Suzanne P. McKee

The pattern of visual deficits in amblyopia

Amblyopia is usually defined as a deficit in optotype (Snellen) acuity with no detectable organic cause. We asked whether this visual abnormality is completely characterized by the deficit in optotype acuity, or whether it has distinct forms that are determined by the conditions associated with the acuity loss, such as strabismus or anisometropia. To decide this issue, we measured optotype acuity, Vernier acuity, grating acuity, contrast sensitivity, and binocular function in 427 adults with amblyopia or with risk factors for amblyopia and in a comparison group of 68 normal observers. Optotype acuity accounts for much of the variance in Vernier and grating acuity, and somewhat less of the variance in contrast sensitivity. Nevertheless, there are differences in the patterns of visual loss among the clinically defined categories, particularly between strabismic and anisometropic categories. We used factor analysis to create a succinct representation of our measurement space. This analysis revealed two main dimensions of variation in the visual performance of our abnormal sample, one related to the visual acuity measures (optotype, Vernier, and grating acuity) and the other related to the contrast sensitivity measures (Pelli-Robson and edge contrast sensitivity). Representing our data in this space reveals distinctive distributions of visual loss for different patient categories, and suggests that two consequences of the associated conditions--reduced resolution and loss of binocularity--determine the pattern of visual deficit. Non-binocular observers with mild-to-moderate acuity deficits have, on average, better monocular contrast sensitivity than do binocular observers with the same acuity loss. Despite their superior contrast sensitivity, non-binocular observers typically have poorer optotype acuity and Vernier acuity, at a given level of grating acuity, than those with residual binocular function.

The detection of motion in the peripheral visual field

To assess the sensitivity of the periphery to motion, we measured differential motion detection and velocity discrimination as a function of eccentricity in the lower visual field. The differential motion threshold, a measure of the ability to detect relative motion (shear) between adjacent visual stimuli, is smaller than the minimum angle of resolution at all retinal loci tested. The target size required to produce the lowest differential motion threshold is surprisingly large, ranging from 1 deg in the fovea to about 20 deg at 40 degrees eccentricity. When the peripheral thresholds for differential motion and for resolution are normalized against the fovea and plotted on linear axes, the eccentricity functions are linear. Velocity discrimination (delta V/V) is as precise in the periphery as it is in the fovea, amounting to about 6% for the optimum velocity range. In the fovea, the minimum Weber fraction is reached at velocities of 5 deg/sec or faster. In the periphery this minimum is found for a faster range of velocities (greater than 30 deg/sec at 40 degrees eccentricity). If target velocity is expressed in the resolution units/second appropriate to each tested eccentricity, the velocity discrimination functions coincide. Thus, while the spatial determinants of velocity discrimination follow the change in resolution found with eccentricity, peripheral temporal sensitivity must be nearly equal to foveal temporal sensitivity.

Precise velocity discrimination despite random variations in temporal frequency and contrast.

Velocity discrimination is not affected by random changes in contrast or temporal frequency. Observers judged the relative velocity of a moving sinusoidal grating when target contrast was varied randomly from trial-to-trial over the range from 5 to 82%. The Weber fraction for the random mixture of interspersed contrast levels was about 0.06, comparable to velocity discrimination for targets presented at a fixed contrast. In a parallel experiment, the spatial frequency of the target was changed randomly from trial-to-trial, a procedure which produced concomitant random changes in the nominal temporal frequency. These variations had little effect on the velocity increment threshold; random changes in temporal frequency ranging from 2.25 to 8.25 Hz increased the Weber fraction from 0.05 to 0.07. Under identical experimental conditions, velocity discrimination was generally more precise than the discrimination of differences in temporal frequency, particularly when temporal frequency thresholds were measured with counterphase gratings. Our results indicate that velocity discrimination depends on velocity.

Statistical properties of forced-choice psychometric functions: implications of probit analysis.

Probit analysis was applied to the problem of threshold estimation from psychometric functions derived from the two-alternative forced-choice (2AFC) method of constant stimuli. Threshold estimates from 2AFC experiments are surprisingly poor: They are about twice as variable as corresponding estimates based on the traditional yes-no method of constant stimuli, and their asymmetrical confidence limits are not readily predicted from conventional standard error formulas. All of these faults are exacerbated in small samples. Computer simulations demonstrated that, for small samples, the probit analysis equations do not give a valid estimate of threshold variability. The variability of staircase estimates of threshold cannot be less than the variability of threshold estimates derived from the method of constant stimuli given an optimum placement of trials. Hence our findings also define the minimum variability of all staircase estimators under the assumptions of probit analysis.

The imprecision of stereopsis

In comparison to lateral judgments of distance, stereoscopic judgments are not precise. Although stereoacuity thresholds for targets presented in the fixation plane can equal the best thresholds for the monocular hyperacuities, i.e. a few sec arc, the increment thresholds for disparity are substantially larger than the increment thresholds for lateral separation (width). We measured the minimum detectable change in the three-dimensional distance separating two features, one presented in the fixation plane, and the other some distance in front of it, i.e. with a significant standing disparity between the two features. For briefly-presented targets (150 msec), the Weber fraction for disparity was 10-20% over the range from 1 to 20 min arc, while the Weber fraction for width was only 2-3% under comparable conditions. The disparity thresholds were substantially improved for a longer duration target (1000 msec), but they were still a factor of two worse than the monocular width thresholds. In a related experiment, the vernier acuity for a standard vernier target was profoundly degraded by pairing the offset upper line presented to one eye with a disparate line in the other eye; the vernier threshold was elevated for disparities ranging from 3 to 30 min arc. This finding shows that the more precise monocular signals are actively suppressed in fused or partially-fused stereoscopic images.

