Long term recordings in the cat motor cortex: unit activity and field potentials from sensory and brain stem stimulation as a means of identifying electrode position

A method of chronically implanting microwires (25 μm diameter, 90% Pt/10% Ir wire) in the cat pericruciate cortex is described. These wires can obtain simultaneous recordings of single and multiunit activity from different sites and multiple recordings from the same site on different postoperative days. The electrodes were permanently implanted in the anaesthetized animal and left in place. Recordings were carried out in the awake, unrestrained cat. Different units were recorded as a result of spontaneous electrode movement in the cortex. Movement was documented with field potential recording from the implanted electrodes and resulted in changes in the latency and polarity of the initial components of the cutaneous field potential and changes in the number of components of the field potential elicited from brain stem stimulation. Changes in the receptive fields of the units, changes in the number of units antidromically invaded from brain stem stimulation, and in the threshold and occurrence of muscle activity elicited from intracortical microstimulation supported the evidence of electrode movement obtained from field potential recordings and could be used to document electrode movements tangential to the plane of the cortical laminae. Movement was a slow gradual process throughout the period of implantation. The longest period from which a single unit could be recorded was 2 days. The need for this neurophysiological analysis is supported by evidence for irregular electrode movement which could not have been reconstructed from histological examination of the cortex.