Biomedical subjects

C A Burbeck

Publications and source records attributed to C A Burbeck.

11 recordsLinked to original sources

PubMed1992

Separation discrimination with embedded targets.

Previous research has shown that separation discrimination thresholds are independent of the internal spatial scale (local spatial frequency) of the targets whose separation is being judged. The experiments reported here tested the generality of this conclusion for separation discrimination of targets that were embedded in an array of identical objects, where crowding could enhance the importance of the scale at which the individual target locations are encoded. No effect of the local spatial scale of the targets was found under these conditions.

Contrast Sensitivity↗

PubMed1990

Spatial-filter selection in large-scale spatial-interval discrimination.

Spatial-interval discrimination thresholds were measured for a pair of bars in the presence of other parallel bars placed far enough from the targets as to be outside the range of neural and optical blurring. Thresholds were elevated when the targets were embedded in an array of four parallel bars (two between and two flanking the targets), but not when there were only two parallels, whether the parallels were between the target bars or flanking them. The threshold elevation was larger with a 100-msec than with a 500-msec exposure duration. Attenuating the high spatial frequencies magnified the threshold elevation. The data indicate that the process responsible for spatial-interval discrimination automatically selects which spatial filters to use; it does not have to scan through all ranges of spatial filters.

Contrast Sensitivity↗

PubMed1990

Two mechanisms for localization? Evidence for separation-dependent and separation-independent processing of position information.

The Weber function for separation--i.e. delta s as a function of separation s--is typically measured using a pair of targets presented roughly symmetrically relative to the fovea. With this paradigm, as the separation increases, the eccentricity of the individual targets increases also. To disentangle the effects of separation and eccentricity on the Weber function for separation, we systematically examined each of these variables and also examined the effects of target size and exposure duration. Separation discrimination thresholds were measured for average separations from 3 to 6 deg across a wide range of eccentricities, and for eccentricities of 2.5-10 deg for a range of separations. The dependence of threshold on target size was measured by varying the length of the stimuli from 1 to 120 min arc; the dependence on exposure duration was measured using durations of 100 and 500 msec at 10 deg eccentricity for comparison with data collected previously at smaller eccentricities. We found that for separations less than the eccentricity of the targets, thresholds depend primarily on separation; for larger separations, thresholds depend solely on eccentricity. In general, unless the targets are very small or quite brief, the spatial and temporal characteristics of the targets are not major contributors to the slope of the Weber function. Two mechanisms are proposed to account for thresholds in the two regions, one separation-dependent and one separation-independent.

Differential Threshold↗

PubMed1990

Spatiotemporal limitations in bisection and separation discrimination.

Exposure duration was found to have a different effect on bisection thresholds than on separation-discrimination thresholds. Bisection thresholds were higher than separation discrimination thresholds between 33 and 150 msec but equal to or lower than them at longer durations. Experiments in which stimulus contrast was manipulated showed that the effect of exposure duration on separation-discrimination and bisection thresholds could not be attributed primarily to temporal contrast integration. The data could be accounted for by a model in which bisection is done by encoding the two separations in bisection sequentially.

Contrast Sensitivity↗

PubMed1989

Spatial interactions in rapid pattern discrimination.

We measured reaction times (RTs) for identification of a target among distracters under stabilized image conditions in which the positions of the target and the distracters were constant within a single experimental session. Under these conditions, the observer need not search for the target because its position is known. We nevertheless found that the presence of even a single distracter could elevate RTs. The magnitude of this effect depended on the distance of the distracter from the target and, for some observers, the distance of the distracter from the fovea. When we added not one but six background elements in a ring around the target, RT increased even more. If, apart from these neighboring distracters, the target was surrounded by more distracters located beyond the nearest neighbors, RT was, in general, not increased further. These findings suggest that adding background elements in a search task can elevate RTs in ways that are not dependent on the positional uncertainty of the target.

Female↗

PubMed1988

Large-scale relative localization across spatial frequency channels.

Large-scale relative localization accuracy is measured with objects that stimulate different ranges of spatial frequencies. The author has previously made measurements using objects that stimulate only high-spatial-frequency channels or only low-spatial-frequency channels and found no effect of spatial frequency. In the present study, relative localization accuracy, i.e. interval discrimination, is measured with an object pair consisting of a low-spatial-frequency object and a high-spatial-frequency object. Relative localization accuracy for this cross-channel stimulus is as high as for the same-channel stimuli used previously, showing that the relative localization mechanism operates effectively across spatial frequency channels.

Humans↗

PubMed1987

Position and spatial frequency in large-scale localization judgments.

