Spatial interactions and models of adaptation.

Adaptation mechanisms can be divided into two classes: multiplicative mechanisms which reduce the gain and subtractive mechanisms which discount or filter out the background signal. This paper investigates the neural basis of subtractive adaptation in photopic vision. Specifically, can the spatial interactions revealed by Westheimer's effect be described as subtractive? The evidence presented here shows that they can. That is, small adapting fields raise threshold more than large ones because they produce more response compression, not because they reduce the gain. As the background is enlarged progressively more of the background signal is subtracted off, reducing the response compression at some later non-linear site. These results indicate that retinal center-surround antagonism is one of the mechanisms mediating subtractive adaptation.

The size of the pool for bleaching adaptation in human rod vision.

We present new psychophysical estimates of the size of the rod pool for bleaching adaptation in the human retina. We estimate that at 5 deg nasal eccentricity in the human retina the size of the adaptation pool for rods is between 5 and 7.5 min arc. This estimate is compatible with the extent of the dendritic spread of rod bipolars located in this region of the primate retina and with the area occupied by roughly 50 rods in this parafoveal region of the human retina. Thus a candidate for the site of adaptation is the bipolar cell whose receptive field is comprised of approx. 50 rods. These estimates represents the lowest measurements to date of the size of the adaptation pool for rods.

Temporal modulation sensitivity in cone dark adaptation.

The temporal modulation sensitivity of cone vision was studied using sinusoidally modulated lights. We found that the effects of bright flash bleaches mirror the effects of light adaptation and raise low frequency threshold most. Prolonged pre-exposures, however, raise high frequency threshold most. Two processes are therefore required to describe the mechanisms of recovery of sensitivity following bleaching exposures, one which acts like a background light and selectively attenuates low frequencies, and another process with a long integration time which selectively attenuates high frequencies.

The role of spatial filtering in sensitivity regulation.

The role of spatial filtering in controlling sensitivity to increments is hard to evaluate under normal viewing conditions because eye movements lead to a confounding of spatial and temporal transients. We measured sensitivity to increments on different sized backgrounds in photopic and scotopic vision when the backgrounds were stabilized on the retina, thus eliminating temporal transients. The saturating effect of small fields on photopic thresholds was preserved under these conditions indicating that spatial filtering by retinal cells is critical in maintaining photopic sensitivity. Some effect of spatial pattern on sensitivity in stabilized vision was also observed in scotopic vision, although it was much smaller than was observed in photopic vision. The interaction effects between rod and cone systems that are observed with small backgrounds were also preserved in stabilized vision, implicating a very peripheral site for the generation of these interactions.

The role of exocentric reference frames in the perception of visual direction.

One classic piece of evidence for an efference copy signal of eye position is that a small, positive afterimage viewed in darkness is perceived to move with the eye. When a small stationary reference point is visible the afterimage appears to move relative to the reference point. However, this is true only when the afterimage is localized to a small area. We have observed that when an extended afterimage of a complex scene is generated by a brief, bright flash it does not appear to move, even with large changes in eye position. When subjects were instructed to maintain their direction of gaze, we observed small saccades (typically < 1 deg) and slow drift movements often totalling more than 10 deg over a 30 sec period. When the instructions were to simply inspect the extended afterimage, subjects made larger saccades (up to 5 deg) which were not accompanied by afterimage movement. The smaller movements observed under the first instructions are greater than those observed in the dark or with small afterimages. When a visible reference is present with these large afterimages, the afterimage appears stationary, while the reference point appears to move. Eye position was monitored following the generation of such afterimages. In general, the perceived motion of the stationary reference point was in a direction opposite to the motion of the eye. Similar drift movements of smaller magnitude were observed with localized afterimages, but the motion was attributed to the afterimage.(ABSTRACT TRUNCATED AT 250 WORDS)

Surface segmentation mechanisms and motion perception.

