Processing Of Visual Features Research Articles

Enantio-selective cognitive and brain activation effects of N-ethyl-3,4-methylenedioxyamphetamine in humans

In a randomised double-blind trial the subjective, neuropsychological and brain activation effects of the two enantiomers of the MDMA (ecstasy-) like drug N-ethyl-3,4-methylenedioxyamphetamine (MDE) were studied in five normal subjects using functional magnetic resonance imaging (fMRI). (S)-MDE produced elevated mood, impairments in conceptually driven cognition and marked right frontal activation. In contrast, (R)-MDE produced increased depression, enhanced visual feature processing, and activation of visual cortical and left frontal areas. Plasma concentrations were higher for the (R)-enantiomer. The so-called entactogenic effects of MDE are likely to be caused by the (S)-enantiomer, whereas (R)-MDE appears to be responsible for neurotoxic effects.

Discrimination of single features and conjunctions by children

Stimuli that are discriminated by a conjunction of features can show more rapid early processing in adults. To determine how this facilitation effect develops, the processing of visual features and their conjunction was examined in 7-12-year-old children. The children completed a series of tasks in which they made a target-non-target judgement as a function of shape only, colour only or shape and colour features, while event-related potentials were recorded. To assess early stages of feature processing the posteriorly distributed P1 and N1 were analysed. Attentional effects were seen for both components. P1 had a shorter latency and P1 and N1 had larger amplitudes to targets than non- targets. Task effects were driven by the conjunction task. P1 amplitude was largest, while N1 amplitude was smallest for the conjunction targets. In contrast to larger left-sided N1 in adults, N1 had a symmetrical distribution in the children. N1 latency was shortest for the conjunction targets in the 9-10-year olds and 11-12-year olds, demonstrating facilitation in children, but which continued to develop over the pre-teen years. These data underline the sensitivity of early stages of processing to both top-down modulations and the parallel binding of non-spatial features in young children. Furthermore, facilitation effects, increased speed of processing when features need to be conjoined, mature in mid-childhood, arguing against a hierarchical model of visual processing, and supporting a rapid, integrated facilitative model. 2003 Elsevier B.V. All rights reserved.

Event-related potentials and audiovisual stimuli: multimodal interactions

In a discrimination task event-related potentials (ERPs) were recorded in human participants to simultaneously presented visual and auditory stimuli with relevant and irrelevant auditory and visual features. Stimuli consisted of combinations of a sine-wave tone (either target or non-target frequency) and a rectangle of particular (target or non-target) orientation. In two conditions either the auditory or the visual discrimination was easier. Stimuli with just one target feature elicited attention-related ERP components (selection negativity and N2b in the visual, and N2b in the auditory modality) only if that feature could be easily discriminated. This finding shows the hierarchical organization of attentional processing of auditory and visual features. Target stimuli in both conditions elicited visual and auditory attention-related components. This finding, together with the similarity of ERPs to stimuli without target features suggest, that these components cannot be considered as on-line correlates of elementary discrimination processes.

Illusion processing in hemispatial neglect

Twelve patients with hemispatial neglect and two control groups were tested to examine the effects of the Müller-Lyer and Judd illusions on bisection behaviour. The studies were designed to investigate whether neglect patients were indeed unaware of the left sides of the illusory figures. In Experiment 1, participants were asked to describe the illusory figures prior to bisection, whereas in Experiment 2, they compared two illusions whose fins, in the critical condition, differed on the left and then performed the bisection. It was found that the illusions worked equally well in all three groups. Interestingly, apart from one exception, almost all neglect patients explicitly reported the left-sided fins in Experiment 1. Only five patients failed to do so but only on an average of 16% of trials. In Experiment 2, six patients made errors in the comparison task but four of these patients did not neglect any left-sided fins in Experiment 1 (with the exception of three overall trials for LC and EdR). This finding seems a good indication that the two tasks differ in their requirements. The comparison task may be perceived as harder as it requires discrimination rather than detection and thus lead to more neglect type errors than the bisection task. In one neglect patient, the illusions consistently failed to work. This patient presented with an occipito-temporal and basal ganglia lesion and the mechanisms responsible for the processing of simple visual features might have possibly been impaired in her case.

Why Vision is More than Seeing

Vision is so closely identified with visual phenomenology that we sometimes forget that the visual system does more than deliver our experience of the world. Vision also plays a critical role in the control of our movements, from picking up our coffee cups to playing tennis. But the visual control of movement has, until recently, been relatively neglected. Indeed, traditional accounts of vision, while acknowledging the role of vision in motor control, have simply regarded such control as part of a larger function – that of constructing an internal model of the external world. Even though such accounts might postulate separate ‘modules’ for the processing of different visual features, such as motion, colour, texture, and form, in most of these accounts there is an implicit assumption that, in the end, vision delivers a single representation of the external world – a kind of simulacrum of the real thing that serves as the perceptual foundation for all visually driven thought and action.