The frequency-channel model and the position, or "local-signs," model that have been proposed to account for hyperacuity (i.e. small-scale relative spatial localization) are examined in the context of large-scale relative spatial localization. As a basis for subsequent experiments, localization accuracy is measured over a large range of object separations, and previous findings that the "Weber fraction for localization" is constant are replicated. The effects on localization accuracy of both high- and low-spatial frequency components in the objects being localized are examined in some detail. Localization accuracy is found not to rely exclusively on either the high- or the low-frequency components. Neither the frequency-channel nor the position hypothesis as defined here is consistent with all of the observed results. However, with a slight modification, the position hypothesis can account qualitatively for all of the observed results, whereas no reasonable modification of the frequency-channel hypothesis appears able to do as well.

Humans↗

PubMed1987

Further evidence for a broadband, isotropic mechanism sensitive to high-velocity stimuli.

Spatial frequency and orientation selectively, the most prominent properties of image-processing in the striate cortex, are not uniform throughout the spatiotemporal frequency domain. Some current models include one "transient" mechanism at very high velocities (i.e. low spatial and high temporal frequencies), and multiple "sustained" mechanisms elsewhere in the spatiotemporal frequency domain, but they do not consider the parameter of orientation. On the basis of earlier, orthogonal masking experiments, we concluded that the high-velocity mechanism is sensitive to a broad band of spatial frequencies, and has little or no orientation selectivity. In the present study we use pattern adaptation to measure the spatiotemporal properties of this mechanism. In other experiments, we attempt to relate it to the direction-selective motion detectors that also respond at high velocities. Finally we compare the pattern-adaptation results to the results of orthogonal subthreshold summation experiments in the same region of high temporal and low spatial frequencies.

Adaptation, Ocular↗

PubMed1987

Locus of spatial-frequency discrimination.

In standard frequency-discrimination experiments either the retinal spatial frequencies (cycles per degree) or the object spatial frequencies (real world) could be compared, because the retinal and object frequency differences are the same. Current models of spatial-frequency discrimination assume that observers compare the retinal frequencies. I test this assumption by presenting gratings at different viewing distances (with strong depth cues). The object frequencies of the gratings bear the same relationship that they do in a standard frequency-discrimination experiment, but the retinal frequency of the more distant grating is always markedly higher than that of the near grating. The observer's task is to compare the object spatial frequencies. This change from one depth to two (with no change in the stimulus object) has a negligible effect on the observer's performance, suggesting that observers compare object frequencies even in standard spatial-frequency-discrimination experiments. This conclusion is supported by the findings that (1) observers appear unable to learn to compare retinal frequencies and (2) the interstimulus interval has no effect (over the range 0-1020 msec), implying long-term storage of the visual information. Suggests are made about why these results are consistent with good system design.

Discrimination, Psychological↗

PubMed1986

Negative afterimages and photopic luminance adaptation in human vision.

Previous studies of the negative afterimage have reported that the process responsible for these aftereffects has a bandpass spatial characteristic. If this finding is correct, then negative afterimages cannot arise from a simple, local, adaptive process. I remeasure the spatial-frequency characteristic of the negative-afterimage process by using an afterimage contrast-matching procedure with retinally stabilized stimuli and find the spatial characteristic to be constant in the low-spatial-frequency region. This finding is consistent with the theory that the negative afterimage results from local luminance adaptation. As a test of the local adaptation explanation of the negative afterimage, the effect of the negative afterimage on the temporal contrast-sensitivity function (CSF) (measured down to 0.062 Hz) is determined. The apparent contrasts of the negative afterimages associated with very slowly (less than 0.5 Hz) flickering, threshold-contrast stimuli are calculated from power-function descriptions of the temporal development of the negative afterimage, and these afterimage contrasts are then subtracted from the temporal CSF's. The resulting curves are constant for temporal frequencies below 1 Hz, indicating that the decline in sensitivity at lower temporal frequencies is due entirely to the negative-afterimage process. Both the spatial and the temporal characteristics of the negative-afterimage process are consistent with its being a component of local luminance adaptation.

Acclimatization↗

PubMed1986

Exposure-duration effects in localization judgments.

The effects on localization accuracy of increasing exposure duration beyond 100 msec are explored for a wide range of object separations. Previous reports that localization accuracy for objects separated by a few minutes of arc increases for exposures up to at least 400 msec are confirmed. I report here that localization of larger objects at larger separations does not improve when the exposure duration is increased beyond 100 msec. This difference between the small- and large-scale results can be explained by the difference in the spatial-frequency content of the objects being localized: When high-frequency objects are substituted for spectrally broadband objects in the large-scale case, the exposure-duration effects for widely separated objects become similar to those obtained in the small-scale case. These results suggest that the exposure-duration effect previously reported in hyperacuity studies is not specific to the localization task per se but rather is a suprathreshold version of the familiar form of spatiotemporal interaction seen in contrast-threshold results. They also suggest that a single type of mechanism underlies small- and large-scale localization.

Differential Threshold↗