Some claustro-cortical connections in the cat and baboon as studied by retrograde horseradish peroxidase transport.

1. The mammalian Claustrum (Cl) is a convergent multisensory structure of unknown function, and disputed ontogenetic origin. Its cortical projections, hitherto unknown, have been studied in cat and baboon by means of the horseradish peroxidase (HRP) technique. HRP was injected into the gyrus proreus (frontal eye field) of cats, and separately into the frontal eye fields, visual areas, and motor-premotor areas of the baboon cortex. 2. Differential retrograde transport to the Cl was demonstrated, such that in the cat the ipsilateral dorsal Cl was shown to be the principal origin of claustroproreate projections. In the baboon, the whole Cl projects onto area 8, while only the posteroventral part of the nucleus sends efferents to the visual cortex. The projection to the motor and premotor areas is present, but does not seem to be "essential." 3. Discussion of the physiological literature, together with anatomical evidence of reciprocal cortico-claustral projections to closely similar regions of the Cl lead to the suggestion that the Cl is concerned with the integration of messages subserving visually-directed movements. Some other functional implications are also discussed.