David C. Burr

Feature Detection in Human Vision: A Phase-Dependent Energy Model

This paper presents a simple and biologically plausible model of how mammalian visual systems could detect and identify features in an image. We suggest that the points in a waveform that have unique perceptual significance as ‘lines’ and ‘edges’ are the points where the Fourier components of the waveform come into phase with each other. At these points ‘local energy’ is maximal. Local energy is defined as the square root of the sum of the squared response of sets of matched filters, of identical amplitude spectrum but differing in phase spectrum by 90°: one filter type has an even-symmetric line-spread function, the other an odd-symmetric line-spread function. For a line the main contribution to the local energy peak is in the output of the even-symmetric filters, whereas for edges it is in the output of the odd-symmetric filters. If both filter types respond at the peak of local energy, both edges and lines are seen, either simultaneously or alternating in time. The model was tested with a series of images, and shown to predict well the position of perceived features and the organization of the images.

Changes in visual perception at the time of saccades

We frequently reposition our gaze by making rapid ballistic eye movements that are called saccades. Saccades pose problems for the visual system, because they generate rapid, large-field motion on the retina and change the relationship between the object position in external space and the image position on the retina. The brain must ignore the one and compensate for the other. Much progress has been made in recent years in understanding the effects of saccades on visual function and elucidating the mechanisms responsible for them. Evidence suggests that saccades trigger two distinct neural processes: (1) a suppression of visual sensitivity, specific to the magnocellular pathway, that dampens the sensation of motion and (2) a gross perceptual distortion of visual space in anticipation of the repositioning of gaze. Neurophysiological findings from several laboratories are beginning to identify the neural substrates involved in these effects.

Functional Implications of Cross-Orientation Inhibition of Cortical Visual Cells. I. Neurophysiological Evidence

Simple and complex cells of striate cortex of anaesthetized and paralysed cats were stimulated with two superimposed one-dimensional grating stimuli of different orientations to investigate inhibitory effects of non-optimally oriented stimuli. We confirmed that a stimulus of orientation orthogonal to a cell’s long axis significantly reduces the cell’s discharge rate. Further experiments revealed the following, (i) The inhibition was typically stronger for simple than for complex cells, (ii) It is very broadly tuned for orientation, all orientations outside the cell’s tuning band having a comparable inhibitory effect. (iii) Similarly, it is broadly tuned for spatial frequency. These last two results suggest that the inhibition arises not from a single cell but from a pool of cells, (iv) The pattern of the discharge of the inhibition in response to stimulation by phase-reversed sinusoidal gratings is consistent with the notion that the inhibition arises from complex cells. A second series of recordings of stimulation by visual noise patterns demonstrated how ‘cross-orientation inhibition’ prevents simple cells from responding to two-dimensional visual noise while allowing them to respond to comparable one-dimensional noise patterns. We suggest that this mechanism may serve to render simple cells selectively sensitive to one-dimensional stimuli, such as the contours or borders of visual objects.

A cortical area that responds specifically to optic flow, revealed by fMRI

The continuously changing optic flow on the retina provides information about direction of heading and about the three-dimensional structure of the environment. Here we use functional magnetic resonance imaging (fMRI) to demonstrate that an area in human cortex responds selectively to components of optic flow, such as circular and radial motion. This area is within the region commonly referrred to as V5/MT complex, but is distinct from the part of this region that responds to translation. The functional properties of these two areas of the V5/MT complex are also different; the response to optic flow was obtained only with changing flow stimuli, whereas response to translation occurred during exposure to continuous motion.

Saccadic eye movements cause compression of time as well as space

There is now considerable evidence that space is compressed when stimuli are flashed shortly before or after the onset of a saccadic eye movement. Here we report that short intervals of time between two successive perisaccadic visual (but not auditory) stimuli are also underestimated, indicating a compression of perceived time. We were even more surprised that in a critical interval before saccades, perceived temporal order is consistently reversed. The very similar time courses of spatial and temporal compression suggest that both are mediated by a common neural mechanism, probably related to the predictive shifts that occur in receptive fields of many visual areas at the time of saccades.

