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J L Zacks

Publications and source records attributed to J L Zacks.

8 recordsLinked to original sources
PubMed1993

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.

Adult↗
PubMed1990

Visual memory and the perception of a stable visual environment.

The visual world appears stable and continuous despite eye movements. One hypothesis about how this perception is achieved is that the contents of successive fixations are fused in memory according to environmental coordinates. Two experiments failed to support this hypothesis; they showed that one's ability to detect a 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 fixations were identical or different, even when the 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.

Attention↗
PubMed1976

Postnatal changes in corneal curvature.

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 animal's age and weight. To facilitate this prediciton 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.

Age Factors↗
PubMed1970

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.

Dark Adaptation↗
PubMed1970

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.2. There is a minimum response duration of 50-70 msec no matter how brief the flash is made. This duration is reached with stimuli lasting 32 msec or shorter.3. Reducing the background illumination obviously increases the latency of responses to stimuli at 4 times threshold intensity (about 10 msec increment per log. unit decrement) but has no obvious effect on the minimum response duration.4. The relation between intensity and duration of a flash for threshold responses closely resembles that in human psychophysical experiments. The Bunsen-Roscoe law is applicable for flash durations up to 64 msec.5. If equal amounts of energy are delivered in the form of a pair of flashes of varying separation rather than by rectangular pulses, the shape of the response changes more abruptly with the temporal factor.6. Non-linear performance is apparent for stimuli as weak as 4 times threshold.7. A method is developed for quantitative analysis of individual responses. It is based upon cross-correlation of the train of impulses with a Gaussian smoothing function and represents local impulse frequency as a smooth function of time. The method also improves the signal-to-noise ratio of post-stimulus time-histograms of the sum of many responses.8. The measurement of variability of individual responses is the main result of the method; its magnitude indicates that it is a significant new factor limiting temporal resolution with suprathreshold stimuli.

Animals↗