Biomedical subjects

Voyko Kavcic

Publications and source records attributed to Voyko Kavcic.

4 recordsLinked to original sources

PubMed2003

Spatial disorientation in Alzheimer's disease: the remembrance of things passed.

BACKGROUND: Patients with Alzheimer's disease (AD) and many older adults become lost even in familiar surroundings. This is commonly attributed to memory impairment, but it may reflect impaired spatial cognition. METHODS: The authors examined the role of memory, perceptual, and cognitive mechanisms in spatial disorientation by comparing the performance of normal young (YN), middle-aged (MA), older adult (OA), and AD subjects on neuropsychological and spatial orientation tests. RESULTS: The tendency to become lost is shared by almost all patients with AD (93%) and some OA subjects (38%). This impairment is not related to memory impairment. Instead, it reflects an inability to link recognized scenes with locations in the environment. CONCLUSIONS: Spatial disorientation reflects the impaired linking of landmarks and routes that should be assessed in conjunction with routine memory testing in elderly patients.

Adult↗

PubMed2003

Attentional dynamics and visual perception: mechanisms of spatial disorientation in Alzheimer's disease.

Visuospatial disorientation forces Alzheimer's disease patients to abandon independent activities. We found previously that limitations of ambulatory and vehicular navigation are linked to impaired visual motion processing in Alzheimer's disease. We now hypothesize that these perceptual impairments reflect temporal constraints on visual attention. We evaluated attentional, perceptual and neuropsychological capacities in 14 Alzheimer's disease patients and 12 age-matched older normal controls. The temporal dynamics of visual attention were measured using rapid serial visual presentation (RSVP) to assess the attentional blink. Visual processing for spatial orientation was assessed using perceptual thresholds for optic flow, the visual motion seen during observer self-movement. Alzheimer's disease patients show an exaggerated attentional blink during RSVP, identifying the first of two targets but missing the second target depending on the number of intervening distractors. They also show a unique form of attentional masking in which they miss the first target but identify the second, again depending on the number of intervening distractors. Both types of RSVP errors are correlated with selectively elevated optic flow thresholds in Alzheimer's disease patients. This suggests that temporal constraints on visual perception might impair optic flow analysis and contribute to spatial disorientation in Alzheimer's disease. These findings are consistent with two-stage models of visual perception, suggesting that the working memory mechanisms in the second stage provide feedback control of input to category-specific perceptual processors in the first stage.

Aged↗

PubMed2003

Frontal cortex, laterality, and memory: encoding versus retrieval.

The cerebral hemispheres differ in their capabilities and response to verbal versus nonverbal visual material. A priori, it might thus be expected that the right hemisphere would be best activated during a mnemonic task with fMRI when using nonverbalizable images, and the left hemisphere with verbal material. However, previous psychological tests had shown a high degree of similarity in measures of memory for these disparate items. It was thus hypothesized that extensive commonality in the areas activated would prevail when this previously tested material was employed with fMRI. Six subjects underwent fMRI with four types of trials in blocks: fixating; passively viewing 12 words and 12 nonverbalizable images; endeavoring to remember (encoding) another set of 12 words and images; endeavoring to recognize (retrieve) previously viewed words or images. Passive viewing produced small islands of activation in left versus right frontal cortex for words and images, respectively. Endeavoring to remember enlarged the areas of activation and produced some bilaterality. Retrieval greatly augmented activation as well as bilaterality, and some 20% of the activated frontal volume was shared by words and images. Thus, on the one hand, the distribution of activation upon retrieval differed substantially for words versus images, but on the other, as predicted, there was considerable commonality. Predominant laterality of activation in some areas shifted between encoding and retrieval (HERA), importantly involving different regions for words versus images. Of course, processes other than memory per se are undoubtedly involved in these distributions of fMRI activation in frontal cortex, yet the nature of the to-be-remembered items is clearly a major factor, in accord with the asymmetric lateralization in their basic representation.

Adult↗

PubMed2002

Efficiency of the forebrain commissures: memory for stimuli seen by the other hemisphere.

How well will one cerebral hemisphere recognize items viewed initially via the other? Nonverbalizable images or words were presented to one visual field and memory for them tested in the same or the other visual field. The initially viewing hemisphere subsequently had no secure advantage in accuracy, and only for images was there a 30-ms (ca 3%) penalty in reaction time for viewing with the "other" hemisphere. Interhemispheric mnemonic communication is thus highly reliable. At longer retention intervals (1-2 min vs. 4-30 s, with accumulating added stimuli), however, recognition of words was asymmetric as to hemisphere, in that initial viewing via the right hemisphere was subsequently (and paradoxically) much better recognized via the left (other) hemisphere than was the converse situation. This suggests that the initial engram with right hemispheric viewing of words ultimately becomes established in the left, and that the right has less accurate access to a previous "left hemispheric view".

Adult↗