Delay-tuned neurons in the midbrain of the big brown bat.

1. The auditory midbrain in Eptesicus contains delay-tuned neurons that encode target range. Most delay-tuned neurons respond poorly to tones or individual frequency-modulated (FM) sweeps and require combinations of FM sweeps. They are combination sensitive and delay tuned. The index of facilitation (IF), a coefficient measuring combination sensitivity for individual delay-tuned neurons, ranged from 0.14 to 1.0, with an average of 0.64 +/- 0.24 (mean +/- SD). Of the 33 facilitated responses from 29 neurons, 23 (70%) exhibited IFs > 0.5, which corresponds to a facilitated response 3 times greater than the sum of the responses to individual pulse and echoes. Thus the responses of midbrain delay-tuned neurons are highly combination sensitive. 2. The response of midbrain delay-tuned neurons is phasic, with an average of 0.7 +/- 0.4 action potentials elicited per optimal pulse-echo pair. Thus midbrain delay-tuned neurons in Eptesicus act as probability encoders. 3. The distribution of best echo delays (BDs) of midbrain delay-tuned neurons ranged from 8 to 30 ms. As an ensemble, midbrain delay-tuned neurons encode target ranges of 138-516 cm. There is a basic correspondence between the physiologically determined span of midbrain BDs between 8 and 30 ms and the behaviorally determined borders of the approach (8- to 17-ms echo delay) and search stages (17- to 30-ms echo delay) of the insect pursuit sequence. Midbrain delay-tuned neurons can be separated into two subpopulations on the basis of the difference in distributions of the echo best amplitude (EBA) tuning at BD. The BDs of one subpopulation correspond to the span of search stage echo delays, and the BDs of the other subpopulation correspond to the span of approach stage echo delays. 4. EBAs of neurons in each subpopulation are tailored to the specific perceptual requirements of the corresponding behavioral stage. EBAs of midbrain neurons tuned to echo delays between 17 and 30 ms (N = 12) correspond to the search stage and are suited to the requirements of target detection. EBAs of midbrain neurons tuned to echo delays between 17 and 30 ms (N = 21) correspond to the approach stage and are suited to the requirements of target size discrimination. 5. The best FM sweeps for the pulse (PFM) and echo (EFM) were determined for each midbrain neuron. PFMs appear to cluster at frequencies corresponding to the three harmonic peaks in the emitted pulse power spectra.(ABSTRACT TRUNCATED AT 400 WORDS)