Adaptive mechanisms in the elasmobranch hindbrain

The suppression of self-generated electrosensory noise (reafference) and ot her predictable signals in the elasmobranch medulla is accomplished in part by an adaptive filter mechanism, which now appears to represent a more uni versal form of the modifiable efference copy mechanism discovered by Bell, It also exists in the gymnotid electrosensory lateral lobe and mechanosenso ry lateral line nucleus in other teleosts, In the skate dorsal nucleus, mot or corollary discharge, proprioceptive and descending electrosensory signal s all contribute in an independent and additive fashion to a cancellation i nput to the projection neurons that suppresses their response to reafferenc e, The form of the cancellation signal is quite stable and apparently well- preserved between bouts of a particular behavior, but it can also be modifi ed within minutes to match changes in the form of the reafference associate d with that behavior. Motor corollary discharge, proprioceptive and electro sensory inputs are each relayed to the dorsal nucleus from granule cells of the vestibulolateral cerebellum. Direct evidence from intracellular studie s and direct electrical stimulation of the parallel fiber projection suppor t an adaptive filter model that places a principal site of the filter's pla sticity at the synapses between parallel fibers and projection neurons.