Biomedical subjects

Bruce Bridgeman

Publications and source records attributed to Bruce Bridgeman.

11 recordsLinked to original sources

PubMed2006

Contributions of lateral inhibition to object substitution masking and attention.

Lateral inhibition, the inhibition of neurons by other neurons at the same level, exists at several levels of the visual system. Implications of lateral inhibition for sensory coding and perception have been investigated with a mathematical model that accounts for many properties of metacontrast masking and brief storage of sensory information. Here that model simulates object substitution masking, where a target and mask appear simultaneously but the mask disappears after a variable delay. The target becomes strongly masked if the mask offset is delayed after target offset, and target visibility does not recover with longer mask offset delays. Object substitution masking is most effective if attention is diverted by the presence of many simultaneous masks, only one of which surrounds a target. The lateral inhibitory model reproduces the effects of attention on object substitution masking by exploiting the longer latency of response to unattended stimuli. Decreasing the interval over which sensory codes are analyzed, reflecting the shorter latency of response to attended stimuli, weakens the masking in a way that reflects the psychophysical effects of attention.

Attention↗

PubMed2006

Efference copy and its limitations.

Efference copy, an internal brain signal informing the visual system of commands to move the eye, was the dominant explanation for visual space constancy for over a century. The explanation is not viable, however; the signal is to small, to slow, and too unreliable to support the perception of perfect constancy. Newer theories recognize that detailed image information does not survive refixation in any case. Efference copy is a viable explanation of static position perception and sensorimotor interaction, but the rich, stable visual world is an illusion.

Eye Movements↗

PubMed2005

Change perception using visual transients: object substitution and deletion.

In three experiments we studied change detection and identification when no extraneous transients were present in the image at the time of change. Each image consisted of 12 different objects, sorted by color into three different levels of probability of change. In Experiment 1, change of one object was detected and identified frequently in objects having the highest probability of change (central interest), which we hypothesize were mainly visited by attention. Changes in other objects with a lower probability of change (marginal interest), however, although detected efficiently were unlikely to be identified. Identification improved for less attended objects if the changed stimulus simply disappeared, allowing visual persistence to hold information about the object until attention could be shifted to it (Experiment 2). Contrary to previous findings showing that response times (RTs) for luminance change detection in a multi-element display are not altered by attention, we found changes in objects of central interest to be detected faster than in objects of marginal interest when objects' identity was to be held in working memory. However, no differences in RTs emerged in the same change detection task when objects' identity was not stored in working memory (Experiment 3).

Adult↗

PubMed2005

Influence of visually induced expectation on perceived motor effort: a visual-proprioceptive interaction at the Santa Cruz Mystery Spot.

It is known that dense objects seem heavier than larger, less dense objects of the same weight. We have investigated a related illusion, in which visual context biases the apparent weight of a single object. The apparatus is a cabin on a steep hillside near Santa Cruz, CA, tilted 17 degrees from vertical. From its ceiling hangs a weight on a chain. The cabin's tilt makes the weight appear suspended at an angle. Pushing the weight toward the visually based vertical is perceived as difficult, whereas pushing it away from the visual vertical is perceived as easy. Seven subjects pushed the weight in both directions, judging required effort on a double-anchored 1-10 scale. All experienced the effort illusion, with no significant subject effect. When subjects' eyes were closed, the effect was smaller but still present. Apparently proprioceptive and skin inputs, equal for both directions, are ignored or underweighted as visually based expectations influence perceived effort.

Humans↗

PubMed2004

The induced Roelofs effect: two visual systems or the shift of a single reference frame?

Cognitive judgments about an object's location are distorted by the presence of a large frame offset left or right of an observer's midline. Sensorimotor responses, however, seem immune to this induced Roelofs illusion, with observers able to accurately point to the target's location. These findings have traditionally been used as evidence for a dissociation of the visual processing required for cognitive judgments and sensorimotor responses. However, a recent alternative hypothesis suggests that the behavioral dissociation is expected if the visual system uses a single frame of reference whose origin (the apparent midline) is biased toward the offset frame. The two theories make qualitatively distinct predictions in a paradigm in which observers are asked to indicate the direction symmetrically opposite the target's position. The collaborative findings of two laboratories clearly support the biased-midline hypothesis.

Adult↗

PubMed2004

Decoupling stimulus duration from brightness in metacontrast masking: data and models.

A brief target that is visible when displayed alone can be rendered invisible by a trailing stimulus (metacontrast masking). It has been difficult to determine the temporal dynamics of masking to date because increments in stimulus duration have been invariably confounded with apparent brightness (Bloch's law). In the research reported here, stimulus luminance was adjusted to maintain constant brightness across all durations. Increasing target duration yielded classical U-shaped masking functions, whereas increasing mask duration yielded monotonic decreasing functions. These results are compared with predictions from 6 theoretical models, with the lateral inhibition model providing the best overall fit. It is tentatively suggested that different underlying mechanisms may mediate the U-shaped and monotonic functions obtained with increasing durations of target and mask, respectively.

