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Biomedical subjects

B Bridgeman

Publications and source records attributed to B Bridgeman.

At least 19 recordsLinked to original sources
PubMed1991

Flicker distorts visual space constancy.

Effects of flicker on space perception were measured by displacing a flickering target during saccadic eye movements. A small target was flickered at 33, 66, 130 or 260 Hz. Using a 2-interval forced-choice design, sensitivity to the displacement was about twice as great when the target was moved in the direction opposite the eye movement as when it was moved in the same direction. This would be expected from a partial breakdown of space constancy--the world should seem to jump in the direction opposite an eye movement. Even if a suppression of displacement detection during saccades prevents this jump from being perceived; it should be easier to detect a target displacement in the direction opposite the eye movement than in the same direction: when movement is opposite, the imposed displacement adds to the illusory displacement, making detection easier. Displacements were more easily detected at lower flicker rates. Results imply that both masking and extraretinal signals are important in suppressing the detectability of target displacements during saccades, and that flicker on video display terminals may distort space perception.

Humans↗
PubMed1991

Ocular proprioception and efference copy in registering visual direction.

We measured the roles of eye muscle proprioception ("inflow") and efference copy ("outflow") in registering eye position. During monocular fixation, pressing on the side of an occluded eye results in a passive rotation, changing the proprioception without affecting oculomotor efference. As we have shown previously, a constant press on the side of the viewing eye induces active resistance to rotation, changing efference because oculomotor innervation compensates for the eyepress; the viewing eye's fixation remains constant. Using these two types of eyepress, both perceived target deviations and pointing biases in an unstructured visual field were measured in 8 subjects under efference copy, proprioception and control (no eyepress) conditions. Eye deviation was measured photoelectrically. Physiological gains of efference copy and proprioception was about 5/8 and 1/4 respectively. There was no statistically significant difference between perceptual judgement and open-loop pointing. The sum of gains of efference copy and proprioception, about 7/8, indicates incomplete registration of eye eccentricity in an unstructured field, and quantitatively accounts for several previously unexplained results in the literature.

Adult↗
PubMed1990

Saccadic suppression of displacement is strongest in central vision.

Perception of target displacement is severely degraded if the displacement occurs during a saccadic eye movement, but the variation of this effect across the visual field is unknown. A small target was displaced from a starting point at the midline, or 10 deg to the right or left, while the eye made a saccade from the 10 deg right position to the 10 deg left position. Saccades were detected and the target displaced on line. Assessed with a signal detection measure, suppression was stronger in central vision than in more peripheral locations for all three subjects. Leftward and rightward displacements yielded equal thresholds. The results complement the findings of others to reveal a picture of perceptual events during saccades, with both deeper saccadic suppression and faster correction of spatial values (the correspondences between retinal position and perceived egocentric direction), favouring more accurate spatial processing in central vision than in the periphery.

Adult↗
PubMed1989

The physiology of attention: participation of cat striate cortex in behavioral choice.

In the awake, behaving cat, we have compared response of striate cortex neurons to informative and to noninformative stimuli. A cat pressed a pedal in response to a flashed pattern repeated every 10 s, receiving a reward if the press was within a 0.5-1.5-s post-stimulus window. There were three trial types: 50% of the trials had only the initial informative flash, 25% had an additional physically identical, but unrewarded, flash 500 ms after the first, and 25% had an unrewarded flash 3-5 s after the informative flash. Cats learned to respond only to the rewarded flashes. Neurons were divided into two categories: 27 neurons defined as "primary" showed an early burst of firing 30-70 ms after stimulus onset, and 17 did not. The distinction was arbitrary, since all cells were exposed to the same stimulus. For stimuli preceding a pedal press, stimulus-synchronized histograms of primary neurons had a smaller early burst and more firing before and after it. Response-synchronized histograms showed an abrupt decrease in firing shortly before the pedal press. The effects were stronger for primary cells in the stimulus-synchronized data and stronger for non-primary cells in the response-synchronized data.

Animals↗
PubMed1989

Influence of mechanical disturbance on oculomotor behavior.

