Biomedical subjects

V Kavcic

Publications and source records attributed to V Kavcic.

4 recordsLinked to original sources

PubMed2000

Hemispheric interaction, metacontrol, and mnemonic processing in split-brain macaques.

These experiments explored the interactions remaining between the cerebral hemispheres in two split-brain macaques. The 'split' was earlier confirmed by showing that one hemisphere was incapable of identifying visual images seen by the other. The critical tests for residual interactions were intermingled with control trials in a continuous recognition task. These tests were of two kinds: 'parallel processing', to determine how simultaneous viewing by both hemispheres affected subsequent recognition by one of them alone; and 'conflict', where opposite responses were demanded from the two hemispheres, thus assessing the issue of metacontrol. Two types of stimuli were also employed: ART, in which each hemisphere saw essentially the same image; and BIPARTITE, in which images were entirely different for each hemisphere. Since, with either type of stimulus, performance was best when viewed by both hemispheres at both encoding and retrieval, 'parallel processing' was highly efficient. However, when both hemispheres viewed initially and only one was subsequently queried, performance was significantly worse than when each hemisphere acted alone on each occasion. It is thus reasoned that when both hemisphere view together, the resultant memory trace somehow reflects the bilaterality, a conclusion concordant with observations of Marcel on blindsight. Processing different images (BIPARTITE) was somewhat more disruptive in this regard than if the same image was viewed by each hemisphere. This was particularly true in the conflict situation, where for one hemisphere the item seen was NEW and for the other it was OLD. A response of 'OLD' was, at first, consistently rewarded. When this well-established protocol was changed, the hemispheres in each animal were gradually able to revise their joint behavior. This, together with the effect of disparate images, and the deficiency evoked when the animals were forced to recognize unilaterally an image first viewed under bilateral conditions, all manifest considerable, and complex, interaction between the hemispheres despite absence of the forebrain commissures. The superior colliculus seems a likely focal point for such interhemispheric effects.

Animals↗

PubMed2000

Temporal cost of switching between kinds of visual stimuli in a memory task.

These experiments measured the extra time required to respond when the type of stimulus was changed from one trial to the next. Heretofore, switching costs have been measured for a change in task, but we wished to isolate the cost of changing the sensorial component per se and its necessary analytical processing, as distinct from changing the question being posed or the type of response to be given. Thus, the task was identical throughout the experiments: continuous recognition in a single, 240-trial session, in which the subject was required to distinguish initial from repeat appearances of a stimulus, the single repetition of each stimulus occurring after 1-31 intervening trials. There were two categories of 200-ms stimuli, linguistic (words and non-words) and images (multiple-colored or gray scale panels, human faces, or butterflies); and two conditions of switching, predictable (alternating on each trial) or unpredictable, in which the switch occurred after three to eight trials of one kind. In the majority of cases, there was a robust switching cost, from 24 to 92 ms. The similarity of costs in the predictable and unpredictable modes suggests that this cost is derived at least as much from terminating the modus operandi for the preceding type of stimulus as from a reconfiguration of processing for the new type of stimulus. In the switch between words and images, the costs were "paradoxical" (asymmetrical), in that the switching from an image to a word was more costly than the reverse, that is, changing from the more to the less difficult required the greater time. This, too, is compatible with the idea that termination of the previous mode of processing is a major component of the cost. Thus, in contemplating the neuronal/cognitive events underlying visual memory, consideration must be given to the inertia of pre-existing linkages.

Adult↗

PubMed2000

Hemispheric interactions during a face--word Stroop-analog task.

A Stroop-analog task with faces and words was developed to investigate intrahemispheric and interhemispheric Stroop effects (SEs). Lateralized faces and words were used in an attempt to invoke right- and left-hemispheric specialization, respectively. A prototypical male face, female face, and baby face and the corresponding words man, woman, and baby were presented as congruent or incongruent face-word pairs either to the same visual field (i.e., unilateral presentations) or to separate visual fields (i.e., bilateral presentations). Bidirectional SEs were found. Word distractors interfered with the identification of face targets, and, somewhat surprisingly, an even greater SE was obtained when words were the target and faces were ignored. Laterality effects were most pronounced for bilateral trials, whereby the SE was larger for right-hemisphere than for left-hemisphere target presentations, irrespective of type of target. This finding suggests that the left hemisphere is generally better shielded than the right from interhemispheric interference effects.

Adolescent↗

PubMed1999

Long-term reversal of hemispheric specialization for visual memory in a split-brain macaque.

Accuracy of response and pattern of ocular fixations in three split-brain macaques were used to evaluate performance of each hemisphere in a continuous visual recognition task. The animal indicated by ocular fixation upon response points whether a displayed image or face was 'NEW' or 'OLD'. An inadvertent lesion of cingulate gyrus severely reduced contralateral fixations and impaired performance of the affected hemisphere in one animal, confirming the inferred relation between hemisphere and laterality of fixations. The hemispheres in the other two animals were initially remarkably similar in accuracy with human faces and with images; but the right hemisphere was significantly superior to the left for macaque faces. Parallel to this, in the one animal tested while simultaneously using both eyes/hemispheres, fixations were made primarily on the left half of human and macaque faces (right hemispheric control), whereas for images the ocular fixations were predominantly focused on the right half. However, after further, extensive training the left hemisphere performed with significantly greater accuracy than the right on all material and this shift was accompanied and further corroborated by a reversal of the fixational pattern to favor the right half of faces, as continued to be the case with images. Thus, over the long term both the pattern of ocular fixations and the accuracy of performance demonstrate a migration from right to left hemispheric dominance as familiarity with the task increased. Performance of the initially superior hemisphere actually diminished with this shift, presenting a uniquely puzzling question of hemispheric balance in the absence of the forebrain commissures.

Animals↗