Is spatial adaptation caused by prolonged inhibition?

The locus of attentional effects in texture segmentation

Cuing covert spatial attention can increase spatial resolution. Here we pinpointed the specific locus of this effect using texture segmentation. At the level of visual cortex, texture segmentation theoretically involves passage of visual input through two layers of spatial linear filters separated by a pointwise nonlinearity. By manipulating the textures to differentially stimulate first- or second-order filters of various scales, we showed that the attentional effect consistently varied with the latter. These psychophysical results further support the hypothesis that attention increases resolution at the attended location and are consistent with an effect of attention at stages as early as the primary visual cortex.

Spatial-frequency adaptation: evidence for a multiple-channel model of short-wavelength-sensitive-cone spatial vision

The frequency selective effects of spatial adaptation were measured with vertically-oriented, cosine stimuli upon an intense long-wavelength (yellow) field, which isolated the short-wavelength-sensitive (S) cones. Consistent with isolated-S-cone spatial threshold and masking results, the adaptation measurements demonstrate S-cones input to multiple, orientation selective, spatial frequency mechanisms. Moreover, the adaptation measurements show the minimum number of S-cone mechanisms is three. The frequency tuning of each mechanism was derived from the S-cone threshold and masking results. Two of the tuning curves are bandpass with peak sensitivities in a vicinity of 0.7 and 1.4 c/deg, respectively. These two closely resemble tuning curves derived from results with luminance-modulated stimuli. Confined to the range of frequencies examined (0.25-2.83 c/deg), the third tuning curve is lowpass with a high-frequency cutoff of roughly 2.0 c/deg. However, subsequent measurements of orientation selectivity demonstrate the third mechanism to have bandpass frequency tuning as well.

Orientation bandwidth: the effect of spatial and temporal frequency

The orientation bandwidths of psychophysically defined channels of human vision were estimated by two techniques for a wide range of spatial and temporal frequencies. The first technique was an adaptation paradigm, where the subjects' ability to see patterns of various orientations was measured before and after adapting to a high contrast pattern. The second technique evaluated subjects' ability to discriminate between two gratings of different orientations in relation to their ability to detect the patterns. Bandwidths were unaffected by temporal frequency at high spatial frequencies but increased with temporal frequency at low spatial frequencies. Bandwidths increased modestly with decreasing spatial frequency at low temporal frequencies but more drastically at high temporal frequencies. Both techniques gave similar results except for patterns with very low spatial and high temporal frequencies. In this region the stimulus appears "spatial-frequency doubled" which may be used as a cue for the orientation discrimination task.

Measurement of visual channels by contrast adaptation

Inspection of a high-contrast grating pattern affects our ability to detect patterns that are similar. This technique can be used to infer the underlying mechanisms of the visual system. By using this technique, measurements of the bandwidth of orientation channels are taken for different levels of adapting contrast and adapting duration. If the threshold elevation is plotted as the difference between the unadapted and adapted threshold in decibels, then the orientation bandwidth is invariant if taken at some fraction of the maximum elevation. This results from the fact that, as the orientation difference between the adapting and test patterns increases, the function relating threshold elevation to adapting contrast reduces in slope. These data contradict the often-used 'equivalent contrast transformation' (in which the fall off in the adaptation effect with respect to orientation is expressed in terms of an equivalent reduction in adapting contrast) as this would produce quite different bandwidths at different adapting contrasts. The data also address the issue of the neuronal mechanisms of adaptation.

Contrast adaptation in striate cortex of macaque

We have characterized the contrast-response relationships for simple and complex cells in striate cortex of macaque monkey, before and during adaptation to high-contrast sinusoidal gratings of the optimal spatial-frequency and orientation. Adaptation brings about systematic changes in the steepness of contrast-response curves and in the effective contrast of stimuli. Adaptation reduces the detectability of low-contrast gratings by almost a factor of three, but by extending the operating range of most cells it appears to improve the discriminability of high-contrast stimuli that previously gave rise to responses of saturating amplitude.

