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Results for “VISUAL PERCEPTION”

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At least 397 records · Page 22Linked to original sources
PubMed2005

Two reference frames for visual perception in two gravity conditions.

The processing and storage of visual information concerning the orientation of objects in space is carried out in anisotropic reference frames in which all orientations are not treated equally. The perceptual anisotropies, and the implicit reference frames that they define, are evidenced by the observation of 'oblique effects' in which performance on a given perceptual task is better for horizontally and vertically oriented stimuli. The question remains how the preferred horizontal and vertical reference frames are defined. In these experiments cosmonaut subjects reproduced the remembered orientation of a visual stimulus in 1g (on the ground) and in 0g, both attached to a chair and while free-floating within the International Space Station. Results show that while the remembered orientation of a visual stimulus may be stored in a multimodal reference frame that includes gravity, an egocentric reference is sufficient to elicit the oblique effect when all gravitational and haptic cues are absent.

Analysis of Variance↗
PubMed2003

Explicit neural representations, recursive neural networks and conscious visual perception.

The fundamental question as to whether the neural correlates of any given conscious visual experience are expressed locally within a given cortical area or more globally within some widely distributed network remains unresolved. We inquire as to whether recursive processing-by which we mean the combined flow and integrated outcome of afferent and recurrent activity across a series of cortical areas-is essential for the emergence of conscious visual experience. If so, we further inquire as to whether such recursive processing is essential only for loops between extrastriate cortical areas explicitly representing experiences such as color or motion back to V1 or whether it is processing between still higher levels and the areas computing such explicit representations that is exclusively or additionally essential for visual experience. If recursive processing is not essential for the emergence of conscious visual experience, then it should also be possible to determine whether it is only the intracortical sensory processing within areas computing explicit sensory representations that is required for perceptual experience or whether it is the subsequent processing of the output of such areas within more anterior cortical regions that engenders perception. The present analysis suggests that the questions posed here may ultimately become experimentally resolvable. Whatever the outcome, the results will likely open new approaches to identify the neural correlates of conscious visual perception.

Animals↗
PubMed2006

An fMRI study of the neural correlates of graded visual perception.

The neural correlates of clearly perceived visual stimuli have been reported previously in contrast to unperceived stimuli, but it is uncertain whether intermediate or graded perceptual experiences correlate with different patterns of neural activity. In this study, the subjective appearance of briefly presented visual stimuli was rated individually by subjects with respect to perceptual clarity: clear, vague or no experience of a stimulus. Reports of clear experiences correlated with activation in a widespread network of brain areas, including parietal cortex, prefrontal cortex, premotor cortex, supplementary motor areas, insula and thalamus. The reports of graded perceptual clarity were reflected in graded neural activity in a network comprising the precentral gyrus, intraparietal sulcus, basal ganglia and the insula. In addition, the reports of vague experiences demonstrated unique patterns of activation. Different degrees of perceptual clarity were reflected both in the degree to which activation was found within parts of the network serving a clear conscious percept, and additional unique activation patterns for different degrees of perceptual clarity. Our findings support theories proposing the involvement of a widespread network of brain areas during conscious perception.

Adult↗
PubMed2005

Neural impact of the semantic content of visual mental images and visual percepts.

The existence of hemispheric lateralization of visual mental imagery remains controversial. In light of the literature, we used fMRI to test whether processing of mental images of object drawings preferentially engages the left hemisphere to compared non-object drawings. An equivalent comparison was also made while participants actually perceived object and non-object drawings. Although these two conditions engaged both hemispheres, activation was significantly stronger in the left occipito-temporo-frontal network during mental inspection of object than of non-object drawings. This network was also activated when perception of object drawings was compared to that of non-object drawings. An interaction was nonetheless observed: this effect was stronger during imagery than during perception in the left inferior frontal and the left inferior temporal gyrus. Although the tasks subjects performed did not explicitly require semantic analysis, activation of this network probably reflected, at least in part, a semantic and possibly a verbal retrieval component when object drawings were processed. Mental imagery tasks elicited activation of early visual cortex at a lower level than perception tasks. In the context of the imagery debate, these findings indicate that, as previously suggested, figurative imagery could involve primary visual cortex and adjacent areas.

Adolescent↗
PubMed2005

Visual perception of touchdown point during simulated landing.

Experiments examined the accuracy of visual touchdown point perception during oblique descents (1.5 degrees -15 degrees ) toward a ground plane consisting of (a) randomly positioned dots, (b) a runway outline, or (c) a grid. Participants judged whether the perceived touchdown point was above or below a probe that appeared at a random position following each display. Although judgments were unacceptably imprecise and biased for moving dot and runway displays, accurate and unbiased judgments were found for grid displays. It is concluded that optic flow per se does not appear to be sufficient for a pilot to land an airplane and that the systematic errors associated with optic flow under sparse conditions may be responsible for the common occurrence of landing incidents in so-called "black hole" situations.

Adult↗
PubMed1993

Unmasking human visual perception with the magnetic coil and its relationship to hemispheric asymmetry.

Visual suppression by a magnetic coil (MC) pulse delivered over human calcarine cortex after a transient visual stimulus 80-100 ms earlier has been used to suppress the representation of a 'masking' visual stimulus and thus to unmask a 'target' visual stimulus given, e.g., 100 ms before the mask. The resulting target unmasking as a function of the interval between mask and MC pulse is approximately the inverse of the visual suppression curve. Arbitrary visual linear patterns can similarly be unmasked. At the long target-mask interval used, the site of masking is deduced to lie beyond calcarine cortex. In several right-handed subjects tested, powerful MC stimulation of the left (but not right) temporo-parieto-occipital cortex also led to (weaker) unmasking.

Functional Laterality↗
PubMed2001

Linking the laminar circuits of visual cortex to visual perception: development, grouping, and attention.

How do the laminar circuits of visual cortical areas V1 and V2 implement context-sensitive binding processes such as perceptual grouping and attention, and how do these circuits develop and learn in a stable way? Recent neural models clarify how preattentive and attentive perceptual mechanisms are intimately linked within the laminar circuits of visual cortex, notably how bottom-up, top-down, and horizontal cortical connections interact within the cortical layers. These laminar circuits allow the responses of visual cortical neurons to be influenced, not only by the stimuli within their classical receptive fields, but also by stimuli in the extra-classical surround. Such context-sensitive visual processing can greatly enhance the analysis of visual scenes, especially those containing targets that are low contrast, partially occluded, or crowded by distractors. Attentional enhancement can selectively propagate along groupings of both real and illusory contours, thereby showing how attention can selectively enhance object representations. Recent models explain how attention may have a stronger facilitatory effect on low contrast than on high contrast stimuli, and how pop-out from orientation contrast may occur. The specific functional roles which the model proposes for the cortical layers allow several testable neurophysiological predictions to be made. Model mechanisms clarify how intracortical and intercortical feedback help to stabilize cortical development and learning. Although feedback plays a key role, fast feedforward processing is possible in response to unambiguous information. Model circuits are capable of synchronizing quickly, but context-sensitive persistence of previous events can influence how synchrony develops.

Animals↗