The Role of the Fusiform Gyrus in Successful Encoding of Face Stimuli

PET was used to measure regional cerebral blood flow (rCBF) while memorizing pictures of unfamiliar human faces presented one at a time (FaceMemory). Other conditions included: (1) FaceRepeat—memorization of four individual faces presented repeatedly; (2) FaceWatching—viewing passively single faces without overt memory demands; and (3) Scrambled—counting dots superimposed on pictures of scrambled faces. After each FaceMemory condition and after the final FaceWatching condition scan, recall was tested by measuring face recognition. Contrasting FaceMemory and Scrambled conditions revealed several temporal activations: right midfusiform and bilateral anterior fusiform gyri. Contrasting FaceWatching and Scrambled conditions showed bilateral activation in the temporal poles and in the anterior fusiform gyri. No hippocampal activation arose from any contrast. Region of interest analyses on the above areas showed correlations with performance: (1) only rCBF in the right midfusiform correlated positively with encoding during the FaceMemory and FaceWatching conditions; (2) in the right temporal polar cortex rCBF decreased during FaceMemory and correlated positively with performance, whereas rCBF increased during FaceWatching and correlated negatively with incidental performance; and (3) activity in the anterior fusiform gyri remained constant across the conditions of FaceMemory, FaceRepeat, FaceWatching, and Scrambled and was uncorrelated with performance. These data suggest an expanded mnemonic role for the right midfusiform in depth of processing/encoding of face information, temporal polar cortex in face perception and recognition, and anterior fusiform activity in featural visual feature processing.

Visual feature processing and nonspatial attention: A combined MEG-EEG analysis

Visual feature processing and nonspatial attention: A combined MEG-EEG analysis

1612 Processing of 3D visual features by neurons in monkey caudal intraparietal sulcus

Image fragments arising from partial occlusion may be perceptually unified by a surface integration process on the basis of similar color or texture. In a new objective measure pitting surface feature similarity against binocular disparity, observers discriminated whether a colored circle had either crossed or uncrossed disparity relative to a surrounding gray rectangle. Sensitivity to disparity was impaired only when (1) the configuration of the other surface fragments in the display supported the integration of a surface behind the rectangle and circle, and (2) matched the color of the central circle. Results were consistent with the hypothesis that a surface integration process integrated similarly-colored surface fragments into a smooth surface, even when those fragments were at different depths. Surface integration caused small and reliable effects on depth perception despite unambiguous disparity information. Perceived depth does not depend solely upon disparity, and may be determined after three-dimensional figural unity is established.

Processing of 3D visual features by neurons in monkey caudal intraparietal sulcus

Processing of 3D visual features by neurons in monkey caudal intraparietal sulcus

Context dependence in visual feature processing

Dependence in visual feature processing was studied with two identification experiments using briefly exposed stimuli. The basic stimuli consisted of two orthogonal line segments which formed either one of the four angles of a square, and the subject had to identify the location of the horizontal and vertical line segment making up a stimulus. In the first experiment, the two orthogonal line segments were sometimes separated by a gap. In the second experiment, either a 45° diagonal or a 135° diagonal or both were added to the right angles. With the observed response frequencies two forms of independence in feature detection were tested. Feature detection is called state independent if the detection of a feature is independent of the detection of another feature. According to context independence the detection of a feature is independent of the orientation and location of other features in the stimulus. Feature detection was shown to be context dependent whereas the hypothesis of state independence was not rejected, and state independence was unrelated to the size of the gap between the two orthogonal line segments in the first experiment. It was argued that the context effects obtained (context dependence) had occurred early in processing during the automatic activation of representational units corresponding to features before controlled search, interpretation and decision processes became involved.

Levels of feature analysis in processing visual patterns.

In this paper, a revised Pandemonium-like model of visual-feature processing is formulated and a preliminary test of its feasibility is reported. The model differentiates visual-feature processing into a series of hierarchical stages organized by increasing complexity, with the output of each stage going both to the next higher stage, and directly to a more central processor. In the experiment, subjects sorted decks of cards into piles according to the presence or absence of a target stimulus which differed from nontargets in a variety of different features; detection of a feature was sufficient for detection of a target. The data generally supported the revised Pandemonium model, in that targets which differed from nontargets in features thought to be low in the hierarchy were processed faster than targets whose difference was in a high level feature. An extension of the revised model did somewhat less well in predicting the results of sorting for targets in which detection of any one of several features was sufficient for target detection.

Visual hierarchical feature processing: The relation of size, spatial position and identity

A feature analysis theory of pattern perception was developed based on physiological properties of the visual system. Specific predictions about the relations of size, spatial position, and identity analyses were made; further, these predictions differed as a function of stimulus parameters. Twenty-four Ss were tested on a task designed to reveal relations among various analyses. The results supported the contention that the level of analysis of a dimension changes relative to other stimulus dimensions as a function of stimulus parameters. Implications for pattern perception are discussed.