Joshua A. Solomon

Facilitation from collinear flanks is cancelled by non-collinear flanks

Detection of a central Gabor pattern is facilitated by the presence of collinear flanking patterns. We find that this facilitation is greatly reduced when the collinear flanks are combined with non-collinear flanks to form a coherent surround. These results are unlikely to be explained by mechanisms that merely transduce local contrast in a nonlinear fashion. A model wherein the outputs of such mechanisms are combined anisotropically provides a better account for these results.

The lateral inhibition of perceived contrast is indifferent to on-center/off-center segregation, but specific to orientation

UNLABELLED When a central test patch C, composed of an isotropic spatial texture, is surrounded by a texture field S, the perceived contrast of C depends substantially on the contrast of the surround S. When C is surrounded by a high contrast texture with a similar spatial frequency content, it appears to have less contrast than when it is surrounded by a uniform field. Here, we employ two novel textures: T+ which is designed to selectively stimulate only the on-center system, and T-, the off-center system. When C and S are of type T+ and T-, the reduction of Cs apparent contrast does not vary with the combination of T+, T-. This demonstrates that the reduction of Cs apparent contrast is mediated by a mechanism whose neural locus is central to the interaction between on-center and off-center visual systems. We further demonstrate orientation specificity: the reduction of grating Cs apparent contrast by a surround grating S, of the same spatial frequency is greatest when C and S have equal orientation. Using dynamically phase-shifting sinusoidal gratings of 3.3, 10 and 20 c/deg, we measured reduction of apparent contrast using different contrast-combinations of C and S. RESULTS (1) S gratings, both parallel and perpendicular to C, cause a reduction in Cs apparent contrast relative to a uniform surround. (2) In all of the viewing conditions, the reduction of apparent contrast induced by the parallel surrounds was at least as great as that induced by the perpendicular surrounds. Often it was much greater. (3) Orientation specificity increases with increasing spatial frequency and with decreasing stimulus contrast.

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 targets 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.

Transducer model produces facilitation from opposite-sign flanks

Small spots, lines and Gabor patterns can be easier to detect when they are superimposed upon similar spots, lines and Gabor patterns. Traditionally, such facilitation has been understood to be a consequence of nonlinear contrast transduction. Facilitation has also been reported to arise from non-overlapping patterns with opposite sign. We point out that this result does not preclude the traditional explanation for superimposed targets. Moreover, we find that facilitation from opposite-sign flanks is weaker than facilitation from same-sign flanks. Simulations with a transducer model produce opposite-sign facilitation.

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°) 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.

Contrast discrimination function: spatial cuing effects

The effects of spatial cuing were measured for discrimination between an increment and a decrement on a targets pedestal contrast. Discrimination thresholds measured in the absence of a spatial cue were always higher than corresponding thresholds measured in the presence of a spatial cue, except when pedestal contrast was near zero. Uncued discrimination thresholds rose monotonically with pedestal contrast; cued discrimination thresholds formed a dipper function of pedestal contrast. A spatial-uncertainty model incorporating a nonlinear transducer produced similar results.

Visibility of DCT basis functions: effects of contrast masking

The discrete cosine transform (DCT) is a standard method by which images may be compressed. Initially, the image is divided into 8/spl times/8-pixel blocks, each of which is then transformed into its DCT, which is an 8/spl times/8 matrix of coefficients. One image-dependent factor influencing the detectability of DCT basis functions is contrast masking. The authors present data which indicate that sensitivity to a particular coefficients quantization error is affected by the magnitudes of other coefficients. They propose a revision of Model O to account for between-coefficient contrast masking.<<ETX>>

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.

The history of dipper functions

Dipper-shaped curves often accurately depict the relationship between a baseline, or “pedestal,” magnitude and a just noticeable difference in it. This tutorial traces the 45-year history of the dipper function in auditory and visual psychophysics, focusing on when they happen and why. Popular theories of both positive and negative masking (i.e., the “handle” and “dip,” respectively) are described. Sometimes, but not always, negative masking disappears with an appropriate redescription of stimulus magnitude.

1st- and 2nd-order motion and texture resolution in central and peripheral vision

STIMULI. The 1st-order stimuli are moving sine gratings. The 2nd-order stimuli are fields of static visual texture, whose contrasts are modulated by moving sine gratings. Neither the spatial slant (orientation) nor the direction of motion of these 2nd-order (microbalanced) stimuli can be detected by a Fourier analysis; they are invisible to Reichardt and motion-energy detectors. METHOD. For these dynamic stimuli, when presented both centrally and in an annular window extending from 8 to 10 deg in eccentricity, we measured the highest spatial frequency for which discrimination between +/- 45 deg texture slants and discrimination between opposite directions of motion were each possible. RESULTS. For sufficiently low spatial frequencies, slant and direction can be discriminated in both central and peripheral vision, for both 1st- and for 2nd-order stimuli. For both 1st- and 2nd-order stimuli, at both retinal locations, slant discrimination is possible at higher spatial frequencies than direction discrimination. For both 1st- and 2nd-order stimuli, motion resolution decreases 2-3 times more rapidly with eccentricity than does texture resolution. CONCLUSIONS. (1) 1st- and 2nd-order motion scale similarly with eccentricity. (2) 1st- and 2nd-order texture scale similarly with eccentricity. (3) The central/peripheral resolution fall-off is 2-3 times greater for motion than for texture.