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David A Leopold

Publications and source records attributed to David A Leopold.

9 recordsLinked to original sources
PubMed2003

Generalized flash suppression of salient visual targets.

A pattern of light striking the retina of an alert observer is normally readily perceived. While a handful of conditions exist in which even salient visual stimuli can be rendered invisible, the mechanisms underlying such suppression remain poorly understood. Here, we describe experiments using a novel stimulation sequence that gives rise to the sudden and reliable subjective disappearance of a wide range of visual patterns. We found that a parafoveal target immediately vanished from perception following the abrupt onset of a surrounding texture. The probability of disappearance was influenced by the ocular configuration of the target and surround, as well as their spatial separation. In addition, suppression was critically dependent upon several hundred milliseconds of stimulus-specific adaptation. These findings demonstrate that the all-or-none disappearance of a salient visual target, which is reminiscent of a high-level selection process, is inextricably linked to topographic stimulus representations, presumably in the early visual cortex.

Adult↗
PubMed2003

Perception of temporally interleaved ambiguous patterns.

BACKGROUND: Continuous viewing of ambiguous patterns is characterized by wavering perception that alternates between two or more equally valid visual solutions. However, when such patterns are viewed intermittently, either by repetitive presentation or by periodic closing of the eyes, perception can become locked or "frozen" in one configuration for several minutes at a time. One aspect of this stabilization is the possible existence of a perceptual memory that persists during periods in which the ambiguous stimulus is absent. Here, we use a novel paradigm of temporally interleaved ambiguous stimuli to explore the nature of this memory, with particular regard to its potential impact on perceptual organization. RESULTS: We found that the persistence of a perceptual configuration was robust to interposed visual patterns, and, further, that at least three ambiguous patterns, when interleaved in time, could undergo parallel, stable time courses. Then, using an interleaved presentation paradigm, we established that the occasional reversal in one pattern could be coupled with that of its interleaved counterpart, and that this coupling was a function of the structural similarity between the patterns. CONCLUSIONS: We postulate that the stabilization observed with repetitive presentation of ambiguous patterns can be at least partially accounted for by processes that retain a recent perceptual interpretation, and we speculate that such memory may be important in natural vision. We further propose that the interleaved paradigm introduced here may be of great value to gauge aspects of stimulus similarity that appeal to particular mechanisms of perceptual organization.

Adolescent↗
PubMed2003

Visual perception: shaping what we see.

A new psychophysical study has examined the free flow of perception as observers viewed stimuli with several possible visual interpretations. The results suggest that our subjective impression of such ambiguous patterns may be more closely linked to the brain's encoding of complex shape than previously appreciated.

Form Perception↗
PubMed2003

Very slow activity fluctuations in monkey visual cortex: implications for functional brain imaging.

We examined fluctuations in band-limited power (BLP) of local field potential (LFP) signals recorded from multiple electrodes in visual cortex of the monkey during different behavioral states. We asked whether such signals demonstrated coherent fluctuations over time-scales of seconds and minutes, and would thus serve as good candidates for direct comparison with data obtained from functional magnetic resonance imaging (fMRI). We obtained the following results. (i) The BLP of the local field displayed fluctuations at many time-scales, with particularly large amplitude at very low frequencies (<0.1 Hz). (ii) These fluctuations exhibited high coherence between electrode pairs, particularly for BLP signals derived from the gamma frequency range. (iii) Coherence in the BLP, unlike that in the raw LFP, did not fall off sharply as a function of cortical distance. (iv) The structure and coherence of BLP changes were highly similar under distinctly different behavioral states. These results demonstrate the existence of widespread coherent activity fluctuations in the brain of the awake monkey over very long time-scales. We propose that such signals may make a significant contribution to the high variability observed in the time course of physiological signals, including those measured with functional imaging techniques. The results are discussed in the context of combined fMRI/electrophysiological recordings.

Animals↗
PubMed2003

Spatial patterns of spontaneous local field activity in the monkey visual cortex.

