Biomedical subjects

Marlene Behrmann

Publications and source records attributed to Marlene Behrmann.

13 recordsLinked to original sources

PubMed2004

Parietal cortex and attention.

The parietal lobe forms about 20% of the human cerebral cortex and is divided into two major regions, the somatosensory cortex and the posterior parietal cortex. Posterior parietal cortex, located at the junction of multiple sensory regions, projects to several cortical and subcortical areas and is engaged in a host of cognitive operations. One such operation is selective attention, the process where by the input is filtered and a subset of the information is selected for preferential processing. Recent neuroimaging and neuropsychological studies have provided a more fine-grained understanding of the relationship between brain and behavior in the domain of selective attention.

Attention↗

PubMed2004

Are Greebles like faces? Using the neuropsychological exception to test the rule.

Which image geometries count as face-like and which do not? Across multiple experiments, novel objects called Greebles have been used to argue that face-specific effects can be obtained with non-face stimuli under certain situations, in particular with expert observers. However, this claim depends on the argument that these non-face stimuli are not a priori treated by the face processing system. To address this question, CK, a neuropsychological patient well-known for exhibiting severe visual object agnosia and dyslexia but intact face processing, was tested with Greebles. CK performed poorly on Greebles, indicating that his intact face-specific abilities do not extend to include Greebles. These results suggest that insofar as CK is relying on face-specific visual processes, these processes do not a priori treat Greebles as faces.

Adolescent↗

PubMed2004

Hemispatial neglect and visual search: a large scale analysis.

Visual search tasks have standardly been divided into two categories: those in which the target is detected through a serial, attention-driven search and those in which the target is detected rapidly in parallel and, apparently, without attentional processing. Several studies have examined this distinction in patients with hemispatial neglect with the clear prediction that the former, but not the latter, should be impaired. These studies, however, have proved inconclusive. We have addressed this issue in a large sample of patients with unilateral hemispheric infarcts to the left or right hemisphere. In addition to measuring the patients' performance on both types of visual search tasks, we documented the presence and severity of neglect and of visual field defects in these same individuals. Patients with brain-damage with or without accompanying neglect were impaired at searching for the contralateral target on both forms of visual search, relative to normal control subjects, although this deficit was magnified in individuals with neglect and was also exacerbated by the presence of hemianopia. This pattern was also more pronounced in individuals with right-than with left-hemisphere lesions. The findings not only clarify the contradictory neuropsychological data but also provide clear evidence for the involvement of attentional processing in all forms of visual search.

Aged↗

PubMed2004

Role of attention and perceptual grouping in visual statistical learning.

Statistical learning has been widely proposed as a mechanism by which observers learn to decompose complex sensory scenes. To determine how robust statistical learning is, we investigated the impact of attention and perceptual grouping on statistical learning of visual shapes. Observers were presented with stimuli containing two shapes that were either connected by a bar or unconnected. When observers were required to attend to both locations at which shapes were presented, the degree of statistical learning was unaffected by whether the shapes were connected or not. However, when observers were required to attend to just one of the shapes' locations, statistical learning was observed only when the shapes were connected. These results demonstrate that visual statistical learning is not just a passive process. It can be modulated by both attention and connectedness, and in natural scenes these factors may constrain the role of stimulus statistics in learning.

Attention↗

PubMed2004

Path integration deficits during linear locomotion after human medial temporal lobectomy.

Animal navigation studies have implicated structures in and around the hippocampal formation as crucial in performing path integration (a method of determining one's position by monitoring internally generated self-motion signals). Less is known about the role of these structures for human path integration. We tested path integration in patients who had undergone left or right medial temporal lobectomy as therapy for epilepsy. This procedure removed approximately 50% of the anterior portion of the hippocampus, as well as the amygdala and lateral temporal lobe. Participants attempted to walk without vision to a previously viewed target 2-6 m distant. Patients with right, but not left, hemisphere lesions exhibited both a decrease in the consistency of path integration and a systematic underregistration of linear displacement (and/or velocity) during walking. Moreover, the deficits were observable even when there were virtually no angular acceleration vestibular signals. The results suggest that structures in the medial temporal lobe participate in human path integration when individuals walk along linear paths and that this is so to a greater extent in right hemisphere structures than left. This information is relevant for future research investigating the neural substrates of navigation, not only in humans (e.g., functional neuroimaging and neuropsychological studies), but also in rodents and other animals.

Adult↗

PubMed2003

What does visual agnosia tell us about perceptual organization and its relationship to object perception?

The authors studied 2 patients, S.M. and R.N., to examine perceptual organization and its relationship to object recognition. Both patients had normal, low-level vision and performed simple grouping operations normally but were unable to apprehend a multielement stimulus as a whole. R.N. failed to derive global structure even under optimal stimulus conditions, was less sensitive to grouping by closure, and was more impaired in object recognition than S.M. These findings suggest that perceptual organization involves a multiplicity of processes, some of which are simpler and are instantiated in lower order areas of visual cortex (e.g., collinearity). Other processes are more complex and rely on higher order visual areas (e.g., closure and shape formation). The failure to exploit these latter configural processes adversely affects object recognition.

Adolescent↗

PubMed2003

Expertise in tactile pattern recognition.

This article explores expertise in tactile object recognition. In one study, participants were trained to differing degrees of accuracy on tactile identification of two-dimensional patterns. Recognition of these patterns, of inverted versions of these patterns, and of subparts of these patterns was then tested. The inversion effect (better recognition of upright than inverted patterns) and the part-whole effect (better recognition of the whole than a part pattern), traditionally considered signatures of visual expertise, were observed for tactile experts but not for novices. In a second study, participants were trained as visual or tactile experts and then tested in the trained and nontrained modalities. Whereas expertise effects were observed in the modality of training, cross-modal transfer was asymmetric; visual experts showed generalization to haptic recognition, but tactile experts did not show generalization to visual recognition. Tactile expertise is not obviously attributable to visual mediation and emerges from domain-general principles that operate independently of modality.