Two Distinct Mechanisms of Suppression in Human Vision

Cortical visual neurons in the cat and monkey are inhibited by stimuli surrounding their receptive fields (surround suppression) or presented within their receptive fields (cross-orientation or overlay suppression). We show that human contrast sensitivity is similarly affected by two distinct suppression mechanisms. In agreement with the animal studies, human surround suppression is tightly tuned to the orientation and spatial frequency of the test, unlike overlay suppression. Using a double-masking paradigm, we also show that in humans, overlay suppression precedes surround suppression in the processing sequence. Surprisingly, we find that, unlike overlay suppression, surround suppression is only strong in the periphery (>1° eccentricity). This result argues for a new functional distinction between foveal and peripheral operations.

Sequential recruitment in the discrimination of velocity.

Human observers can discriminate a 5% difference in velocity for a wide range of velocities. Using an apparent-motion stimulus, we demonstrated that velocity discrimination depends on the detection of small changes in asynchrony, changes of the order of 1 msec or less. The simplest component of an apparent-motion stimulus is a pair of spatially separate lines presented asynchronously. Generally the incremental asynchrony threshold for a single pair of lines is much too large to account for velocity discrimination. A sequence of five to eight asynchronously presented targets, equivalent to continuous motion viewed for a duration of 80-100 msec is required to reach asymptotic velocity discrimination. Our experiments rule out probability summation as the explanation for the enhanced temporal sensitivity observed with the sequential presentation of multiple asynchronous targets. Sequential recruitment, a descriptive term for this enhanced temporal sensitivity, depends on the summation of a velocity-specific signal within the physiological network responding to motion.

Constraints on long range interactions mediating contour detection

Contour detection may be mediated by lateral interactions between neighboring cortical neurons whose receptive fields have collinear axes of preferred orientation. This hypothesis was tested in psychophysical experiments and computer simulations using a contour detection task in which observers searched for groups of Gabor patches that followed spatially extended contour paths embedded in noise consisting of several hundred Gabor patches with random positions and orientations. The orientation-selective units in the simulated neural network were linked by facilitatory interconnections whose strength depended on the geometry (distance, curvature, change in curvature) of smooth curves connecting the orientation axes of units in a pairwise fashion. Psychophysical detection performance was much higher for contour signal groups that followed closed rather than open-ended paths. However, just two sudden changes in orientation of neighboring Gabor patch elements in closed-path contours reduced detection performance to the same levels obtained with open-ended contours. These psychophysical data agreed with the results of the neural network simulations. Furthermore, the simulations also accounted for previous findings that removal of a single Gabor patch element from a closed-path contour group significantly degraded detection performance. We conclude that closure alone is not sufficient to enhance the visibility of a contour. However, if a closed contour meets certain geometric constraints, then lateral interactions based on these constraints can generate facilitation that reverberates around the closed path, thereby enhancing the contours visibility.

The use of an implicit standard for measuring discrimination thresholds.

We measured thresholds for comparing the separation between lines, using either the method of constant stimuli (MCS) or the method of single stimuli (MSS). In the MCS an explicit standard is presented on each trial, whereas in the MSS the standard is the mean of the set. The thresholds for the MSS procedure were nearly identical to those with the MCS procedure, whether or not feedback was used. A statistical model is presented showing how the threshold error estimated by MSS varies according to the number of past stimuli used by the observer to calculate the mean of the set. If the model is an accurate representation of human processing, our observers were averaging over the last 10-20 trials to estimate the implicit standard. Our results show that the explicit standard in the MCS procedure is generally superfluous. Provided that the test range is small, and that the observer is given some practice trials, thresholds measured with MSS procedure are just as precise as those measured with the traditional MCS procedure.

The effect of spatial configuration on surround suppression of contrast sensitivity.

Contrast sensitivity is known to be strongly influenced by the target surround, yet the role of the surround interaction in visual processing remains unclear. Previously, we have shown that the surround strongly suppresses contrast sensitivity in the periphery when the surround spatial frequency and orientation match those of the target (Petrov, Carandini, & McKee, 2005). Here, we explore how various spatial characteristics of the iso-oriented and frequency-matched surround, such as surround phase and spatial layout, affect suppression. We manipulated surround geometry (annulus ring, half annulus, and bow tie) and its separation from the target (both laterally and in depth) and varied the position of the half-annulus and bow-tie surrounds with respect to Gabor targets orientation and with respect to its location in the visual field (i.e., radial vs. tangential surrounds). We also compared monoptic, dichoptic, and binocular surround suppression. Except for a significant radial-tangential anisotropy, only the area of the surround and the lateral separation between the surround and target had a significant effect on the magnitude of suppression. We showed that, although suppression amplitude remains constant with stimulus eccentricity, the lateral extent of suppression scales in proportion to the eccentricity. The most surprising finding was that the extent of surround suppression does not scale with stimulus size or spatial frequency. We suggest that the properties of surround suppression are best explained by a mechanism that selects salient targets for subsequent saccades.