1,721,084 research outputs found
Large receptive fields for optic flow detection in humans
No full textWe used a psychophysical summation technique to study the propel-ties of detectors tuned to radial, circular and translational motion, and to determine the spatial extent of their receptive fields. Signal-to-noise motion thresholds were measured for patterns curtailed spatially in various ways. Sensitivity for radial, circular and translational motion increased with stimulus area at a rate predicted by an ideal integrator. When sectors of noise were added to the stimulus; sensitivity decreased at a rate consistent with an ideal integrator. Summation was tested for large annular stimuli, and shown to hold up to 70 degrees in some cases, suggesting very large receptive fields for this type of motion (consistent with the physiology of neurones in the dorsal region of the medial superior temporal area (MSTd)). This is a far greater area than observed for summation of contrast sensitivity to gratings (Anderson SJ and Burr DC, Vis Res 1987;29:621-635, and to this type of stimuli (Morrone MC, Burr DC and Vaina LM, Nature 1995;376:507-509, consistent with the suggestion that the two techniques examine different levels of motion analysis. (C) 1998 Elsevier Science Ltd. All rights reserved
THE ROLE OF FEATURES IN STRUCTURING VISUAL IMAGES
No full textEdges and lines carry much information about images and many models have been developed to explain how the human visual system may process them. One recent approach is the local energy model of Morrone and Burr. This model detects and locates both lines and edges simultaneously, by taking the Pythagorean sum of the output of pairs of matched filters (even- and odd-symmetric operators) to produce the all-positive local energy function. Maxima of this function signal the presence of all image features that are then classified as lines or edges (or both) and as positive or negative, depending on the strength of response of the even- and odd-symmetric operators. If the feature is an edge, it carries with it a brightness description that extends over space to the next edge. The model successfully explains many visual illusions, such as the Craik-O'Brien, Mach bands and a modified version of the Chevreul. Features can structure the visual image, often creating appearances quite contrary to the physical luminance distributions. In some examples the features dictate totally the image structure, 'capturing' all other information; in others the features are seen in transparence together with an alternate image. All cases can be predicted from the rules for combination of local energy at different scales
IMPULSE-RESPONSE FUNCTIONS FOR CHROMATIC AND ACHROMATIC STIMULI
No full textThresholds were measured for detecting pairs of briefly flashed stimuli displayed successively at variable onset asynchronies. The stimuli were 1 cycle/deg vertical sinusoidal gratings, modulated either in luminance (Yellow-black) or in color (red-green). The successive presentations were either of the same contrast (positive) or of opposite contrast (negative), yielding four separate summation curves: positive and negative summation for color and for luminance. Both the positive and the negative curves followed a shorter time course for luminance than for color, implying a faster response at threshold. To calculate impulse response functions from the summation data, we assumed that the neural impulse response from two successive stimuli sum linearly at threshold, that thresholds are determined by probability summation of the combined impulse response over time, and that the impulse response can be described by an exponentially damped frequency-modulated sinusoidal function with four free parameters. The predicted impulse responses for luminance and for color are quite different, being biphasic for luminance and monophasic for color. Fourier transform of these functions yielded estimates of the amplitude and the phase functions of hypothetical visual detectors: the amplitude functions predicted well the contrast sensitivity of counterphased gratings (as a function of temporal frequency) both for luminance and for chromatic stimuli
Feature detection in human vision: a phase-dependent energy model.
No full textThis 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 degrees: 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
Temporal impulse response functions for luminance and colour during saccades
No full textPrevious work has shown that during saccadic eye movements, contrast sensitivity for low spatial frequency patterns modulated in luminance is selectively reduced by up to one logarithmic unit, while high spatial frequency patterns, and equiluminant patterns of all spatial frequencies are not suppressed at all: [Burr et al. (1994). Nature, 371, 511-513]. Here we study the temporal characteristics for sensitivity to luminance and chromatic patterns during saccades, using the two-pulse summation technique. Sensitivity was measured for detecting two successive pulses as a function of stimulus-onset asynchrony, during normal viewing and during saccades, Impulse response functions were estimated from the summation data, for all conditions. For equiluminance, the functions were monophasic during normal viewing and saccades. For luminance modulation, the impulse response functions were di-phasic in both normal viewing and saccades. However, during saccades the impulse responses were faster in normal viewing. This result is consistent with the suggestion that saccadic suppression is mediated by contrast gain control mechanisms, known to occur in M-cells but not P-cells. Copyright (C) 1996, Published by Elsevier Science Ltd
- …
