Inferior olivary neurons in the awake cat: detection of contact and passive body displacement.

We have recorded from 306 neurons in the inferior olive of six alert cats. Most of the cats were trained to perform a simple task with the forelimb. We observed the neural responses to a wide variety of cutaneous and proprioceptive stimuli, as well as responses during spontaneous and learned active movements. Neurons responsive to somatosensory stimulation were found in all parts of the inferior olive, and they were roughly evenly divided between those responsive to cutaneous stimulation and those responsive to proprioceptive stimulation. In the dorsal accessory olive all neurons were responsive to somatosensory stimulation. In the medial accessory nucleus 88% and in the principal olive 74% of cells were responsive to somatosensory stimulation. Cells responsive to cutaneous stimulation usually had small receptive fields, commonly on the paw. These cells had low-threshold responses to one or more forms of cutaneous stimulation and typically fired one spike at the onset of the stimulus on 80% or more of stimulus applications. Cells responsive to proprioceptive stimulation most commonly responded to passive displacements of a limb. These cells were often very sensitive, responding to linear displacements of less than 1 cm in one specific direction. No cells in our sample responded reliably during active movement by the animal. Only 21% of cells responding to passive proprioceptive stimulation showed any modulation during active movement, and the modulation was weak. Likewise, cells responsive to cutaneous stimulation generally failed to respond when a similar stimulus was produced by an active movement by the animal. Exceptions to this were stimuli produced during exploratory movements or when the receptive field unexpectedly made contact with an object during active movement. Electrical stimulation applied in the inferior olive failed to evoke movements or to modify ongoing movement. Our results are consistent with the hypothesis that inferior olivary neurons function as somatic event detectors responding particularly reliably to unexpected stimuli.