Two studies are presented which explore how depth information is determined from conditions of transparency and whether this information combines with other depth information to determine the segmenting of motion information on the basis of likely surface boundaries. The first study explored how binocular disparity combines with monocular depth cues associated with transparency to determine whether subjects see one or multiple surfaces in depth in static displays. When transparency provided a depth cue that was consistent with binocular disparity, depth discrimination thresholds were at normal levels. However, if transparency was inconsistent with the binocular disparity, depth discrimination thresholds were elevated, indicating that subjects had difficulty seeing distinct surfaces lying in separate depth planes. Moreover, threshold elevations were found to correspond to the reductions in contrast between the intersecting contours of the stimulus, suggesting that the strength of perceived depth from transparency is the result of attenuated responses from competing contrast sensitive T-junction mechanisms responsible for the detection of opaque occlusion. A second experiment explored whether the grouping of local motion signals relied on surface interpretations that result from the interaction of transparency and disparity. Surface interpretations were manipulated in moving plaids by combining transparent layering and binocular disparity to show that the motion arising from contours is grouped together (pattern motion) when these cues support the existence of a single surface, and is segregated (component motion) when they support separate surfaces. When these cues were consistent, only small disparity differences were required for the gratings to appear as separately moving surfaces. However, when they were inconsistent, greater disparities were required (about a factor of 2 greater). Taken together, these studies demonstrate that the grouping of local motion information is not resolved within the motion system alone. Information seemingly unrelated to motion processing, namely surface segmentation cues, is used to determine whether or not motion information arising from various contours is pooled together to determine a single motion.

Stabilized vision through a bleaching window.

The visibility of stimuli flickering at low temporal frequencies was studied with images viewed normally and when stabilized by a new technique (bleached-window stabilization) that produces perfectly stabilized images without attachment to the eye. Contrary to expectation, sensitivity to low-frequency flicker was higher in the stabilized than in the unstabilized case. Several controls exclude the possibility that the paradoxical increase in sensitivity is an artefact of the new technique, and suggest that, in normal viewing, transient activity created by fixational eye-movements can mask the signals arising from the low-frequency flicker.

What controls attention in natural environments?

The highly task-specific fixation patterns revealed in performance of natural tasks demonstrate the fundamentally active nature of vision, and suggest that in many situations, top-down processes may be a major factor in the acquisition of visual information. Understanding how a top-down visual system could function requires understanding the mechanisms that control the initiation of the different task-specific computations at the appropriate time. This is particularly difficult in dynamic environments, like driving, where many aspects of the visual input may be unpredictable. We therefore examined drivers' abilities to detect Stop signs in a virtual environment when the signs were visible for restricted periods of time. Detection performance is heavily modulated both by the instructions and the local visual context. This suggests that visibility of the signs requires active search, and that the frequency of this search is influenced by learnt knowledge of the probabilistic structure of the environment.

The time-course of multiplicative and subtractive adaptation process.

This paper examines, for foveal cone vision, the processes which mediate the transition to a steady state of adaptation following a change of illumination. In the steady state, the signal from an adapting field is attenuated not only by a multiplicative factor (reduction in gain) but also by a subtractive signal. We show that the multiplicative change is accomplished very rapidly following the onset of an adapting field (within about 50 msec). Much of the subtractive change is also accomplished rapidly, but it takes several sec to complete. At the offset of the field, the multiplicative process takes over 200 msec to recover. This slower time-course at offset may be a consequence of receptoral persistence.

Subtractive processes in light adaptation.

We measured the time course of light adaptation in foveal vision following the onset of an adapting background. Several adaptational steps in the low to mid photopic range were examined. The time course of multiplicative and subtractive components of the adaptation were extracted from the data. Unlike previous findings there were no subtractive changes for several hundred milliseconds following light onset, and the process took 10-15 sec to reach steady state. It seems likely that the fast component previously observed results from effectively instantaneous center-surround antagonism, and that our measurements reflect a second subtractive process involving the slow loss of the d.c. signal over time.

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.

The spread of adaptation in human foveal and parafoveal cone vision.

We investigated the spread of bleaching adaptation for human cone vision in the central fovea and at an eccentricity of 5 deg in the nasal retina. Cone thresholds measured after adaptation to a grating bleach were compared to those measured after a uniform bleach. We conclude that the foveal and parafoveal cone systems show excellent localization of the effects of adaptation. For areas 2.5-5 min removed from the bleach, our measurement show only small sensitivity losses amounting to between 0.10 and 0.25 log unit elevation in threshold, after taking account of optical scatter.

Task constraints in visual working memory.

This paper examines the nature of visual representations that direct ongoing performance in sensorimotor tasks. Performance of such natural tasks requires relating visual information from different gaze positions. To explore this we used the technique of making task relevant display changes during saccadic eye movements. Subjects copied a pattern of colored blocks on a computer monitor, using the mouse to drag the blocks across the screen. Eye position was monitored using a dual-purkinje eye tracker, and the color of blocks in the pattern was changed at different points in task performance. When the target of the saccade changed color during the saccade, the duration of fixations on the model pattern increased, depending on the point in the task that the change was made. Thus different fixations on the same visual stimulus served a different purpose. The results also indicated that the visual information that is retained across successive fixations depends on moment by moment task demands. This is consistent with previous suggestions that visual representations are limited and task dependent. Changes in blocks in addition to the saccade target led to greater increases in fixation duration. This indicated that some global aspect of the pattern was retained across different fixations. Fixation durations revealed effects of the display changes that were not revealed in perceptual report. This can be understood by distinguishing between processes that operate at different levels of description and different time scales. Our conscious experience of the world may reflect events over a longer time scale than those underlying the substructure of the perceptuo-motor machinery.

