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At least 1,171 records · Page 65Linked to original sources
PubMed1993

Comparison of behavioural and morphological measures of visual acuity during ontogeny in a teleost fish, Forsterygion varium, tripterygiidae (Forster, 1801).

Ocular morphology was examined in larval, juvenile and adult F. varium. There was a 26-fold increase in eye size from 0.28 mm in the smallest larva (5.0 mm in length) to a maximum diameter of 7.2 mm in a 110 mm long adult. Larval fish had pure cone retinae at hatching, however, putative rod precursor cells were also present. Juvenile and adult fish had a duplex retina with cones arranged in a square mosaic in which 4 equal double cones surrounded a central single cone. Hypertrophy of cone ellipsoids with increasing eye size resulted in maintenance of a closely packed array in fishes of all sizes. Theoretical sensitivity, assessed in terms of convergence of rods:bipolars, rod density, and photoreceptor outer segment length, increased during the juvenile phase but was constant across the adult size range. Angular density of cones increased with increasing eye size such that theoretical spacial acuity was poor in smallest fish (1 degree 8') and improved to an asymptotic value of about 9' in adults. Behavioural acuity of a 1-day-old larva determined using the optokinetic response (29 degrees), was very much poorer than histological estimates (1 degree 8'). Behavioural acuity improved to 4 degrees 18' at 14 days of age, compared to a theoretical value of 54'. An estimate of Matthiessen's ratio based on histological measurements suggests that the larval eye is initially strongly myopic, and grows into focus. Development of the retractor lentis muscle was first apparent 7 days after hatching with the result that larval eyes are incapable of accommodative lens movements to correct for a refractive error. This apparent myopia is thought to account for at least part of the mismatch between theoretical and behavioural spatial acuity.

Animals↗
PubMed1996

A theory of the visual motion coding in the primary visual cortex.

This paper demonstrates that much of visual motion coding in the primary visual cortex can be understood from a theory of efficient motion coding in a multiscale representation. The theory predicts that cortical cells can have a spectrum of directional indices, be tuned to different directions of motion, and have spatiotemporally separable or inseparable receptive fields (RF). The predictions also include the following correlations between motion coding and spatial, chromatic, and stereo codings: the preferred speed is greater when the cell receptive field size is larger, the color channel prefers lower speed than the luminance channel, and both the optimal speeds and the preferred directions of motion can be different for inputs from different eyes to the same neuron. These predictions agree with experimental observations. In addition, this theory makes predictions that have not been experimentally investigated systematically and provides a testing ground for an efficient multiscale coding framework. These predictions are as follows: (1) if nearby cortical cells of a given preferred orientation and scale prefer opposite directions of motion and have a quadrature RF phase relationship with each other, then they will have the same directional index, (2) a single neuron can have different optimal motion speeds for opposite motion directions of monocular stimuli, and (3) a neuron's ocular dominance may change with motion direction if the neuron prefers opposite directions for inputs from different eyes.

Color Perception↗
PubMed1992

Visual search for oriented lines: the role of angular relations between targets and distractors.

Subjects can perform parallel visual search for an item of unique orientation when it is presented on a background texture composed of lines of two other orientations. A number of cues can be used to speed this search. It helps if the target has a unique categorical status (e.g., it is the only 'steep' item) (Wolfe et al., J. Exp. Psychol. Hum. Percept. Perf. 18, 34-49 1992). It helps if the two background orientations are symmetrical about a vertical axis (Wolfe and Friedman-Hill, Psychol. Sci. 3, 194-198, 1992). The experiments presented here show that it helps if the angles formed by the target with one of the distractor types are smaller than the angles formed by the two distractor orientations. These results illustrate that stimulus similarity is a complex concept even for a simple feature like orientation.

Adaptation, Ocular↗
PubMed1994

Segmenting textured 3D surfaces using the space/frequency representation.

Segmenting 3D textured surfaces is critical for general image understanding. Unfortunately, current efforts of automatically understanding image texture are based on assumptions that make this goal impossible. Texture-segmentation research usually assumes that the textures are flat and viewed from the front, while shape-from-texture work assumes that the textures have already been segmented. This deadlock means that none of these algorithms will work reliably on images of 3D textured surfaces. An algorithm has been developed by the authors that can segment an image containing nonfrontally viewed, planar, periodic textures. The spectrogram (local power spectrum) is used to compute local surface normals from small regions of the image. This algorithm does not require unreliable image feature detection. Based on these surface normals, a 'frontalized' version of the local power spectrum is computed that shows what the region's power spectrum would look like if viewed from the front. If neighboring regions have similar frontalized power spectra, they are merged. The merge criterion is based on a description length formula. The segmentation is demonstrated on images with real textures. To the authors' knowledge, this is the first program that can segment 3D textured surfaces by explicitly accounting for 3D shape effects.

Algorithms↗
PubMed2003

A modeling approach to the human spatial orientation system.

The human spatial orientation system is highly complex and nonlinear. It is difficult, therefore, to arrive at an unequivocal model of the underlying processing by merely combining the known elementary mechanisms ("bottom-up" approach); additional "top-down" concepts are required to narrow the choice between several formally equivalent solutions. We here suggest a concept in which sensorimotor control is based on a meta-level that provides an internal representation of the physical stimuli acting upon a subject (e.g., tilt of the support surface), whereas the classic reflex concept essentially proceeds from a direct coupling between physiological stimuli, sensors and actuators. At the hypothesized meta-level, the axial body segments are represented as a stack of superimposed platforms with the lowermost platform (generally the feet) riding on a support surface that acts as the buttress for the subject's active movements. From the sensory point of view, this stack constitutes a system of nested references. This concept explains data from various experiments dealing with self- and object motion perception and body stabilization in a more exhaustive way than does the classic concept. In our view, it provides a robust, flexible, and modular framework for perception and action in space.

