Analysis of acoustic elements and syntax in communication sounds emitted by mustached bats

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.

Syntax processing by auditory cortical neurons in the FM–FM area of the mustached batPteronotus parnellii

“…From a detailed analysis of communication call structure in P. parnellii, the presence of strong constraints on the use of simple syllables as components of composites is obvious (8). Thus, of the 342 disyllabic combinations in composites possible in theory, less than 15 have been found to occur (8).…”

“…From a detailed analysis of communication call structure in P. parnellii, the presence of strong constraints on the use of simple syllables as components of composites is obvious (8). Thus, of the 342 disyllabic combinations in composites possible in theory, less than 15 have been found to occur (8). The present observations that neuronal responses to these composite communication calls were highly vulnerable to (i) reversal of order of syllables within a natural composite, (ii) introducing a silent period between the syllables, and (iii) playing the stimulus in reverse provide independent lines of evidence at the single unit level that such syntax in communication sounds is processed by neurons in the mammalian nonprimary auditory cortex.…”

“…2) are named according to the simple syllables (e.g., bent Upward FM-short True Constant Frequency) from which they are composed (for complete nomenclature, see ref. 8). For determining a unit's spontaneous activity a no-stimulus interval (i.e., no.…”

“…Thus, we hypothesized that FM-FM neurons may also show combination-sensitivity for heterosyllabic composites (8), providing a neuronal correlate for syntax of communication sounds in a mammalian species.…”

“…First, in the context of intraspecific acoustic communication, mustached bats frequently combine otherwise independently emitted simple syllables to form either isosyllabic trains or heterosyllabic composites. The syntactical rules for the generation of the last-mentioned higher order constructs have been revealed in detail (8). Second, the auditory cortex of P. parnellii is arguably the most intensively studied and best understood of all mammals (9).…”