Biomedical subjects

M Behrmann

Publications and source records attributed to M Behrmann.

At least 19 recordsLinked to original sources

PubMed2000

Intact spatial updating during locomotion after right posterior parietal lesions.

One function of the posterior parietal cortex (PPC) is to monitor and integrate sensory signals relating to the current pointing direction of the eyes. We investigated the possibility that the human PPC also contributes to spatial updating during larger-scale behaviors. Two groups of patients with brain injuries either including or excluding the right hemisphere PPC and a group of healthy subjects performed a visually-directed walking task, in which the subject views a target and then attempts to walk to it without vision. All groups walked without vision accurately and precisely to remembered targets up to 6 m away; the patient groups also performed similarly to the healthy controls when indicating egocentric distances using non-motoric responses. These results indicate that the right PPC is not critically involved in monitoring and integrating non-visual self-motion signals, at least along linear paths. In addition, visual perception of egocentric distance in multi-cue environments is immune to injury of a variety of brain areas.

Adult↗

PubMed2000

Occlusion, symmetry, and object-based attention: reply to Saiki (2000).

J. Saiki (2000) argued that, because the stimuli used by M. Behrmann, R. S. Zemel, and M. C. Mozer (1998) were confounded by symmetry, conclusions about whether amodally completed objects can benefit from object-based attention are unwarranted. Here, the authors examine J. Saiki's claim further and expand on their view of the mechanisms underlying object-based attention, suggesting that perceptual organization is the process whereby features from a single object are selectively attended. In light of this, they claim that heuristics such as symmetry and collinearity play an important role in the facilitation of features from a single object. In support of this claim, they present data from a further experiment using displays that exploit common fate, another grouping heuristic, and show that, under these conditions, the hallmark of object-based attention, a single-object advantage, is obtained for the occluded (amodally completed) shapes.

Adult↗

PubMed2000

Selective attention to the parts of an object.

Recent empirical results suggest that there is a decrement in dividing attention between two objects in a scene compared with focusing attention on a single object. However, objects can be made of individual parts. Is there a decrement for dividing attention across different parts of a single object? We addressed this question in two experiments. In Experiment 1, we demonstrated that attention can exhibit part-based selection--that is, the subjects were more accurate in reporting two attributes from the same part of an object than they were in reporting attributes from different parts of an object. In Experiment 2, we demonstrated that part-based attentional decrements occurred simultaneously with object-based attentional decrements. The results from Experiment 2 demonstrated that part-based attention is evident at the same time as objects are processed as coherent whole. Our results imply that there is an attentional mechanism that can select either objects or their parts.

Adult↗

PubMed1999

Slowing of reaction time in Parkinson's disease: the involvement of the frontal lobes.

This study investigated the possibility that the previously mixed findings relating to cognitive deficits in Parkinson's disease might be attributable to inhomogeneity within the patients sampled, with attentional deficits occurring only for those Parkinson's patients who also have additional frontal lobe impairment. Twenty-five patients with idiopathic Parkinson's disease were classified as showing frontal dysfunction, or not, on the basis of their performance on the Wisconsin Card Sorting Test and the picture arrangement subtest of the WAIS. The two groups, and a control group of normal elderly subjects matched for age and IQ, undertook tests of visual attention designed to dissociate baseline response speed from central information processing speed. Error rates did not differ between the groups. Performance of the non-frontally impaired Parkinson's group was indistinguishable from that of the controls. By contrast, the 'frontally impaired' Parkinson's group responded significantly more slowly than the controls. Further analyses indicated that for the frontally-impaired Parkinson's group, information processing and automatic functions were unimpaired but there was a generalised slowing (as reflected by increased baseline response time) which may represent a non-specific global cognitive impairment. These findings suggest that the frontal lobes may be implicated in slowed response speed in Parkinson's disease.

Adult↗

PubMed1999

The cognitive neuroscience of visual attention.

In current conceptualizations of visual attention, selection takes place through integrated competition between recurrently connected visual processing networks. Selection, which facilitates the emergence of a 'winner' from among many potential targets, can be associated with particular spatial locations or object properties, and it can be modulated by both stimulus-driven and goal-driven factors. Recent neurobiological data support this account, revealing the activation of striate and extrastriate brain regions during conditions of competition. In addition, parietal and temporal cortices play a role in selection, biasing the ultimate outcome of the competition.

