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.

Horseradish peroxidase tracing and extracellular electrophysiological recording techniques were employed to delineate prosencephalic connections of the gustatory system in ictalurid catfishes. The isthmic secondary gustatory nucleus projects rostrally to several areas of the ventral diencephalon including the nucleus lobobulbaris and the nucleus lateralis thalami. Injections of HRP in the vicinity of the nucleus lobobulbaris reveal an ascending projection to the telencephalon terminating in the area dorsalis pars medialis (Dm) and the medial region of area dorsalis pars centralis (Dc). Conversely, injections of HRP into the gustatory region of area dorsalis pars medialis label small neurons in the nucleus lobobulbaris. Gustatory neurons in the telencephalon send descending projections via the medial and lateral forebrain bundles to several nuclei in the anterior and ventroposterior diencephalon. The nucleus lateralis thalami, a diencephalic nucleus, receives ascending gustatory projections from the secondary gustatory nucleus but does not project to the telencephalon. Neurons in both the nucleus lateralis thalami and the telencephalic gustatory target exhibit multiple extraoral and oral receptive fields and complex responses to chemical (taste) and tactile stimulation.

Syntax denotes a rule system that allows one to predict the sequencing of communication signals. Despite its significance for both human speech processing and animal acoustic communication, the representation of syntactic structure in the mammalian brain has not been studied electrophysiologically at the single-unit level. In the search for a neuronal correlate for syntax, we used playback of natural and temporally destructured complex species-specific communication calls-socalled composites-while recording extracellularly from neurons in a physiologically well defined area (the FM-FM area) of the mustached bat's auditory cortex. Even though this area is known to be involved in the processing of target distance information for echolocation, we found that units in the FM-FM area were highly responsive to composites. The finding that neuronal responses were strongly affected by manipulation in the time domain of the natural composite structure lends support to the hypothesis that syntax processing in mammals occurs at least at the level of the nonprimary auditory cortex.From a linguist's point of view (1-3), ''syntax'' denotes a rule system that accounts for the ability to produce an infinite variety of sequences (i.e., words and sentences) from a fixed number of phonemes (i.e., vowels and consonants). Apart from human speech, rule systems for the sequencing of speciesspecific vocalizations have been found repeatedly in both birds (4, 5) and nonhuman mammals (6, 7). Thus, more generally, ''syntax'' can be understood as any system of rules that allows one to predict the sequencing of communication signals (3).Our present report of syntax processing by auditory cortical neurons in the mustached bat Pteronotus parnellii is founded on several previous studies in this species by employing both neurophysiological and behavioral methods. 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). Thus, established representational maps (e.g., ref. 10) can be used as a reference to record from defined areas and functional subtypes of auditory cortical neurons. Third, neurons in the FM-FM area of the mustached bat auditory cortex were shown recently to respond facilitatively to isosyllabic pairs (11). Presumably, these neurons mediate acoustic communication (11) in addition to their primary (i.e., first discovered) function in echolocation (12).FM-FM neurons exhibit heteroharmonic combinationsensitivity for paired stimuli mimicking the FM components of the bats' echolocation sounds (pulse and echo) (12) and often respond well to communication calls (11). Thus, we hypothesized that FM-FM neurons may also show combination-sensit...