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Biomedical subjects

M Behrmann

Publications and source records attributed to M Behrmann.

At least 37 records · Page 2Linked to original sources
PubMed1997

Spatial attention does not require preattentive grouping.

Does spatial attention follow a full preattentive analysis of the visual field, or can attention select from ungrouped regions of the visual field? We addressed this question by testing an apperceptive agnosic patient, J. W., in tasks involving both spatial selection and preattentive grouping. Results suggest that J.W. had intact spatial attention: He was faster to detect targets appearing at cued location relative to targets appearing at uncued locations. However, his preattentive processes were severely disrupted. Gestalt grouping and symmetry perception, both thought to involve preattentive processes, were impaired in J. W. Also, he could not use gestalt grouping cues to guide spatial attention. These results suggest that spatial attention is not completely dependent on preattentive grouping processes. We argue that preattentive grouping processes and spatial attention may mutually constrain one another in guiding the attentional selection of visual stimuli but that these 2 processes are isolated from one another.

Adult↗
PubMed1997

Patterns of eye movements during parallel and serial visual search tasks.

Eye movements were monitored while subjects performed parallel and serial search tasks. In Experiment 1a, subjects searched for an "O" among "X"s (parallel condition) and for a "T" among "L"s (serial condition). In the parallel condition in Experiment 1b, "[symbol: see text]" was the target, and "O"s were distractors; in the serial condition these stimuli switched roles. Displays contained 1, 12, or 24 stimuli, with both target-present and target-absent trials. RT and eye-movement measures (number of fixations, saccadic error, and latency to move) indicated that search efficiency was greatest in the parallel conditions, followed by the serial condition of experiment 1a and, finally, by the serial condition of Experiment 1b. This suggests that eye movements are correlated with the attentional processes underlying visual search.

Attention↗
PubMed1997

Spatial- and object-based attentional deficits in Alzheimer's disease. Relationship to HMPAO-SPECT measures of parietal perfusion.

The purpose of this study was to investigate the ability of patients with Alzheimer's disease to shift attention between spatial locations and between objects, and to examine the brain regions involved in these cognitive operations using single photon emission computed tomography (SPECT) imaging. A recent study of patients with focal lesions provided evidence that the right and left parietal lobes are differentially involved in shifting selective attention from invalidly cued spatial locations and objects, respectively (Egly et al. J Exp Psychol Gen 1994; 123: 161-77). Accordingly, in Alzheimer's disease patients, we hypothesized that right parietal hypoperfusion on SPECT would be associated with deficits on the spatial-based component of a cued reaction time task, and left parietal hypoperfusion would be associated with the deficits on the object-based component. Attentional performance of Alzheimer's disease patients (n = 29) was compared with aged-matched normal controls (n = 17) using a cued reaction time task based on Egly et. al. (1994). Regions of interest were defined semi-automatically on SPECT, and were anatomically localized with the aid of co-registered MRI. As hypothesized, in Alzheimer's disease patients, reaction time costs of invalid targets eliciting shifts of attention between spatial locations were selectively correlated with SPECT hypoperfusion in the right superior parietal lobe; while reaction time costs of between-object shifts of attention were correlated with hypoperfusion in the left inferior parietal lobe. These results provide evidence for the specialized roles of the right and left parietal regions in the spatial and object components of attentional shifting respectively, and suggest that the cognitive profile associated with Alzheimer's disease includes both spatial- and object-based attentional impairments.

Aged↗
PubMed1997

Long-term semantic priming: a computational account and empirical evidence.

Semantic priming is traditionally viewed as an effect that rapidly decays. A new view of long-term word priming in attractor neural networks is proposed. The model predicts long-term semantic priming under certain conditions. That is, the task must engage semantic-level processing to a sufficient degree. The predictions were confirmed in computer simulations and in 3 experiments. Experiment 1 showed that when target words are each preceded by multiple semantically related primes, there is long-lag priming on a semantic-decision task but not on a lexical-decision task. Experiment 2 replicated the long-term semantic priming effect for semantic decisions with only one prime per target. Experiment 3 demonstrated semantic priming with much longer word lists at lags of 0, 4, and 8 items. These are the first experiments to demonstrate a semantic priming effect spanning many intervening items and lasting much longer than a few seconds.

Adult↗
PubMed1996

Object-centered not scene-based visual neglect.

Whether visuospatial attention accesses object-centered representations, in addition to location-based coordinates, was investigated in patients with hemispatial neglect who detected a target on the left or right of a single object (2 connected circles or barbell) or of 2 objects (2 unconnected circles). The object or objects either remained static (left circle in left space) or rotated by 180 degrees (left circle now in right space). Relative to the static condition, in the rotating condition, detection times are facilitated on the left (contralateral) and inhibited on the right (ipsilateral) of space even when eye movements are controlled. This modulation of neglect was only observed for the single object, but not for the 2-object displays. The findings suggest that attention operates on object-centered as well as on location-based representations, and thus accesses multiple reference frames.

Aged↗
PubMed1995

Spatial attention in the mental architecture: evidence from neuropsychology.

