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W S Geisler

Publications and source records attributed to W S Geisler.

At least 19 recordsLinked to original sources
PubMed2001

Edge co-occurrence in natural images predicts contour grouping performance.

The human brain manages to correctly interpret almost every visual image it receives from the environment. Underlying this ability are contour grouping mechanisms that appropriately link local edge elements into global contours. Although a general view of how the brain achieves effective contour grouping has emerged, there have been a number of different specific proposals and few successes at quantitatively predicting performance. These previous proposals have been developed largely by intuition and computational trial and error. A more principled approach is to begin with an examination of the statistical properties of contours that exist in natural images, because it is these statistics that drove the evolution of the grouping mechanisms. Here we report measurements of both absolute and Bayesian edge co-occurrence statistics in natural images, as well as human performance for detecting natural-shaped contours in complex backgrounds. We find that contour detection performance is quantitatively predicted by a local grouping rule derived directly from the co-occurrence statistics, in combination with a very simple integration rule (a transitivity rule) that links the locally grouped contour elements into longer contours.

Bayes Theorem↗
PubMed1999

Motion streaks provide a spatial code for motion direction.

Although many neurons in the primary visual cortex (V1) of primates are direction selective, they provide ambiguous information about the direction of motion of a stimulus. There is evidence that one of the ways in which the visual system resolves this ambiguity is by computing, from the responses of V1 neurons, velocity components in two or more spatial orientations and then combining these velocity components. Here I consider another potential neural mechanism for determining motion direction. When a localized image feature moves fast enough, it should become smeared in space owing to temporal integration in the visual system, creating a spatial signal-a 'motion streak'-oriented in the direction of the motion. The orientation masking and adaptation experiments reported here show that these spatial signals for motion direction exist in the human visual system for feature speeds above about 1 feature width per 100 ms. Computer simulations show that this psychophysical finding is consistent with the known response properties of V1 neurons, and that these spatial signals, when appropriately processed, are sufficient to determine motion direction in natural images.

Computer Simulation↗
PubMed1995

Adaptation mechanisms in spatial vision--I. Bleaches and backgrounds.

To examine how the mechanisms of bleaching and background adaptation affect spatial pattern vision, contrast detection thresholds were measured in the fovea for sinusoidal (increment-Gabor) targets, during long-term dark adaptation following full bleaches, and against steady adapting backgrounds of various intensities. The dark-adaptation curves were found to be invariant in shape over the range of spatial frequencies tested (1-15 c/deg); in other words, the amplitude sensitivity functions were invariant during dark adaptation. These results support the hypothesis that bleaching adaptation is local and multiplicative. On the other hand, the background-adaptation curves measured for different spatial frequencies were found to converge as background intensity increased; the amplitude sensitivity functions became flatter. These results reject the equivalent-background hypothesis.

Adaptation, Ocular↗
PubMed1995

Adaptation mechanisms in spatial vision--II. Flash thresholds and background adaptation.

To examine how the mechanisms of light adaptation affect spatial pattern vision, contrast detection thresholds were measured for sinusoidal (increment-Gabor) probes on flashed backgrounds in the presence of steady adapting backgrounds. The thresholds for all spatial frequencies (1-12 c/deg), flashed-background intensities (dark to 4 log td) and adapting-background intensities (dark to 4 log td) were adequately described by a simple model consisting of a compressive nonlinearity (a modified Naka-Rushton function), a subtractive adaptation factor, and a multiplicative adaptation factor. For all five subjects the compressive nonlinearity was found to vary systematically with spatial frequency; for all but one subject, the subtractive and multiplicative factors were found to be relatively constant.

Adaptation, Ocular↗
PubMed1995

Bayesian analysis of identification performance in monkey visual cortex: nonlinear mechanisms and stimulus certainty.

The identification performance of single neurons in the primary visual cortex was quantified by measuring how accurately one could know the stimulus based upon the neuron's response. We found that for a typical neuron a response of 10 action potentials, following one brief stimulus presentation, was sufficient to classify the stimulus as belonging to a relatively small region in stimulus space, with a high degree of confidence. The performance was better than that which could be attained through linear summation of excitation and inhibition alone. The results suggest that the enhanced performance is a consequence of two nonlinear mechanisms: contrast gain control and expansive response exponent.

