PubMed Health⌕ Search
Home/Archive/PubMed

PubMed · 7740772

Visual processing of motion boundaries.

W L Sachtler·Q Zaidi
View original source ↗Export metadata ↓

Abstract

Psychometric functions for motion detection were measured for various spatial velocity profiles made of independently moving lines of random dots. In the first experiment, sensitivity was greater for square-wave velocity profiles than for sine waves of the same fundamental spatial frequency. Sensitivity for square waves depended on the phase of the waveform with respect to the fixation point, which precludes a characterization of the processes underlying the detection of shearing motion as a translation-invariant system. The second experiment, using velocity fields created by spatial super-position of sine waves, showed that motion boundaries facilitate detection of motion due to the steepness of the velocity gradient, and not simply because of added power at higher harmonics. In the third experiment, fluted velocity waveforms were created by subtracting the fundamental sinusoidal component from square waves, retaining sharp motion boundaries between opposing directions but removing the regions of uniform motion. Subtracting the fundamental from low-frequency square waves did not lower sensitivity to motion, indicating that sensitivity was largely determined by the presence of motion boundaries. In the final section of this article, a model is presented that can account for the data by using linear center-surround velocity mechanisms whose sizes increase with eccentricity while their sensitivity for shearing motion decreases.

Explore related subjects

Fixation, OcularHumansMaleMathematicsModels, NeurologicalMotion PerceptionPattern Recognition, VisualPsychometricsTime FactorsJournal ArticleResearch Support, U.S. Gov't, P.H.S.

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

W L Sachtler, Q Zaidi. 1995. Visual processing of motion boundaries.. https://doi.org/10.1016/0042-6989(94)00160-n

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related citations

PubMed1995

The effect of a masked stimulus on the response to the masking stimulus.

Four experiments are reported in which the subjects had to respond to a target that masked a preceding prime via metacontrast masking. In one part of Experiment 1, the subjects discriminated the target's shape (square or diamond) by a motor-choice reaction, and in another part they had to respond with a simple reaction. The prime was neutral (circular) with respect to the target's shape. The data showed a facilitation effect. In both tasks the reaction time was reduced by the masked prime. However, the reduction was more pronounced with simple reaction than with choice reaction. In the other experiments, additional primes were used with the same angular shapes as the targets. In Experiments 2 and 3, after discriminating the target's shape by a choice reaction, the subjects had to judge the prime's shape in a signal-detection task. While neither the d' value for discriminating the angular primes from the circular ones (Exp. 2) nor the d' value for distinguishing between the angular primes (Exp. 3) was different from zero, the choice-reaction data showed a congruency effect. With a congruent prime (i.e., a prime that had the same shape as the target), the reaction times were reduced. With an incongruent prime, the reaction times grew. In Experiment 4 the errors were investigated. The facilitation effect was present in the RT, but not in the number of errors, whereas the congruency effect was present in the number, but not in the RT of errors. While the facilitation effect can be attributed either to an unspecific activation by the masked prime or to an influence of the prime on attentional processes, the congruency effect can be explained by the assumption that the masked prime directly activates the specific response, which corresponds to the prime's shape.

Fixation, Ocular↗
PubMed1995

Context effects on perceived position: sustained and transient temporal influences on spatial interactions.

The effects of sustained and transient responses to inducing lines on the perceived position of nearby test lines were measured as constant errors in the vernier alignment of the test lines. The sustained response to the continued presence of the inducing lines had no effect on the perceived position of the test lines for very small inducing/test-line distances, and a repulsive effect for larger distances. Transient responses to the onset of the inducing lines had attractive effects for small distances, whereas transient responses to the offset of the inducing lines had repulsive effects for all distances tested. Attraction and repulsion effects on perceived position are accounted for by facilitating and inhibiting interactions that influence the relative sensitivities of an ensemble of position-selective detecting units.

Fixation, Ocular↗
PubMed1995

Does saccadic undershoot minimize saccadic flight-time? A Monte-Carlo study.

Visually guided saccades to single targets undershoot by about 10% of the target distance, and require additional secondary saccades to foveate the target. We examine theoretically the hypothesis that undershooting is an economical strategy for maximizing the time for clear vision by minimizing saccadic flight-time. Using a simple stochastic model, Monte-Carlo simulations show that when the standard deviation of saccadic error is about 10% of the target distance the optimal gain of primary saccades is about 0.93-0.97 depending on the main sequence for saccade duration. When the standard deviation of saccadic error is large, as occurs in the human infant, the optimal gain decreases to about 0.6, which agrees with empirical observations. We conclude that saccadic undershoot is consistent with an adaptive controller that attempts to minimize total saccadic flight-time during sequences, rather than retinal error. The ethology and physiology of such a controller is discussed in the context of visual scanning and visual development.

Fixation, Ocular↗