James L. Zacks

Responses of cat retinal ganglion cells to brief flashes of light

1. The responses of cat retinal ganglion cells to brief flashes of light have been illustrated and described with a view to providing material for comparison with psychophysical experiments in the scotopic (rod‐dominated) range of performance.

Visual memory and the perception of a stable visual environment

The visual world appears stable and continuous despite eye movementa. One hypothesisabout how this perception is achieved is that the contents of succesaivatienn~arefusedinmemory according to environmental coordinates. Two experiments failed to support this hypothesis; they showed that one’s ability to detecta grating presented after a saccade is unaffected by the presentation of a grating with the same spatial frequency in the same spatial location before the saccade. A third experiment tested an alternative explanation of perceptual stability that claims that the contents of successive fixations are compared, rather than fused, across-saccades, allowing one to determine whether the world has remained stable. This hypothesis was supported: Experienced subjects could accurately determine whether two patterns viewed in successive fixationswere identical or different, evenwhenthe two patterns appeared in different spatial positions across the saccade. Taken together, these results suggest that perceptual stability and information integration across saccades rely on memory for the relative positions of objects in the environment, rather than on the spatiotopic fusion of visual information from successive fixations.

Temporal Summation Phenomena at Threshold: Their Relation to Visual Mechanisms

Threshold energies were determined for brief flashes as a function of their duration in order to determine the maximum duration for which the flash intensity and duration could be varied reciprocally without affecting detectability (the Bunsen-Roscoe effect). A pair of threshold-level flashes for which reciprocity obtained in the determination of threshold were shown to be discriminable from each other at several imperfectly detectable energy levels. Thus equal detectability of flashes of equal energy does not imply identical neural responses to such stimuli. It is suggested that the summation reflects primarily the operation of the detection mechanism rather than of the peripheral visual mechanism. Some general implications for the interpretation of threshold measures are also discussed.

Visual search times assessed without reaction times: a new method and an application to aging

We consider several ways in which the interpretation of reaction time (RT) data might confound differences in visual search rates with non-search-related factors. To determine whether estimates of search rates for groups differing in age suffered this problem, we compared estimates provided by the RT method with those obtained using a forced-choice method with limited-duration stimuli. The forced-choice method provided faster estimates of search rates. The effects of age, the variable in which we were interested, were comparable, but the difference between results obtained using the two methods suggests the need for caution in using the RT method. We discuss how the forced-choice method can be used, under appropriate circumstances, to provide an independent test of whether subjects are carrying out serial searches and, if they are, to provide search rate estimates even from data obtained using only a single array size.

The effects of psychophysical procedure and stimulus duration in the measurement of Bezold-Brücke hue shifts.

Abstract Recent studies of Bezold-Brucke hue shifts have yielded conflicting data on the existence of invariant hues and on the nature of the hue shifts which are obtained. Two apparent differences in these experiments have been the duration of the stimulus and the psychophysical procedure used to assess the hue shifts. These differences in experiments were examined systematically with the conclusion that the stimulus duration was the more important variable in accounting for the differences between various experiments.

Estimation of the variability of the latency of responses to brief flashes

Abstract Human observers discriminated which of two spatially separated 1 msec flashes came first. Performance was measured as a function of the difference between the onsets of the flashes. The data are interpreted within the framework of a model which makes it possible to estimate the variability of the physiological responses evoked by these stimuli.

Effects of Active and Passive Movement upon Auditory Function during Prolonged Atypical Stimulation

The ability to discriminate minute time differences between paired clicks presented to the two ears is significantly impaired if observers walk about during 1 hr. of exposure to dichotic white noise. Performance does not deteriorate if bodily movements are restricted. Comparison of active and passive movement conditions for both translation and rotation demonstrated that self-induced movement of the observer is necessary to produce this effect. These findings support the notion that active movement of the observer with its concurrent sensory feedback is essential for the maintenance of stable and effective perceptual-motor behavior or its alteration.

The Stiles-Crawford effect: two models evaluated.

Abstract Models explaining the Stiles-Crawford Effect typically characterize individual receptors as either narrowly or broadly “tuned” (i.e. reactive to light incident through a narrow or broad angle, respectively). Makous (1968) has shown that he retina contains narrowly-tuned channels, but his data do not require channels that are tuned differently to reside in different cones. In the present study we (1) bolster the evidence for narrowly-tuned channels by performing a replication of the Makous (1968) experiment using narrow-band stimuli. (2) show that increment threshold limiting factors operate only after the separate channel outputs have been combined and (3) on the basis of data from a brightness-matching experiment argue that channel input-output functions and channel output combinations are linear. If cone input-output functions are sufficiently non-linear, the demonstrated linear combination of channel outputs suggests that narrowly-tuned channels co-exist within broadly-tuned cones.

Postnatal changes in corneal curvature.

&NA; Corneal curvature was measured in 10 kittens from three litters and found to change significantly during postnatal development. From our data analysis, corneal curvature can be predicted from an animals age and weight. To facilitate this prediction for use in fitting contact lenses as occluders or diffusers in developmental studies, prediction equations are given as are expected weights of kittens as a function of age.

Another new type of cat retinal ganglion cell.

Abstract A group of cat retinal ganglion cells, termed “triggered response cells”, responded to normal visual stimuli with elevations of their firing rates lasting tens or hundreds of seconds. These cells seem different from other cells whose maintained discharges undergo long-lasting changes. In many other respects “triggered response cells” resemble Y-type cells.