Threshold Elevation Aftereffect Research Articles

Predictive feature remapping before saccadic eye movements.

Saccadic eye movements cause rapid and dramatic displacements of the retinal image of the visual world, yet our conscious perception of the world remains stable and continuous. A popular explanation for this remarkable ability of our visual system to compensate for the displacements is the predictive feature remapping theory. The theory proposes that, before saccades, the representation of a visual stimulus can be predictively transferred from neurons that initially encode the stimulus to neurons whose receptive fields will encompass the stimulus location after the saccade. Visual adaptation aftereffect experiments performed by Melcher (2007) provided psychophysical evidence for this theory. However, it was argued that the visual aftereffects were not measured at the "appropriate" remapped location (Rolfs, Jonikaitis, Deubel, & Cavanagh, 2011). Therefore, whether the remapped representation contains feature information (e.g., orientation, motion direction, or contrast) is still a subject of intense debate. Here, to explore the nature of the predictive transfer during trans-saccadic perception, we measured visual aftereffects (tilt aftereffect, motion aftereffect, and threshold elevation aftereffect) at the appropriate remapped location of adapting stimuli before saccades. We observed a significant tilt aftereffect and motion aftereffect, but little threshold elevation aftereffect. Furthermore, the tilt aftereffect and motion aftereffect exhibited spatial specificity. These findings provide strong evidence for the predictive feature remapping theory and suggest that intermediate visual processing stages (i.e., extrastriate visual cortex) might be critical for feature remapping. Finally, we propose that the feature remapping process might also contribute to the spatiotopic representation of visual features.

A monocular contribution to stimulus rivalry

When corresponding areas of the two eyes view dissimilar images, stable perception gives way to visual competition wherein perceptual awareness alternates between those images. Moreover, a given image can remain visually dominant for several seconds at a time even when the competing images are swapped between the eyes multiple times each second. This perceptual stability across eye swaps has led to the widespread belief that this unique form of visual competition, dubbed stimulus rivalry, is governed by eye-independent neural processes at a purely binocular stage of cortical processing. We tested this idea by investigating the influence of stimulus rivalry on the buildup of the threshold elevation aftereffect, a form of contrast adaptation thought to transpire at early cortical stages that include eye-specific neural activity. Weaker threshold elevation aftereffects were observed when the adapting image was engaged in stimulus rivalry than when it was not, indicating diminished buildup of adaptation during stimulus-rivalry suppression. We then confirmed that this reduction occurred, in part, at eye-specific neural stages by showing that suppression of an image at a given moment specifically diminished adaptation associated with the eye viewing the image at that moment. Considered together, these results imply that eye-specific neural events at early cortical processing stages contribute to stimulus rivalry. We have developed a computational model of stimulus rivalry that successfully implements this idea.

Opposite Modulation of High- and Low-Level Visual Aftereffects by Perceptual Grouping

A fundamental task of visual perception is to group visual features - sometimes spatially separated and partially occluded - into coherent, unified representations of objects. Perceptual grouping can vastly simplify the description of a visual scene and is critical for our visual system to understand the three-dimensional visual world. Numerous neurophysiological and brain imaging studies have demonstrated that neural mechanisms of perceptual grouping are characterized by the enhancement of neural responses throughout the visual processing hierarchy, from lower visual areas processing grouped features to higher visual areas representing objects and shapes from grouping. In a series of psychophysical adaptation experiments, we made the counterintuitive observation that perceptual grouping amplified the shape aftereffect but meanwhile, reduced the tilt aftereffect and the threshold elevation aftereffect (TEAE). Furthermore, the modulation of perceptual grouping on the TEAE showed a partial interocular transfer. This finding suggests a 2-fold effect of perceptual grouping - enhancing the high-level shape representation and attenuating the low-level feature representation even at a monocular level. We propose that this effect is a functional manifestation of a predictive coding scheme and reflects an efficient code of visual information across lower and higher visual cortical areas.

