Projections of the feline inferior colliculus were studied using the Nauta-Laidlaw method to demonstrate degenerating axons. A subtentorial stereotaxic approach was used to avoid corticofugal degeneration. Direct connections from the inferior colliculus to the anterior midline cerebellar cortex were observed. There is a topographical relationship of fibers of the brachium of the inferior colliculus and parabrachial region to the superior colliculus. A definite projection to the superior colliculi including a pathway through its commissure was found from the parabrachial region. A projection exists from the inferior colliculus to the dorsolateral portion of the central gray as far as the pretectum. There is a projection along the medial portion of the superior colliculus to the pretectum. Projections to the midbrain reticular formation, central gray and superior colliculi were substantial. Thalamic projections include a distribution of fibers to the magnocellular and rostral portion of the principal divisions of the medial geniculate body and to the lateral posterior thalamic nucleus. The rostral connections of the inferior colliculus with areas other than the medial geniculate body indicate that it may function in roles in addition to that of a mass somatomotor reflex center. Possible roles of the inferior colliculus in attention, habituation, and integration of corticovisual and auditory impulses are suggested.Neuroanatomical studies have been made which describe projections of the inferior colliculus. Ascending projections of the inferior colliculus have been reported by Woollard and Harpman ('40) as ending only in the principal division of the medial geniculate body via the ipsilateral brachium. Recently, workers have observed projections entering the magnocellular and rostral part of the principal divisions (Barnes, Magoun and Ranson, '43; Rasmussen, '61 ; Moore and Goldberg, '63). Whitfield ('67) states that comparatively little work seems to have been done on the details of the afferent fiber connections to the medial geniculate body from the inferior colliculus and other structures. Some projections to the medial geniculate body and inferior colliculus from limbic structures have been described (Powell and Hoelle, '67; Akagi and Powell, '68). Rasmussen ('61) and Moore and Goldberg ('63) in turn, found that the reticular formation of the midbrain receives terminal projections from the inferior colliculus through the parabrachial region. Some projections coursing mediad from the inferior colliculus to the midbrain central gray have been described in the cat by Woollard and Harpman ('40) and Moore and Gddberg ('63). Rose, Greenwood, Goldberg and J. COMP. NEUR., 136: 183-192.Hind ('63) have shown that the inferior colliculus is highly tonotopic. Hoelle ('67) placed lesions in the inferior colliculi bilaterally to study their role in frequency discrimination. He concluded that the observed effects were due to the interruption of fibers to midline thalamic nuclei. Such evidence implies that the inferior...

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