Response properties of FM-FM combination-sensitive neurons in the auditory cortex of the mustached bat

Summary For echolocation, the mustached bat,Pteronotus parnellii rubiginosus, emits orientation sounds (pulses) and listens to echoes. Each pulse is made up of 8 components, of which 4 are constant frequencies (CF_1–4) and 4 are frequency-modulated (FM_1–4). Target-range information, conveyed by the time delay of the echo FM from the pulse FM, is processed in this species by specialized neurons in a part of the auditory cortex known as the FM-FM area. These cortical neurons are responsive to pulse-echo pairs at specific echo delays (Fig. 1). The essential components in the sound pair include the pulse FM₁ followed by an echo FM_n (n=2, 3 or 4). Downward sweeping FM₁-FM_n sounds that are similar to those the animal naturally hears during echolocation are the most effective in evoking facilitative responses. Most FM-FM neurons, however, still exhibit facilitative responses to stimulus pairs consisting of upward sweeping FM sounds and/or pure tones at frequencies found in FM sweeps (Figs. 2 and 3). The magnitude of facilitation is altered by changes in echo rather than pulse amplitude (Figs. 5 and 6). Neurons characterized by shorter best delays (or echoes from closer targets) do not require larger best echo amplitudes for facilitation.Abbreviations CF constant frequency - FM frequency modulation - H_n CF — FM harmonics of the mustached bat biosonar signal - CF_n CF components of the harmonics - FM_n FM components of the harmonics - PCF_n pulse CF_n - ECF_n echo CF_n - PFM_n pulse FM_n - EFM_n echo FM_n - PH_n pulse H_n - EH_n echo H_n - BA best amplitude for facilitation - BD best delay for facilitation - PST peri-stimulus-time - PSTC peri-stimulus-time-cumulative - dB SPL dB re 20 Pa