Using a binocular search coil technique, we measured oculomotor behavior during gentle pressing with a finger on the outer canthus of one eye. With a steadily pressed viewing eye, and the fellow eye occluded, the occluded eye deviates while the pressed eye does not. If the eye is rapidly pressed and released, the compensation of rotational force in the pressed eye becomes incomplete, so that both eyes move. At high frequencies of press (greater than 1 Hz) the pressed eye is deviated and the contralateral eye no longer moves. In darkness the pressed eye always rotates but the contralateral eye never does, demonstrating that any inflow from proprioceptors sensing rotation of the pressed eye does not affect oculomotor posture as measured in the fellow eye. With binocular viewing the results are more variable. On some trials neither eye moves, while on others both move. The results can be interpreted as a Hering's law controlled attempting to reconcile disparate inputs from the two eyes. The results confirm and extend, with objective measures, earlier conclusions from subjective experiments.

Darkness↗
PubMed1989

Effect of context and efference copy on visual straight ahead.

Bias in efferent commands to the eye changes the apparent straight ahead direction in an unstructured visual field, but has little effect in a normal visual environment. Naive subjects set a visible marker to appear straight ahead under monocular viewing conditions and while pressing on the viewing eye. Three background conditions were used: a naturalistic landscape photograph, a blank field, and a repeating checkerboard texture that provides strong contours but no information about visual direction. Effect of eyepress on straight-ahead judgments was small but significant with the landscape background, and larger with the blank field; the checkerboard texture yielded a bias halfway between the magnitudes of bias in the other two conditions. A visual capture theory predicts that the textured field should work like a blank one, while an oculomotor theory predicts that it should work like a natural one. Interpreted in this context, the results show the two theories to be about equally important in judging straight ahead. A second experiment with experienced observers and moving backgrounds gave the same result.

Eye Movements↗
PubMed1989

The psychophysics of the pursuit oculomotor system.

When a fixation point moves under a row of identical targets at a speed of one target for each flash of a strobe, smooth apparent movement of the targets is seen (the "picket-fence illusion"). When the fixation point is removed, the eye continues to pursue the apparent target movement. Pursuit continues through small changes in target configuration, but is interrupted by a change to a very dissimilar target (such as 1 vs. x) in the middle of a row. This new method, the "pursuit-interruption method," showed that large differences in the number of pixels in a line did not interrupt tracking if the end points of the line were preserved. Pursuit interruption by changes in line orientation (such as /vs./) corresponded to the orientation bandwidth of orientation-sensitive cortical neurons. The maximum number of consecutive missing targets that does not interrupt pursuit depends on frequency of target presentation as well as on parameters of the pursuit system.

Attention↗
PubMed1987

Oculomotor and skeletal motor systems share one map of visual space.

Previous research has shown that cognitive and motor visual systems have separate maps of visual space because spatial values in them can be manipulated independently. Here we ask whether the motor map in turn can be divided into separate representations of space. Forty large saccadic eye movements in darkness resulted in significant ocular undershoot of a remembered eccentric target location, and a comparable amount of undershoot with manual pointing to the same remembered target. Saccadic and manual measures were also highly correlated within sessions as well as between sessions. Because pointing could be changed by manipulating only saccades, we conclude that the two systems share a single map of space.

Eye Movements↗
PubMed1979

Interaction of stimulus size and retinal eccentricity in metacontrast masking.

Metacontrast masking occurs both at the fovea and in the retinal periphery; foveally, the smallest stimulus elicited the strongest masking, whereas peripherally the reverse was the case. An analysis of variance showed a significant size effect, eccentricity effect, and size-eccentricity interaction. As stimulus size increased, the stimulus onset asynchrony of maximum masking shifted to greater values. Both foveal metacontrast and peak shifts contradicted predictions made by the hypothesis that metacontrast is mediated by an interaction of sustained and transient channels in the visual system. The data are consistent, however, with a lateral inhibitory model of metacontrast masking and stimulus coding.

Discrimination Learning↗
PubMed1979

Relation between cognitive and motor-oriented systems of visual position perception.

Although subjects failed to detect a target displacement if it occurred near the time of a saccadic eye movement (a cognitive visual task), they were still able to point to the center of the target with an unseen pointer (a motor visual task). Pointing performance was not affected by detecting or failing to detect a stimulus displacement. The experiments demonstrate that some information that is available to a motor-oriented visual system is unavailable to the cognitive visual system, under conditions simulating normal perception.

Cognition↗