Attention↗

PubMed1999

Attention accesses multiple reference frames: evidence from visual neglect.

Research with normal participants has demonstrated that mechanisms of selective attention can simultaneously gain access to internal representations of spatial information defined with respect to both location- and object-based frames of reference. The present study demonstrates that patients with unilateral spatial neglect following a right-hemisphere lesion are poorer at detecting information on the contralateral left side in both location- and object-based spatial coordinates simultaneously. Moreover, the extent of the neglect is modulated by the probability of a target's appearing in either reference frame; as the probability of sampling a target in a particular frame of reference increases, so does the severity of neglect in the frame. These findings suggest that attention can be flexibly and strategically assigned to a reference frame depending on the contingencies of the task.

Aged↗

PubMed1999

Visual attention deficits in Alzheimer's disease: simple versus conjoined feature search.

The authors investigated selective attention in patients with Alzheimer's disease (AD), using a well-known visual search procedure. In simple feature search, the deficit observed in AD patients represented a baseline shift in the median hit reaction time (RT). On the conjoined feature search task, the median hit RT for AD patients increased disproportionately with increasing array size, indicating an additional cognitive impairment on this task. Of particular importance, the cognitive deficit observed in conjunction search was more profound than that predicted on the basis of previous reports of global cognitive slowing in AD. There was some evidence that the performance of AD patients improved more than the performance of controls over the duration of the experimental test session. Patients also had more difficulty in detecting targets on the right side of hemispace and in more peripheral locations.

Adolescent↗

PubMed1999

Can face recognition really be dissociated from object recognition?

We argue that the current literature on prosopagnosia fails to demonstrate unequivocal evidence for a disproportionate impairment for faces as compared to nonface objects. Two prosopagnosic subjects were tested for the discrimination of objects from several categories (face as well as nonface) at different levels of categorization (basic, subordinate, and exemplar levels). Several dependent measures were obtained including accuracy, signal detection measures, and response times. The results from Experiments 1 to 4 demonstrate that, in simultaneous-matching tasks, response times may reveal impairments with nonface objects in subjects whose error rates only indicate a face deficit. The results from Experiments 5 and 6 show that, given limited stimulus presentation times for face and nonface objects, the same subjects may demonstrate a deficit for both stimulus categories in sensitivity. In Experiments 7, 8 and 9, a match-to-sample task that places greater demands on memory led to comparable recognition sensitivity with both face and nonface objects. Regardless of object category, the prosopagnosic subjects were more affected by manipulations of the level of categorization than normal controls. This result raises questions regarding neuropsychological evidence for the modularity of face recognition, as well as its theoretical and methodological foundations.

Adolescent↗

PubMed1998

Requirements for the translocation of elongation-arrested, ribosome-associated OmpA across the plasma membrane of Escherichia coli.

An oligodeoxynucleotide-dependent method to generate nascent polypeptide chains was adopted for use in a cell-free translation system prepared from Escherichia coli. In this way, NH2-terminal pOmpA fragments of distinct sizes were synthesized. Because most of these pOmpA fragments could be covalently linked to puromycin, precipitated with cetyltrimethylammonium bromide, and were enriched by sedimentation, they represent a population of elongation-arrested, ribosome-associated nascent chains. Translocation of these nascent pOmpA chains into inside-out membrane vesicles of E. coli required SecA and (depending on size) SecB. Whereas their translocation was strictly dependent on the H+-motive force of the vesicles, no indication for the involvement of the bacterial signal recognition particle was obtained. SecA and SecB, although required for translocation, did not mediate binding of the ribosome-associated pOmpA to membrane vesicles. However, SecA and SecB cotranslationally associated with nascent pOmpA, since they could be co-isolated with the ribosome-associated nascent chains and as such catalyzed translocation subsequent to the release of the ribosome. These results indicate that in E. coli, SecA also functionally interacts with preproteins before they are targeted to the translocase of the plasma membrane.

Adenosine Triphosphatases↗

PubMed1998

Visuomotor processing in unilateral neglect.