Opponent-movement mechanisms in human vision

A vertical grating that sinusoidally reverses contrast can be synthesized from two identical component gratings that move with equal velocities in opposite directions (leftward and rightward). Such a counterphase grating is used as a suprathreshold masking pattern. When the mask is of low spatial frequency and is modulated rapidly, a test pattern consisting of an increment of the rightward component and an equivalent simultaneous decrement of the leftward component is highly detectable compared with simultaneous increments or decrements of both components. The visibility of the opponent-movement test signal is strongly facilitated by high-contrast masks. This facilitation is accompanied by a high sensitivity for judging the direction of motion of the test. These results show that certain detection mechanisms are highly sensitive to the difference of the rightward and leftward components. However, when the mask is of threshold contrast, the rightward- and leftward-moving test components appear to be detected independently. A high-contrast grating that rapidly moves in one direction strongly masks gratings moving in the same or opposite direction; this shows that moving patterns are not detected by unidirectional mechanisms when contrast is clearly suprathreshold. The results may be explained by a model with mechanisms that are excited by one direction of motion and inhibited by the opposite direction.

Facilitatory and inhibitory after-effect of spatially localized grating adaptation

Aftereffects of spatially localized grating adaptation were measured for different locations of the adaptation grating relative to test grating. When the adaptation grating was located on or near the retinal area occupied by the test grating, contrast sensitivity was markedly reduced. When the adaptation grating was spatially separated from the test grating, contrast sensitivity was significantly increased. This aftereffect of spatially localized grating adaptation suggests that spatial-frequency-selective detectors are not spatially independent, but tonically inhibited by spatially contiguous mechanisms. Thus the adaptation of these mechanisms might cause an increase in contrast sensitivity of detectors subserving the test grating.

Adaptive processes controlling sensitivity of short-wave cone pathways to different spatial frequencies

The sensitivity of the short-wave cone pathways was measured with violet interference gratings on monochromatic adapting fields. Spectral field sensitivity functions for test gratings of 2, 4 and 8 c/deg approximately matched Stiles' pi 3 function, which presumably reflects light-adaptation of the short-wave cones. Previous work had suggested that field sensitivity varies as a function of test spatial frequency, but this now seems an artifact of spatial adaptation. (The present study shows the pathway is highly susceptible to spatial adaptation induced by fine patterns.) For various spatial frequencies that affect the short-wave pathway, signals also pass through a cancellate adaptation site where signals from the short-wave and the middle- and long-wave cones act in opposition. Transient inputs in either opponent direction polarize or desensitize the opponent site. Additional transient inputs of the opposite sign then reduce the polarization and increase sensitivity. Such effects were demonstrated with test gratings as high as 12 c/deg. Transient effects at the opponent site are particularly evident at higher spatial frequencies.

The cone threshold: spatial interactions of rod and cone adapting signals

Steady small blue adapting fields that stimulated only rods produced large threshold elevations for a superimposed tiny red flash that stimulated cones. The threshold elevation differed considerably between observers, confirming the results of Buck (Topical Meeting on Recent Advances in Vision, abstract in Technical Digest, 1980; Invest. Ophthal, visual Sci., Suppl. 20, 207, 1981). A red annulus that stimulated cones reduced slightly the threshold elevation produced by the small rod adapting field. Similarly, when the cone threshold was elevated by a small red field that stimulated only cones, a blue annulus that stimulated rods slightly reduced the threshold of the red cone test flash. These effects, although weak, demonstrate lateral sensitizing interactions between a cone center field and rod annulus, and vice versa, when the cone threshold is assessed with a tiny red flash.

