1. The little brown bat, Myotis lucifugus, employs biosonar pulses containing broadband frequency -modulated (FM) sounds of only one harmonic during the initial phases of echolocation. Neurons throughout the auditory cortex exhibit delay-dependent facilitation to artificial pulses and echoes at particular echo delays. Extracellular unit recordings of these delay-sensitive neurons determined the essential frequency components in the sound pair and their relative timing for evoking maximum facilitation. 2. The entire 60-kHz sweep of both the simulated pulse and echo were divided into four equal spectral quarters (Ist, IInd, IIIrd, and IVth), each linearly sweeping 15 kHz downward in 1 ms, to determine the spectral parts essential for maximal facilitation. Maximal facilitation was evoked equally by pulse-echo pairs in which the sound components consisted of either the entire 60-kHz FM sweeps or only the essential quarters. Most neurons required the IVth quarter of the pulse and the echo for delay sensitivity. This is consistent with the hypothesis that the essential quarters swept excitatory frequencies just above inhibitory frequencies. 3. The spectral and temporal contributions to delay sensitivity were examined independently. The spectral content for each spectral quarter of echo was varied in echo delay, and the sound-pair responses were compared. Maximal facilitation in individual delay-sensitive neurons required both a specific part of the echo spectrum and a specific echo delay. 4. The FM sweeps of the essential pulse and echo quarters were further narrowed to their minimum bandwidth, and the essential pulse frequencies (EPFs) and essential echo frequencies (EEFs) were determined. Both the EPFs and EEFs averaged approximately 8 kHz in FM bandwidth and represented different spectral parts of the echolocation pulse emitted by this FM bat. All neurons showed delay sensitivity to search stimuli in which pulse-echo stimuli consisted of 15-kHz FM pairs. 5. Delay sensitivity in virtually all neurons required pulse and echo components whose essential frequencies differed. However, some spectral overlap was found between the pulse and echo in 39% of these neurons. The majority of neurons (81%) required a pulse and echo in which their mean frequencies differed by>or = 16 kHz. This includes neurons with pulse and echo overlapping spectrally and those with sound components showing no overlap but separated by a relatively small frequency range. 6. The facilitative frequency-tuning curves of individual neurons were measured with their essential pulse and echo frequencies.(ABSTRACT TRUNCATED AT 400 WORDS)
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