Hugh R. Wilson

Computational evidence for a rivalry hierarchy in vision

Cortical-form vision comprises multiple, hierarchically arranged areas with feedforward and feedback interconnections. This complex architecture poses difficulties for attempts to link perceptual phenomena to activity at a particular level of the system. This difficulty has been especially salient in studies of binocular rivalry alternations, where there is seemingly conflicting evidence for a locus in primary visual cortex or alternatively in higher cortical areas devoted to object perception. Here, I use a competitive neural model to demonstrate that the data require at least two hierarchic rivalry stages for their explanation. This model demonstrates that competitive inhibition in the first rivalry stage can be eliminated by using suitable stimulus dynamics, thereby revealing properties of a later stage, a result obtained with both spike-rate and conductance-based model neurons. This result provides a synthesis of competing rivalry theories and suggests that neural competition may be a general characteristic throughout the form-vision hierarchy.

fMRI evidence for the neural representation of faces.

fMRI (functional magnetic resonance imaging) studies on humans have shown a cortical area, the fusiform face area, that is specialized for face processing. An important question is how faces are represented within this area. This study provides direct evidence for a representation in which individual faces are encoded by their direction (facial identity) and distance (distinctiveness) from a prototypical (mean) face. When facial geometry (head shape, hair line, internal feature size and placement) was varied, the fMRI signal increased with increasing distance from the mean face. Furthermore, adaptation of the fMRI signal showed that the same neural population responds to faces falling along single identity axes within this space.

Selective preference in visual fixation away from negative images in old age? An eye-tracking study.

Recent studies have suggested that older individuals selectively forget negative information. However, findings on a positivity effect in the attention of older adults have been more mixed. In the current study, eye tracking was used to record visual fixation in nearly real-time to investigate whether older individuals show a positivity effect in their visual attention to emotional information. Young and old individuals (N = 64) viewed pairs of synthetic faces that included the same face in a nonemotional expression and in 1 of 4 emotional expressions (happiness, sadness, anger, or fear). Gaze patterns were recorded as individuals viewed the face pairs. Older adults showed an attentional preference toward happy faces and away from angry ones; the only preference shown by young adults was toward afraid faces. The age groups were not different in overall cognitive functioning, suggesting that these attentional differences are specific and motivated rather than due to general cognitive change with age.

An fMRI study of the selective activation of human extrastriate form vision areas by radial and concentric gratings

The ventral form vision pathway of the primate brain comprises a sequence of areas that include V1, V2, V4 and the inferior temporal cortex (IT) [1]. Although contour extraction in the V1 area and responses to complex images, such as faces, in the IT have been studied extensively, much less is known about shape extraction at intermediate cortical levels such as V4. Here, we used functional magnetic resonance imaging (fMRI) to demonstrate that the human V4 is more strongly activated by concentric and radial patterns than by conventional sinusoidal gratings. This is consistent with global pooling of local V1 orientations to extract concentric and radial shape information in V4. Furthermore, concentric patterns were found to be effective in activating the fusiform face area. These findings support recent psychophysical [2,3] and physiological [4,5] data indicating that analysis of concentric and radial structure represents an important aspect of processing at intermediate levels of form vision.

Dynamics of travelling waves in visual perception

Nonlinear wave propagation is ubiquitous in nature, appearing in chemical reaction kinetics, cardiac tissue dynamics, cortical spreading depression and slow wave sleep. The application of dynamical modelling has provided valuable insights into the mechanisms underlying such nonlinear wave phenomena in several domains. Wave propagation can also be perceived as sweeping waves of visibility that occur when the two eyes view radically different stimuli. Termed binocular rivalry, these fluctuating states of perceptual dominance and suppression are thought to provide a window into the neural dynamics that underlie conscious visual awareness. Here we introduce a technique to measure the speed of rivalry dominance waves propagating around a large, essentially one-dimensional annulus. When mapped onto visual cortex, propagation speed is independent of eccentricity. Propagation speed doubles when waves travel along continuous contours, thus demonstrating effects of collinear facilitation. A neural model with reciprocal inhibition between two layers of units provides a quantitative explanation of dominance wave propagation in terms of disinhibition. Dominance waves provide a new tool for investigating fundamental cortical dynamics.