The extent to which visual information on the contralateral, unattended side influences the performance of patients with hemispatial neglect was studied in a visuomotor reaching task. We replicated the well-established finding that, relative to target-alone trials, normal subjects are slower to reach to targets in the presence of visual distractors which appear either ipsilateral or contralateral to the target, with greater interference in the former condition. Six patients with hemispatial neglect showed even greater interference than did the normal subjects when the distractor appeared ipsilaterally but showed no significant interference from contralateral distractors. This pattern of performance was qualitatively similar for patients with lesions restricted to posterior regions and for patients with more extensive lesions involving both posterior and anterior brain regions. These findings suggest that, in the visuomotor domain, information on the contralateral side is processed minimally, if at all, in patients with hemispatial neglect.

Adult↗

PubMed1998

Visuospatial neglect in normal subjects: altered spatial representations induced by a perceptual illusion.

Previous studies have suggested that the representation of space is distorted in patients with hemispatial neglect. To examine the nature of the altered spatial representation, we developed an analog of neglect in normal subjects using bisections of the Judd visual illusion (e.g., <---<) whose direction (facing left/right/both) and fin angle (14 degrees/45 degrees/76 degrees) were orthogonally varied. Subjects made bisection errors that deviated significantly in a direction opposite to that in which the fins pointed, and these deviations decreased as fin angle increased. We replicated these findings in a second task in which subjects were given the midpoint and placed the two fins at the ends of an imaginary shaft (e.g., < x <, note: dot = true center). These findings suggest that the geometric properties of the display have a strong influence on the representation of space, and thus may be used to induce a distorted perception of space analogous to that observed in patients with visuospatial neglect.

Adolescent↗

PubMed1998

Visual complexity in letter-by-letter reading: "pure" alexia is not pure.

Standard accounts of pure alexia have favoured the view that this acquired disorder of reading arises from damage to a left posterior occipital cortex mechanism dedicated to the processing of alphanumeric symbols. We challenge these accounts in two experiments and demonstrate that patients with this reading deficit are also impaired at object identification. In the first experiment, we show that a single subject, EL, who shows all the hallmark features of pure alexia, is impaired at picture identification across a large set of stimuli. As the visual complexity of pictures increases, so EL's reaction time to identify the stimuli increases disproportionately relative to the control subjects. In the second experiment, we confirm these findings with a larger group of five pure alexic patients using a selected subset of high- and low-visual complexity pictures. These findings suggest that the deficit giving rise to pure alexia is not restricted to orthographic symbols per se but, rather, is a consequence of damage to a more general-purpose visual processing mechanism.

Adult↗

PubMed1998

Object-based attention and occlusion: evidence from normal participants and a computational model.

One way of perceptually organizing a complex visual scene is to attend selectively to information in a particular physical location. Another way of reducing the complexity in the input is to attend selectively to an individual object in the scene and to process its elements preferentially. This latter, object-based attention process was examined, and the predicted superiority for reporting features from 1 relative to 2 objects was replicated in a series of experiments. This object-based process was robust even under conditions of occlusion, although there were some boundary conditions on its operation. Finally, an account of the data is provided via simulations of the findings in a computational model. The claim is that object-based attention arises from a mechanisms that groups together those features based on internal representations developed over perceptual experience and then preferentially gates these features for later, selective processing.

Adolescent↗

PubMed1998

Ocular search during line bisection. The effects of hemi-neglect and hemianopia.

We examined ocular fixations during line bisection in five patients with left hemianopia, two patients with right hemianopia, nine patients with left hemi-neglect and nine normal control subjects. Compared with measures in control subjects, the median fixation, and left- and rightmost fixations were shifted contralaterally in patients with hemianopia alone and ipsilaterally in patients with hemi-neglect. The fixation with the longest duration and the bisection point were also shifted contralaterally with hemianopia and ipsilaterally with hemi-neglect. However, the number of fixations and the spatial range spanned by fixations did not differ between the groups, showing that ocular exploration was not truncated in any group. Only some patients showed a previously reported directional search bias. Overall, there was no directional bias in saccadic number or amplitude. The distribution of fixations was most dense at the centre of the line in normal subjects, while hemianopic patients fixated most frequently at the ends of lines in their contralateral (blind) hemispace and at a central locus that was biased slightly contralaterally, as was their bisection judgement. This contralateral bias may reflect either an adaptive contralateral attentional gradient or a non-veridical spatial representation within the remaining normal hemifield. Hemi-neglect patients had a broad distribution of fixation peaks in the ipsilateral hemispace. Of two hemi-neglect patients with many fixations, one clustered fixations at a position right of centre, as if a normal fixation pattern was shifted rightward, while the other had two fixation peaks: one to the far right and the other near the centre of the line, reminiscent of the dual peaks of activity seen in some recent hemi-neglect models. These data reveal a heterogeneity in the routes by which right-biased judgements of spatial centre are reached by hemi-neglect patients.

