Stimulus specific adaptation in the auditory system of insects: Can a single neuron learn?

Abstract

Stimulus-specific adaptation (SSA) is the suppression of a neuron's activity to repetitive stimuli while maintaining responsiveness to infrequent signals. In Neoconocephalus katydids, one auditory interneuron (TN-1) shows strong SSA in oddball paradigms, when standard and oddball pulses differ in carrier frequency. SSA occurred for pulse rates from >140 Hz down to 1 Hz. At fast repetition rates (>100 Hz), responses to the common pulses ceased, while oddballs elicit single spikes. At slower rates (<50 Hz), both standard and oddball pulses elicited spiking, with responses to oddballs being significantly larger, comparable to SSA described in vertebrate hearing systems. At slow rates, SSA also occurred when the shape of standard and oddball pulses differed, while having identical spectral properties. We identified at least two dendritic mechanisms that contribute to SSA at fast pulse rates (>100 Hz). The mechanisms underlying SSA at slow pulse rates are less well understood; likely, dendritic Ca2 + -gated currents contribute to SSA at slow pulse rates. At slow rates, response reduction to repeated pulses resembles habituation as oddball pulses cause dishabituation, i.e., the response to the following standard pulse is larger than that to the preceding one. Whether this habituation is generated within TN-1 or by its synaptic inputs remains an open question.