Action Potentials↗
PubMed1995

Separation of low-level and high-level factors in complex tasks: visual search.

A method for assessing the role of low-level factors in complex tasks is described. The method, which involves comparing simple-discrimination performance and complex-task performance for the same stimuli, was used to assess the role of low-level factors in multiple-fixation visual search. In one experiment, the target and background were composed of line segments that differed in color, orientation, or both; in another, target and background were composed of filtered-noise textures that differed in spatial frequency, orientation, or both. Most of the variance in search time was found to be predictable from the discrimination data, suggesting that low-level factors often play a dominant role in limiting search performance. A signal-detection model is presented that demonstrates how current psychophysical models of visual discrimination might be generalized to obtain a theory that can predict search performance for a wide range of stimulus conditions.

Attention↗
PubMed1992

Cortical neurons: isolation of contrast gain control.

The selectivity of cortical neurons remains invariant with contrast, even though the contrast-response function saturates. Both the invariance and the saturation might be due to a contrast-gain control mechanism. To test this hypothesis, a drifting grafting was used to measure the contrast-response function, while a counterphase grating was simultaneously presented at the null position of the receptive field (where it evokes no response at any contrast). When the contrast of the counterphase grating increased, the contrast-response function shifted primarily to the right. This result is consistent with the hypothesis that there is a fast-acting gain-control mechanism which effectively scales the input contrast by the average local contrast.

Adaptation, Ocular↗
PubMed1992

Stereopsis at isoluminance in the absence of chromatic aberrations.

Stereo (front-back) discrimination thresholds were measured in a two-interval forced-choice paradigm for chromatic (red-green) random-dot stereograms that had all detectable longitudinal and transverse aberrations removed by low-pass filtering. The thresholds were measured as a function of the luminance ratio of the red and the green stereo elements. Although individual differences were apparent, three subjects were able to fuse all the stimuli, including those at isoluminance. A quantitative, ideal-observer analysis was used to determine the neural efficiency with which color and luminance information was used in this stereo task. For two subjects, efficiency was constant as a function of the red-to-green ratio; for the third subject, efficiency was less near isoluminance.

Color Perception↗
PubMed1991

The relative contributions of pre-neural and neural factors to areal summation in the fovea.

In order to determine the relative contributions of pre-neural and neural factors to areal summation in the fovea, measurements of Ricco's area were made. These were compared to the results of an ideal-observer analysis which incorporated only pre-neural factors, up to the level of the photoreceptor. The comparison indicated that Ricco's area in the fovea is largely (if not completely) accounted for by pre-neural factors. Thus, our results, in agreement with the recent analysis of contrast sensitivity by Banks, Geisler and Bennett (1987; Vision Research, 27, 1915-1924), are consistent with the hypothesis that a neural pathway exists which consists of units whose center mechanisms sum over only a single cone or a single row of cones. Our results also imply that the quantum efficiency for detection is constant for test areas up to 256 min2, once the effects of the optics and receptor aperture are factored out.

Adaptation, Ocular↗
PubMed1991

Motion selectivity and the contrast-response function of simple cells in the visual cortex.

The responses of simple cells were recorded from the visual cortex of cats, as a function of the position and contrast of counterphase and drifting grating patterns, to assess whether direction selectivity can be accounted for on the basis of linear summation. The expected responses to a counterphase grating, given a strictly linear model, would be the sum of the responses to the two drifting components. The measured responses were not consistent with the linear prediction. For example, nearly all cells showed two positions where the responses approached zero (i.e. two "null phase positions"); this was true, even for the most direction selective cells. However, the measured responses were consistent with the hypothesis that direction selectivity is a consequence of the linear spatiotemporal receptive-field structure, coupled with the nonlinearities revealed by the contrast-response function: contrast gain control, halfwave rectification, and expansive exponent. When arranged in a particular sequence, each of these linear and nonlinear mechanisms performs a useful function in a general model of simple cells. The linear spatiotemporal receptive field initiates stimulus selectivity (for direction, orientation, spatial frequency, etc.). The expansive response exponent enhances selectivity. The contrast-set gain control maintains selectivity (over a wide range of contrasts, in spite of the limited dynamic response range and steep slope of the contrast-response function). Rectification conserves metabolic energy.