Cerebral Cortex↗

PubMed2004

Different effects of eyelid blinks and target blanking on saccadic suppression of displacement.

Displacements of visual stimuli during saccadic eye movements are often not noticed. We have demonstrated that saccadic suppression of image displacement can be eliminated by blanking the stimulus for a short period during and after the saccade (Deubel, Schneider, & Bridgeman, 1996). Here we report an experiment in which target visibility was interrupted after the saccade, either by distal target blanking or by voluntary eyeblink. The data show that the effect of blinking is different from blanking; interruption of vision due to a blink did not enable subjects to detect target displacements any better than they had done in the no-blank condition. The results provide evidence for an extraretinal signal that distinguishes between endogenous and exogenous sources of temporary object disappearance after the saccade.

Adult↗

PubMed2003

Space-independent modality-driven attentional capture in auditory, tactile and visual systems.

Extending previous evidence for attentional shifts across auditory and visual modalities without the confound of the two modalities originating at different locations (Turatto et al. 2002), we investigated attention shifts between auditory and tactile modalities, and between tactile and visual modalities. Two stimuli (S1 and S2), either in the same or in different modalities, were delivered from the same spatial source and were separated by a variable temporal gap. S1 was task irrelevant, whereas S2 required a speeded discrimination. Results showed that modality switching is detrimental independently of the stimulated modality as long as the temporal lag between S1 and S2 is short enough that there is not time to switch attention before S2 is delivered. We observed automatic, modality-driven, attentional capture, with ipsimodal trials leading to faster response times than crossmodal trials. The present results cannot be accounted for by spatial artifacts, response priming or criterion shifts, and are interpreted as the consequence of a space-independent attentional shift across sensory modalities.

Acoustic Stimulation↗

PubMed2002

Transsaccadic memory of position and form.

Why and how people perceive the visual world as continuous and stable, despite the gross changes of its retinal projection that occur with each saccade, is one of the classic problems in perception. In the present paper, we argue that an important factor of visual stability and transsaccadic perception is formed by the reafferent visual information, i.e., the visual display that is present when the eyes land. After a review of some of the relevant theoretical, behavioural and physiological research on space constancy, saccadic suppression and transsaccadic memory, three experiments are presented. In a first experiment, we study the effect of an extended horizontal bar covering the target area for a short period after the saccade on saccadic suppression of image displacement. The results show that the bar acts just like a temporary blanking of the saccade target, leading to a strong reduction of saccadic suppression. In the second experiment, we show that any object that is present immediately after the saccade can establish a spatial reference, even if it is dissimilar to the saccade target. In a third experiment we study, with a similar approach, the effect of blanking and postsaccadic information on transsaccadic integration of form information. The data demonstrate that a localized postsaccadic object tends to replace the content of transsaccadic memory.

Form Perception↗

PubMed2001

An unbiased measure of the contributions of chroma and luminance to saccadic suppression of displacement.

Perception of image displacement is suppressed during saccadic eye movements. We probed the source of saccadic suppression of displacement by testing whether it selectively affects chromatic- or luminance-based motion information. Human subjects viewed a stimulus in which chromatic and luminance cues provided conflicting information about displacement direction. Apparent motion occurred during either fixation or a 19.5 degree saccade. Subjects detected motion and discriminated displacement direction in each trial. They reported motion in over 90% of fixation trials and over 70% of saccade trials. During fixation, the probability of perceiving the direction carried by chromatic cues decreased as luminance contrast increased. During saccades, subjects tended to perceive the direction indicated by luminance cues when luminance contrast was high. However, when luminance contrast was low, subjects showed no preference for the chromatic- or luminance-based direction. Thus magnocellular channels are suppressed, while stimulation of parvocellular channels is below threshold, so that neither channel drives motion perception during saccades. These results confirm that magnocellular inhibition is the source of saccadic suppression.

Adult↗

PubMed2001

A Comparison of Two Lateral Inhibitory Models of Metacontrast.

Metacontrast, an apparent reduction in brightness of a target that is followed by a non-overlapping mask, has been modeled with simulated neural nets incorporating either recurrent lateral inhibition or forward and backward inhibition with lateral components. A one-layer lateral inhibitory model (B. Bridgeman, 1971, Psychological Review 78, 528-539) and a six-layer model (G. Francis, 1997, Psychological Review 104, 572-594) both simulate the basic metacontrast effect, showing that stimulus-dependent activity that reverberates for some time in the model after stimulus offset is essential to simulate metacontrast. The six-layer model does not simulate monotonic masking with low response criterion, an essential property of metacontrast; the lateral inhibitory model uses duration of reverberation to simulate the criterion. Each model simulates several variations of masking, such as changing the relative energy of target and mask, but neither can handle effects of practice or attention that apparently engage higher processing levels. Copyright 2001 Academic Press.

Journal Article↗