Facilitatory and inhibitory frequency tuning of combination-sensitive neurons in the primary auditory cortex of mustached bats

Mustached bats, Pteronotus parnellii parnellii, emit echolocation pulses th at consist of four harmonics with a fundamental consisting of a constant fr equency (CF1-4) component followed by a short, frequency-modulated (FM1-4) component. During flight, the pulse fundamental frequency is systematically lowered by an amount proportional to the velocity of the bat relative to t he background so that the Doppler-shifted echo CF2 is maintained within a n arrowband centered at similar to 61 kHz. In the primary auditory cortex, th ere is an expanded representation of 60.6- to 63.0-kHz frequencies in the " Doppler-shifted CF processing" (DSCF) area where neurons show sharp, level- tolerant frequency tuning. More than 80% of DSCF neurons are facilitated by specific frequency combinations of similar to 25 kHz (BFlow) and similar t o 61 kHz (BFhigh). To examine the role of these neurons for fine frequency discrimination during echolocation, we measured the basic response paramete rs for facilitation to synthesized echolocation signals varied in frequency , intensity, and in their temporal structure. Excitatory response areas wer e determined by presenting single CF tones, facilitative curves were obtain ed by presenting paired CF tones. All neurons showing facilitation exhibit at least two facilitative response areas, one of broad spectral tuning to f requencies centered at BFlow corresponding to a frequency in the lower half of the echolocation pulse FM1 sweep and another of sharp tuning to frequen cies centered at BFhigh corresponding to the CF2 in the echo. Facilitative response areas for BFhigh are broadened by similar to 0.38 kHz at both the best amplitude and 50 dB above threshold response and show lower thresholds compared with the single-tone excitatory BFhigh response areas. An increas e in the sensitivity of DSCF neurons would lead to target detection from fa rther away and/or for smaller targets than previously estimated on the basi s of single-tone responses to BFhigh. About 15% of DSCF neurons show obliqu e excitatory and facilitatory response areas at BFhigh so that the center f requency of the frequency-response function at any amplitude decreases with increasing stimulus amplitudes. DSCF neurons also have inhibitory response areas that either skirt or overlap both the excitatory and facilitatory re sponse areas for BFhigh and sometimes for BFlow. Inhibition by a broad rang e of frequencies contributes to the observed sharpness of frequency tuning in these neurons. Recordings from orthogonal penetrations show that the bes t frequencies for facilitation as well as excitation do not change within a cortical column. There does not appear to be any systematic representation of facilitation ratios across the cortical surface of the DSCF area.