Animals↗
PubMed1991

Discrimination performance of single neurons: rate and temporal-pattern information.

1. A new method of measuring the performance of neurons in sensory discrimination tasks was developed and then applied to single-neuron responses recorded in the auditory nerve of chinchilla and in the striate visual cortex of cat. 2. Most previous methods of measuring discrimination performance have employed decision rules that involve comparing the total counts of action potentials (spikes) produced by two different stimuli. Such measures ignore response pattern and hence may not reflect all the information transmitted by a neuron. The proposed method attempts to measure all (or most) of the transmitted information by constructing descriptive models of the neuron's response to each stimulus in the discrimination experiment; these descriptive models consist of measured probability distributions of the spike counts in small time bins. The measured probability distributions are then used to define an optimal decision rule (an ideal observer) for discriminating the two stimuli. Finally, discrimination performance is measured by applying this decision rule to novel presentations of the same two stimuli. 3. Intensity and temporal-phase discrimination were measured for three neurons in the auditory nerve of chinchilla. The discrimination stimuli were low-frequency pure tones of 70-ms duration. Intensity thresholds were found to be 5-20 dB lower at low intensities using the new pattern method compared with the traditional counting method. The pattern method led to better performance because it utilized both rate and temporal pattern information. Phase discrimination performance using the counting method was at chance because the average spike rate did not change with phase. On the other hand, using the pattern method, phase discrimination thresholds were found to decrease with intensity, often reaching values equivalent to 30-40 microseconds of temporal offset. These thresholds are as good as or better than behavioral thresholds in chinchilla. 4. Contrast and temporal-phase discrimination were measured for three neurons in the striate visual cortex of cat. The discrimination stimuli were drifting sine-wave gratings of 100- to 160-ms duration. Contrast discrimination functions measured by the pattern method and the counting method were found to be essentially identical. Phase discrimination using the counting method was at chance. However, using the pattern method, phase thresholds were found to decrease with contrast, reaching values equivalent to 7 ms of temporal offset for the two simple cells. 5. Our results suggest that temporal response pattern carries substantial information for intensity and phase discrimination in the auditory nerve and for phase discrimination in the striate visual cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1990

Color as a source of information in the stereo correspondence process.

Although previous research has shown that depth perception is weak for isoluminant stereograms, the possibility remains that color plays an important role in stereopsis when luminance variations are present. To examine this possibility, we measured the relative contribution of chromatic and luminance cues in solving the correspondence problem for ambiguous "wallpaper" stereograms composed of vertical bars. Using an ideal-observer analysis, we found that chromatic cues were used much more efficiently than luminance cues in disambiguating these stereograms when the patterns were presented on a dark background but were used with about equal efficiency when presented on a light background. Another experiment (using the same wallpaper patterns) showed that chromatic and luminance cues were also used with about equal efficiency in a standard stereo detection task. Some of the implications of these results for theories of stereo vision are discussed.

Color Perception↗
PubMed1989

Visual cortical receptive fields in monkey and cat: spatial and temporal phase transfer function.

The response amplitude of simple cortical cells to spatiotemporal sine-wave patterns has been thoroughly documented in both cat and monkey. However, comparable measurements of response phase are not available even though phase measurements are essential for estimating the complete transfer function of a cell, and thus its spatiotemporal receptive field. This report describes a simple procedure for measuring both the amplitude and the phase transfer functions of striate cells. This technique was applied to 15 monkey and 27 cat simple cells. The spatiotemporal phase response functions were found to be adequately described by linear equations in four parameters. Both the amplitude and phase responses were found to satisfy several strong constraints implied by the class of linear quadrature models proposed recently in theories of biological motion sensitivity. Because the data satisfied these constraints, it was possible to determine four important receptive field properties from the phase data: the spatial symmetry, the temporal symmetry, the response latency, and the spatial position. The receptive fields were found to have a wide range of spatial symmetries, but a more narrow range of temporal symmetries. Spatiotemporal receptive fields reconstructed from complete transfer functions are used to illustrate some of the differences between direction selective and nondirection selective cells. Finally, the effects of linear and nonlinear mechanisms on amplitude, phase, and direction selective responses are considered.

Animals↗
PubMed1989

Sequential ideal-observer analysis of visual discriminations.

