Midbrain structures controlling vocalization in the domestic chick

Bilateral lesions confined to the anterior intercollicular area (ICo) produce muting in domestic chicks. Calling is lost both in open field test, and to novel stimuli which evoke pecking. Lesions of the medial edge of the ICo also mute, perhaps by cutting connections with the central mesencephatic grey. Partial destruction of the ICo allows full intensity calling but with greatly increased latency. If an area ventral to the ICo is also destroyed, one type of calling reappears: it is still not readily evoked by particular stimuli which are effective in normal chicks but it often occurs during locomotion. Crowing survives ICo lesions and presumably depends on a different route to lower centres. Totally deaf birds show quite normal calling, so that hearing deficits are probably not involved in the muting. Finally, mammalian homologues of the ICo and its connections are identified.