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Abstract
The generation of expectations about sensory input and the subtraction of such expectations
from actual input appear to be important features of sensory processing. This paper
describes the generation of sensory expectations within cerebellum-like structures
of four distinct groups of fishes: Mormyridae; Rajidae; Scorpaenidae; and Apteronotidae.
These structures consist of a sheet-like array of principal cells. Apical dendrites
of the principal cells extend out into a molecular layer where they are contacted
by parallel fibers. The basilar regions of the arrays receive primary afferent input
from octavolateral endorgans, i.e., elec-troreceptors, mechanical lateral line neuromasts,
or eighth nerve endorgans. The parallel fibers in the molecular layer convey various
types of information, including corollary discharge signals associated with motor
commands, sensory information from other modalities such as proprioception, and descending
input from higher stages of the sensory modality that is processed by the structure.
Associations between the signals conveyed by the parallel fibers and particular patterns
of sensory input to the basal layers lead to the generation of a negative image of
expected sensory input within the principal cell array. Addition of this negative
image to actual sensory input results in the subtraction of expected from actual input,
allowing the unexpected or novel input to stand out more clearly. Intracellular recording
indicates that the negative image is probably generated by means of anti-Hebbian synaptic
plasticity at the parallel fiber to principal cell synapse. The results are remarkably
similar in the different fishes and may generalize to cerebellum-like structures in
other sensory systems and taxa.