Aspects of the electrical activity in the cerebral cortex of various mammals.
thesisposted on 19.11.2015, 08:43 by Laurence. Howard
The first Section of this work concerns the origin of the potential waves evoked at the cerebral cortex of the cat by contralateral forepaw stimulation. Initially positive deflexions have been recorded in somatic Area I and initially negative waves in the region medial to this. It is suggested that there is one region, in the upper limb of the cortex at the base of the coronal sulcus, which receives an afferent projection from the dorsal tips of the contralateral digits. The current and voltage fields generated by the depolarisation of pyramidal cells in this region have been calculated from volume conductor theory and account for the different waves recorded from the cortical surface. Depth records of both unitary and mass extracellular potentials support this hypothesis. Work has also been undertaken on the species variation in the response evoked by contralateral forepaw stimulation. Those species with a flat cortical surface (rat, rabbit and coypu) yield similar evoked potentials, which are usually initially positive. This is in contrast with the responses observed on the convoluted cortex of the cat. These results suggest that cortical architecture is an important factor in the variety of responses observed in the cat. Some components of both mass and unitary potentials evoked at the cerebral cortex, by contralateral cortical stimulation are affected by the absence of spontaneous cortical activity at the instant of arrival of the afferent volley. In the absence of spontaneous activity the response is a pure positive wave, whilst in periods of activity there is a small positive deflexion followed by a large negativity. Evidence is put forward that this large surface negativity is the reflection of a superficial pocket of negativity, within the cortex, which is observed only during cortical activity.