Regular Impulse Activity of Single Units in the Cat Striate Cortex

Abstract

IN the course of an investigation into the spontaneous and evoked activity of single cells in the visual cortex of unanæsthetized, unrestrained cats, a number of cells have been discovered, remarkable for their regularity and rate of discharge. Each of these cells, of which five have so far been observed, discharged a single spike whenever it fired, and each spike was followed by a long silent period lasting many seconds. As may be seen in Fig. 1, the inter-spike intervals for a given cell are, with few exceptions, distributed over a very narrow range. In calculating the mean value of the inter-spike intervals of cells 2, 3 and 4, certain intervals were omitted. In cell 2, for example, the interval which was omitted lasted 20.29 sec., that is, approximately twice the modal inter-spike interval (9.5–9.99 sec.) for this unit. Similar prolonged periods of silence occurred occasionally between action potentials from cells 3 and 4, but always these gaps were, to a close approximation, a multiple of the modal interval. It is possible that the missing spikes were lost in the base-line of the record; but there were instances where the failure to fire appeared genuine, for careful examination of those records where the signal/noise ratio was particularly good failed to reveal the missing spike. That this property of maintaining the overall rhythm is real is suggested from the behaviour of cell 4. The mean value of the regular inter-spike intervals of this cell was 9.42 sec.; but of the three intervals excluded in calculating this value, one was 18.39 sec. (approximately twice the expected interval), one was 6.60 sec. and one was 3.21 sec. These last two intervals occurred in succession so that the overall regularity of discharge was maintained.