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PubMed1996

Cortical dynamics of lateral inhibition: visual persistence and ISI.

Psychophysical studies show that increasing the interstimulus interval (ISI) between two stimuli decreases persistence of the first stimulus. While some researchers account for these results with interactions of transient and sustained inhibition, this paper describes an alternative explanation. In a neural-network model of boundary detection called the boundary contour system, persistence is the result of feedback-generated reverberations. Mechanisms to control these reverberations include lateral inhibition, which computer simulations show allows persistence in the network to qualitatively match the psychophysical data. Additional simulations predict that increasing the duration of the second stimulus should cause persistence to increase with ISI. The model links psychophysical data on visual persistence with computational requirements of spatial vision and properties of cells in visual cortex.

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PubMed1996

Cortical dynamics of visual persistence and temporal integration.

In a temporal integration experiment, subjects must integrate two visual stimuli, presented at separate times, to perform an identification task. Many researchers have assumed that the persistence of the leading stimulus determines the ability to integrate the leading and trailing stimuli. However, recent studies of temporal integration have challenged that hypothesis by demonstrating that several theories of persistence are incompatible with data on temporal integration. This paper shows that an account of visual persistence given by a neural network model of preattentive vision, called the boundary contour system, explains data on temporal integration. Computer simulations of the model explain why temporal integration becomes more difficult when the display elements are separated by longer interstimulus intervals or are of longer duration or of higher luminance, or are spatially closer together. The model suggests that different mechanisms underlie the inverse duration effects for leading and for trailing elements. The model further predicts interactions of spatial separation, duration, and luminance of the trailing display.

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PubMed1989

Negative afterimages and the McCollough effect.

Four experiments were conducted to test earlier claims about the relationship between the negative afterimage and the McCollough effect. The first claim (Hansel & Mahmud, 1978) is that long-lasting afterimages occur when induced by the same alternating-stimulus procedure as that used to induce the McCollough effect. The second claim (Murch & Hirsch, 1972) is that afterimages can themselves induce McCollough effects if they are induced and paired sequentially with grating patterns. In testing these claims, a reliable computer-controlled color-cancellation technique developed earlier was used to measure the apparent color of both afterimages and McCollough effects objectively. No support was found for the first claim following alternative presentation of two homogeneously colored regions for total periods of 5 min (Experiment 1) and 20 min (Experiment 2). The second claim was fully supported: After an induction period of 7.3 min, a McCollough effect occurred for a red-vertical pairing but not for a green-horizontal pairing (Experiment 3); but after an induction period of 20 min, McCollough effects occurred strongly for both pairings (Experiment 4). The theoretical implications of these outcomes are considered in the context of recent theories of color and pattern processing in the visual system.

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PubMed1995

Using the standard staircase to measure the point of subjective equality: a guide based on computer simulations.

In experiments that measure a point of subjective equality, it is necessary to employ a psychophysical technique that measures the 50% point (P50) on the psychometric function. The standard staircase is an attractive candidate, because its simplicity makes it easy to understand and implement. However, Pentland (1980, Perception & Psychophysics, 28, 377-379) found his own maximum likelihood method (Best PEST) to be considerably less variable than the standard staircase. Here, computer simulations were used to study the effects of manipulating several parameters of the standard staircase in order to try to find a variant that would be competitive with the performance of Best PEST. A graphical summary of some of the standard staircase implementations that were found to be most stable is presented in order that investigators may choose easily the parameters best suited to their requirements. A comparison with an implementation of Pentland's Best PEST failed to replicate Pentland's finding, suggesting that investigators should have no misgivings about the standard staircase in experiments that measure a point of subjective equality.

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PubMed2000

[Recovery of 3D structure from motion and its statistical efficiency].

In order to investigate characteristics of the "structure from motion" in noise, we compared performances of recovering a three-dimensional structure from motion (3D condition) with those of recovering a two-dimensional structure (2D condition). In addition, we adopted a statistical efficiency approach to analyze the results. We conducted two experiments where we presented to observers a pair of three dots corresponding to three apexes of triangles rotating around an axis projected onto the fronto-parallel plane. A gaussian noise was added to the area of each triangle for each stimulus frame. Observers were required to discriminate areas of two triangles perceived with moving dots. Main results were as follows: (a) Significant difference was found between the efficiencies for 3D condition and 2D condition. We obtained about 50% or less recovery (i.e., a ratio of the 3D efficiency to the 2D one) in Experiment 2. (b) When we removed the constraints, the human efficiency and the percent recovery decreased dramatically.

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