Spontaneous activity among visually responsive neurons is often considered to consist of random neural events, or perhaps to reflect an irrelevant by-product of brain homeostasis. However, recent studies have emphasized that such ongoing activity is strongly synchronized over large cortical distances, and can have a marked impact on the responsiveness of neurons to visual stimuli, suggesting that such activity may indeed be highly relevant to the brain's interpretation of its sensory input. In the current study, we examined the spatiotemporal nature of local field potential (LFP) fluctuations in the visual cortex of two macaque monkeys that were awake, but in a state of relaxation with minimal visual stimulation. Using an array of 16 electrodes spaced by several millimeters, we simultaneously monitored the LFP at many sites over a large region of the visual cortex. In agreement with the literature, we found that the coherence in the raw LFP signal fell off quickly with both frequency and distance. However, when we examined slower fluctuations in the LFP power, we found that power signals, including those derived from the high y-range frequencies, had high coherence that fell off only very slowly with cortical distance. Finally, we performed an additional experiment, with several electrodes placed on either side of a sulcus, to demonstrate that the decline in local field synchrony with cortical distance was so reliable that the interruption in the cortical sheet corresponding to the opening of the sulcus could be easily identified by monitoring just a few minutes of spontaneous LFP activity. These experiments reveal that a significant portion of spontaneous LFP fluctuations in the visual cortex is contributed by global mechanisms, imposing synchrony that is, first and foremost, a function of cortical separation between any two points.

Animals↗
PubMed2002

Visual neurophysiology: recordings from the human primate.

Neurophysiological studies in human patients, with experiments of the kind traditionally reserved for monkeys, are beginning to provide valuable insight into the workings of the brain. Taking center stage is the question of which neurons lie at the heart of perception itself.

Amygdala↗
PubMed2002

Visual processing in the ketamine-anesthetized monkey. Optokinetic and blood oxygenation level-dependent responses.

We used optokinetic responses and functional magnetic resonance imaging (fMRI) to examine visual processing in monkeys whose conscious state was modulated by low doses (1-2 mg/kg) of the dissociative anesthetic ketamine. We found that, despite the animal's dissociated state and despite specific influences of ketamine on the oculomotor system, optokinetic nystagmus (OKN) could be reliably elicited with large, moving visual patterns. Responses were horizontally bidirectional for monocular stimulation, indicating that ketamine did not eliminate cortical processing of the motion stimulus. Also, results from fMRI directly demonstrated that the cortical blood oxygenation level-dependent (BOLD) response to visual patterns was preserved at the same ketamine doses used to elicit OKN. Finally, in the ketamine-anesthetized state, perceptually bistable motion stimuli produced patterns of spontaneously alternating OKN that normally would be tightly coupled to perceptual changes. These results, taken together, demonstrate that after ketamine administration cortical circuits continue to processes visual patterns in a dose-dependent manner despite the animal's behavioral dissociation. While perceptual experience is difficult to evaluate under these conditions, oculomotor patterns revealed that the brain not only registers but also acts upon its sensory input, employing it to drive a sensorimotor loop and even responding to a sensory conflict by engaging in spontaneous perception-related state changes. The ketamine-anesthetized monkey preparation thereby offers a safe and viable paradigm for the behavioral and electrophysiological investigation of issues related to conscious perception and anesthesia, as well as neural mechanisms of basic sensory processing.

Anesthetics, Dissociative↗
PubMed2002

Stable perception of visually ambiguous patterns.

During the viewing of certain patterns, widely known as ambiguous or puzzle figures, perception lapses into a sequence of spontaneous alternations, switching every few seconds between two or more visual interpretations of the stimulus. Although their nature and origin remain topics of debate, these stochastic switches are generally thought to be the automatic and inevitable consequence of viewing a pattern without a unique solution. We report here that in humans such perceptual alternations can be slowed, and even brought to a standstill, if the visual stimulus is periodically removed from view. We also show, with a visual illusion, that this stabilizing effect hinges on perceptual disappearance rather than on actual removal of the stimulus. These findings indicate that uninterrupted subjective perception of an ambiguous pattern is required for the initiation of the brain-state changes underlying multistable vision.

Adolescent↗