Perceptual grouping weakens local feature representation

Previous studies showed that shape perception could reduce activity in human primary visual cortex (Murray et al., 2002; Fang et al., 2008). But the functional role of the reduction is still unclear. Here we performed psychophysical adaptation experiments to address this issue. Subjects adapted to a thin diamond with its four corners hidden by three horizontal occluders. The diamond translated with a circular trajectory. Its speed was 3.38°/s and its direction (clockwise or counter-clockwise) reversed every 5 s. Due to the existence of the occluders, only four bars (part of the sides of the diamond) were visible, but subjects were able to group them into a translating diamond (the diamond stimulus). The width of the bars was 1.80° and their orientations were 25° tilted (left or right). Their luminance was modulated spatially by a sine function (2 cycles/°) and their Michelson contrast was 1. A second adapting stimulus (the non-diamond stimulus) was generated by slightly changing the positions of the occluders in the diamond stimulus, which then could not be perceived as a moving diamond. Both the diamond stimulus and the non-diamond stimulus were presented in the left visual field. We measured two local aftereffects induced by adaptation to one of the four bars—tilt aftereffect (TAE) and threshold elevation aftereffect (TEAE). We also measured high-level shape aftereffect (SAE) by presenting normal or close-to-normal diamonds (their aspect ratios were around 1) in the right visual field. Compared with adaptation to the non-diamond stimulus, adaptation to the diamond stimulus induced a stronger SAE, but weaker TAE and TEAE. These results suggest that perceptual grouping enhances shape representation in high-level shape selective areas (e.g. LOC), but meanwhile, weakens local feature representation in early visual areas (e.g. V1), which provides psychophysical evidence for the predictive coding theory (Mumford, 1992).

The effect of crowding on orientation-selective adaptation in human early visual cortex

Crowding is the identification difficulty for a target in the presence of nearby flankers. Based on psychophysical findings, many theories have been proposed to explain crowding at multiple levels. However, little is known about its neural mechanism. In this study, we combined psychophysical and fMRI adaptation techniques to search for the cortical locus of crowding. In the psychophysical experiment, when subjects' attention was controlled, we found that the threshold elevation aftereffect (TEAE) was not affected by crowding, regardless of the contrast level of adapting stimulus. In the fMRI experiment, the orientation-selective fMRI adaptation in V1 was not affected by crowding either. However, downstream from V1, we found that crowding weakened the adaptation effect in V2 and V3. Our results demonstrate that crowding occurs beyond V1 and provide one of the first pieces of direct evidence supporting the two-stage model of crowding (D. M. Levi, 2008).

Strength of early visual adaptation depends on visual awareness

We measured visual-adaptation strength under variations in visual awareness by manipulating phenomenal invisibility of adapting stimuli using binocular rivalry and visual crowding. Results showed that the threshold-elevation aftereffect and the translational motion aftereffect were reduced substantially during binocular rivalry and crowding. Importantly, aftereffect reduction was correlated with the proportion of time that the adapting stimulus was removed from visual awareness. These findings indicate that the neural events that underlie both rivalry and crowding are inaugurated at an early stage of visual processing, because both the threshold-elevation aftereffect and translational motion aftereffect arise, at least in part, from adaptation at the earliest stages of cortical processing. Also, our findings make it necessary to reinterpret previous studies whose results were construed as psychophysical evidence against the direct role of neurons in the primary visual cortex in visual awareness.

Spatial-frequency adaptation affects spatial-probability summation.

The effects of spatial-frequency adaptation on spatial-probability summation were investigated by two techniques: the measurement of the dependence of cosine grating sensitivity on the number of cycles presented and the determination of the frequency-of-seeing curve for a cosine grating. Both measurements indicate that the contribution of spatial-probability summation to adapted grating thresholds is reduced in comparison with unadapted conditions. This reduction in the effects of spatial-probability summation, together with the decrease in local sensitivity, which depends on the spatial inhomogeneity of the visual system, is sufficient to account for the threshold elevation aftereffect measured when adapting and testing with cosine gratings.

Evidence against saturation of contrast adaptation in the human visual system.

In many experiments on visual pattern perception, it has been assumed that the effects of adaptation to contrast saturate within a few minutes. However, we show here that such saturation does not necessarily occur. Instead, both the buildup and recovery of the threshold elevation aftereffect fit closely a power function.

An investigation into hemisphere differences in adaptation to contrast.

