Hierarchical detection structures in vision.

A simple example shows that a hierarchy of visual cells can display different behavior for different stimuli. Stimuli with spatial characteristics similar to the receptive fields of a given level in the hierarchy can display summation behavior revealing the organization of that level; but other stimuli may show the organization of cells at other levels. On the other hand, such behavior may not actually occur in vision because the required nonlinearity between levels is very specific and may not be physiologically realistic.

Orientation selectivity in detection of chromatic gratings.

Vertical and horizontal equiluminance gratings (red-green and blue-yellow at 1 or 3 cycles per degree) act independently in contrast-threshold measurements. This suggests that detection of such patterns takes place in color-sensitive neural structures with orientational specificity.

Random pattern generation using coherent optical spatial filtering.

Coherent optical spatial filtering is used to generate 2-D random patterns for use in studies of image noise perception. Some of the problems encountered are: small filter dimensions due to low spatial frequencies required; obtaining a noise source with a suitable Wiener spectrum; and the nonlinearity of the recording process. A procedure that overcomes these difficulties is discussed, and an example using a circularly symmetric bandpass filter is presented.

Spatial summation effects on two-component grating thresholds.

Some experimental studies of subthreshold summation between sinusoidal grating components have been interpreted as showing very narrow channel bandwidths in human visions. This paper discusses an alternative interpretation of these experiments based on consideration of probability-summation effects among spatially distributed detectors. We conclude that frequency-selective channels must still be hypothesized in order to fit the data, but the channel bandwidth may be much wider than earlier interpretations suggest.