Classification of unit responses in the auditory cortex of the unanaesthetized and unrestrained cat

Although there have been many detailed studies of neural unit responses at lower levels of the auditory pathways, there have been relatively few at the cortical level. The majority of these have been carried out under anaesthesia (e.g. Thomas, 1952; Davies, Erulkar & Rose, 1954; Erulkar, Rose & Davies, 1956; Hind, Davies, Woolsey, Benjamin, Welkes & Thompson, 1960). The deleterious effect of anaesthesia on the responses of auditory cortical units has been well shown by Thomas (1952), Erulkar et al. (1956), and Katsuki, Murata, Suga & Takenaka (1959). Anaesthesia reduces the number of units encountered by a micro-electrode (Katsuki, Murata, et al. 1959) and reduces the capacity of units to respond to auditory stimuli (Thomas, 1952; Erulkar et al. 1956). Similar conclusions have been drawn from observations on other regions of the cortex (e.g. Renshaw, Forbes & Morison, 1940; Woldring & Dirken, 1950; Li & Jasper, 1953). Katsuki, Watanabe & Maruyama (1959) and Katsuki, Watanabe & Suga (1959) obtained data on the unanaesthetized immobilized cat, but only two series of studies appear to have been carried out on the intact unrestrained unanaesthetized animal: those of Katsuki, Murata et al. (1959), and Katsuki, Murata, Suga & Takenaka (1960) on the cat and monkey, respectively, and those of Hubel, Henson, Rupert & Galambos (1959), and Bogdanski & Galambos (1960) on the cat. The findings of these two groups of workers differ somewhat, especially in regard to the distribution of types of unit responses among the total population. The experiments we have carried out afford a further study, on a quantitative basis, in which we have tried to account for some of the differences which have been reported.