Isabelle Mareschal

Sensitivity to contrast modulation depends on carrier spatial frequency and orientation.

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.

Humans have an expectation that gaze is directed toward them.

Summary Many animals use cues from another animal’s gaze to help distinguish friend from foe [1–3]. In humans, the direction of someone’s gaze provides insight into their focus of interest and state of mind [4] and there is increasing evidence linking abnormal gaze behaviors to clinical conditions such as schizophrenia and autism [5–11]. This fundamental role of another’s gaze is buoyed by the discovery of specific brain areas dedicated to encoding directions of gaze in faces [12–14]. Surprisingly, however, very little is known about how others’ direction of gaze is interpreted. Here we apply a Bayesian framework that has been successfully applied to sensory and motor domains [15–19] to show that humans have a prior expectation that other people’s gaze is directed toward them. This expectation dominates perception when there is high uncertainty, such as at night or when the other person is wearing sunglasses. We presented participants with synthetic faces viewed under high and low levels of uncertainty and manipulated the faces by adding noise to the eyes. Then, we asked the participants to judge relative gaze directions. We found that all participants systematically perceived the noisy gaze as being directed more toward them. This suggests that the adult nervous system internally represents a prior for gaze and highlights the importance of experience in developing our interpretation of another’s gaze.

Contrast gain control in natural scenes

Behavioral and electrophysiological studies of visual processing routinely employ sine wave grating stimuli, an approach that has led to the development of models in which the first stage of cortical visual processing acts as a bank of narrowband local filters whose responses vary with the contrast of preferred structure falling within their receptive fields. The relevance of this approach to natural vision is currently being challenged. We examine the contrast response of the human visual system to natural scenes. The results support a narrowband approach to visual processing but require its elaboration. Unlike grating patterns, the contrast response to natural scenes depends on the phase structure at remote spatial scales, but over a limited spatial region. The results suggest that contrast gain control acts within, but not across, cortical hypercolumns and serves to reduce the difference between the responses of detectors in regions of high and low contrast. This process tends to normalize the response of the visual system across natural scenes, which contain uneven contrast distributions.

An oblique effect for local motion: Psychophysics and natural movie statistics

Human perception of visual motion is thought to involve two stages--estimation of local motion (i.e., of small features) and global motion (i.e., of larger objects)--identified 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) paradigm--where one measures direction discrimination thresholds in the presence of directional variability--to 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.

Dual-route model of the effect of head orientation on perceived gaze direction.

Previous studies on gaze perception have identified 2 opposing effects of head orientation on perceived gaze direction—1 repulsive and the other attractive. However, the relationship between these 2 effects has remained unclear. By using a gaze categorization task, the current study examined the effect of head orientation on the perceived direction of gaze in a whole-head condition and an eye-region condition. We found that the perceived direction of gaze was generally biased in the opposite direction to head orientation (a repulsive effect). Importantly, the magnitude of the repulsive effect was more pronounced in the eye-region condition than in the whole-head condition. Based on these findings, we developed a dual-route model, which proposes that the 2 opposing effects of head orientation occur through 2 distinct routes. In the framework of this dual-route model, we explain and reconcile the findings from previous studies, and provide a functional account of attractive and repulsive effects and their interaction.

Gaze constancy in upright and inverted faces

The dual-route model (Otsuka, Mareschal, Calder, & Clifford, 2014) posits that constancy in the perception of gaze direction across lateral head rotation depends on the integration of information from the eye region and information about head rotation. Incorporation of information about head rotation serves to compensate for the change in eye-region information when viewing a rotated head. We tested the ability of this model to predict the magnitude of Wollastons effect: When eyes from a frontal pose are inserted into an angled face, the perceived direction of gaze appears attracted towards the direction of the head. The framework of the dual-route model explains Wollastons effect as a result of the misapplication of this same integration operation without any change in eye-region information. To test this explanation, we compared the magnitude of the integration occurring for Wollastons effect to that for normal faces. Here, participants performed categorical judgment of gaze direction across head rotation poses in three image conditions: normal face, eyes-only, and Wollaston. Integration of eye and head information was inferred by comparing the effect of pose between the eyes-only condition and the normal face condition, and by examining the effect of pose in the Wollaston condition. Consistent with the dual-route model, the magnitude of integration was similar between the normal face condition and the Wollaston condition. Further, upright and inverted faces yielded similar levels of gaze constancy, showing that the dual-route model applies to the perception of gaze direction in inverted faces as well as in upright faces.

Linking lower and higher stages of motion processing

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 Bex et 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.

Perceived pattern regularity computed as a summary statistic: implications for camouflage

Why do the equally spaced dots in figure 1 appear regularly spaced? The answer ‘because they are’ is naive and ignores the existence of sensory noise, which is known to limit the accuracy of positional localization. Actually, all the dots in figure 1 have been physically perturbed, but in the case of the apparently regular patterns to an extent that is below threshold for reliable detection. Only when retinal pathology causes severe distortions do regular grids appear perturbed. Here, we present evidence that low-level sensory noise does indeed corrupt the encoding of relative spatial position, and limits the accuracy with which observers can detect real distortions. The noise is equivalent to a Gaussian random variable with a standard deviation of approximately 5 per cent of the inter-element spacing. The just-noticeable difference in positional distortion between two patterns is smallest when neither of them is perfectly regular. The computation of variance is statistically inefficient, typically using only five or six of the available dots.

A psychophysical correlate of contrast dependent changes in receptive field properties

Recent physiological investigations have demonstrated that a neurons 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 neurons 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 patterns 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 surrounds 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.

Dynamics of unconscious contextual effects in orientation processing

Contextual effects abound in the real world; how we perceive an object depends on what surrounds it. A classic example of this is the tilt illusion (TI) whereby the presence of a surround shifts the perceived orientation of a target. Surprisingly, the magnitude and direction of this shift depend on the orientation difference between the target and surround: when their orientations are similar, the perceived difference is amplified and the target appears repelled in orientation from the surround (i.e., the TI). However, when their orientations are close to perpendicular, the difference is decreased and the target appears attracted in orientation toward the surround (i.e., the indirect TI). These misperceptions of orientation have revealed much about the underlying detectors involved in visual processing and how they interact with each other. What remains at stake are the levels of processing involved. To examine this, we designed a reverse-correlation technique whereby observers are blind to the orientation of the surround. We find that the TI and indirect TI occur reliably and over a similar time course, supporting the role of a single mechanism underlying orientation biases that operates in the early stages of visual processing before the conscious extraction of the surround orientation.