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Biomedical subjects

S B Stevenson

Publications and source records attributed to S B Stevenson.

8 recordsLinked to original sources
PubMed1992

Disparity tuning in mechanisms of human stereopsis.

The change in sensitivity across some stimulus dimension which follows adaptation to a particular stimulus can reveal a great deal about the tuning characteristics of underlying sensory/perceptual mechanisms. In this study, a psychophysical adaptation paradigm was employed to characterize the disparity tuning of perceptual mechanisms involved in stereopsis. The stimulus was a dynamic random-dot stereogram (DRDS) portraying a surface which varied in interocular correlation (IOC) and retinal disparity. Adaptation to a fully correlated DRDS surface produced an elevation in IOC threshold over a relatively narrow range of disparities, with maximum effect at the disparity of the adapting stimulus. The width of these disparity tuning functions varied from 5 arc min for adaptation at the horopter to 20 arc min for adaptation at 20 arc min disparity. Frequently, IOC sensitivity was enhanced for disparities on either side of the adapted disparity, suggesting that an opponent center-surround organization operates at an early level of disparity processing. A model of underlying channel structure consistent with these data is presented.

Adaptation, Ocular↗
PubMed1991

Depth attraction and repulsion in random dot stereograms.

Previous studies of perceived attraction or repulsion of adjacent visual targets have used local targets whose positions were varied in both depth and direction. We have measured these effects in three subjects using dynamic random-dot stereograms to isolate depth-axis effects. Results show that both attraction and repulsion effects can occur for overlapping, positively correlated, random-dot surfaces. The results were quantitatively similar to those reported previously for local targets. Manipulation of interocular correlation confirmed that the effects are produced by binocular interactions. Results are explained as accurate judgments based on the stimulus at the cyclopean level.

Depth Perception↗
PubMed1991

Interocular correlation, luminance contrast and cyclopean processing.

We have investigated the nature and viability of interocular correlation as a measure of signal strength in the cyclopean domain. Thresholds for the detection of interocular correlation in dynamic random element stereograms were measured as a function of luminance contrast, a more traditional measure of stimulus strength. At high contrasts, correlation thresholds were independent of contrast. At low contrasts, correlation thresholds were inversely proportional to the square of contrast. Stereothresholds were also measured as a function of both contrast and interocular correlation. At low contrasts, stereoacuity was inversely proportional to both interocular correlation and the square of contrast. These results are consistent with an inherently multiplicative mechanism of binocular combination, such as a cross-correlation of the two eye's inputs.

Contrast Sensitivity↗
PubMed1990

Discrimination of jittered sonar echoes by the echolocating bat, Eptesicus fuscus: the shape of target images in echolocation.

1. Behavioral experiments with jittering echoes examined acoustic images of sonar targets in the echolocating bat, Eptesicus fuscus, along the echo delay or target range axis. Echo phase, amplitude, bandwidth, and signal-to-noise ratio were manipulated to assess the underlying auditory processes for image formation. 2. Fine delay acuity is about 10 ns. Calibration and control procedures indicate that this represents temporal acuity rather than spectral discrimination. Jitter discrimination curves change in phase when the phase of one jittering echo is shifted by 180 degrees relative to the other, showing that echo phase is involved in delay estimation. At an echo detectability index of about 36 dB, fine acuity is 40 ns, which is approximately as predicted for the delay accuracy of an ideal receiver. 3. Compound performance curves for 0 degrees and 180 degrees phase conditions match the crosscorrelation function of the echoes. The locations of both 0 degrees and 180 degrees phase peaks in the performance curves shift along the time axis by an amount that matches neural amplitude-latency trading in Eptesicus, confirming a temporal basis for jitter discrimination.

Acoustic Stimulation↗
PubMed1989

Hyperacuity, superresolution and gap resolution in human stereopsis.

Different types of stereoscopic acuity were studied with tasks adapted from studies of visual direction acuity. Dynamic, random-element stereograms portraying multiple surfaces in depth and a temporal 2AFC procedure were used for all measurements. The three tasks required detection of a depth offset (Hyperacuity task), a depth-axis thickening (Superresolution task), and a depth-axis gap between surfaces (Gap Resolution task). Thresholds for the three tasks were on the order of 3 sec arc, 30 sec arc and 200 sec arc of retinal disparity, respectively. These results are comparable to those for the analogous visual direction tasks on which they were patterned, suggesting that the underlying judgments involved are similar. Results are used to estimate the intrinsic noise of horizontal disparity processing.

Depth Perception↗
PubMed1989

Clutter interference and the integration time of echoes in the echolocating bat, Eptesicus fuscus.

The ability of the echolocating bat, Eptesicus fuscus, to detect a sonar target is affected by the presence of other targets along the same axis at slightly different ranges. If echoes from one target arrive at about the same delay as echoes from another target, clutter interference occurs and one set of echoes masks the other. Although the bat's sonar emissions and the echoes themselves are 2 to 5 ms long, echoes (of approximately equal sensation levels--around 15 dB SL) only interfere with each other if they arrive within 200 to 400 microseconds of the same arrival time. This figure is an estimate of the integration time of the bat's sonar receiver for echoes. The fine structure of the clutter-interference data reflects the reinforcement and cancellation of echoes according to their time separation. When clutter interference first occurs, the waveforms of test and cluttering echoes already overlap for much of their duration. The masking effect underlying clutter interference appears specifically due to overlap, not between raw echo waveforms, but between the patterns of mechanical excitation created when echoes pass through bandpass filters equivalent to auditory-nerve tuning curves. While the time scale of clutter interference is substantially shorter than the duration of echo waveforms, it still is much longer than the eventual width of a target's range-axis image expressed in terms of echo delay.

Animals↗
PubMed1986

Dependence of visual suppression on the amplitudes of saccades and blinks.

Visual suppression accompanying voluntary saccades and eyeblinks was measured for a range of amplitudes of both. Saccade amplitudes varied from 2 to 32 degrees and blink amplitudes varied from a slight movement to a full closure of the eye. In every case, thresholds for detecting full-field luminance decrements were determined with the method of constant stimuli and a two alternative forced choice procedure. Results from three subjects show a monotonic increase in the amount of suppression produced by saccades and blinks of increasing amplitude. Data are discussed with respect to theories about the origin of visual suppression.

Adult↗