Effect of early exposure to patterned sound on unit activity in rat inferior colliculus.

1. Young rats were exposed to one of two patterns of sounds for 5 h daily during the first 4 mo of life. The up pattern consisted of a tone swept from 6 to 9 kHz in 1 s alternated with a 1-s noise burst. The down pattern differed in that the sweeps were from 9 to 6 kHz. 2. A pattern evoked more spikes, on the average, from units in the inferior colliculus of rats exposed to that pattern than from units in animals exposed to the other pattern. 3. The exposure effect was most pronounced in the unit responses to the noise-burst segment within a pattern suggesting a long-lasting, malleable influence of the tone sweep which defined the pattern. Responses to pattern noise components were less for both exposed groups than for the unexposed controls, suggesting that inhibitory mechanisms were responsible for the pattern discrimination evident in the responses of the exposed unit population. 4. Unit responses in unexposed rats were somewhat more selective for the down pattern so that the resulting shift in response selectivity was relatively more apparent for exposure to the up pattern. 5. While control and down-exposed units generally responded more to both the tone sweep and noise burst in one pattern, a large proportion of the up-exposed unit population continued to favor the down over the up-swept tone, but responded more to the up-pattern noise bursts. This suggests that the unit responses to the noise bursts did not simply reflect prolonged responses to the tone sweeps. 6. No similar effects were seen for units from mothers similarly exposed to the same patterns.