Biomedical subjects

Joshua A Solomon

Publications and source records attributed to Joshua A Solomon.

7 recordsLinked to original sources

PubMed2006

Stochastic re-calibration: contextual effects on perceived tilt.

The human visual system exaggerates the difference between the tilts of adjacent lines or grating patches. In addition to this tilt illusion, we found that oblique flanks reduced acuity for small changes of tilt in the centre of the visual field. However, no flanks--regardless of their tilts--decreased sensitivity to contrast. Thus, the foveal tilt illusion should not be attributed to orientation-selective lateral inhibition. Nor is it similar to conventional crowding, which typically does not impair letter recognition in the fovea. Our observers behaved as though the reference orientation (horizontal) had a small tilt in the direction of the flanks. We suggest that the extent of this re-calibration varies randomly over trials, and we demonstrate that this stochastic re-calibration can explain flank-induced acuity loss in the fovea.

Contrast Sensitivity↗

PubMed2005

Monocular texture segmentation and proto-rivalry.

When the right eye's target is the left eye's distracter and vice versa, orientation-defined search is impossible unless, as we show here, the elements are close together. More than 1s was required to find inverse-cyclopean texture boundaries when elements were arranged on a 16 x 16 grid. Less than 250 ms was required for a 24 x 24 grid covering the same area. The conventional view is that binocular rivalry requires at least 200 ms to develop, but our results suggest a more rapid access to monocular signals. We call this rapid form of access "proto-rivalry."

Eye Movements↗

PubMed2005

Stimulus contrast and the Reichardt detector.

The direction of a drifting grating can become more easily identified when a stationary, flickering grating, with the same spatial and temporal frequencies, is added to it. This amplification has been accepted as evidence that motion perception depends on the product of visual signals elicited before and after a target changes position, as computed by a Reichardt detector. However, amplification is also consistent with a model in which direction identification depends on the product of detection probabilities before and after the position shift. In this paper, we compare the Reichardt detector with a model of Probability Multiplication. For 2-frame sequences, similar results are predicted by Probability Multiplication and a Reichardt model, in which the performance-limiting noise is early (i.e. it is added prior to signal multiplication). Many new and previously published results are consistent with these predictions. Other results are documented in which the amplification is too large to be consistent with Probability Multiplication. To explain these latter results, Reichardt detectors must have both early and late noises.

Discrimination, Psychological↗

PubMed2005

The role of target salience in crowding.

We studied 'crowding' in the parafovea using orientation identification of a Gabor target as the task, and flanking Gabors on an isoeccentric circle as distractors. Orientation-discrimination thresholds were raised by nearby flanking distractors. This crowding effect was increased by the number of distractors and decreased by the spatial separation between target and distractors. Crowding was greatest when the target was in the centre of the distractor array and smallest when the target was on the edge of the array. A cue indicating the position of the target improved performance when the position was otherwise unknown and the spatial separation between target and distractors was large, but the cue had no significant effect when separation was small. Increasing the contrast of the target relative to the distractors reduced crowding, but targets of smaller contrast than that of the distractors are even harder to identify than those of the same contrast. Putting the target and distractors in different depth planes decreased crowding for some observers, but there were qualitative individual differences. A large (say, 45 degrees) difference in orientation between target and distractors caused the target to 'pop out' in a presence/absence task, despite the evidence from other studies that crowding is still found in these conditions. We conclude that salience has, at best, modest effects on crowding.

Cues↗

PubMed2004

Crowding and the tilt illusion: toward a unified account.

Crowding, the difficult identification of peripherally viewed targets amidst similar distractors, has been explained as a compulsory pooling of target and distractor features. The tilt illusion, in which the difference between two adjacent gratings' orientations is exaggerated, has also been explained by pooling (of Mexican-hat-shaped population responses). In an attempt to establish both phenomena with the same stimuli--and account for them with the same model--we asked observers to identify (as clockwise or anticlockwise of vertical) slightly tilted targets surrounded by tilted distractors. Our results are inconsistent with the feature-pooling model: the ratio of assimilation (the tendency to perceive vertical targets as tilted in the same direction as slightly tilted distractors) to repulsion (the tendency to perceive vertical targets as tilted away from more oblique distractors) was too small. Instead, a general model of modulatory lateral interaction can better fit our results.

Female↗

PubMed2004

The effect of spatial cues on visual sensitivity.

Although once doubted, a consensus has emerged from the literature that visual sensitivity can be heightened locally with an appropriate precue. Experiments with partially and totally valid precues suggest an increase in sensitivity of less than one-half log unit at the precued position, as compared with other positions. New experiments with non-informative precues demonstrate that most of this small enhancement is not due to focal attention. Sensitivity can be heightened at eight positions simultaneously, just as much as when a single position is precued. Sensitivities produced by single, totally valid precues and single, non-informative precues were similar. Thus there seems to be no capacity limit for the effect of precues on visual sensitivity.

Attention↗

PubMed2002

Noise reveals visual mechanisms of detection and discrimination.

When performance is limited by stochastically defined masks, (psychophysical) reverse correlation has proven to be an especially efficient tool for estimating the templates used by detection and discrimination mechanisms. Here I describe a maximum-likelihood approach to quantifying the significance of differences between estimates of template. Four methodologically related experiments illustrate the versatility of reverse correlation. Experiment 1 shows significant differences between the templates used by different observers when detecting a bright Gaussian blob. The results of Experiment 2 are consistent with observers not using information about the phase of a parafoveal wavelet when detecting it. Experiments 3 and 4 reveal not only the templates used by detection mechanisms but also aspects of their response functions. Both results are consistent with a sensory threshold. Experiment 3 shows that 2-alternative forced-choice detection errors are caused when the target's effective contrast is reduced, not when the mask looks more like the expected target+mask than the actual target+mask. Experiment 4 suggests that observers use optimally tuned detection templates for orientation discrimination.

Choice Behavior↗