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this article may be addressed to either Michael Kahana or Robert Sekuler, Volen National Center for Complex Systems, MS 013, Brandeis University, Waltham, MA 02254-9110. E-mail may be sent to kahana @brandeis.edu or sekuler@brandeis.edu plex multidimensional stimulus spaces (Nosofsky, 1992; Maddox & Ashby, 1996; Ashby & Perrin, 1988), with decision rules that can predict performance in a variety of classification paradigms (Nosofsky & Palmeri, 1998; Nosofsky & Alfonso-Reese, 1999; Maddox & Ashby, 1996). Although models of classification and models of visual discrimination share many assumptions about stimulus representation and subjects' decision rules, models of classification have been primarily developed to explain subjects' classification of combinations of simple geometric forms, whereas models of discrimination have been developed to explain subjects ' discrimination of elemental visual stimuli, including sinusoidal luminance gratings. Because such stimuli can be combined to synthesize more complex images such as textures and natural scenes, they represent a natural test-bed for assessing theories' power and generalizability

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Psychophysical contrast increment thresholds were compared with neuronal responses, inferred from functional magnetic resonance imaging (fMRI) to test the hypothesis that contrast discrimination judgements are limited by neuronal signals in early visual cortical areas. FMRI was used to measure human brain activity as a function of stimulus contrast, in each of several identifiable visual cortical areas. Contrast increment thresholds were measured for the same stimuli across a range of baseline contrasts using a temporal 2AFC paradigm. FMRI responses and psychophysical measurements were compared by assuming that: (1) fMRI responses are proportional to local average neuronal activity; (2) subjects choose the stimulus interval that evoked the greater average neuronal activity; and (3) variability in the observer's psychophysical judgements was due to additive (IID) noise. With these assumptions, FMRI responses in visual areas V1, V2d, V3d and V3A were found to be consistent with the psych...

Substantial evidence supports a relationship between gait perception and gait synthesis. Furthermore, passive mechanical systems demonstrate that the jointed leg systems of humans have innate oscillations that form a gait. These observations suggest that systems may perceive gaits by synchronizing an internal oscillating model to observed oscillations. We present such a system in this paper that uses phase-locked loops to synchronize an internal oscillator with oscillations from a video source. Arrays of phase-locked loops, called video phase-locked loops, synchronize a system with the oscillations in pixel intensities. We then test the perception of the resulting synchronized-oscillator model in various gait recognition tasks. Tools based on Procrustes analysis and directional statistics provide the computational mechanism to compare patterns of oscillations. We discuss the possibility of an alternative model for motion perception based on synchronization with the transient oscillations of temporal band-pass filters that is consistent with other proposed models for human perception. Synchronization of a kinematic model to oscillations also suggests a path to bridge the gap between the model-free and model-based domains.

this article should be addressed to W. Todd Maddox, Department of Psychology, Mezes Hall 330 Mail Code B3800, University of Texas, Austin, Texas, 78712. E-mail: maddox@psy.utexas.edu

We report contrast detection, contrast increment, contrast masking, orientation discrimination, and spatial frequency discrimination thresholds for spatially localized stimuli at 4 ° of eccentricity. Our stimulus geometry emphasizes interactions among overlapping visual filters and differs from that used in previous threshold measurements, which also admits interactions among distant filters. We quantitatively account for all measurements by simulating a small population of overlapping visual filters interacting through divisive inhibition. We depart from previous models of this kind in the parameters of divisive inhibition and in using a statistically efficient decision stage based on Fisher information. The success of this unified account suggests that, contrary to Bowne [Vision Res. 30, 449 (1990)], spatial vision thresholds reflect a single level of processing, perhaps as early as primary visual cortex. © 2000 Optical Society of America [S0740-3232(00)02311-5] OCIS codes: 330.0330, 330.1800, 330.4060, 330.5510, 330.6100, 330.7310. 1.

Within natural images there is substantial spatial variation in both local contrast and local luminance. Understanding the statistics of these variations is important for understanding the dynamics of receptive field stimulation that occur under natural viewing conditions and for understanding the requirements for effective luminance and contrast gain control. Local luminance and contrast were measured in a large set of calibrated 12-bit gray-scale natural images, for a number of analysis patch sizes. For each image and patch size we measured the range of contrast, the range of luminance, the correlation in contrast and luminance as a function of the distance between patches, and the correlation between contrast and luminance within patches. The same analyses were also performed on hand segmented regions containing only ‘‘sky’’, ‘‘ground’’, ‘‘foliage’’, or ‘‘backlit foliage’’. Within the typical image, the 95 % range (2.5–97.5 percentile) for both local luminance and local contrast is somewhat greater than a factor of 10. The correlation in contrast and the correlation in luminance diminish rapidly with distance, and the typical correlation between luminance and contrast within patches is small (e.g., 0.2 compared to 0.8 for 1/f noise). We show that eye movements are frequently large enough that there will be little correlation in the contrast or luminance on a receptive field from one fixation to the next, and thus rapid contrast and luminance gain control are essential. The low correlation between local luminance and contrast implies that efficient contrast gain control mechanisms can operate largely independently of luminance gain control mechanisms. Ó 2005 Elsevier Ltd. All rights reserved. 1.

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RF-LISSOM, a self-organizing model of laterally connected orientation maps in the primary visual cortex, was used to study the psychological phenomenon known as the tilt aftereffect. The same self-organizing processes that are responsible for the long-term development of the map are shown to result in tilt aftereffects over short time scales in the adult. The model permits simultaneous observation of large numbers of neurons and connections, making it possible to relate high-level phenomena to low-level events, which is difficult to do experimentally. The results give detailed computational support for the long-standing conjecture that the direct tilt aftereffect arises from adaptive lateral interactions between feature detectors. They also make a new prediction that the indirect effect results from the normalization of synaptic efficacies during this process. The model thus provides a unified computational explanation of self-organization and both the direct and indirect tilt aftereffect in the primary visual cortex.