Biomedical subjects

M M Hayhoe

Publications and source records attributed to M M Hayhoe.

At least 19 recordsLinked to original sources

PubMed2001

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.

Attention↗

PubMed2000

Spatio-temporal organization of behavior.

One of the most widely used terms in the study of human performance is attention. Yet it can also be argued that it is one of the most confusing and misunderstood. Huge variations in performance, from not noticing large changes in images or natural situations, to differences of tens of milliseconds have all been described as attentional effects. We argue that the large disparity in results can be more easily understood in the context of a fairly complete model of human performance that describes the execution of a set of complex natural tasks via a collection of visual routines that extract crucial information from the optical array. The description of visual routines is hierarchical. At the most abstract level, a scheduler must pick a small set of programs for the current tasks. Each program contains steps which are keyed to information in the scene. This is extracted by visual routines which run during a single fixation and extract pertinent information. The library of routines themselves are designed to execute quickly, but their actual performance depends on signal-to-noise characteristics of the imaged scene. The hierarchical description of behavior shows that questions about attention make sense in the context of the descriptive level in which they are embedded. We illustrate these principles with examples of driving behaviors.

Attention↗

PubMed1998

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.

Color Perception↗

PubMed1997

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.

Adult↗

PubMed1997

Deictic codes for the embodiment of cognition.

To describe phenomena that occur at different time scales, computational models of the brain must incorporate different levels of abstraction. At time scales of approximately 1/3 of a second, orienting movements of the body play a crucial role in cognition and form a useful computational level--more abstract than that used to capture natural phenomena but less abstract than what is traditionally used to study high-level cognitive processes such as reasoning. At this "embodiment level," the constraints of the physical system determine the nature of cognitive operations. The key synergy is that at time scales of about 1/3 of a second, the natural sequentiality of body movements can be matched to the natural computational economies of sequential decision systems through a system of implicit reference called deictic in which pointing movements are used to bind objects in the world to cognitive programs. This target article focuses on how deictic binding make it possible to perform natural tasks. Deictic computation provides a mechanism for representing the essential features that link external sensory data with internal cognitive programs and motor actions. One of the central features of cognition, working memory, can be related to moment-by-moment dispositions of body features such as eye movements and hand movements.

Brain↗

PubMed1996

Goal-directed arm movements change eye-head coordination.

We compared the head movements accompanying gaze shifts while our subjects executed different manual operations, requiring gaze shifts of about 30 degrees. The different tasks yielded different latencies between gaze shifts and hand movements, and different maximum velocities of the hand. These changes in eye-hand coordination had a clear effect on eye-head coordination: the latencies and maximum velocities of head and hand were correlated. The same correlation between movements of the head and hand was also found within a task. Therefore, the changes in eye-head coordination are not caused by changes in the strategy of the subjects. We conclude that head movements and saccades during gaze shifts are not based on the same command: head movements depend both on the actual saccade and on possible future gaze shifts.

Arm↗

PubMed1995

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)

Afterimage↗

PubMed1993

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.

Cues↗

PubMed1992

Hand-eye coordination during sequential tasks.

The small angle subtended by the human fovea places a premium on the ability to quickly and accurately direct the gaze to targets of interest. Thus the resultant saccadic eye fixations are a very instructive behaviour, revealing much about the underlying cognitive mechanisms that guide them. Of particular interest are the eye fixations used in hand-eye coordination. Such coordination has been extensively studied for single movements from a source location to a target location. In contrast, we have studied multiple fixations where the sources and targets are a function of a task and chosen dynamically by the subject according to task requirements. The task chosen is a copying task: subjects must copy a figure made up of contiguous coloured blocks as fast as possible. The main observation is that although eye fixations are used for the terminal phase of hand movements, they are used for other tasks before and after that phase. The analysis of the spatial and temporal details of these fixations suggests that the underlying decision process that moves the eyes leaves key decisions until just before they are required.

Eye Movements↗

PubMed1992

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.

Adaptation, Ocular↗

PubMed1990

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.

Adaptation, Ocular↗

PubMed1990

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.

Accommodation, Ocular↗

PubMed1990

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.

Adaptation, Ocular↗

PubMed1989

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.

Adaptation, Ocular↗

PubMed1987

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.

Adaptation, Ocular↗

PubMed1986

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.

Dark Adaptation↗

PubMed1984

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.

Afterimage↗

PubMed1982

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.

Fixation, Ocular↗