Suggestions that the cerebral hemispheres differ in their ability to adapt to high-contrast visual stimuli were investigated using vertical sinusoidal gratings. In one experiment, subjects set thresholds for stimuli to the left or right of fixation before and after adaptation to high contrast. In another experiment, subjects detected the appearance and compared the relative suprathreshold contrasts of stimuli presented simultaneously to left and right of fixation before and after adaptation. In a third experiment, signal detection parameters were calculated. No systematic hemisphere differences were found in baseline sensitivity, in apparent suprathreshold contrast, in the magnitude or time-course of the threshold elevation aftereffect, or in apparent contrast after adaptation. Possible explanations for the hemisphere differences reported by other workers are discussed, and task complexity is suggested as a major factor which distorted previous results.

Localized effects of spatial frequency adaptation.

The spatially localized threshold-elevation aftereffect of spatial-frequency adaptation was measured by using localized, aperiodic test patterns that have bandpass Fourier transforms. At a given retinal location, the threshold-elevation curves are consistent with the fatigue of size-turned mechanisms with center-surround sensitivity profile. Only a few different sizes of such mechanisms were required to fit the local results. The local aftereffect was also measured as a function of eccentricity near the fovea. The results indicate that the threshold-elevation aftereffect of spatial-frequency adaptation is not spatially homogeneous.

A study of interocular transfer of spatial adaptation.

The threshold-elevation aftereffect was measured ipsiocularly and interocularly following grating adaptation of one eye. The functions relating aftereffect magnitude to adapting contrast and adaptation time were similar under the two testing conditions, with interocular transfer remaining fairly constant; decay times were similar for ipsiocular and interocular aftereffects of comparable magnitude, and their frequency selectivities were the same. It is concluded that the stimulus-response characteristics of monocular and binocular spatial channels are fundamentally similar.

Binocular interactions in normal and anomalous binocular vision.

Binocular interactions for grating patterns were investigated in humans with normal binocular vision and in humans with abnormal binocular visual experience due to strabismus and/or amblyopia via 1) comparison of monocular and binocular contrast thresholds; 2) interocular transfer of the threshold elevation aftereffect; and 3) dichoptic masking. Whereas the normal observers showed improved binocular over monocular contrast sensitivity (i.e., binocular summation) and substantial interocular transfer of the threshold elevation aftereffect, the abnormal observers showed an absence of binocular summation and no significant interocular transfer. The dichoptic masking experiments showed that a suprathreshold masking grating presented to one eye elevated the contrast threshold for gratings presented to the fellow eye, within a narrow range of spatial frequencies (about 1 octave wide at half height) and orientations, centered about the spatial frequency and orientation of the mask. The magnitude and bandwidth of this masking effect was similar in subjects with normal and abnormal binocular vision, occurring even when the masking grating was presented to the amblyopic eye. These effects depend upon the contrast of the masking grating. In individuals with normal binocular vision, a grating with subthreshold contrast presented to one eye reduces the contrast threshold for detection of gratings of similar spatial frequency and orientation presented to the fellow eye. No such subthreshold summation is evident in the amblyopic observers. We conclude that while strabismus and/or amblyopia disrupted the normal excitatory interactions between the two eyes, cortical inhibitory binocular connections were not disrupted.

The site of binocular rivalry suppression.

Two experiments were performed to localize the site of binocular rivalry suppression in relation to the locus of grating adaptation. In one experiment it was found that phenomenal suppression of a high-contrast adaptation grating presented to one eye had no influence on the strength of the threshold-elevation aftereffect measured interocularly. Evidently information about the adaptation grating arrives at the site of the aftereffect (presumably binocular neurons) even during suppression. In a second experiment 60 s of grating adaptation was found to produce a short-term reduction in the predominance of the adapted eye during binocular rivalry. These findings provide converging lines of evidence that suppression occurs at a site in the human visual system after the locus of grating adaptation and, hence, after the striate cortex.

Storage of spatially specific threshold elevation.

The decay of several visual aftereffects may be prolonged by interposing a period of light-free or pattern-free viewing between adaptation and testing. We demonstrate that this storage phenomenon can be observed using the threshold elevation aftereffect that follows inspection of a high-contrast grating pattern. Control experiments comparing thresholds for vertical and horizontal gratings after adaptation to a vertical grating reveal that the stored aftereffect, like its unstored counterpart, is pattern-selective. Storage is equally pronounced with stimuli that are detected by pattern-analyzing or movement-analyzing visual channels. Unlike other aftereffects, the threshold-elevation aftereffect requires that the storage period be light-free; no storage is seen if a blank field is inspected between adaptation and testing. The results are discussed with respect to the nature of visual aftereffects, and possible cognitive or physiological models of storage.