Using neuropsychological evidence, this paper examines whether spatial attention functions as a domain-specific module or as a more general-purpose central processor. Data are presented from two spatial attention cuing tasks completed by subjects, with an acquired attentional deficit, and control subjects. In both tasks, an arrow indicated with high probability the side of response (response task) or the side of space on which the stimulus would appear (visuospatial task). In the response task, the stimuli appeared foveally and the response component was lateralized, and in the visuospatial task, the stimuli were lateralized and the response component remained constant in the midline. Only the neglect subjects showed a disproportionate increase in reaction time on both the response and visuospatial tasks when the arrow cued the subject to the ipsilateral side and the stimulus or response was on the side of space contralateral to the lesion. The substantial association across the two tasks suggests that a common underlying internal spatial representation subserves perception and action. While this finding is consistent with Fodor's view of a cross-domain processor, it does not meet all of his criteria of a central processor. We conclude, therefore, that the posterior attentional mechanism is strictly neither a module nor a central processor. Rather, these results suggest that a common attentional mechanism may subserve behavior in domains that are tightly coupled.

Adult↗
PubMed1994

Intact visual imagery and impaired visual perception in a patient with visual agnosia.

Although it is now well accepted that visual mental imagery and visual perception share common underlying mechanisms, there are several reports in which they are dissociated. Evidence for the separability of these processes is provided by a patient, C.K., who has a profound visual object recognition deficit attributable to an impairment in grouping or segmenting visual images. Despite this perceptual deficit, C.K. was able to draw objects in considerable detail from memory, and his knowledge of the visual appearance of objects was preserved on a variety of mental imagery tasks. Together with previous cases, these findings confirm the double dissociation between object recognition and perception. Interestingly, C.K. could also recognize newly constructed objects in his internal imagery. To accommodate these results, we propose a model in which imagery and perception are strongly associated but are also functionally specialized.

Adult↗
PubMed1992

Dissociation between mental imagery and object recognition in a brain-damaged patient.

Visual imagery is the creation of mental representations that share many features with veridical visual percepts. Studies of normal and brain-damaged people reinforce the view that visual imagery and visual perception are mediated by a common neural substrate and activate the same representations. Thus, brain-damaged patients with intact vision who have an impairment in perception should have impaired visual imagery. Here we present evidence to the contrary from a patient with severely impaired object recognition (visual object agnosia) but with normal mental imagery. He draws objects in considerable detail from memory and uses information derived from mental images in a variety of tasks. In contrast, he cannot identify visually presented objects, even those he has drawn himself. He has normal visual acuity and intact perception of equally complex material in other domains. We conclude that rich internal representations can be activated to support visual imagery even when they cannot support visually mediated perception of objects.

Adult↗
PubMed1990

The evolution of pure alexia: a longitudinal study of recovery.

This case report documents the partial recovery, over a 12-month period, of pure alexia in an adult female following a left occipital infarction. Measures of speed and accuracy were obtained on an oral reading and a lexical decision task immediately postonset and then on 10 subsequent occasions. Explicit letter-by-letter reading was observed only during the first week poststroke but a significant effect of word length was seen in all testing sessions. Reading accuracy was relatively good at all stages and reading latency showed a remarkable decrease over time but did not reach normal reading rates. The inability to use higher-order orthographic knowledge, as manifest in the absence of a word superiority effect, was still noted at one year postonset. We therefore concluded that the change in behavior was attributable to increased proficiency in the use of the adaptive letter-by-letter procedure rather than to the resolution of the underlying deficit. It is suggested that longitudinal neurobehavioral studies add to our understanding of the alexic deficit and provide insight into the recovery process.

Adult↗
PubMed1990

Perceptual and conceptual mechanisms in neglect dyslexia. Two contrasting case studies.

The contribution of peripheral, data-driven effects is contrasted with conceptual, 'top-down' effects to the reading performance of 2 subjects with neglect dyslexia following a single right hemisphere lesion. Several tasks were administered, manipulating the physical, lexical or morphemic properties of the stimuli in an attempt to establish whether the attentional deficit disrupts reading at an early or late stage of processing. Both subjects were impaired at detecting elementary stimulus features on the left side of the display but were even more impaired at identifying conjoined features. One subject's performance was influenced by structural manipulations which altered the low-level representation of the stimulus. The other was less affected by structural changes of the stimuli but was influenced by the lexical and morphemic status of the words. This apparent double dissociation is interpreted as arising from a graded attentional deficit at a single locus, early in the reading process where low-level information is detected. When the deficit is not severe sufficient information may be picked up and may interact with higher order lexical knowledge to offset partially the peripheral malfunction. For a severe attentional deficit, top-down knowledge is not engaged as insufficient information is processed on the left-hand side. This hybrid view of attention provides insight into the mechanisms underlying neglect dyslexia and bears on the role of attention in normal visual processing.

Aged↗
PubMed1989

Category-specific treatment of a lexical-semantic deficit: a single case study of global aphasia.

The aim of this study was to improve the comprehension of single items in a subject with global aphasia. Existing models of semantic organisation were used to inform and guide the treatment programme. Detailed pre-therapy testing suggested a severe comprehension deficit with an inability to obtain a precise semantic specification of the items, irrespective of modality of input. A category-specific hierarchical treatment programme including generic and specific details about items was implemented through various exercises and drills. Post-therapy testing revealed a significant improvement on treated and untreated items of treated categories but limited generalisation to items in untreated categories. Improvement in the subject's general semantic abilities was also documented following treatment. The results, which are shown to be a direct consequence of the intervention programme, lend support to the categorical and hierarchical view of the organisation of semantics and provide a basis for future rehabilitation studies in this area.

Aphasia↗