No abstract

Mustached bats, Pteronotus parnellii parnellii spend most of their lives in the dark and use their auditory system for acoustic communication as well as echolocation. The sound spectrograms of their communication sounds or "calls" revealed that this species produces a rich variety of calls. These calls consist of one or more of the 33 different types of discrete sounds or "syllables" that are emitted singly and/or in combination. These syllables can be further classified as 19 simple syllables, 14 composites, and three subsyllables. Simple syllables consist of characteristic geometric patterns of CF (constant frequency), FM (frequency modulation), and NB (noise burst) sounds that are defined quantitatively using statistical criteria. Composites consist of simple syllables or subsyllables conjoined without any silent interval. Most syllable types exhibit a large intrinsic variation in their physical structure compared to the stereotypic echolocation pulses. Syllable domains are defined on the basis of multiple parameters, although these can be collapsed onto three dimensions that capture 99% of the measured variation among different types of syllables. Temporal analysis of multisyllabic constructs reveals several syntactical rules for syllable transitions.

No abstract

The time course of acoustic events is a critical element for the recognition of biologically rneaningful sounds. Echolocating bats analyze the time intervals between their emitted biosonar pulses and the echoes returning from objects to assess target distance (range). In this study, we have explored the auditory cortex of the mustached bat, Pteronotus parnellii rubiginosus, using pairs of acoustic stimuli mimicking the multiharmonic biosonar signals (pulses) used by this species and their echoes.A discrete field of auditory cortex dorsorostral to the tonotopicaJly organized primary field contains neurons which are insensitive to pure tone, frequency-modulated (FM), or noise stimuli presented singly. Rather, they respond strongly to pairs of stimuli, specifically, the fundamental FM component of the pulse paired with an FM component of one of the higher harmonics of the echo. We call these neurons FM1-FM, facilitation neurons. There are three separate longitudinal clusters in this cortical area containing FM1-FM2, FM1-FM3, and FMI-FM4 neurons, respectively. Moreover, FMI-FM, neurons are specifically sensitive to the time delay between the two FM components, i.e., the time delay of the echo from the pulse. Thus, they can decode target range.Two types of delay-sensitive neurons were found. Tracking neurons, whose response to echo delay varied according to repetition rate and stimulus duration, were found rarely. Delay-tuned neurons, which were tuned to specific time delays (best delays) of the echo from the pulse, were much more evident. Both types of neurons are organized into columns with similar best delays, and the best delay of delay-tuned neurons was found to increase systematically along the cortical surface in the rostrocaudal direction. This area, therefore, contains a neural representation of target range along this best delay axis. Such an axis exists in each of the clusters of FMI-FMz, FM]-FM3, and FMI-FM4 neurons. This is a new type of cortical organization which is not tonotopic but which represents an important acoustic cue related to the time course of acoustic events. Acoustic signals can be described in terms of physical parameters, such as frequency, amplitude, and time. time to convey information. Although the temporal patterns within phonemes and their sequences in the acoustic stream carry much of the total information in vocalizations, research in mammalian audition has focused mainly on the analysis and representation of frequency in the auditory system. That the cochlea is primarily a frequency analyzer (von B6k6sy, 1960) probably accounts for much of this emphasis. Another factor in favor of this bias is the relative ease of generation and the physical simplicity of pure tone stimuli for investigating the properties of the auditory system. A major result of this endeavor has been to show that central auditory organization is dominated by the recapitulation of the morphology of the sensory epithelium, i.e., the organ of Corti. Mapping of peripheral organization onto the central ner...

Peripheral auditory neurons are tuned to single frequencies of sound. In the central auditory system, excitatory (or facilitatory) and inhibitory neural interactions take place at multiple levels and produce neurons with sharp level-tolerant frequency-tuning curves, neurons tuned to parameters other than frequency, cochleotopic (frequency) maps, which are different from the peripheral cochleotopic map, and computational maps. The mechanisms to create the response properties of these neurons have been considered to be solely caused by divergent and convergent projections of neurons in the ascending auditory system. The recent research on the corticofugal (descending) auditory system, however, indicates that the corticofugal system adjusts and improves auditory signal processing by modulating neural responses and maps. The corticofugal function consists of at least the following subfunctions. auditory system ͉ descending system ͉ learning and memory ͉ plasticity ͉ tonotopic map T he central auditory system creates many physiologically distinct types of neurons for auditory signal processing. Their response properties have been interpreted to be produced by divergent and convergent interactions between neurons in the ascending auditory system. Until recently, the contribution of the descending (corticofugal) system to the shaping (or even creation) of their response properties has hardly been considered. Recent findings indicate that the corticofugal system plays important roles in shaping or even creating the response properties of central auditory neurons and in reorganizing cochleotopic (frequency) and computational (e.g., echo-delay) maps. Therefore, the understanding of the neural mechanisms for auditory signal processing is incomplete without the exploration of the functional roles of the corticofugal system. In this article, we first enumerate several types of neurons and computational maps created in the bat's central auditory system and then describe the anatomy and physiology of the corticofugal system. Neurons Tuned to Acoustic Parameters Characterizing Biosonar Signals and Cochleotopic and Computational MapsAll peripheral neurons are tuned to single frequencies (best frequencies, BFs). In the central auditory system, excitatory, inhibitory, and facilitatory neural interactions take place at multiple levels and produce neurons with sharp level-tolerant (the width of a frequency tuning curve is narrow regardless of sound levels) frequency tuning curves (1) and also neurons tuned to specific values of parameters other than frequency. Some of these neurons apparently are related to the processing of biosonar signals. They are latency-constant, phasic on-responding neurons (2, 3); paradoxical latency-shift neurons (4); durationtuned neurons (5) amplitude coordinates for the fine spatio-temporal representation of periodic frequency and amplitude modulations of echoes from flying insects (9). In the superior colliculus of the big brown bat, there is a space map, and some neurons are tuned to a sound source at ...