Biological and Experimental Psychology Group

We consider how the detection of second-order contrast structure depends on the orientation and spatial frequency of first-order luminance structure. For patterns composed of a bandpass noise carrier multiplied by a contrast envelope function, we show that sensitivity to the envelope varies in proportion to the spatial frequency of the carrier. For oriented carriers at low spatial-frequencies, detection of the contrast envelope is easier when the envelope and carrier are perpendicular, but this dependency diminishes as the spatial frequency of the carrier increases. These differences are not attributable to either the detection of side-bands, or the presence of spurious contrast structure in unmodulated carrier images. A final experiment measured envelope detection in the presence of noise masks. Results indicate that orientationally and spatially-band pass filtering precedes the detection of second-order structure.

The motion aftereffect (MAE) was used to study the temporal and spatial frequency selectivity of the visual system at supra-threshold contrasts. Observers adapted to drifting sine-wave gratings of a range of spatial and temporal frequencies. The magnitude of the MAE induced by the adaptation was measured with counterphasing test gratings of a variety of spatial and temporal frequencies. Independently of the spatial or temporal frequency of the adapting grating, the largest MAE was found with slowly counterphasing test gratings (at approximately 0.125–0.25 Hz). The largest MAEs were also found when the test grating was of similar spatial frequency to that of the adapting grating, even at very low spatial frequencies (0.125 c/deg). These data suggest that MAEs are dominated by a single, low-pass temporal frequency mechanism and by a series of band-pass spatial frequency mechanisms. The band-pass spatial frequency tuning even at low spatial frequencies suggests that the “lowest adaptable channel” concept [Cameron et al. (1992). Vision Research, 32, 561–568) may be an artifact of disadvantaged low spatial frequencies using static test patterns.

We sought to determine how local and global features within an image interact by examining whether orientation discrimination thresholds could be modified by contextual information. In particular, we investigated how local orientation signals within an image are pooled together, and whether this pooling process is dependent on the global orientation content present in the image. We find that observers' orientation judgments depend on surround contextual information, with performance being optimal when the center and surround stimuli are clearly distinct. In cases where the center and surround were not clearly segregated, we report two sets of results. If there was an ambiguity regarding the perception of a global structure (i.e. a small mismatch between local cues), observers’ performance was impaired. If there was no mismatch and local and global cues were consistent with the perception of a single surface, observers performed as well as in the distinct surfaces case. Although some of our results can be largely accounted for by interactions between differently oriented filters, other aspects are more difficult to reconcile with this explanation. We suggest that low level filtering constrains observers' performance, and that influences arising from image segmentation modify how local orientation signals are pooled together.

Motivated by the recent physiological finding that a neuron's receptive field can increase in size by a factor of 2-4-fold at low contrast [Nat. Neurosci. 2 (1999) 733, Proc. Natl. Acad. Sci. USA 96 (1999 12073], we sought to examine whether a psychophysical task might reflect the contrast dependent changes in the size/structure of a receptive field. We postulate that since spatial summation is not contrast invariant, a task that relies on the spatial structure of a receptive field, such as orientation discrimination, should also be affected by changes in contrast. Previously, orientation discrimination thresholds have been reported to be roughly independent of the contrast of a stimulus for most of the visible range of contrasts [i.e. Here, we found large improvements in orientation discrimination with contrast that were dependent on stimulus area. Furthermore, the apparent constancy of orientation discrimination for large area stimuli is possibly a result of a floor effect on the threshold. Therefore we conclude that there is not strong evidence for contrast invariant orientation discrimination. We interpret these results in the context of recent neurophysiological results about the expansion of cortical cells' receptive fields at low contrast.

The spatial frequency selectivity of motion detection mechanisms can be measured by comparing the magnitude of motion aftereffects (MAEs) as a function of the spatial frequency of the adapting and test gratings. For static test gratings, narrow spatial frequency tuning has been reported in a number of studies. However, for dynamic test patterns, reports have been conflicting. Ashida & Osaka [(1994).Perception, 23, 1313–1320] found no tuning whereas Bexet al.(1996)Vision Research, 36, 2721–2727] reported a narrow tuning. The main difference between the two studies was the temporal frequency of the test pattern. In this study we measured the spatial frequency tuning of the MAE using test patterns for a range of temporal frequencies. The results confirmed that there was narrow spatial frequency tuning when the test pattern was counterphasing at a low temporal frequency. However, the spatial frequency selectivity broadened as the temporal frequency of the test pattern was increased.

