Neuroscience Letters, 20 (1980) 5-10 5 © Elsevier/North-Holland Scientific Publishers Ltd. AFFERENT AND EFFERENT CONNECTIONS BETWEEN THE CLAUSTRUM AND PARIETAL ASSOCIATION CORTEX IN CAT: A HORSERADISH PEROXIDASE AND AUTORADIOGRAPHIC STUDY D.R.F. IRVINE* and J.F. BRUGGE Department of Neurophysiology and Waisman Center on Mental Retardation and Human Development, University of Wisconsin, Madison, WI 53706 (U.S.A.) (Received June l lth, 1980; Revised version received July 18th, 1980; Accepted July 24th, 1980) Combined injections of [3H]proline and horseradish peroxidase (HRP) were made into acoustically responsive cortical association areas on the middle suprasylvian gyrus (MSA) and anterior lateral gyrus (ALA) in the cat. Sources and terminations of cortical axons were located within the claustrum. There are three findings: (1) MSA and ALA project to and receive fibers from restricted areas of the dorsal claustrum. The locations of the projection fields within the claustrum are different for the two cortical areas; (2) the afferent and efferent projection fields in the claustrum are coextensive suggesting a reciprocal claustrabcortical relationship and (3) the projections are bilateral with the ipsilateral projection predominating. There have been many descriptions of projections from cerebral cortex to the claustrum [e.g. 2, 3, 5, 9, 11, 17]. Until recently the retrograde degeneration studies of Narkiewicz [14] provided the only evidence for a widespread cortical afferent pathway from this nucleus. However, the use of exogenous tracers has now clearly shown such connections, mainly with visual cortex and the frontal eye fields, in primates [1, 10, 16, 18] and in the cat [6, 12, 15, 16]. In the course of our studies on the connectivity patterns of acoustically responsive association areas in cat neocortex, we were struck by what appear to be strong reciprocal connections between the claustrum and two parietal association fields. One of these areas is on the middle suprasylvian gyrus (MSA), the other is on the anterior lateral gyrus (ALA) [8]. Here we describe the results of 4 experiments in which horseradish peroxidase (HRP) and tritiated [3H]proline were together injected into physiologically defined, acoustically responsive association areas in cat. Under chloralose anesthesia a Davies chamber was positioned over one of these association fields. The distribution of averaged surface-evoked responses to noise- * On leave from Neuropsychology Laboratory, Department of Psychology, Monash University, Clayton, Vic. 3168, Australia - address for correspondence.