Adult↗

PubMed1997

Surface dyslexia in nonfluent progressive aphasia.

This article presents the case of a 59-year-old male, JH, with a 6-year history of primary progressive aphasia (PPA), a disorder characterized by isolated language deterioration with relative preservation of other cognitive abilities. JH also shows typical features of surface dyslexia, a reading disorder exemplified by the selective preservation of phonological reading. One recent theory is that surface dyslexia in individuals with PPA results from a loss of semantic knowledge. In this paper we consider an additional possibility and present data supporting the notion that surface dyslexia may also arise from the malfunction in the links between semantic representations and phonology. JH has remarkably preserved lexical semantic knowledge when assessed on tasks that do not require verbal output. Further, item-by-item comparisons of his oral reading and comprehension ability show no significant correspondence between his reading and semantic knowledge. These findings lead us to conclude that, in JH's case, the surface dyslexia is attributable not to a semantic deficit per se but rather to the inability to access phonological information from semantics. JH's language profile is considered in relation to potential sources of surface dyslexia and other cases of progressive aphasia.

Aphasia, Broca↗

PubMed1997

The removal of binocular cues disrupts the calibration of grasping in patients with visual form agnosia.

The present study tested the idea that the visuomotor systems mediating prehension do not have independent access to pictorial cues processed by perceptual mechanisms. Individuals with visual form agnosia, whose perceptual systems are compromised but who have intact visuomotor control, were examined to determine whether they could use pictorial scene cues to calibrate manual prehension when binocular information was removed. The removal of binocular cues produced considerable disruptions in size-constancy of grip aperture, which, combined with earlier observations in normal subjects, suggests that binocular cues are of primary importance in calibration of grasping. In the absence of binocular vision, normal subjects can use pictorial information, information that is severely compromised in individuals with visual form agnosia, to compute the distance (and thus the size) of the goal object. Thus, individuals with visual form agnosia must rely on a retinal image that remains uncalibrated, leading to inaccurate calibrations of grip aperture. The fact that these individuals scaled their grasp much less accurately under the monocular viewing condition, despite showing normal binocular grasping, suggests that pictorial cues to depth, which are presumably processed by mechanisms mediating our perception of objects and events in the world, can be accessed by visuomotor mechanisms only indirectly. These results, together with others, suggest that the visuomotor system 'prefers' to use binocular information and uses pictorial cues only as a last resort.

Adult↗

PubMed1997

Impaired visual search in patients with unilateral neglect: an oculographic analysis.

The attentional deficit underlying hemispatial neglect was examined through a detailed analysis of the eye movement performance of a group of neglect patients. Relative to normal subjects and to patients with hemianopia without neglect, patients with left neglect make fewer fixations and have shorter inspection time on the contralesional left side. They also start their search to the right of the midline and make significantly more fixations and longer fixations on the ipsilesional right side. A positive linear relationship between horizontal location and frequency of fixations was noted for the neglect group as a whole, as well as for most of the individual patients. These findings strongly endorse the view that the attentional deficit in neglect follows a left right gradient. The peak of the maximum fixations, however, is not on the extreme right, as might be predicted by a strict gradient account, and is more consistent with recent views that the midsagittal plane of the viewer is redirected rightwards. These findings provide a detailed analysis of the eye movement patterns in neglect patients and demonstrate the robustness of oculographic analysis for examining their altered spatial representation.

Aged↗

PubMed1997

Frequency and consistency effects in a pure surface dyslexic patient.

Data are presented from a neurological patient (M.P.) with an acquired deficit for naming words with atypical spelling-sound correspondences. In Experiment 1, the degree of consistency within neighborhoods of orthographically similar words had a parallel impact on M.P.'s pronunciations of regular and irregular words and nonwords. This result is more compatible with models in which the same basic procedure, sensitive in a graded fashion to both frequency and consistency, computes pronunciations for all types of letter strings than it is with models postulating separate lexical and nonlexical mechanisms. In Experiment 2, both correct and regularized pronunciations of exception words yielded response times significantly modulated by word frequency, a finding not predicted by an current model. Neuropsychological observations provide an important additional source of evidence regarding models of cognitive function.

Dyslexia↗