Multiple combination-sensitive neurons in the auditory cortex of the mustached bat

Abstract

The mustached bat, Pteronotus parnellii, emits biosonar pulses consisting of four constant-frequency (CF1–4) and four frequency-modulated (FM1–4) components. The FM–FM area of its auditory cortex consists of three subdivisions, containing either FM1–FM2, FM1–FM3 or FM1–FM4 combination-sensitive neurons. The FM–FM area also contains ‘multiple combination-sensitive’ neurons: FM1–FM2,3, FM1–FM3,4, FM1–FM2,4, and FM1–FM2,3,4 neurons. All FM–FM neurons are tuned to a time delay (echo delay) of FMn (n=2–4) from FM1. In the present study, we made the following four major findings. (1) Multiple combination-sensitive neurons show the strongest response to a combination of more than two signal elements. (2) Multiple combination-sensitive neurons are located in about 100 μm wide bands at the boundaries between two adjacent subdivisions of the FM–FM area. (3) Iso-best-delay contour lines across the three single combination-sensitive subdivisions are not interrupted by multiple combination-sensitive bands. (4) Each subdivision of the FM–FM area has frequency-vs.-frequency coordinates in terms of best FM1 and best FMn frequencies for facilitation, although such coordinates were not obtained with single tone bursts.