Lateral interactions in human cone dark adaptation.

1. The course of cone dark adaptation after exposure to a strong bleaching light depends on the size of the bleached region. Threshold for brief, tiny test flash centred in the bleached region is elevated more, and recovery is retarded by a small bleach. This effect has its parallel in the sensitization effect observed with steady backgrounds. 2. Previous results, that a similar sensitization effect is not observed in rod dark adaptation, are confirmed. 3. This sensitization effect in cone dark adaptation does not transfer binocularly, and is unaffected by pressure blinding during the bleaching exposure. 4. Threshold following a small bleach may be lowered by adding a steady annular background to the region surrounding the bleached patch. Conversely, bleaching the area surrounding a small, steady background can lower threshold for a test flash centred on the background. 5. These interactions between backgrounds and bleaches may be explained if bleaches produce long-lasting signals from neurones in the bleached area, which then lead into a spatially opponent stage of processing. 6. It is likely that the persisting signals come from the cone receptors, since the Bunsen-Roscoe Law (intensity-time reciprocity) holds for small bleaches as well as large, for durations up to about 3 sec.

After-effects of small adapting fields.

1. Sensitivity to a small test probe in the centre of a small, steady background is less than when the background is large (sensitization). When an equiluminous steady annulus is added to the region surrounding a small background, rod threshold takes several minutes to stabilize at its new, lower level. The after-effects of the small background follow a time course characteristic of cortical adaptation. 2. The sensitivity loss and time course of recovery after intense bleaching lights in the cone system depend markedly on the size of the retinal region bleached, although no such effect is observed in the rod system. If a steady annular surround is added to the region surrounding the bleached patch, threshold falls rapidly to the value it would have after a large-area bleach of the same intensity. 3. The interaction between bleaches and steady surrounds suggests that bleaches produce long-lasting signals in the cone receptors. 4. The different temporal properties of sensitization on rod backgrounds and sensitization after cone bleaches suggest that different mechanisms underlie the two phenomena. 5. In cone vision, if light is added to the area surrounding a small, steady background, the subsequent readjustment takes minutes to complete, as it does in rod vision. But in addition, for cones, a large proportion of the sensitivity loss caused by the small background can be rapidly restored, as it is with cone bleaches. 6. The above results, together with the known absence of sensitization in rod dark adaptation, are consistent with the hypothesis that sensitization occurs at least partly at the retinal level in the cone system, but not (or only weakly) in the rod system, and that there is an additional, probably cortical elevation, common to rod and cone systems, for small backgrounds, but not for small, brief bleaches.

Disappearance of afterimages at 'impossible' locations in space.

An eccentrically positioned afterimage, viewed in the dark, will disappear if the eye is positioned so that the afterimage now projects to a more extreme location relative to straight ahead. It was found that the afterimage disappeared when it projected to a location which corresponded to the edge of the visual field defined by the brow, cheek, and nose. This suggests that visibility of stimuli from those retinal regions shadowed by the head is influenced by eye-position information.

Reference frames in saccadic targeting.

We attempt to determine the egocentric reference frame used in directing saccades to remembered targets when landmark-based (exocentric) cues are not available. Specifically, we tested whether memory-guided saccades rely on a retina-centered frame, which must account for eye movements that intervene during the memory period (thereby accumulating error) or on a head-centered representation that requires knowledge of the position of the eyes in the head. We also examined the role of an exocentric reference frame in saccadic targeting since it would not need to account for intervening movements. We measured the precision of eye movements made by human observers to target locations held in memory for a few seconds. A variable number of saccades intervened between the visual presentation of a target and a later eye movement to its remembered location. A visual landmark that allowed for exocentric encoding of the memory target appeared in half the trials. Variable error increased slightly with a greater number of intervening saccades. The landmark aided targeting precision, but did not eliminate the increase in variable error with additional intervening saccades. We interpret these results as evidence for a representation that relies on knowledge of eye position with respect to the head and not one that relies solely on updating in a retina-centered frame. Our results allow us to set an upper bound on the standard deviation of an eye position signal available to the saccadic system during short memory periods at 1.4 degrees for saccades of about 10 degrees.