vary according to the attention being paid to the spatial region within which the target stimulus is presented.     

Apparent swinging motion from a 2-D sinusoidal pattern

A new motion illusion is reported that is observed on a 2-D sinusoidal pattern composed of two 1-D sinusoids, in which the constituent elements of the middle column appear to swing relative to the two flanking columns when the point of fixation is slowly moved back and forth about the middle column. To better understand the underlying mechanisms of the apparent swinging motion, the spatial properties of a 2-D sinusoidal pattern were examined in terms of spatial frequency, orientation, and contrast. Thirty-four subjects rated the magnitude of the motion. The apparent swinging was greatest when the two 1-D components had spatial frequencies of 1-2 cycles deg-1, relative orientations between 15 degrees and 30 degrees, and high contrasts. A spatiotemporal interaction between spatially overlapping visual units differing in polarity (Khang and Essock, 1997 Perception 26 585-597, 831-846) and the resultant shift in the global-motion signal was proposed as a likely cause of the apparent swinging motion.