Adaptive changes in firing of primary auditory cortical neurons following illumination shift from light to dark in freely moving guinea pigs.

Some animals are forced to rely more on non-visual signals, such as audition or olfaction, than on vision when a bright environment becomes dark. By recording from a primary-like auditory cortex (field A) in freely moving guinea pigs, possible changes in the responsiveness of single units were explored in association with illumination changes. For a subset of units, we found that robust decreases (off-decrease) or increases (off-increase) in baseline discharge (BsD) were initiated soon after room light was silently extinguished. These neuronal changes were accompanied by the initiation of explorative locomotion, possibly reflecting a changed internal brain state. Preferred acoustic stimuli evoked salient excitatory responses against the reduced BsD level in the dark for the off-decrease units, and salient inhibitory responses against the increased BsD level for the off-increase units. Histological verification indicated that the units showing such BsD changes were located predominantly in layer V or its vicinity. These results are discussed in the context of the effects of the brainstem neuromodulatory systems that are activated during behavioral adaptation to new environments.