Recent physiological studies indicate that the tuning properties of neurons under acute preparation in primary visual cortex can change over time. We used a psychophysical reverse correlation paradigm to examine the potential repercussions of this neuronal property for human observers' ability to discriminate the orientation of targets over time. Observers were required to identify the orientation of a Gabor target presented within dynamic white noise. Frames from the noise movies were pooled to compute dynamic classification images (CIs) associated with the observers' discrimination performance, which then were fit with a weighted difference-of-Gabor function. Best-fitting templates were temporally bandpass, tuned to more oblique orientations than the stimulus but, crucially, did not change over time. The results suggest that the template for orientation discrimination is selected within the first 50 ms of stimulus onset and that, unlike the response of single cells, there is no measurable dynamic component to either orientation or spatial frequency tuning of human orientation discrimination.

Outside the fovea, the visual system pools features of adjacent stimuli. Left or right of fixation the tilt of an almost horizontal Gabor pattern becomes difficult to classify when horizontal Gabors appear above and below it. Classification is even harder when flankers are to the left and right of the target. With all four flankers present, observers were required both to classify the target’s tilt and perform a spatial frequency task on two of the four flankers. This dual task proved significantly more difficult when attention was directed to the horizontally aligned flankers. We suggest that covert attention to stimuli can increase the weights of their pooled features.

Recent physiological investigations have demonstrated that a neuron's area of spatial summation can vary depending on stimulus contrast. Specifically, when the same stimulus is presented to a neuron at a low contrast, the area of summation (or neuron's receptive field) can increase by at least a factor of two, compared to that estimated with a high contrast stimulus. We sought to examine this phenomenon psychophysically by using an orientation discrimination task carried out in the presence of contextual stimuli. We have found previously that orientation discrimination thresholds for a sine-wave grating are elevated by the presence of a surround pattern of similar orientation (with an offset) and spatial frequency. However, when these patterns were separated by a gap of mean luminance exceeding roughly 1 deg, thresholds dropped to the level measured using the center pattern alone. Here, we examined the surround pattern's effect on orientation thresholds as a function of the contrast of the center and surround. We find that when both are presented at a low contrast, the detrimental influence of the surround on orientation thresholds is maintained over larger gap separations. We also find that the spatial frequency and orientation selectivity of the surround's masking effect on orientation thresholds is broader at low contrast than at high contrast. Although the results support the idea of a spatial reorganization of the mechanisms involved in the task at low contrast, these changes are insufficient, in and of themselves, to account for the data. We suggest that additional influences possibly reflecting image segmentation also affect performance.

Visual processing has been widely investigated with narrow band stimuli at low contrasts. We used a masking paradigm to examine how visual sensitivity under these conditions compares with the perception of the direction of heading in real scenes (i.e., with dynamic natural images at high contrasts). We first confirmed and extended previous studies showing biases in the amplitude distribution for spatial frequency, temporal frequency, speed and direction in dynamic natural movies. We then measured contrast thresholds for identification of the direction of motion for an observer traveling at various speeds. In spite of differences in contrast sensitivity and large non-uniformities in the amplitude content of the stimuli, contrast thresholds were relatively invariant of spatial frequency and completely invariant of temporal frequency, speed and direction. Our results suggest that visual processing normalises responses to supra-threshold structure at different spatial and temporal frequencies within natural stimuli and so equates their effective visibility.

When a peripherally viewed stimulus is presented with flankers, observers' acuity for shape generally decreases. We wondered whether a change in the locus of information accrual accompanied these performance deficits and employed psychophysical reverse correlation to find out. Surrounding the target (a near-vertical Gabor patch) with a vertical grating caused a slight elongation and a rotation in the decision templates for orientation identification. We also found that the contrast required to maintain criterion performance in this condition was actually lower than it was in a target-alone condition. However, this facilitation decreased with practice, due to perceptual learning in the target-alone condition. Unlike a continuous surround, isolated flanks elevated contrast thresholds, but decision templates were similar with both of these contexts. The rotation of decision templates (off-orientation looking) suggests that performance is limited by additive internal noise. We speculate that this noise can be reduced when the target is easily segregated from its surround.

Visual sensitivity is reduced in the periphery for many discrimination tasks. Previously it has been reported that motion coherence thresholds are higher for dot stimuli presented in the periphery, a finding that could arise either from (a) impaired motion integration or (b) from motion integrators inheriting more noisy local directional signals. We sought to disentangle these factors using an equivalent noise paradigm. We report a deterioration in discrimination thresholds in the periphery that does not result from reduced visibility and is fully accounted for by an increase in local directional uncertainty with no change in sampling efficiency. Changes in motion coherence thresholds with stimulus eccentricity, measured using similar stimuli, exhibit a high degree of inter-subject variability.

