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PubMed1983

Contribution of two movement detecting mechanisms to central and peripheral vision.

Two mechanisms, one for the detection of fast, and the other for slow movement of a sinusoidal grating are identified, and investigated under central, parafoveal, and peripheral viewing conditions. The fast movement data is considered in terms of the Reichardt model, in which signals from two adjacent inputs are cross-correlated leading to halving of the spatial resolving power for movement detection, compared with that for pattern detection. The mechanism underlying slow movement detection is regarded as being closely related to pattern detection, probably at the single unit level. The characteristics of this mechanism are discussed in the light of recent electrophysiological experiments describing clusters of simple cells in the visual cortex with "directional preference" properties.

Form Perception↗
PubMed1983

Detection of visual forms in space and time.

Stationary and moving target forms were composed of 5 equally spaced dots embedded in a background of 600 noise dots; the spatial and temporal separations between the target dots were varied independently. Target detectability decreased linearly with both spatial and temporal separations between the target dots. Detectability of both stationary and moving targets obeyed the same quantitative dependence on total separations, invariant under orientation in space-time. Detection also depended primarily on the relative density of the target and noise rather than on the absolute spatial or temporal separations between target dots. Thus, space and time had interchangeable effects on the detection of both stationary and moving targets.

Female↗
PubMed1983

Visual masking by flickering surrounds.

Observers detected drifting sine-wave gratings presented in a circular 3 dia test field which was surrounded by a 3.25 degrees wide annulus. Forced choice contrast thresholds were measured with surrounds consisting of a steady field of light or uniform sinusoidal flicker. The flickering surround raised detection thresholds only for gratings with spatial frequencies below 2-4 c/deg. Variations on the basic experiment revealed that: (1) low spatial frequency gratings drifting through the surround masked detection of uniform flicker presented to the center; (2) masking did not depend greatly on the drift rate of the test grating but could not be obtained with stationary targets; (3) flicker restricted to either the top or side borders of the test field was a sufficient condition to produce masking; (4) the size of the masking effect decreased with center-surround separation. These results suggest a destructive interaction between transient mechanisms subserving neighboring regions of the visual field.

Form Perception↗
PubMed1984

Binocular simple cells for local stereopsis: comparison of receptive field organizations for the two eyes.

If a cell is to serve as a depth detector in a local stereopsis mechanism, it could indicate the depth of a specific object feature by responding only when that feature is located at the cell's preferred depth and being silent at other depths, the preferred depth varying from cell to cell over a small range. In order to assign a depth value to a particular object feature, the two receptive fields of the cell should respond to one and the same feature in the visual field. This can be done only if the organizations of the two receptive fields are identical or nearly so. Out of 31 cells in the simple family in the cat striate cortex, 15 were selected as having a monocular response from each eye sufficient to be able to examine their receptive field organizations in quantitative detail. The two receptive fields of each cell were remarkably similar in respect to the number, spatial sequence and position disparities of the response peaks to moving light and dark bars, as well as in respect to the relative ocular dominances, peak separations and direction selectivities of the response peaks to the two kinds of bar.

Action Potentials↗
PubMed1984

An inherent nonlinearity in near-threshold contrast detection.

We measured an essentially normal pedestal effect using stationary gaussian targets and slowly moving pedestal gratings. Since these conditions greatly reduce the information provided by the pedestal, we question whether uncertainty about the stimulus can be the main cause of the pedestal effect.

Form Perception↗
PubMed1985

Spatial contrast sensitivity: effects of peripheral field stimulation during monocular and dichoptic viewing.

This report examines whether a radial grating with a blank 2 deg central aperture viewed with one eye can affect contrast sensitivity for foveally-viewed, counterphasing or stationary, sine-wave gratings seen with the other eye. We find that the moving radial grating preferentially raises the threshold for the low spatial frequencies of the counterphasing but not the stationary foveal stimulus. These results closely parallel recent primate electrophysiological work which suggests that visual stimulation of the peripheral field with a moving radial grating can activate inhibitory corticofugal influences on lateral geniculate neurons. The current data are evaluated in terms of a model which suggests that the peripheral stimulus activates corticofugal mechanisms.

Form Perception↗
PubMed1986

Why have multiple cortical areas?

Image processing requires free access to information about all parts of an image, but a nerve cell in V1 can only interact directly with a tiny fraction of the other cells in V1. The problem this poses might be alleviated by forming secondary "neural images" in which information is re-arranged, and some possible rules of projection for forming such images are explored. It is also suggested that all parts of the cerebral cortex detect, and subsequently signal, suspicious coincidences in their inputs. Acquiring knowledge of the associative structure of sensory messages, in the form of the unexpected coincidences that occur, may represent the beginning of the formation of a working model, or cognitive map, of the environment.

Brain Mapping↗