Processing of Species-Specific Vocalizations in the Inferior Colliculus and Medial Geniculate Body of the Guinea Pig

Abstract

Speech sounds are complex signals that are characterized by rapid variations in spectral distribution and temporal pattern. In order to understand the neurophysiological basis for the discrimination of speech sounds, several approaches have been used in the past, among them the study of responses of single nerve cells in the auditory system of birds or mammals to natural species-specific vocalizations. Already the first studies performed in the auditory cortex of awake squirrel monkeys suggested that neurons may exist in the cortex which extract specific features of the calls, similar to visual cortex neurons performing a feature extraction (Wollberg and Newman, 1972; Winter and Funkenstein, 1973; Newman and Wollberg, 1973; Manley and Muller-Preuss, 1978; Newman, 1978; Newman and Symmes, 1979). Later these studies were continued in the auditory cortex of the squirrel monkey by Pelleg-Toiba and Wollberg (1991) and most recently in the macaque auditory cortex by Rauschecker et al. (1995). In birds a systematic study of neurons specialized to recognize species-specific calls was performed for example in guinea fowl (Bonke et al., 1979; Scheich et al., 1979). In the auditory neostriatum of this bird neurons were described which respond in a highly selective way to so-called Iambus-like calls and which distinguish these calls from other calls of the guinea fowl. The results of these studies led authors to the conclusion that the band of strongest energy in such wide-band calls plays a key role for neuronal recognition. A specific type of feature extraction exists in bats where the detection of emitted biosonar signals serves for perception of space and detection of prey (as reviewed by Suga, 1996). The auditory periphery of mustached bat shows specializations for detection and frequency analysis of species-specific complex biosonar signals. It contains neurons which are used for fine level-tolerant frequency analysis and others that are used for fine time analysis. The bat’s central auditory system processes different types of auditory information in a parallel and hierarchical way and represents these in different areas of the auditory cortex. Specialization of neurons in the anterior auditory cortical fields for detection of self vocalizations was also confirmed in the rufous horseshoe bat (Radtke-Schuller and Schuller, 1995).