Human perception of visual motion is thought to involve two stagesVestimation of local motion (i.e., of small features) and global motion (i.e., of larger objects)Videntified with cortical areas V1 and MT, respectively. We asked if poor discrimination of oblique compared to cardinal directions (the oblique effect for motion; OEM) reflects a deficit in local or in global motion processing. We used an equivalent noise (EN) paradigmVwhere one measures direction discrimination thresholds in the presence of directional variabilityVto quantify local and global limits. We report that the OEM diminishes with increasing directional variability, indicating that global motion processing (the number of local motion signals pooled) is equal across all directions and that the OEM is attributable to anisotropies in local motion processing. To investigate the origin of this effect, we measured local motion statistics from natural movies (filmed from the point of view of a walking observer). This analysis reveals that the distribution of local directional energy on the oblique directions tends to be broader, and frequently more asymmetric, than on the cardinal directions. If motion detectors are optimized to deal with our visual world then such anisotropies likely explain the local nature of the OEM.

The accurate perception of another person's gaze direction underlies most social interactions and provides important information about his or her future intentions. As a first step to measuring gaze perception, most experiments determine the range of gaze directions that observers judge as being direct: the cone of direct gaze. This measurement has revealed the flexibility of observers' perception of gaze and provides a useful benchmark against which to test clinical populations with abnormal gaze behavior. Here, we manipulated effective signal strength by adding noise to the eyes of synthetic face stimuli or removing face information. We sought to move beyond a descriptive account of gaze categorization by fitting a model to the data that relies on changing the uncertainty associated with an estimate of gaze direction as a function of the signal strength. This model accounts for all the data and provides useful insight into the visual processes underlying normal gaze perception.

Contextual effects are ubiquitous in vision and provide a means for detectors with localized receptive fields to encode global properties of a stimulus. Although the nature of the neural connections is complex, the majority of evidence supports the Gestalt idea of collinearity; interactions are greatest when the target and surround orientations are spatially aligned to form a contour. Here we create a novel stimulus that simultaneously probes all areas around a detector to determine which spatial positions influence perception in human observers. We find that the surrounding spatial areas that contribute most to contextual effects for our perception of orientation and motion are not confined to a specific location. Rather our results reveal that human perception displays some interobserver variability in the weighting of detector interactions that is largely independent of collinear structure. We propose that these more extensive surround stimuli reveal how complex visual structure may modulate performance in a manner that is not easily predictable using more conventional stimuli.

The fear of being scrutinised is a core feature of social anxiety disorder and socially anxious individuals overestimate being 'looked at'. A recent development in the vision sciences is a reliable psychophysical index of the range of eye gaze angles judged as being directed at oneself (Cone of Direct Gaze: CoDG). We tested the CoDG as a measure of "being looked at" in social anxiety. Participants were stratified into high/low social anxiety groups and asked to judge whether they were being 'looked at' by computerised male faces varying in eye gaze deviation and facial emotion. High socially anxious males had a wider CoDG than low socially anxious males; high and low socially anxious females did not differ. Fearful faces elicited narrower cones than neutral or angry faces; however, the effect size was small and not evident for the high socially anxious males. Measures of subjective reactions to the study, and to being looked at in general, indicated that the results may be in part due to males suffering more discomfort when being looked at. The results show that measures derived from psychophysics, in this case, the CoDG, have potential as clinical and research tools for measuring anxiety about being scrutinised.

Gaze is an important social cue in regulating human and non-human interactions. In this study, we employed an adaptation paradigm to examine the mechanisms underlying the perception of another's gaze. Previous research has shown that the interleaved presentation of leftwards and rightwards gazing adaptor stimuli results in observers judging a wider range of gaze deviations as being direct. We applied a similar paradigm to examine how human observers encode oblique (e.g. upwards and to the left) directions of gaze. We presented observers with interleaved gaze adaptors and examined whether adaptation differed between congruent (adaptor and test along same axis) and incongruent conditions. We find greater adaptation in congruent conditions along cardinal (horizontal and vertical) and non-cardinal (oblique) directions suggesting gaze is not coded alone by cardinal mechanisms. Our results suggest that the functional aspects of gaze processing might parallel that of basic visual features such as orientation.