Sound source position discrimination in rats: acquisition and performance as a function of continuous and pulsed stimuli

Abstract

The aim of the present study was to account for the observation that temporal characteristics of a novel acoustic stimulus can affect an organism's ability to acquire a discrimination of that stimulus. In a preliminary go-right go-left auditory discrimination study, two groups of rats were trained under a novel 3 kHz to 25 kHz, 80 dB (SPL) noise signal. In one group the noise was presented in pulsed trains of 0.5 sec noise, 0.5 sec silence. In the other group the noise was presented continuously. Discrimination of the pulsed signal was acquired rapidly, reaching 90 percent correct responses or better within the first few trials or by the second session. In the continuous noise group, a slower rate of acquisition and an overall lower level of accuracy was acheived. Experiment 1 examined whether the continuous-pulsed effect could be replicated in a larger apparatus. Results did not differ from the preliminary study. Experiment 2 examined the role of stimulus intensity on acquisition of stimulus control using the large enclosure. Rats were trained under continuous and pulsed noise with intensities either 95 dB, 65 dB or 61 dB (SPL). Together with the 80 dB data, the results are consistent with the Stimulus Intensity Dynamism phenomenon in that within each condition, highest percent correct performance and shortest mean latencies were obtained at the highest intensities. Experiment 3 investigated approach and exploratory behavior to the novel sounds independent of lever press and magazine training. Five second trains of 80 dB, 65 dB or 61 dB (SPL) novel pulsed or continuous noise were presented for three sessions. Differences in response topography were observed. Animals exposed to the pulsed stimulus showed a clearly superior ability to accurately approach and explore the sound source. Higher intensity continuous noise produced immediate and rapid approach behavior, but these animals did not successfully make contact or align the head and body with the sound source. It was concluded that although high intensity stimuli evoke faster responding, faster responding does not necessarily correspond with improved accuracy. The pulsed feature of the signal appears essential to produce accurate approach and exploration of novel sound sources, and may account for the continuous-pulsed effect.