Looking While Unhappy Mood-Congruent Gaze in Young Adults, Positive Gaze in Older Adults

Recent findings that older adults gaze toward positively valenced stimuli and away from negatively valenced stimuli have been interpreted as part of their attempts to achieve the goal of feeling good. However, the idea that older adults use gaze to regulate mood, and that their gaze does not simply reflect mood, stands in contrast to evidence of mood-congruent processing in young adults. No previous study has directly linked age-related positive gaze preferences to mood regulation. In this eye-tracking study, older and younger adults in a range of moods viewed synthetic face pairs varying in valence. Younger adults demonstrated mood-congruent gaze, looking more at positive faces when in a good mood and at negative faces when in a bad mood. Older adults displayed mood-incongruent positive gaze, looking toward positive and away from negative faces when in a bad mood. This finding suggests that in older adults, gaze does not reflect mood, but rather is used to regulate it.

Local and global contributions to shape discrimination

Humans are remarkably sensitive in detecting small deviations from circularity. In tasks involving discrimination between closed contours, either circular in shape or defined by sinusoidal modulations of the circle radius, human performance has been shown to be limited by global processing. We assessed the amount of global pooling for different pattern shapes (different radial modulation frequencies, RF) when circular deformation was restricted to a fraction of the contour. The results show that the improvement in performance depends on the modulation frequency (the pattern shape) when increasing the number of cycles of an RF pattern. Global processing only extends up to modulation frequencies between 5 and 10. For higher frequencies, performance can be predicted by probability summation. Position uncertainty cannot explain these effects. In a circumstance where global pooling exceeds probability summation (RF=5), we split the pattern up into five identical segments conserving the total amount of information presented. Thresholds are significantly affected by different global arrangements of these segments: (a) Occluding small parts of the pattern shows a significant effect on the position of occluders with performance lowest when gaps are placed at the points of maximum curvature. (b) Shifting segments away from the pattern centre (exploded condition) or displaying them out of concentric context (spiral condition) shuts down global processing. (c) Jittering segments radially disrupts both global and local processing. We conclude that RF patterns in the global processing range are analysed by detecting the points of maximum curvature and that, in this range, the visual system can only reliably process up to about 5 local curvature extrema.

Minimal physiological conditions for binocular rivalry and rivalry memory.

Binocular rivalry entails a perceptual alternation between incompatible stimuli presented to the two eyes. A minimal explanation for binocular rivalry involves strong competitive inhibition between neurons responding to different monocular stimuli to preclude simultaneous activity in the two groups. In addition, strong self-adaptation of dominant neurons is necessary to enable suppressed neurons to become dominant in turn. Here a minimal nonlinear neural model is developed incorporating inhibition, self-adaptation, and recurrent excitation. The model permits derivation of an equation for mean dominance duration as a function of the underlying physiological variables. The dominance duration equation incorporates an explicit representation of Levelts second law. The same equation also shows that introduction of a simple compressive response nonlinearity can explain Levelts fourth law. Finally, addition of brief, recurrent synaptic facilitation to the model generates properties of rivalry memory.

Quantifying facial expression recognition across viewing conditions.

Facial expressions are key to social interactions and to assessment of potential danger in various situations. Therefore, our brains must be able to recognize facial expressions when they are transformed in biologically plausible ways. We used synthetic happy, sad, angry and fearful faces to determine the amount of geometric change required to recognize these emotions during brief presentations. Five-alternative forced choice conditions involving central viewing, peripheral viewing and inversion were used to study recognition among the four emotions. Two-alternative forced choice was used to study affect discrimination when spatial frequency information in the stimulus was modified. The results show an emotion and task-dependent pattern of detection. Facial expressions presented with low peak frequencies are much harder to discriminate from neutral than faces defined by either mid or high peak frequencies. Peripheral presentation of faces also makes recognition much more difficult, except for happy faces. Differences between fearful detection and recognition tasks are probably due to common confusions with sadness when recognizing fear from among other emotions. These findings further support the idea that these emotions are processed separately from each other.

Sensitivity to global form in glass patterns after early visual deprivation in humans.

To compare the effects of early monocular versus early binocular deprivation on the perception of global form, we assessed sensitivity to global concentric structure in Glass patterns with varying ratios of paired signal dots to noise dots. Children who had been deprived by dense congenital cataracts in one (n=10) or both (n=8) eyes performed significantly worse than comparably aged children without eye problems. Consistent with previous results on sensitivity to global motion [Vision Research 42 (2002) 169], thresholds in the deprived eyes were significantly better after monocular deprivation than after binocular deprivation of comparable duration, even when there had been little patching of the nondeprived eye after monocular deprivation. Together, the results indicate that the competitive interactions between a deprived and nondeprived eye evident in the primary visual cortex can co-occur with complementary interactions in extrastriate cortex that enable a relative sparing of some visual functions after early monocular deprivation.