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The blue-blindness (tritanopia) of the human foveola normally goes unnoticed but can be directly visualized by having observers view a flickering, monochromatic, short-wavelength field. The blue scotoma appears as a tiny dark spot in central vision, the visibility of which depends upon the wavelength of the field and the temporal frequency of modulation. Comparisons of fading times as a function of flicker frequency for the blue scotoma, foveal afterimages and optically stabilized images indicate a common time course, consistent with the hypothesis that perceptual filling-in of the foveal blue scotoma reflects the operation of neural processes similar to those involved in fading and regeneration of stabilized images.

Looking at the world with one eye, we do not notice a scotoma in the receptor-free area of the visual field where the optic nerve leaves the eye. Rather we perceive the brightness, color, and texture of the adjacent area as if they were actually there. The mechanisms underlying this kind of perceptual filling-in remain controversial. To better understand these processes, we determined the minimum region around the blind spot that needs to be stimulated for filling-in by carefully mapping the blind spot and presenting individually fitted stimulus frames of different width around it. Uniform filling-in was observed with frame widths as narrow as 0.05 degrees visual angle for color and 0.2 degrees for texture. Filling-in was incomplete, when the frame was no longer contiguous with the blind spot border due to an eye movement. These results are consistent with the idea that perceptual filling-in of the blind spot depends on local processes generated at the physiological edge of the cortical representation.

The absence of short-wave-sensitive (S-) cones in the human foveola normally goes unnoticed, but the resulting foveal S-cone, or blue, scotoma can be visualized as the negative afterimage of a short-wavelength adapting field on a larger white background. The afterimage has an annular shape with a lighter inner region that corresponds to Maxwell's spot, and a small bright spot in the center corresponding to the foveal blue scotoma. We have shown that the visibility of the center spot in the afterimage approximately follows the spectral sensitivity curve of the S-cones. We further demonstrate that the central bright spot subtends a retinal area that is coincident with the tritanopic region of the foveola. The macular pigment distribution measured for the same observers also peaks in the central fovea, but has a relatively high density over a broader retinal region than the bright spot in the negative afterimage, and more closely corresponds to the lighter annular region of the afterimage. The results support the hypothesis of an active post-receptoral process for filling-in of chromatic scotomas.

Discrimination accuracy decreases when viewers simultaneously monitor two perceptually distinct stimulus components for changes in a common property, e.g. contrast [Magnussen & Greenlee (1997). Journal of Experimental Psychology: Human Perception and Performance, 23, 1603-1616; Olzak & Wickens (1997). Perception, 26, 1101-1120]. We ask whether the limitation is in monitoring two components or in making dual decisions about a single property. Using the same uncertainty paradigm as Magnussen and Greenlee, we find no evidence of a processing limitation when viewers simultaneously monitor one component (1.25 c/d) for a possible change in contrast and a second component (5 c/d) for a possible change in spatial frequency, regardless of whether the components are spatially separated or superimposed. The limitation is in making dual decisions about a single property.

Discrimination thresholds of spatial frequency and choice reaction times (RT) were measured in three subjects who performed a dual-judgment delayed discrimination task. Two reference gratings were presented side-by-side with a 0-800 msec stimulus onset asynchrony (SOA), which were followed after a 5-sec retention interval by two test gratings. Subjects judged which component changed and which interval had the higher spatial frequency (SF). Thresholds in the dual-judgment task were four to six times higher than thresholds in single-judgment tasks. The SOA had only a moderate effect on discrimination thresholds, whereas the difference between the spatial frequencies of the two components had a highly significant effect. The discrimination thresholds increase with increasing spatial frequency difference for the lower SF component, while they decrease for the higher SF component. An analysis of the distribution of possible error types indicated that all subjects tended to respond more frequently that the higher SF component changed. This tendency led to more errors on trials where the low SF component changed. A posthoc analysis revealed, in two of the three subjects, a significant correlation between delta f/f and RT such that higher delta f/f values were associated with lower RTs and vice versa.

The visual acuity of twelve multi-handicapped, mentally retarded subjects, diagnosed as deaf-blind, was measured on two occasions with the Teller Acuity Cards (TAC). Eight subjects scored above the criterion for legally blind and the results of six of these indicated various degrees of poor to approaching-normal eyesight. To evaluate high-level vision four subjects were tested with the Fagan Test, assessing visual recognition memory for faces subsequent to familiarization with the preferential looking technique. The results for three subjects showed evidence for perceptual recognition. It is concluded that TAC combined with the Fagan Test may detect perceptual capacities unnoticed by clinical observation.

We explored the buildup and decay of threshold elevation during and after adaptation to sinewave gratings in a series of experiments investigating the effects of adapting time, adapting contrast, spatial frequency and retinal eccentricity. Contrast thresholds for vertical sinewave gratings truncated in space by a one-dimensional Gaussian envelope were measured before and after adaptation to a full-field suprathreshold grating of the same spatial frequency and orientation. Thresholds were measured intermittently after adaptation in a "seen/not-seen" single presentation procedure until these thresholds returned to baseline values. The first test grating was presented 300 msec after the offset of the adapting stimulus, and thereafter at regular intervals. At different times after adaptation, contrast thresholds were estimated by off-line analysis of the data using the QUEST algorithm. Adapting time was either 1, 10, 100 or 1000 sec and adapting contrast was either 9, 19, 29 or 39 dB (re. 1%). The test gratings were presented centered either at the fixation point or at 5 and 10 deg eccentricity along the horizontal meridian. The results suggest that up to the saturation level the buildup and the decay of adaptation to contrast is well described by a power function of time. The slope of the best fitting line on log-log axes is fairly constant for the adaptation times tested. As reported earlier, thresholds increased with adapting contrast and these contrast-dependent differences were evident 300 msec after the termination of adaptation. Adaptation at 10 deg eccentricity yielded slightly higher threshold elevations than for central vision. Based on these results, a description is given of the dynamic response of the underlying neural mechanisms.

