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Biomedical subjects

J R Bergen

Publications and source records attributed to J R Bergen.

10 recordsLinked to original sources
PubMed1992

Directionally selective complex cells and the computation of motion energy in cat visual cortex.

We applied a set of 1- and 2-bar tests to directionally selective (DS) complex cells in the cat's striate cortex, and compared the responses with those predicted by two computational models. Single-bar responses and 2-bar interactions produce distinctive patterns that are highly diagnostic. The observed responses are quite similar to those predicted by a basic (non-opponent) motion-energy model [Adelson & Bergen (1985) Journal of the Optical Society of America A, 2, 284-299]. However, they are not consistent with an opponent combination of energy models, nor are they consistent with any stage of the classic Reichardt model. In particular, the Reichardt model (as well as opponent combinations of energy models) predicts a separable space-time symmetry in the 2-bar interaction that is not observed in our measurements, while the non-opponent energy model predicts an inseparable, oriented interaction very similar to the measured cortical responses. Comparisons between model and measurements suggest possible mechanisms of spatial receptive-field organization and of nonlinear transformations.

Action Potentials↗
PubMed1991

Texture segregation and orientation gradient.

Rapid texture segregation is examined using filtered noise textures. The stimuli consist of a foreground region of filtered noise with one dominant texture orientation against a background region with a different dominant orientation. Shape discrimination of the foreground region is measured as a function of the difference in orientation between the two regions (delta theta), the distance over which the dominant orientation rotates from the background to the foreground value (delta chi), and the dominant spatial frequency of the textures (f). Performance declines with smaller delta theta, larger delta chi, and lower f. These effects are partially independent of viewing distance, which implies that it is the relative or object spatial frequency, not retinal spatial frequency, which determines performance in this task. We present a model consisting of channels tuned for orientation and spatial frequency which compute local oriented energy, followed by (texture) edge detection and a cross-correlator which performs the shape discrimination. Monte Carlo simulations of this model are in accord with the degradation in performance with increased delta chi and decreased delta theta.

Humans↗
PubMed1988

Early vision and texture perception.

Texture perception has frequently been studied using textures constructed by repeated placement of micropatterns or texture elements. Theories have been developed to explain the discriminability of such textures in terms of specific features within the micropatterns themselves. For example, Beck observed that a region filled with vertical Ts is readily distinguished from one filled with tilted Ts but not from one filled with vertical Ls. He attributed this to the different distribution of oriented line segments present in the former case but not in the latter. However, Bergen and Julesz found that a region of randomly oriented Xs segregated from one filled with randomly oriented Ls, in spite of the identical distribution of oriented line segments in the two cases. They suggested that this discrimination might be based on the density of such features as terminators, corners, and intersections within the patterns. We note here that simpler, lower-level mechanisms tuned for size may be sufficient to explain this discrimination. We tested this by varying the relative sizes of the Xs and the Ls; when they produce equal responses in size-tuned mechanisms they are hard to discriminate, and when they produce different size-tuned responses they are easy to discriminate.

Form Perception↗
PubMed1985

Prediction of flicker sensitivities from temporal three-pulse data.

At low spatial frequencies it is possible to accurately predict sinusoidal flicker sensitivities from thresholds for a series of three temporal pulses. This is accomplished using a model comprising a linear temporal filter followed by an essential nonlinearity and temporal probability summation. At high spatial frequencies accurate predictions still require temporal probability summation, but not the essential nonlinearity.

Flicker Fusion↗
PubMed1985

Spatiotemporal energy models for the perception of motion.

A motion sequence may be represented as a single pattern in x-y-t space; a velocity of motion corresponds to a three-dimensional orientation in this space. Motion sinformation can be extracted by a system that responds to the oriented spatiotemporal energy. We discuss a class of models for human motion mechanisms in which the first stage consists of linear filters that are oriented in space-time and tuned in spatial frequency. The outputs of quadrature pairs of such filters are squared and summed to give a measure of motion energy. These responses are then fed into an opponent stage. Energy models can be built from elements that are consistent with known physiology and psychophysics, and they permit a qualitative understanding of a variety of motion phenomena.

Humans↗
PubMed1979

Further evidence for four mechanisms mediating vision at threshold: sensitivities to complex gratings and aperiodic stimuli.

It is now generally accepted that several mechanisms mediate contrast detection at threshold. Many attempts to measure the bandwidths of these mechanisms using subthreshold summation have yielded estimates suggesting very narrow frequency tuning. We have measured thresholds for three classes of stimuli: gratings containing two sinusoidal components, sums of "difference of Gaussian" (DOG) patterns, and square waves. The results of the subthreshold summation experiments with gratings are consistent with those of other studies. All of these thresholds can be predicted by a model requiring only four center-surround-type mechanisms at each retinal locus. These mechanisms have a full bandwidth at a half-height of about 1.75 octaves. Quantitative prediction of sensitivities depends critically upon spatial probability summation. It is this spatial nonlinearity that yields these somewhat conterintuitive results. The much narrower bandwidths inferred by others are due to the assumption of linearity used in their analyses.

Computers↗
PubMed1975

Comparison of 3,4-dimethoxyphenylethylamine treated plasma from chronic schizophrenics and controls.

Plasma samples were collected from chronic schizophrenic patients not on drug therapy and from non-psychotic hospital volunteet controls and incubated with 3,4-dimethoxywhenylethylamine (DMPEA) when injected into monoamine oxidase inhibitor (MAOI) pretreated mice who were then aggregated (groups of 5 ea.) for four hours, the two kinds of plasma produced results so similar that no differences between them could be observed. Doubling the amount of plasma and DMPEA likewise failed to differentiate plasma origin. In our hands no differentiation between plasma samples from control subjects and schizophrenic patients could be determined under the conditions employed.

Amphetamine↗