Averaged potentials evoked from the brain during cognitive tasks, in the normal subject and in pathological states

Small, Marian (1986). Averaged potentials evoked from the brain during cognitive tasks, in the normal subject and in pathological states. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000fcd8

Abstract

Investigation of patients with cerebral lesions and tachistoscopic studies in both normal subjects and commissurectomised patients has shown that the right hemisphere has a distinctive role in the processing of faces. The initial aim of this study was to assess whether these indications of cerebral asymmetry, found with traditional methods, could be mirrored by electrophysiological techniques. Such as approach would help to define not only lateralisation of non-verbal processing but also localisation of function within a hemisphere.

Normal, right handed subjects (assumed to be left hemisphere dominant for language) were presented with four visual conditions: slides of known and unknown faces in order to observe any effect of recognition and two non-verbal control series which provided (i) a complex stimulus other than a face and (ii) a physical stimulus without cognitive content. Both P100 and P300 of the visual evoked response were recorded during all conditions. The similar mean amplitude and latency values obtained across subjects proved the technique to be capable of producing replicable and consistent results although minor changes were apparent in relation to the subjects' sex and age. The dissimilar topographies of P100 and P300 implied different origins for these two components, within the occipital cortex and association areas respectively.

The amplitude of P300 was larger over the right than the left hemisphere in response to all types of stimuli. However, this emphasis was significantly greater over the right side (p< 0.001) with face slides than during the control conditions, inferring that the asymmetry was associated with facial factors rather than stimulus complexity. Absence of this right sided superiority from the same group of subjects in response to inverted face slides strongly suggested that the asymmetry was specific to vertical orientation, i.e. upright faces. The fact that there was no difference between the response to known and unknown faces showed that the right sided P300 superiority occurred without relation to aspects of facial memory; it was present regardless of whether or not a face had been viewed before.

With left handed normal subjects, assumed to be less lateralised for non verbal processing, the right hemisphere P300 amplitude emphasis was not apparent suggesting that the asymmetry previously recorded in dextrals represented organisation of cerebral function.

Investigation of language lateralisation, using this evoked potential technique, was also carried out in right handed subjects. In response to slides of words there was no converse (left greater than right) P300 amplitude asymmetry except for an increase over the left temporal region in females. Word stimuli, unlike non-verbalv material, showed P300 to be of equal amplitude at all electrode sites which, together with the absence of an asymmetry, implied that either verbal processing takes places in structures inaccessible to surface recording or that such function is not so clearly localised.

The opportunity was also taken to investigate patients with cerebral dysfunction in order to observe any associated alteration in waveform. They were presented with the same types of stimuli described above. The findings in a patient with prosopagnosia suggested the presence of bilateral cortical damage at an early stage in visual processing and his dissociation of P100 latency across conditions also implied that stimulus specificity may occur in man at an early perceptual level. The results from the final experiment, which involved patients with missile injuries to the brain, similarly provided evidence of stimulus specificity. Furthermore, men from this group with right sided wounds showed variations in P100 latency depending on the type of stimulus, providing evidence of functions differentiation within the right hemisphere at an early stage in visual processing.