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Biomedical subjects

Alex O Holcombe

Publications and source records attributed to Alex O Holcombe.

9 recordsLinked to original sources
PubMed2005

Time and the brain: how subjective time relates to neural time.

Most of the actions our brains perform on a daily basis, such as perceiving, speaking, and driving a car, require timing on the scale of tens to hundreds of milliseconds. New discoveries in psychophysics, electrophysiology, imaging, and computational modeling are contributing to an emerging picture of how the brain processes, learns, and perceives time.

Animals↗
PubMed2004

Rapid global form binding with loss of associated colors.

Distributed neural processing creates a binding problem: the representations of the various features of an object are distributed across brain areas but must be associated with, or bound to, the same object. Here we determine the limits that binding imposes on the perception of global form in static flow fields defined by oriented dot pairs. The binding of local orientation signals into global form is shown to operate at rapid 20-Hz rates, implying that visual signals at the point of global form extraction retain precise temporal registration. Binding global form with color is limited to rates of 3-5Hz, showing that binding across attributes can impose a severe temporal limit on perception. Judgment of the temporal sequence of the global structures is also limited to slow rates. These results point to a substantial loss of temporal resolution in the visual system following the extraction of global form but preceding visual awareness.

Awareness↗
PubMed2004

Attentional pursuit is faster than attentional saccade.

How quickly can we shift the focus of visual attention? We compared the rates of two types of attentional shifts: attentional saccades (shifts between objects) and attentional pursuit (shifts along with a moving object). Instead of measuring the time required for a single shift, which confounds shift time with cue interpretation time, we measured the pace at which observers could make multiple successive shifts in a predictable order. We find that successive attentional saccades between objects are quite slow (300-500 ms). The object-based theory of attention predicts that attention should shift between locations more quickly when in pursuit of a moving object. Our results support this theory. Attentional pursuit is substantially faster--taking only 200-250 ms to cover the same distance. "Indexing" a moving object (keeping track of one object) can be done at even faster rates, supporting a distinction between attending to and indexing objects.

Attention↗
PubMed2004

Illusory motion reversal is caused by rivalry, not by perceptual snapshots of the visual field.

In stroboscopic conditions--such as motion pictures--rotating objects may appear to rotate in the reverse direction due to under-sampling (aliasing). A seemingly similar phenomenon occurs in constant sunlight, which has been taken as evidence that the visual system processes discrete "snapshots" of the outside world. But if snapshots are indeed taken of the visual field, then when a rotating drum appears to transiently reverse direction, its mirror image should always appeared to reverse direction simultaneously. Contrary to this hypothesis, we found that when observers watched a rotating drum and its mirror image, almost all illusory motion reversals occurred for only one image at a time. This result indicates that the motion reversal illusion cannot be explained by snapshots of the visual field. The same result is found when the two images are presented within one visual hemifield, further ruling out the possibility that discrete sampling of the visual field occurs separately in each hemisphere. The frequency distribution of illusory reversal durations approximates a gamma distribution, suggesting perceptual rivalry as a better explanation for illusory motion reversal. After adaptation of motion detectors coding for the correct direction, the activity of motion-sensitive neurons coding for motion in the reverse direction may intermittently become dominant and drive the perception of motion.

Humans↗
PubMed2004

Repetition priming in visual search: episodic retrieval, not feature priming.

Previous research has shown that when the targets of successive visual searches have features in common, response times are shorter. However, the nature of the representation underlying this priming and how priming is affected by the task remain uncertain. In four experiments, subjects searched for an odd-sized target and reported its orientation. The color of the items was irrelevant to the task. When target size was repeated from the previous trial, repetition of target color speeded the response. However, when target size was different from that in the previous trial, repetition of target color slowed responses, rather than speeding them. Our results suggest that these priming phenomena reflect the same automatic mechanism as the priming of pop-out reported by Maljkovic and Nakayama (1994). However, the crossover interaction between repetition of one feature and another rules out Maljkovic and Nakayama's (1994) theory of independent potentiation of distinct feature representations. Instead, we suggest that the priming pattern results from contact with an episodic memory representation of the previous trial.

Association Learning↗
PubMed2003

Occlusion cues resolve sudden onsets into morphing or line motion, disocclusion, and sudden materialization.

An abrupt appearance of a new stimulus, or sudden onset, has several possible perceptual interpretations. The change may reflect an object new to the scene or instead be caused by disocclusion of a pre-existing object. Alternatively, the sudden onset may be interpreted as the morphing of a pre-existing figure (as in "line motion"). Previous work has focused on the morphing percept to the exclusion of other interpretations of sudden onsets. This paper supports the idea that morphing, and the other interpretations of sudden onsets, reflect occlusion cues indicating the most likely cause of the stimulus. Consider a line segment that appears abruptly. The data herein show that when the segment has already been represented as present in the scene(via amodal completion), its onset is likely to be perceived as a disocclusion event, with no appearance of morphing. Even when individual frames do not support amodal completion, dynamic (although motionless) cues can favor the disocclusion interpretation, again vetoing the perception of line motion. Some final demonstrations address sudden materialization, in which previously unseen objects suddenly appear. Again there is ambiguity in that sudden materialization and disocclusion can be caused by image changes that are locally identical. Remote cues to occlusion are shown to give these stimuli distinct appearances. The existence of these ambiguities, and the role of occlusion cues in resolving them, has implications for theories of motion perception and attentional capture.

Attention↗