Symmetry detection across the visual field.

Humans are extremely sensitive to symmetry when it is foveated but sensitivity drops as a symmetrical region of a fixed size is moved into the periphery. A psychophysical study was undertaken to determine if eccentricity dependent sensitivity loss could be overcome by magnifying stimuli at each eccentricity (E) by a factor F = 1 + E/E2, where E2 indicates the eccentricity at which the size of a stimulus must be doubled, relative to a foveal standard, to achieve equivalent performance. The psychophysical task required subjects to decide on each trial in which of two intervals a symmetrical stimulus had been presented. Stimuli were presented at a range of sizes and eccentricities (0 to 8 degrees) and the probability of a correct discrimination was computed for each condition. In Experiment 1, thresholds were measured with stimuli set to maximum available contrast and, in Experiment 2, stimuli were presented at a constant multiple of contrast detection threshold. In both experiments, a single scaling function removed most of the eccentricity dependent variation from the data. However, the E2 value recovered for one subject tested in both experiments was larger by about 65% when stimuli were not equated for visibility. We conclude that symmetry detection can be equated across a range of eccentricities by scaling stimuli with an E2 in the range of 0.88 to 1.38 degrees. Failure to equate for visibility across all viewing conditions may result in an inflated estimate of E2.

Texture space.

Many previous studies have examined the ease with which two spatially adjacent textures can be segmented. Our goal is to examine the representational system that determines the appearance of isolated patches of visual texture. To this end, similarity judgments from three subjects were obtained for 20 artificial textures comprising filtered noise. Multidimensional scaling (MDS) revealed that three perceptual dimensions explain most of the variance in subjects' similarity judgments. In addition, the three subjects' similarity judgments and MDS solutions were highly correlated. A computational model utilizing the energy responses in seven bandpass filters explains an average of 80% of the variability in the original similarity scores of individual subjects. In the model, energy responses are mapped to the perceptual space through a linear transformation that can be decomposed into two components. The first component decorrelates initial filter responses and the second component maps the decorrelated filter responses to a perceptual space. These latter transformations show remarkable agreement between the three subjects.

Detection and discrimination of subjective contours defined by offset gratings.

Three experiments were conducted to refine our understanding of the mechanisms that encode subjective contours. In Experiment 1, discrimination thresholds (stimulus onset asynchronies [SOAs] yielding 81% correct) were measured in a backward masking paradigm for subjective contours defined by offset gratings. For large apertures, thresholds increased as carrier frequency increased. For the smallest aperture, thresholds were a U-shaped function of carrier frequency. Experiment 2 showed that these threshold results were generally consistent with the rated strength of the subjective contours. Experiment 3 showed that detection thresholds (SOAs yielding 81% correct) again increased with carrier spatial frequency, increased for obliquely oriented carriers, and, for a particular frequency and orientation of the carrier, were lower when the subjective contour was orthogonal to the carrier. All of these results are well explained by a two-stage process in which a second-layer filter integrates the responses of end-stopped mechanisms to the terminators defining the subjective contour. In the model, the end-stopped mechanisms have low-pass sensitivity to carrier spatial frequency, and the sizes of the second-layer filters are proportional to the scale of the end-stopped mechanisms from which they draw their input.

Second-order motions contribute to vection.

First- and second-order motions differ in their ability to induce motion aftereffects (MAEs) and the kinetic depth effect (KDE). To test whether second-order stimuli support computations relating to motion-in-depth we examined the vection illusion (illusory self motion induced by image flow) using a vection stimulus (V, expanding concentric rings) that depicted a linear path through a circular tunnel. The set of vection stimuli contained differing amounts of first- and second-order motion energy (ME). Subjects reported the duration of the perceived MAEs and the duration of their vection percept. In Experiment 1 both MAEs and vection durations were longest when the first-order (Fourier) components of V were present in the stimulus. In Experiment 2, V was multiplicatively combined with static noise carriers having different check sizes. The amount of first-order ME associated with V increases with check size. MAEs were found to increase with check size but vection durations were unaffected. In general MAEs depend on the amount of first-order ME present in the signal. Vection, on the other hand, appears to depend on a representation of image flow that combines first- and second-order ME.

The effects of eccentricity and spatial frequency on the orientation discrimination asymmetry.

This study investigated the effects of eccentricity and spatial frequency on the discrimination of vertical and oblique (10 deg from vertical) Gabor patches. Within a display stimuli were scaled by a factor F = 1 + E/E2 at each eccentricity (E) in an attempt to equate either the number of photoreceptors (E2 = 2.5) or cortical area (E2 = 0.77) engaged at each eccentricity. The task was to detect a differently oriented target among eleven distractors. Orientation discrimination asymmetries (ODAs) were found such that an oblique stimulus was easier to detect in a background of vertical stimuli than vice versa. Subjects were equally sensitive to the two highest frequency Gabor patches and less sensitive to the lowest frequency Gabors. When stimuli were scaled with E2 = 2.5 sensitivity was constant at all eccentricities and the ODA magnitude was unaffected. When stimuli were magnified with E2 = 0.77 both sensitivity and ODA magnitude increased with eccentricity. Generally, we may conclude that the ODA effect is not a strictly foveal phenomenon nor is it a strictly high frequency effect.

