Stimulus-specific adaptation in specialized neurons in the inferior colliculus of the big brown bat, Eptesicus fuscus.

The inferior colliculus (IC) of the big brown bat (Eptesicus fuscus) contains specialized neurons that respond exclusively to highly specific spectrotemporal patterns such as sinusoidally frequency modulated (SFM) signals or directional frequency modulated sweeps (FM). Other specialized cells with I-shaped frequency response areas (FRAs) are tuned to very narrow frequency bands (1-2 kHz) in an amplitude-tolerant manner. In contrast, non-specialized neurons respond to any stimulus with energy in their frequency response area. IC neurons in several mammalian species, including bats, demonstrate stimulus-specific adaptation (SSA), a reduction in response to a high-probability stimulus. To evaluate the relation between stimulus selectivity and SSA, we presented sounds using an oddball stimulus paradigm and recorded extracellular responses of IC neurons. SFM-selective cells (n = 10), FM-selective cells (n = 7), and cells with I-shaped FRAs (n = 13) did not show SSA under any of the conditions tested (NSSI = 0.009, 0.033, 0.020 respectively). However, non-specialized neurons (n = 52) exhibited various levels of SSA (NSSI = 0.163), with a subset of these cells displaying strong adaptation. These findings suggest that SSA is not a ubiquitous characteristic of all neurons in the bat IC, but is present only in a subset of non-specialized neurons.