Recognition of Modified Conditioning Sounds by Competitively Trained Guinea Pigs

The guinea pig (GP) is an often-used species in hearing research. However, behavioral studies are rare, especially in the context of sound recognition, because of difficulties in training these animals. We examined sound recognition in a social competitive setting in order to examine whether this setting could be used as an easy model. Two starved GPs were placed in the same training arena and compelled to compete for food after hearing a conditioning sound (CS), which was a repeat of almost identical sound segments. Through a 2-week intensive training, animals were trained to demonstrate a set of distinct behaviors solely to the CS. Then, each of them was subjected to generalization tests for recognition of sounds that had been modified from the CS in spectral, fine temporal and tempo (i.e., intersegment interval, ISI) dimensions. Results showed that they discriminated between the CS and band-rejected test sounds but had no preference for a particular frequency range for the recognition. In contrast, sounds modified in the fine temporal domain were largely perceived to be in the same category as the CS, except for the test sound generated by fully reversing the CS in time. Animals also discriminated sounds played at different tempos. Test sounds with ISIs shorter than that of the multi-segment CS were discriminated from the CS, while test sounds with ISIs longer than that of the CS segments were not. For the shorter ISIs, most animals initiated apparently positive food-access behavior as they did in response to the CS, but discontinued it during the sound-on period probably because of later recognition of tempo. Interestingly, the population range and mean of the delay time before animals initiated the food-access behavior were very similar among different ISI test sounds. This study, for the first time, demonstrates a wide aspect of sound discrimination abilities of the GP and will provide a way to examine tempo perception mechanisms using this animal species.

Recognition of non-harmonic natural sounds by small mammals using competitive training

Animals recognize biologically relevant sounds, such as the non-harmonic sounds made by some predators, and respond with adaptive behaviors, such as escaping. To clarify which acoustic parameters are used for identifying non-harmonic, noise-like, broadband sounds, guinea pigs were conditioned to a natural target sound by introducing a novel training procedure in which 2 or 3 guinea pigs in a group competed for food. A set of distinct behavioral reactions was reliably induced almost exclusively to the target sound in a 2-week operant training. When fully conditioned, individual animals were separately tested for recognition of a set of target-like sounds that had been modified from the target sound, with spectral ranges eliminated or with fine or coarse temporal structures altered. The results show that guinea pigs are able to identify the noise-like non-harmonic natural sounds by relying on gross spectral compositions and/or fine temporal structures, just as birds are thought to do in the recognition of harmonic birdsongs. These findings are discussed with regard to similarities and dissimilarities to harmonic sound recognition. The results suggest that similar but not identical processing that requires different time scales might be used to recognize harmonic and non-harmonic sounds, at least in small mammals.

Interplay of excitation and inhibition elicited by tonal stimulation in pyramidal neurons of primary auditory cortex

Tonal responses of neurons in the primary auditory cortex are a function of frequency, intensity and ear of stimulation. These responses occasionally display suppression. This review discusses how excitatory and inhibitory synaptic inputs interact to form suppressive responses and how changes in stimulus attributes affect the magnitude and timing of those responses. Stimulation at the characteristic frequency evokes a stereotyped sequence of depolarization (excitatory) and then hyperpolarization (inhibitory), as predicted from the canonical circuitry. Some neurons stimulated at higher sound intensities display a prominent increase in the magnitude of hyperpolarization or a decrease in its latency, both enabling counteraction with the preceding excitation. These interactions, in part, underlie the non-monotonic suppression. Furthermore, monaural non-dominant ear stimulation elicits such a powerful hyperpolarization as to cancel out the depolarization elicited at dominant ear stimulation, suggesting a linear mechanism for the binaural suppression. Alternatively, it elicits a depolarization almost equal in magnitude and time course to that elicited at binaural stimulation, suggesting a nonlinear interaction responsible for the suppression. Laminar differences are also noted for these inhibitory interactions.

Triadic synaptic interactions of large corticothalamic terminals in non-lemniscal thalamic nuclei of the cat auditory system

Large corticothalamic (CT) terminals, presumed to originate from cortical layer 5 pyramidal cells, are distributed predominantly in non-specific thalamic nuclei in mammals. In the auditory system, little is known about whether these CT projections participate in the synaptic aggregation referred to as the triad. We studied synaptic interactions of these terminals with neuronal elements in one of the auditory non-lemniscal thalamic nuclei, the dorsal nucleus of the medial geniculate complex (MGC), in cats. After injections of an anterograde tracer in the primary auditory cortex, areas containing labeled large terminals were examined using an electron microscope. It was revealed that a fraction of large CT terminals participated in complicated synaptic arrangements: labeled terminals making synaptic contacts with vesicle-free dendrites, probably of thalamic principal neurons, and/or vesicle-filled neuronal profiles, probably of presynaptic dendrites (PSDs) of interneurons. In reconstructions or even in single sections, we found that these synaptic connections participated in triadic arrangements. Thus, PSDs postsynaptic to the labeled CT terminals were in turn presynaptic to the vesicle-free dendrites.