Bilateral symmetry embedded in noise is detected accurately only at fixation.

Bilateral or mirror symmetry is a ubiquitous feature of biological forms that the visual system could exploit for segmenting an object from a cluttered background. If this is so, the visual system may be prepared to detect symmetry at all retinal locations in parallel. Indeed, a biologically plausible model that responds optimally at axes of symmetry is quite easy to construct. Our data show, however, that if such a mechanism exists, it works with high efficiency only at the fovea. The detection of vertical bilateral symmetry embedded in random noise is very poor unless the axis of symmetry is very close to the point of fixation. This leads to the conclusion that symmetry does not play an important role in image segmentation and that it is important to the visual system only after it is fixated.

Illusory contour-motion arising from translating terminators.

Periodic grating patterns were created by phase shifting or eliminating vertical columns of a fine line carrier grating oriented 45 deg. Motion was created by translating the patterns parallel to the carrier grating. This veridical motion was seen when terminators (i) were created in low-frequency carriers; (ii) terminated short lines; and (iii) moved slowly. In the complementary conditions an illusory contour-motion was seen perpendicular to the orientation of the terminator-defined contours. A model involving a competition between second-layer filters (encoding the orientation and motions of the terminator defined contours) and double endstopped mechanisms (signalling the presence of terminators) was developed and found to be in quantitative agreement with these data. Experiments with plaids composed of two such patterns were generally consistent with the results of the one-dimensional cases. Coherent "subjective contour plaid" motion was almost always seen when the two subjective contours had the same orientation and were perfectly phase aligned.

Texture segmentation along the horizontal meridian: nonmonotonic changes in performance with eccentricity.

In 3 experiments, subjects were required to detect the presence of a small region of disparate texture embedded in a larger background at a range of eccentricities. Detection performance always peaked several degrees from fixation. Experiment 1 showed that the location of the peak was not retinally specific; scaling the display changed the location of the performance peak. Experiment 2 showed that poor foveal performance could not be explained by cross-frequency interference; filtering out high spatial frequencies did not lead to improved foveal performance. Experiment 3 showed that the effect is not unique to textures comprising left and right oblique line segments. A parsimonious account of these data is that, at the fovea, there is a mismatch between the scale of the texture and the scale of the mechanisms responsible for encoding texture differences. This mismatch diminishes as the textures are moved further into the periphery.

There is no evidence that Kanizsa-type subjective contours can be detected in parallel.

Davis and Driver presented evidence suggesting that Kanizsa-type subjective contours could be detected in a visual search task in a time that is independent of the number of nonsubjective contour distractors. A linking connection was made between these psychophysical data and the physiological data of Peterhans and von der Heydt which showed that cells in primate area V2 respond to subjective contours in the same way that they respond to luminance-defined contours. Here in three experiments it is shown that there was sufficient information in the displays used by Davis and Driver to support parallel search independently of whether subjective contours were present or not. When confounding properties of the stimuli were eliminated search became slow whether or not subjective contours were present in the display. One of the slowest search conditions involved stimuli that were virtually identical to those used in the physiological studies of Peterhans and von der Heydt to which Davis and Driver wish to link their data. It is concluded that while subjective contours may be represented in the responses of very early visual mechanisms (eg in V2) access to these representations is impaired by high-contrast contours used to induce the subjective contours and nonsubjective figure distractors. This persistent control problem continues to confound attempts to show that Kanizsa-type subjective contours can be detected in parallel.

An examination of colour-contingent pattern aftereffects.

The McCollough effect (ME) is an example of a pattern-contingent colour aftereffect. This study describes some characteristics of another visual aftereffect linking pattern and colour here called a colour-contingent pattern aftereffect (CCPA). After inducing with, for example, a magenta and black radial pattern and a green and black pattern of concentric circles, presentation of a green homogeneous field evoked a faint image of a radial pattern superimposed on the field, whereas presentation of a magenta homogeneous field produced a faint image of concentric circles. The pattern was blurred and fleeting, occurring with the onset of the homogeneous field, but nevertheless was evoked reliably. Various properties of these colour-contingent pattern aftereffects are reported. Although the aftereffects have some of the characteristics of the ME, the CCPA is not as long lasting as the ME, and, unlike the usual ME, it is abolished if eye-movements are made during induction.

Parallel discrimination of subjective contours defined by offset gratings.