The retention of spatial information in visual short-term memory was assessed by measuring spatial frequency discrimination thresholds with a two-interval forced-choice task varying the time interval between the two gratings to be compared. The memory of spatial frequency information was perfect across 10-sec interstimulus intervals. Presentation of a "memory masker" grating during the interstimulus interval may interfere with short-term memory. This interference depends on the relative spatial frequency of the test and masker gratings, with maximum interference at spatial frequency differences of 1-1.5 octaves and beyond. This range of interference with short-term memory is comparable to the bandwidth of sensory masking or adaptation. A change of the relative orientation of test and masker gratings does not produce interference with spatial frequency discrimination thresholds. These results suggest stimulus-specific interactions at higher-level representations of visual form.

Interactions between size and orientation-specific mechanisms in the human visual system were investigated using a sequential adaptation technique. Subjects adapted to a vertical, 4 c/deg high-contrast (0.7) sinewave grating that was interleaved at a rate of 0.5 Hz with another adapting grating differing either in (1) spatial frequency or (2) orientation. Before and after adaptation contrast thresholds were measured for a vertical 4 c/deg sinewave test grating. The resultant elevation in contrast threshold was plotted as a function of the (1) spatial frequency or (2) orientation differences between the first and second adapting gratings. Maximum threshold elevation was found when both adapting gratings shared the same spatial frequency and orientation. Minimum elevations were found when the second grating's spatial frequency or orientation differed by approx. 1.5 octaves or 45 deg, respectively. Beyond these values threshold elevations reapproached the baseline value measured in a control condition, where the 4.0 c/deg adapting grating was interleaved with a blank. The minimum threshold elevations were 0.2-0.3 log units below the baseline level. The results suggest the existence of inhibitory interactions between neural mechanisms tuned to the size and orientation of retinal images.

The tilt aftereffect increases as a logarithmic function of adapting time, reaches saturation after approx 1 hr and decays on a symmetric, logarithmic time-course. This is similar to the time-course of contrast threshold elevation, suggesting that threshold and suprathreshold aftereffects are based on similar type of adaptation processes.

Contrast thresholds for a 6 c/deg sinewave grating were measured following continuous and interrupted adaptation of 10 min duration to a high-contrast (0.6) sinewave grating of the same spatial frequency. Interrupted adaptation was administered as five 2-min segments, and the interadaptation interval (IAI) was varied from 10 to 180 sec. The results indicate that adaptation to spatial contrast can be described by a two-staged process, each stage having a different time constant of adaptation decay.

Adaptation to a high contrast sinewave grating of 1 c/deg spatial frequency causes a large increase in the contrast threshold for a 1 c/deg test grating, but fails to raise the threshold for a squarewave grating of 0.33 c/deg, although the sensitivity of the "channel" tuned to both the third and fifth harmonic components of the squarewave test grating should be thoroughly suppressed. Following sequential adaptation to sinewave gratings of 1 and 3 c/deg spatial frequency, detection of squarewave gratings at 0.33 c/deg likewise remains unaffected. In contrast, after adaptation to a 0.33 c/deg squarewave grating with missing fundamental the contrast threshold for a squarewave test grating of the same frequency is increased by 0.25 log unit, although the higher harmonic component frequencies are less affected than by sequential sinewave adaptation. The results suggest that independent spatial frequency channels detecting harmonic components are not alone sufficient to account for the visibility of low frequency squarewaves.

Growth and decay characteristics of the tilt aftereffect were studied for aftereffects induced by normal or continuous adaptation routines, and for aftereffects induced by successive or spaced adaptation to the same or different orientations on an adapt-partial decay-readapt schedule. In the continuous adaptation condition, growth and decay of the aftereffect were logarithmic functions of time. There was no evidence for saturation after 30 min adaptation. Aftereffect decay following spaced adaptation progresses as by continuous adaptation, but an adapting stimulus introduced during recovery from previous adaptation is more effective on the time scale than when introduced to a fully recovered system, summing approximately linearly with the residual aftereffect and off-setting the recovery process to zero. A second adapting stimulus whose orientation is of opposite sign (ccw vs cw) induces a two-phased decay process consisting of an early cancellation and a later enhancement of the original aftereffect. A two-stage model of adaptation is proposed.

The contrast thresholds for detecting a 6.0 c/deg vertical sinusoidal test grating were tracked during and after 3 hr inspection of a high-contrast adapting grating of the same spatial frequency and orientation. Log contrast threshold increased linearly with log adaptation time, attaining a final stable value after approximately 30 and 60 min of adaptation for the two subjects tested. The recovery function was likewise linear on double logarithmic axes. The results further suggest that adaptation beyond the saturation point had no influence on the subsequent rate of recovery.