Periodic components in steady-state activity of cuneate neurones and their possible role in sensory coding.

1. Individual cuneate touch, touch-hair and proprioceptive neurones often display periodic components in their steady-state resting or evoked discharges. The periods may be visible in the interspike interval distribution or may be revealed only by the expectation density function. Several levels of complexity were identified; from one to four mutually prime, periodic components may be present, with or without an aperiodic component.2. The periodic components usually depend on the peripheral input, as shown by their introduction or modification by peripheral stimulation and by their disappearance following deafferentation.3. The coefficient of variation of periodic discharges by touch and touch-hair neurones was 0.024-0.23 (aperiodic, driven discharges usually had CV's of 0.64-0.77).4. The occasional recording extracellularly of a periodically occurring, unitary prepotential preceding the spike and the intracellular recording of a periodic, ;giant', unitary e.p.s.p. imply that an individual periodic impulse in an afferent fibre may elicit a post-synaptic discharge. Even with threshold stimulation, the timing of post-synaptic discharge was quite precise.5. The periodicity did not appear to reflect a cycle of subnormality intrinsic to the post-synaptic neurone, because a current pulse injected through a micro-electrode lying just external to the membrane could block the next expected periodic discharge, but did not continuously vary its waiting time.6. A mathematical model that fits some discharge patterns predicts the interval distribution from an over-all probability of response to the periodic input: each periodic afferent impulse is treated as an independent trial resulting in success (post-synaptic discharge) or failure. Some other interval distributions are fitted by a ;conditioned' model in which a success reduces the probability of response at the next periodic trial.7. The discussion includes a hypothesis of tactile discrimination, in which the output of a sensory relay, e.g. cuneate neurone, signals both the number of afferent fibres converging upon it and the afferent period in each.