Recent physiological studies (von der Heydt & Peterhans, 1989) suggest that the orientation of subjective contours is encoded very early in the visual system (V2 in monkey). This result is seemingly at odds with existing psychophysical data which suggest that the detection of subjective contours involves selective attention. It is argued that certain subjective contours are registered in a reflexive (bottom-up) manner by the visual system but that selective attention may be needed to gain access to this representation. To assess this suggestion, a visual-search task was used in which subjects were to detect the presence of a horizontal (vertical) subjective contour (defined by offset gratings) in a variable number of vertical (horizontal) subjective contours (also defined by offset gratings). When there were no competing organizations within the display, detection was indeed independent of the number of nontarget distractors, that is, selective attention was unnecessary. In a second experiment, we found that a curved form (a crescent defined by subjective contours) was easier to detect in a background of vertical bars (also defined by subjective contours) than vice versa, namely, a search asymmetry paralleling those found by Treisman and Gormican (1988). A final experiment showed that when the horizontal and vertical bars of the first experiment formed textured regions, they could be discriminated at very brief display durations (30-120 msec). However, when the line terminations aligned along the subjective contour were tapered rather than abrupt, discrimination dropped off with the degree of tapering. The latter result is consistent with the assumption that the registration of subjective contours in V2 involves the integration of responses from aligned, end-stopped cells found in V1 (von der Heydt & Peterhans, 1989).

Texture discrimination with and without abrupt texture gradients.

A common assumption is that effortless, visual texture discrimination relies on the detection of gradients between two textures. This assumption was assessed in two experiments with manipulations that smoothed (Experiment 1) or interrupted (Experiment 2) the gradient between textures comprising L- and X-type micropatterns. Compared to discrimination performance when there was an abrupt discontinuity between juxtaposed textures, performance declined moderately (about 10 percent) when the texture boundary was smoothed. In this case the two textures were asymmetrically discriminated but there was no interaction of this asymmetry with the abruptness of the texture gradient. Abrupt texture gradients, therefore, are not a necessary condition for the asymmetrical discrimination of two textures. A comparison of discrimination performance with juxtaposed textures--having an abrupt gradient--and discrimination performance when the textures were separated into distinct regions--by non-textured areas--yielded very similar results across several texture pairs. Taken together these results indicate that, in certain instances, texture discrimination may involve pattern classification-like processes that are operative in the absence of texture gradients.

Rapid discrimination of McCollough effects.

The McCollough effect is a colour aftereffect that is contingent on pattern orientation. Three experiments were conducted to establish whether such aftereffect colours could serve as a basis for discrimination in several rapid discrimination tasks. In the first experiment it was investigated whether aftereffect colours could act like a simple 'feature' in a visual search task involving a difficult orientation discrimination. Without McCollough adaptation, the time taken to detect a 'target' among 'distractors' increased substantially as the number of distractors increased. With adaptation, detection time was essentially independent of the number of distractors, indicating that the nature of the task changed from a difficult orientation discrimination to a simple discrimination based on differences in aftereffect colours. The second and third experiments employed a difficult four-alternative forced-choice procedure in which subjects were required to discriminate a monochromatic patch of square-wave grating oriented at 45 degrees from three others oriented at 135 degrees (and vice versa). The gratings were presented very briefly (67-333 ms) followed by a 500 ms mask. Subjects performed the task with and without McCollough adaptation. Performance was strikingly better after adaptation: colour aftereffects could be used to make the discrimination even at exposure durations as short as 67 ms. The third experiment demonstrated that this enhanced performance was indeed due to perceived colour differences (rather than a possible contrast difference). The results of the three experiments are discussed in relation to proposals about the locus of the McCollough effect.

Asymmetries in visual texture discrimination.

Recent results have shown that texture discrimination is an asymmetrical process; texture A within texture B may be much easier to detect than texture B within texture A. Two questions regarding discrimination asymmetries are addressed: (i) what sorts of textural properties are associated with discrimination asymmetries; and (ii) what sort of architecture would yield asymmetries. Two experiments show that discrimination asymmetries obtain when textures comprise circles of different sizes (large circles are easier to detect in small than vice versa) and when circles differ only in the regularity of their placement (irregularly placed circles are easier to detect in a background of regularly placed circles than vice versa). A plausible account of texture discrimination would involve the decomposition of images via a set orientation and scale selective filters followed by a second layer of filtering to detect energy differences between adjacent regions in the original convolutions. Discrimination asymmetries provide prima facie evidence against such a model because it involves only local measurements and comparisons. We propose that discrimination asymmetries are elegantly explained if it is assumed that the responses of the orientation and scale selective filters are normalized by the degree to which similarly tuned operators are responding elsewhere in the image; viz., global normalization of filter responses. However, there are cases where such global normalization is not required to explain asymmetrical discrimination.