Parallel single unit and retrograde tracing experiments were carried out in the anesthetized cat to elucidate the representation of the auditory and somatosensory systems in the external nucleus of the inferior colliculus (ICX). Units responding to tonal stimuli were more commonly encountered in ICX and the adjacent intercollicular area (ICA) than were units with identified tactile receptive fields. Concomitantly, a larger number of retrogradely labeled cells were identified in midbrain auditory structures, following injections of horseradish peroxidase (HRP) into CRX, than were found in the combined dorsal column nuclei. Microelectrode recording revealed that the entire accessible body surface was represented in ICX, and receptive fields of individual units were usually large and mostly located on the contralateral side. HRP-labeled cells were scattered throughout the contralateral cuneate, gracile, and spinal trigeminal nuclei. tations of tone pips. Responses to complex sound were commonly observed. Binaural stimuli influenced the firing of the majority of auditory units. Labeled auditory neurons following ICX injections were found mainly in the inferior colliculus of both sides. Spread of tracer into the central and pericentral nuclei was associated with labeling of many neurons in hind brain auditory structures. Only vague suggestions of somatotopy or tonotopy were observed in the electrophysiological experiments; similarly, no topographical relationship between HRP injection site and locus of retrograde label in a given projecting nucleus could be discerned. Speculations were made, in the light of the dual convergent sensory representation in ICX, about the role of this structure in acoustico-motor mechanisms.

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The organization and connections of the brain stem auditory nuclei were studied in the marsupial brush-tailed possum, Trichosurus vulpecula, and compared with commonly studied placental mammals. Although some gross morphological differences may be observed, including a shifted location of the cochlear nuclei to a medial position in the brain stem and a diffusely organized superior olivary complex, the pathways for information flow from periphery to auditory midbrain and the organization of the midbrain are similar in Trichosurus and placentals.

Some anatomical and physiological aspects of the peripheral auditory system were examined in anesthetized brush-tailed possums (Trichosurus uulpecula) and compared with those in the cat. Cochlear microphonic potentials (CM) in response to tones of varying frequency and intensity were recorded a t the round window.The cochlea of Trichosurus has a typically mammalian appearance, with three and one-half turns and a basilar membrane approximately 17 mm in length upon which sit three rows of outer and one of inner hair cells. Three middle ear ossicles, including a delicate stapes of columelliform appearance, are housed in a cavity adjacent to a spongy mass of bone. The latter surrounds the round window and is quite unlike the bullae of many eutherian mammals.Round window recordings revealed that CM potentials of the possum rarely attained magnitudes, for any tone, in excess of 500 pV and were always 10-25 dB less in amplitude than CM of the cat a t a given frequency and intensity. The threshold intensities required to elicit a 50 pV CM at frequencies between 0.4 kHz and 10 kHz were smaller for the cat than the possum, but a slightly greater sensitivity was shown by the possum between 0.1 kHz and 0.4 kHz.Peripheral auditory characteristics of the marsupial brush-tailed possum thus appear to have some features in common, and others a t variance, with commonly-studied placental mammals.The brush-tailed possum, Trichosurus vulpecula, is a marsupial of widespread rural and suburban distribution in Australia. It has a well-developed vocal capacity (Winter, '76) and possesses midbrain auditory neural mechanisms which are qualitatively similar to those of placental mammals (Aitkin et al., '78). However, aside from a few studies of Didelphis virginiana (Fernandez and Schmidt, '63; McCrady et al., '37, '40; Ravizza et al., '69) little information exists about peripheral aspects of audition in metatherian species. The purpose of the present report is t o describe some of the peripheral auditory characteristics of Trichosurus. Studies of marsupials are important to our understanding of mammalian evolution; furthermore, their relative immaturity a t birth (Tyndale-Biscoe, '73) also makes them potentially useful for research into the ontogeny of the auditory system. METHODSSeven brush-tailed possums (10 ears) and three cats (6 ears) were used in this study. Cats were anesthetized with sodium pentobarbital (30 mg/kg i.p.). This anesthetic, so reliable for cats, was found to produce highly variable effects with possums -the same dose in this species could cause death with some specimens and have little effect with others. After some experimentation, possums were anesthetized with ketamine hydrochloride (30 m g k g i.m.1 followed 30 minutes later by an injection of 1% chloralose -10% urethane (5 m l k g i.p. in aqueous solution). Atropine (0.4 mg/kg) was administered subcutaneously to control the salivation which ketamine sometimes induced.After tracheotomy, animals of both species were fixed to a holding frame with an adjusta...