Spatial sensitization and desensitization with small adapting fields: interactions of signals from different classes of cones

Thresholds for brief, tiny flashes were measured on small adapting fields at 5 degrees retinal eccentricity. Rod influences was eliminated by bleaching. Green and red small adapting fields raised the threshold for a green test flash by stimulating both middle and long wave cones-thus spatial desensitization by small adapting fields is not con-specific. However, a small adapting field that strongly stimulated only short wave cones did not affect the visibility of a long wave flash. Spatial sensitization was also measured, using green or red annuli that surrounded a small red field and green test probe. Sensitization was not cone-specific. Possible mechanisms producing desensitization with small fields and annular sensitization are discussed.

An action spectrum for spatial-frequency adaptation

The mechanism of spatial-frequency adaptation, and channel-bandwidth estimates derived from adaptation, were examined in a series of three experiments. The first two experiments measured adaptation as a function of both the frequency and modulation of the adapt stimulus. The threshold for producing adaptation at different adapt frequencies is considered to be an action spectrum whose half-amplitude bandwidth is found to lie between 1/3 and 2/3 octave. The third experiment employed a novel adaptation paradigm--varying the adapt contrast until the test grating was no longer perceptible--but the results supported those of the earlier experiments. It is concluded that adaptation is probably due to a fatigue-like mechanism, and that a sensitivity measure of adaptation (action spectrum) is probably a better representation of underlying channel tuning than is the more customary response measure.

Lateral interactions in the control of visual sensitivity

Threshold vs intensity curves for cone vision, measured in the parafoveal retina, quickly saturate if the adapting background is made small (e.g. 19' at 5 degrees eccentricity). Log increment threshold increases at a rate of about 3:1 with log background illuminance at levels as low as 10 td. This shows that lateral interactions are an important process in preserving differential sensitivity in cone vision across the wide range of illuminances over which it normally operates. Parallels between light and dark adaptation in the effect of field size were explored, since effects of comparable magnitude are observed in both. Backgrounds and bleaches equated for their effects at one field size do not have equal effects on threshold at other field sizes, however, with small-area bleaches raising threshold more than predicted. This failure of equivalence was also revealed in a second experiment, in which recovery of sensitivity following small area bleaches was measured in the presence of large steady background fields, which have the effect of lowering threshold. Thresholds following the small bleach were lowered less than expected on the basis of the "equivalent background" hypothesis, a result which we take to mean that signals from bleached cones exceed those produced by a background which has an equivalent effect on threshold (the "equivalent background"). Control experiments examined whether rods contribute to the overloading of cone response by small fields and the possible contribution of such central adaptation processes as spatial frequency adaptation.

Adaptation to polarized light in humans

Recent investigators reported large adaptational effects in human vision dependent on the plane of linearly polarized light. In the present study, such adaptational effects were observed to be small or insignificant. Our observers adapted to intense homogenous, foveal fields of linearly polarized, orange-red light produced by a beam that entered the side of the pupil. Periodically the plane of polarization of the beam suddenly shifted 90 degrees without there being any change in the spectral distribution of the light or position of the beam in the pupil plane. The change of radiance of the beam could be adjusted to be as low as approx. 0.5%. Signal detection experiments revealed that observers typically could not detect a change of polarization of the field. Results obtained with the method of adjustment showed that the threshold of an increment flash presented on the intense field was not affected by changing the polarization of the field. Physiological implications of these results are discussed.

Low spatial-frequency channels in human vision: adaptation and masking

Previous work showed that adapting to low spatial frequency gratings (below 1.5 cycles/degree) may cause maximal spatial adaptation at a significantly higher spatial frequency. It has been suggested that there are no adaptable spatial-frequency channels tuned to below 1.5 c/deg. Contrary to this view, we found that adaptation and masking with low spatial frequencies (0.12-1.0 c/deg) produced maximal threshold elevations when the test patterns were the same spatial frequency as the adapting or masking pattern. These results were obtained using test patterns that turned on and off gradually or sharply. The results suggest that there are form mechanisms optimally sensitive to very low spatial frequencies. Adaptation was selective to position (phase) and orientation at low spatial frequencies; masking was observed to be selective to orientation at a spatial frequency as low as 0.2 c/deg. A clear dichotomy between transient, motion channels and sustained, form channels at low spatial and temporal frequencies may represent an unrealistic simplification. There may exist directionally-selective motion mechanisms sensitive to very slow motion, and these may play a role in the discrimination of form. The discussion considers the bandwidths of the low spatial frequency mechanisms.