Humans↗
PubMed1982

Inferring local surface orientation from motion fields.

The problem of inferring local surface orientation from changing images is studied computationally by deriving conditions under which the motion information is sufficient for an information processing system, biological or otherwise, to infer unique descriptions of the local surface orientation. The analysis is based on shape-from-motion proposition, which states, that given the first spatial derivatives of the orthographically projected velocity and acceleration fields of a rigidly rotating regular surface, then the angular velocity and the surface normal at each visible point on that surface are uniquely determined up to a reflection. The proof proceeds in two steps. First it is shown that surface tilt and one component of the angular velocity are uniquely determined by the first spatial derivatives of the velocity field. Then it is shown that surface slant and the remaining two components of the angular velocity are uniquely determined if the first spatial derivatives of the acceleration field are also available.

Humans↗
PubMed1983

Sensitivity to countermodulating gratings following spatiotemporal adaptation.

Contrast sensitivities to countermodulating gratings were measured with a two-alternative temporal forced-choice procedure following adaptation to a static grating of the same spatial frequency, a homogeneous flickering field of the same temporal frequency, or a countermodulating grating of identical spatial and temporal frequencies. At high spatial frequencies, the temporal-frequency content of the adaptation was not critical, that is, a countermodulating adaptation grating was only slightly more effective at raising threshold than was a static adaptation grating. At low spatial frequencies, the sensitivity to countermodulating test gratings could not be reduced by either a high-contrast stimulus matching the test in the spatial domain only or by one matching the test in the temporal domain only. Adapting to a high-contrast stimulus matching the countermodulating test grating in both spatial- and temporal-frequency domains was effective at reducing test sensitivity for one observer but not for another.

Adaptation, Ocular↗
PubMed1983

Suprathreshold spatiotemporal response characteristics of the human visual system.

The apparent contrasts of suprathreshold stationary gratings, countermodulated gratings, and homogeneous flickering fields were assessed with a contrast-matching procedure. Results show that, as stimulus amplitude is increased relative to threshold, variations in apparent contrast with spatiotemporal-frequency content become much less pronounced. In other words, the contrast-matching functions are more uniform across both spatial and temporal frequency at levels of contrast well above threshold. These data are interpreted in terms of a compensatory stage in the visual system that varies its gain characteristics according to the detectability of the stimulus.

Differential Threshold↗
PubMed1983

Independence of orientation and size in spatial discriminations.

This study of form vision explores the relationships between orientation and spatial frequency in suprathreshold discrimination tasks. Orientation discrimination thresholds for sine-wave gratings were 0.3-0.5 deg, much less than the roughly 10-24-deg orientational bandwidth of channels; spatial-frequency discrimination thresholds were 3-7%, much less than the roughly 1.2-octave spatial-frequency bandwidth of channels. We find that spatial-frequency discrimination between two gratings was as acute when the two gratings were orthogonal as when they were parallel. Orientation discrimination between two gratings was as acute when the two gratings had the same spatial frequencies as when they had different spatial frequencies. Thus orientation and spatial frequency are independent dimensions at the discrimination stage of spatial information processing.

Discrimination, Psychological↗
PubMed1985

Visual processing of moving stimuli.

This review covers a period bracketed by two OSA Symposia--"Vision under Stabilized Image Conditions," held in San Francisco in 1978, and "Motion and Vision in Man and Machine," held in New Orleans in 1983--during which image stabilization came of age as a standard technique of visual psychophysics. It seems clear that the first spatiotemporal signal-processing step in the visual system depends on image motion; e.g., a constant velocity effectively provides the mathematical convolution of stimulus patterns with retinal receptive fields. Thus the motion of the stimulus pattern at the retina is an important experimental variable over which we have only recently gained full control. We can now replace the haphazard behavior of the subject's natural eye movements by simpler forms of motion that are known and controlled by the experimenter. Results obtained in our laboratory with such stabilized fixation control include threshold measurements for moving lines and gratings, comparison of contrast sensitivities with natural and artificial eye movements, effects of orientation on moving-grating thresholds, models of the stabilized spatiotemporal threshold surface, and effects of retinal inhomogeneity on selectivity for spatial frequency and velocity. Other experiments, too numerous to include here, have also used these techniques.

Eye Movements↗
PubMed1985

Spatial and temporal parameters of motion detection in the peripheral visual field.

We present evidence that motion detectors in the peripheral visual field react to spatiotemporal structure in moving spatial white-noise patterns in a qualitatively similar fashion to those located near the center of gaze. Reichardt-type correlator mechanisms provide a simple theoretical framework in which all observed phenomena can be discussed. Two basic parameters of the correlator model are the time lag and the span. We have devised paradigms in which these parameters may be measured as a function of velocity and position in the visual field. Our results indicate that these parameters are primarily a function of the magnitude of the velocity (the span increasing and the lag decreasing as the velocity is increased) rather than of the retinal eccentricity. In another experiment, we determined the spatial resolution for apparent segregation of the visual image that is due to differential motion of detail in stationary rectangular gratings in which the pixels moved one way in the even bars and the other way in the odd bars. The limit of resolution occurs when the pixels traverse a bar of the grating in a given time, irrespective of the velocity or the retinal eccentricity. For higher velocities, the resolution is uniform over the entire visual field. For slower motion, the region of uniform (and relatively high) resolution shrinks to a region around the center of gaze. Resolution for segregation due to differential movement is at least 10 times worse than typical contrast-grating acuity.

Female↗