Adult↗

PubMed2002

Correlations between the fMRI BOLD signal and visual perception.

Using fMRI and a psychophysical task involving letter identification, Kleinschmidt et al. (2002) (this issue of Neuron) delineate two patterns of neural activation, which manifest in different cortical regions: a transient activation, correlated with the change of a percept, and a longer-term hysteresis, correlated with the maintenance of the percept. These findings are provocative and suggest that neural hysteresis is mediated by visual structures that interact with higher-order regions to support longer-term maintenance of a percept.

Brain Mapping↗

PubMed2002

Eccentricity bias as an organizing principle for human high-order object areas.

We have recently proposed a center-periphery organization based on resolution needs, in which objects engaging in recognition processes requiring central-vision (e.g., face-related) are associated with center-biased representations, while objects requiring large-scale feature integration (e.g., buildings) are associated with periphery-biased representations. Here we tested this hypothesis by comparing the center-periphery organization with activations to five object categories: faces, buildings, tools, letter strings, and words. We found that faces, letter strings, and words were mapped preferentially within the center-biased representation. Faces showed a hemispheric lateralization opposite to that of letter strings and words. In contrast, buildings were mapped mainly to the periphery-biased representation, while tools activated both central and peripheral representations. The results are compatible with the notion that center-periphery organization allows the optimal allocation of cortical magnification to the specific requirements of various recognition processes.

Adult↗

PubMed2002

Mechanisms underlying spatial representation revealed through studies of hemispatial neglect.

The representations that mediate the coding of spatial position were examined by comparing the behavior of patients with left hemispatial neglect with that of nonneurological control subjects. To determine the spatial coordinate system(s) used to define "left" and "right," eye movements were measured for targets that appeared at 5 degrees, 10 degrees, and 15 degrees to the relative left or right defined with respect to the midline of the eyes, head, or midsaggital plane of the trunk. In the baseline condition, in which the various egocentric midlines were all aligned with the environmental midline, patients were disproportionately slower at initiating saccades to left than right targets, relative to the controls. When either the trunk or the head was rotated and the midline aligned with the most peripheral position while the eyes remained aligned with the midline of the environment, the results did not differ from the baseline condition. However, when the eyes were rotated and the midline aligned with the peripheral position, saccadic reaction time (SRT) differed significantly from the baseline, especially when the eyes were rotated to the right. These findings suggest that target position is coded relative to the current position of gaze (oculocentrically) and that this eye-centered coding is modulated by orbital position (eye-in-head signal). The findings dovetail well with results from existing neurophysiological studies and shed further light on the spatial representations mediated by the human parietal cortex.

Aged↗

PubMed2002

Spatial and temporal influences on extinction.

This study investigated the spatial and temporal characteristics of the attentional deficit in patients exhibiting extinction to determine the extent to which these characteristics can be explained by a theory of an underlying gradient resulting from the differential contribution of interacting cell populations. The paradigm required the identification of two letters whose spatial location was varied both within and across hemifields. Additionally, the interval between the appearances of the two stimuli was manipulated by changing the stimulus onset asynchrony (SOA). A final variable, that of expectancy, was introduced by making the stimulus location more or less predictable and examining the effect of this top-down contingency on performance. The findings were consistent across two patients and indicated the joint contribution of both spatial and temporal factors: the contralesional stimulus was maximally extinguished when it was preceded by the ipsilesional stimulus by 300-900 ms, but this extinction was reduced when the stimuli appeared further ipsilesionally. Interestingly, there was increased extinction of the contralesional stimulus when location was predictable. These findings support the hypothesis that the attentional deficit in extinction patients arises from a contralesional-to-ipsilesional gradient of cell populations that interact in a mutually inhibitory manner.

Aged↗

PubMed2002

Impact of learning on representation of parts and wholes in monkey inferotemporal cortex.

Here we investigated the impact of visual discrimination training on neuronal responses to parts of images and to whole images in inferotemporal (IT) cortex. Monkeys were trained to discriminate among 'baton' stimuli consisting of discrete top and bottom parts joined by a vertical stem. With separate features at each end, we were able to manipulate the two parts of each baton independently. After training the monkeys, we used single-cell recording to compare neuronal responses to learned and unlearned batons. Responses to learned batons, though not enhanced in strength, were enhanced in selectivity for both individual parts and for whole batons. Whole-baton selectivity arose from a form of conjunctive encoding whereby two parts together exerted a greater influence on neuronal activity than predicted by the additive influence of each part considered individually. These results indicate a possible neural mechanism for holistic or configural effects in expert versus novice observers.

Animals↗

PubMed2002

Probability cuing of target location facilitates visual search implicitly in normal participants and patients with hemispatial neglect.

We explored how variability in the probability of target locations affects visual search in normal individuals and in patients with hemispatial neglect, a deficit in attending to the contralesional side of space. Young and elderly normal participants responded faster when targets appeared in the more probable region than when targets appeared in the less probable region. Similarly, patients were sensitive to the distribution of targets, even in the neglected field. Although the attentional gradient that characterizes neglect was not eliminated, the response facilitation due to the probability distribution was proportionate to that of control participants and equal in magnitude across the neglected field. All participants exploited the uneven distribution of targets to enhance task performance without explicit instructions to do so or awareness of biases in their behavior. These results suggest that attentional orientation and sensitivity to external probabilities are possibly dissociable. An early sensory and a late motor mechanism are postulated as possibly being involved in the observed probability-matching behavior of participants.

Adolescent↗