Visual stimuli contain a limited amount of information that could potentially be used to perform a given visual task. At successive stages of visual processing, some of this information is lost and some is transmitted to higher stages. This article describes a new analysis, based on the concept of the ideal observer in signal detection theory, that allows one to trace the flow of discrimination information through the initial physiological stages of visual processing, for arbitrary spatio-chromatic stimuli. This ideal-observer analysis provides a rigorous means of measuring the information content of visual stimuli and of assessing the contribution of specific physiological mechanisms to discrimination performance. Here, the analysis is developed for the physiological mechanisms up to the level of the photoreceptor. It is shown that many psychophysical phenomena previously attributed to neural mechanisms may be explained by variations in the information content of the stimuli and by preneural mechanisms.

Attention↗
PubMed1987

The physical limits of grating visibility.

We examined the extent to which pre-neural factors constrain the detectability of sinusoidal gratings of different spatial frequencies and luminances. Contrast sensitivity functions were measured in two observers for foveally-presented grating patches. Spatial extent of the patches was inversely proportional to grating frequency. The observers' contrast sensitivity functions were then compared to the performance of an ideal discriminator (Geisler, 1984) which incorporated the effects of quantal fluctuations, optical transfer, ocular media transmittance, and the aperture, quantum efficiency, and spatial distribution of foveal photoreceptors. The sensitivity of the ideal discriminator was roughly 20-fold greater than that of the human observers, but the shapes of the ideal and human CSFs were quite similar from 5 to 40 c/deg and from 3.4 to 340 cd/m2. The similarity of shapes demonstrates that the high-frequency rolloff of the foveal CSF for gratings with a fixed number of cycles can be explained by the operation of pre-neural factors alone. Previous research has shown that grating summation area is inversely proportional to the square of spatial frequency. Thus, for gratings with fixed spatial extent the high-frequency rolloff can be explained by the pre-neural factors plus variations in grating summation area. These conclusions imply in turn that the neural transfer function is much flatter than previously thought and that private line connections from foveal photoreceptors to higher visual centers are common.

Eye Movements↗
PubMed1986

Sampling-theory analysis of spatial vision.

Spatial-vision research has been largely concerned with measuring and understanding the consequences of receptive-field properties measured by single-unit recording. However, in order to understand spatial-information processing in the visual system, it is equally essential to know the densities and the distribution patterns of the receptive fields. If the receptive fields are not arrayed properly across the visual field, spatial information will be lost. It has been argued, on the basis of the Whittaker-Shannon sampling theorem, that the receptors of the fovea sample the retinal image at a high enough rate to preserve essentially all the available spatial information. In this paper we show how two-dimensional sampling theory can be used to determine which combinations of receptive-field shapes and sampling patterns would preserve all spatial information from the receptors. This analysis will prove useful for determining, in conjunction with electrophysiological and anatomical evidence, what spatial information is or is not being transmitted by a given stage of the visual pathway. It may also prove useful for developing and testing theories of spatial vision.

Humans↗
PubMed1985

Ideal discriminators in spatial vision: two-point stimuli.

An earlier paper [J. Opt. Soc. Am. A 1, 775 (1984)] described a general ideal discriminator (a Stimuli-Defined-Exactly observer) whose sensitivity is limited only by the initial sequence of mechanisms in the visual system--the optics of the eye, the receptor lattice, receptor optics, and photopigment spectral sensitivities. In the present paper, further properties of the model are derived, and a similar model (a Stimuli-Defined-Statistically observer) is developed for conditions with stimulus uncertainty. To test the predictions of the models, two-point intensity discrimination, resolution, and separation discrimination were measured as a function of point-source energy and background luminance. Three important differences between resolution and separation discrimination were found: (1) At moderate to high intensity levels, separation threshold is much smaller than resolution threshold. (2) At low intensity levels, resolution threshold is smaller than separation threshold. (3) Separation threshold decreases more rapidly than resolution threshold as a function of intensity. All three properties were predicted by the models. However, as expected, the quantitative fits are not accurate. The value of the models is that they allow one to determine, for almost arbitrary visual-discrimination tasks, what aspects of performance are accounted for by preneural factors. Furthermore, the models provide a precise metric of the discrimination information available at the receptors. This permits meaningful comparison of human performance across different tasks.

Adult↗