A Visual Sense of Number

Evidence exists for a nonverbal capacity for the apprehension of number, in humans [1] (including infants [2, 3]) and in other primates [4-6]. Here, we show that perceived numerosity is susceptible to adaptation, like primary visual properties of a scene, such as color, contrast, size, and speed. Apparent numerosity was decreased by adaptation to large numbers of dots and increased by adaptation to small numbers, the effect depending entirely on the numerosity of the adaptor, not on contrast, size, orientation, or pixel density, and occurring with very low adaptor contrasts. We suggest that the visual system has the capacity to estimate numerosity and that it is an independent primary visual property, not reducible to others like spatial frequency or density of texture [7].

Spatial and temporal selectivity of the human motion detection system.

Measurements were made of spatial frequency, orientation and temporal frequency selectivity of the visual motion system. The results suggest: (1) There exists in the motion system mechanisms selective for spatial frequency. The preferred spatial frequency varies considerably and extends down to at least 0.06 c/deg. (2) At all spatial frequencies (from 0.1 to 10 c/deg) there exist detectors selective for orientation which vary in (directed) orientation tuning to encompass 360 degrees. (3) The bandwidth of both spatial frequency and orientation selectivity vary inversely with spatial frequency: the lower the spatial frequency, the broader the bandwidth. (4) There exist two classes of temporally tuned detectors, one lowpass (sustained) and one bandpass (transient), of preferred temporal frequency of 7-13 Hz (depending on spatial frequency).

Seeing biological motion

One of the more stunning examples of the resourcefulness of human vision is the ability to see ‘biological motion’, which was first shown with an adaptation of earlier cinematic work: illumination of only the joints of a walking person is enough to convey a vivid, compelling impression of human animation, although the percept collapses to a jumble of meaningless lights when the walker stands still. The information is sufficient to discriminate the sex and other details of the walker,, and can be interpreted by young infants. Here we measure the ability of the visual system to integrate this type of motion information over space and time, and compare this capacity with that for viewing simple translational motion. Sensitivity to biological motion increases rapidly with the number of illuminated joints, far more rapidly than for simple motion. Furthermore, this information is summed over extended temporal intervals of up to 3 seconds (eight times longer than for simple motion). The steepness of the summation curves indicates that the mechanisms that analyse biological motion do not integrate linearly over space and time with constant efficiency, as may occur for other forms of complex motion, but instead adapt to the nature of the stimulus.

Contrast sensitivity at high velocities

Measurements were made of the contrast required to see the direction of motion of drifting gratings (Part 1) and of moving bars (Part 2). The spatial frequency at which least contrast is required to see sinusoidal gratings decreases as their velocity increases, but peak sensitivity is identical at all velocities up to 800 deg/sec. Similarly, the wider a single bar, the higher the velocity at which it is best visible. A bar 80 deg wide is best seen when moving at 300-500 deg/sec, and can be seen, and its direction of motion identified, even when moving at 10(4) deg/sec. These results show that motion does not diminish the visual passband, but instead slides the spatial frequency window along the spatial frequency scale, maintaining peak sensitivity at a temporal frequency of about 10 Hz (at photopic luminances).

Young Children Do Not Integrate Visual and Haptic Form Information

Several studies have shown that adults integrate visual and haptic information (and information from other modalities) in a statistically optimal fashion, weighting each sense according to its reliability [1, 2]. When does this capacity for crossmodal integration develop? Here, we show that prior to 8 years of age, integration of visual and haptic spatial information is far from optimal, with either vision or touch dominating totally, even in conditions in which the dominant sense is far less precise than the other (assessed by discrimination thresholds). For size discrimination, haptic information dominates in determining both perceived size and discrimination thresholds, whereas for orientation discrimination, vision dominates. By 8-10 years, the integration becomes statistically optimal, like adults. We suggest that during development, perceptual systems require constant recalibration, for which cross-sensory comparison is important. Using one sense to calibrate the other precludes useful combination of the two sources.