Contrast masking in human vision

Contrast masking was studied psychophysically. A two-alternative forced-choice procedure was used to measure contrast thresholds for 2.0 cpd sine-wave gratings in the presence of masking sine-wave gratings. Thresholds were measured for 11 masker contrasts spanning three log units, and seven masker frequencies ranging +/- one octave from the signal frequency. Corresponding measurements were made for gratings with horizontal widths of 0.75 degrees (narrow fields) and 6.0 degrees (wide fields). For high contrast maskers at all frequencies, signal thresholds were related to masking contrast by power functions with exponents near 0.6. For a range of low masking contrasts, signal thresholds were reduced by the masker. For the wide fields, high contrast masking tuning functions peaked at the signal frequency, were slightly asymmetric, and had approximately invariant half-maximum frequencies that lie 3/4 octave below and 1 octave above the signal frequency. The corresponding low contrast tuning functions exhibited peak threshold reduction at the signal frequency, with half-minimum frequencies at roughly +/- 0.25 octaves. For the narrow fields, the masking tuning functions were much broader at both low and high masking contrasts. A masking model is presented that encompasses contrast detection, discrimination, and masking phenomena. Central constructs of the model include a linear spatial frequency filter, a nonlinear transducer, and a process of spatial pooling that acts at low contrasts only.

Adapting to two orientations: disinhibition in a visual aftereffect

The tilt aftereffect of adapating to two different orientations simultaneously is weaker than the aftereffect of adapting to the more effective of the two orientations alone. This finding is consistent with explanations of orientational after-effects in terms of lateral inhibition between cortical orientation detectors, but not with explanations in terms of neural "fatigue" from excitation.

Spatial adaptation of short-wavelength pathways in humans

Color-selective spatial adaptation of the short-wavelength, or blue-sensitive, pathway was demonstrated. The adaptation was orientation selective and strongly monocular. Adaptation was assessed by measuring visibility thresholds for monochromatic gratings in subjects adapted to high-contrast violet gratings designed to stimulate only blue-sensitive cones. The results showed spatially selective, adaptable channels within the short-wavelength pathway.

Response of visual mechanisms to stimulus onsets and offsets

Transient and sustained visual mechanisms were studied with single, flickering bars of various widths. Wide bars were largely detected on the basis of temporal luminance transients whereas thin bars were detected on the basis of the sustained contrast. A rapidly flickering uniform field selectively masked wide flickering bars, which suggests that different mechanisms detect wide versus thin flickering bars. For coarse spatial patterns, stimulus onsets were slightly more visible than stimulus offsets, and the response to onsets and offsets approximately summated.

Movement-selective mechanisms in human vision sensitive to high spatial frequencies

Evidence for motion-selective mechanisms sensitive to high spatial frequencies (e.g., 15 c/deg) was obtained via direction-specific adaptation and measurements of the threshold ratios for moving and counterphase flickering gratings.

Spatial-frequency masking with briefly pulsed patterns

Spatial-frequency masking was studied with briefly pulsed (25 ms) vertical gratings. The mask was a noise grating, and the test pattern was a sinusoidal grating. A low-frequency band of noise masked a low- but not high-spatial-frequency test grating when the patterns were presented simultaneously. A high-frequency band of noise did not mask a low-frequency test grating when the patterns were presented simultaneously or when the mask was presented after the test pattern (backward masking). Masking was, however, observed when the mask or test pattern was of sufficiently high contrast so that the stimuli had nonlinear distortion and thus produced DC shifts of the field luminance.