Combination-sensitive neurons in the medial geniculate body of the mustached bat: encoding of relative velocity information

Abstract

1. Orientation sounds (pulses) emitted by the mustached bat (Pteronotus parnellii) consist of up to four harmonics (H1-4); each harmonic contains a constant frequency (CF) component and a terminal frequency modulated (FM) component, so that there are eight components in total (CF1-4 and FM1-4). By referring the echo from a target to the emitted pulse, the mustached bat derives velocity information from Doppler shift and distance information from echo delay. In this study, the responses of single neurons in the medial geniculate body (MGB) to synthetic biosonar signals were investigated. Stimuli consisted of CF, FM, and CF-FM sounds. Paired CF-FM sounds were used to mimic any two harmonics of pulse-echo pairs. The dorsal and medial divisions of the MGB were found to contain combination-sensitive neurons. These neurons responded poorly to individual sounds regardless of frequency and amplitude and were facilitated by paired sounds presented at particular frequencies, amplitudes and inter-component intervals (simulated echo delay). Combination-sensitive neurons were tuned to the frequencies that characterize particular components of natural biosonar signals and were classified according to the components of pulse-echo pairs that best matched the spectral selectivity of the neuron. Two classes of combination-sensitive neurons were found, CF/CF and FM-FM. This paper focuses on CF/CF combination-sensitive neurons, which extract velocity information from paired CF components, and on CF2 and CF3 neurons, which, although not combination-sensitive, are tuned to the frequencies of the CF2 and CF3 components of biosonar signals. 2. CF2 and CF3 neurons were sharply tuned in frequency. The best frequencies of the most sharply tuned CF2 neurons were all approximately equal to 61.17 kHz (SD = 370 Hz), which closely matches the frequency at which P. parnellii stabilizes the CF2 component of an echo when compensating for Doppler shift. Thus CF2 neurons are specialized for a fine analysis of Doppler-compensated echoes. 3. Tuning curves of CF2 and CF3 neurons remained narrow regardless of stimulus level. When compared at high stimulus levels (30 and 50 dB above minimum threshold), bandwidths of tuning curves of CF2 and CF3 neurons were much smaller than those of peripheral auditory neurons turned to CF2 or CF3 frequencies but were about the same as those of cortical neurons tuned to CF2 or CF3 frequencies. Thus the sharpening of neural tuning curves by the bat's central auditory system occurs within or before the MGB.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses to Social Vocalizations in the Inferior Colliculus of the Mustached Bat Are Influenced by Secondary Tuning Curves

“…Because many of the mustached bat social vocalizations contain energy within this low-frequency range (Kanwal et al 1994), this low-frequency tuning caused the 60-kHz neurons to respond to many social vocalizations. This is a significant finding because the 60-kHz region of auditory nuclei in the mustached bat is hypertrophied and has always been thought to primarily be involved in encoding echo information (Olsen and Suga 1991;Suga 1989;Suga and Jen 1976;Suga et al 1975). For instance, the sharp tuning of these neurons to 60 kHz makes them ideally suited to process echo information related to Doppler shift, which allows the bat to determine its relative velocity (Suga and Jen 1976;Suga et al 1975).…”

On the prediction of sweep rate and directional selectivity for FM sounds from two-tone interactions in the inferior colliculus

“…However, a growing number of two-tone studies have uncovered rich facilitatory interactions that are dependent on either the spectral or temporal relationship between 'masker' and probe (Leroy and Wenstrup, 2000;Mittmann and Wenstrup, 1995;Olsen and Suga, 1991;Schreiner, 1997, 2000;Brosch et al, 1999). The facilitatory interactions described in these studies exert their maximal effect when probe and masker are widely separated in frequency, typically when the tones are related harmonically.…”

Biosonar navigation above water II: exploiting mirror images

“…To determine whether a unit was delay-tuned, we tested whether the response elicited by stimulus pairs was significantly stronger (t-test, P Ͻ 0.05) than the sum of the responses elicited by the two stimuli presented alone, a criterion indicating facilitative interactions (e.g., Olsen and Suga 1991).…”