Christina A. Burbeck

Independence of orientation and size in spatial discriminations

This study of form vision explores the relationships between orientation and spatial frequency in suprathreshold discrimination tasks. Orientation discrimination thresholds for sine-wave gratings were 0.3-0.5 deg, much less than the roughly 10-24-deg orientational bandwidth of channels; spatial-frequency discrimination thresholds were 3-7%, much less than the roughly 1.2-octave spatial-frequency bandwidth of channels. We find that spatial-frequency discrimination between two gratings was as acute when the two gratings were orthogonal as when they were parallel. Orientation discrimination between two gratings was as acute when the two gratings had the same spatial frequencies as when they had different spatial frequencies. Thus orientation and spatial frequency are independent dimensions at the discrimination stage of spatial information processing.

Spatiotemporal characteristics of visual mechanisms: excitatory-inhibitory model

The stabilized spatiotemporal threshold response surface can be modeled as the linear difference between the threshold response surfaces of two mechanisms, each of which is simply the product of a spatial and temporal frequency response curve. With no free parameters, the resulting model is shown to be a good fit to available data.

Contrast gain measurements and the transient/sustained dichotomy

We measure threshold for a vertical test grating superimposed on a fixed-contrast horizontal background grating of the same spatial and temporal frequency. The rate of change of this threshold with increasing contrast of the background grating is a measure of the contrast gain of the responding mechanism. Large slopes (high contrast gains) occur when spatial frequency is low and temporal frequency is high; small slopes (low contrast gains) occur when both spatial and temporal frequencies are low and when spatial frequency is high. This division of the spatiotemporal frequency domain into low- and high-gain regions is consistent with the transient/sustained dichotomy found in previous psychophysical studies. Furthermore, our results suggest that the mechanism responsible for detecting low spatial frequencies has a gain characteristic similar to that of cat retina Y cells and that the mechanism responsible for detecting high spatial frequencies has a gain characteristic similar to that of cat retina X cells, as found by Shapley and Victor [J. Physiol. (London) 285, 275-298 (1978)].

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.

Criterion-free pattern and flicker thresholds

Previous measurements of pattern and flicker thresholds in man have used the psychophysical method of adjustment and have required that the subject change criteria. Our new approach measures pattern and flicker thresholds by a criterion-free procedure that requires that the subject distinguish a test stimulus from control stimuli that differ from the test in only one dimension, the spatial dimension in the pattern experiments and the temporal dimension in the flicker experiments. By using this technique, we find that the subjects pattern sensitivity is equal to or greater than his flicker sensitivity for all flickering gratings except those that have extremely low spatial and moderate or high temporal frequencies (depending on the subjects). Furthermore, spatiotemporal interaction is evident in both the pattern and the flicker data. These results differ substantially from those obtained by the method of adjustment.

Role of local adaptation in the fading of stabilized images

We provide evidence that the fading of stabilized images and the formation of negative afterimages result from the same local adaptive process. We measure thresholds for stabilized, static, sine-wave gratings and for stabilized flickering sine-wave gratings. We then measure the contrasts of the negative afterimages formed by the threshold-contrast stabilized, static stimuli. (The threshold-contrast flickering gratings produce no visible afterimages.) We find that the difference between the thresholds for stabilized, static gratings and the thresholds for slowly flickering gratings is equal to the contrasts of the afterimages produced by the stabilized, static gratings. We conclude that the fading of these stabilized gratings can be accounted for completely by local adaptation (the process underlying the formation of negative afterimages.

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 observers 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 observers 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.

Motion and vision. III. Stabilized pattern adaptation.

It has been suggested that local variations of retinal sensitivity may be responsible for elevating the threshold in pattern-adaptation experiments of the Blakemore-Campbell type. Subjects are unable to scan high-contrast gratings uniformly enough to eliminate this possibility. To control this effect, we performed grating-adaptation experiments under stabilized-image conditions, while both adapting and test targets were moved at retinal velocities determined by the experimenter. By means of an afterimage technique, we also measured the strength of the retinal sensitivity mask that forms under these conditions. Varying the spatial frequency and velocity of the adapting stimulus, we inferred the spatial and temporal properties of the principal mechanism that contributes to the afterimage. We found that the Blakemore-Campbell effect persists at adapting velocities that are fast enough to rule out local variations of retinal sensitivity. More surprisingly, even the clearly visible afterimages that occur at a retinal velocity of 0.1 deg/s seem to have no effect on pattern adaptation. (Sensitivity masking can raise the adapted threshold, but only at adapting velocities slower than normal eye movements). By manipulating the image velocity, we were able to shift the spatial frequencies of some threshold-elevation curves, but these shifts were not great enough to suggest that velocity tuning plays important role in pattern adaptation.

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.

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.