Injections of horseradish peroxidase (HRP) were made into the optic radiations just above the lateral geniculate nucleus of four cats to trace the anterograde filling of corticofugal axons terminating in the perigeniculate and lateral geniculate nuclei. The different types of axons were classified according to axonal diameter and the morphology of the terminal appendages. Judging from their morphological organization we suggest that the corticofugal axons are, in the main, slowly conducting and that they have a restricted terminal distribution which extends, however, to a multiplicity of levels in both perigeniculate and lateral geniculate nuclei. These morphological characteristics may have physiological implications in determining the role of the corticofugal pathway.

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In paralyzed and anaesthetized cats, the pathway running from the claustrum to the striate cortex was characterized from the trans-synaptic latencies of responses that were initiated by electrical stimulation in the claustrum (CL) and recorded extra-cellularly in single striate neurons. A second stimulating electrode (OR1) in the primary visual pathway provided information on the input coming to the recorded cell from the lateral geniculate nucleus. An analysis of the classified striate neurons receiving a claustral drive revealed that 68% were C cells and 26% were S cells. For the C cells, 81% had CL latencies of less than 2.5 ms (mean = 1.8 ms) and the potential to receive a direct drive from a fast conducting input; the remaining 19% had latencies around 3.0 ms (mean = 3.0 ms), a value consistent with a disynaptic input from the same type of input. From their CL latencies, the S cells also could be subdivided into two subgroups; one, made up of 36% of the sample had CL latencies of less than 2.5 ms (mean = 1.9 ms) and the capacity, like the majority of C cells, to receive a direct, fast-conducting input; the second subgroup, consisting of 74% of the S cells, had CL latencies longer than 3.0 ms (mean = 5.4 ms). The majority of cells with a claustral-drive (85%) were encountered either in laminae 4 or 6. Claustral-driven cells belonging to both S and C categories were found in the two laminae (4 and 6) and there was no observed predisposition for a particular cell type to cluster in either of these lamina. From a comparison of CL and OR1 latencies, justified on the grounds of independent stimulation, a strict correlation was found for signal conduction properties in the claustral and LGN pathways running to a given striate neuron. From a quantitative evaluation of receptive field properties the claustral-driven striate neurons were found to resemble cells in the general population. As a group, however, they were distinctive in that both end-zone inhibition and direction selectivity were either weak or absent from the cell's response. This finding held for cells in both the C and the S categories. It is concluded from the high incidence of claustral-driven C cells, that the claustral loop from the striate cortex is involved in an aspect of motion detection.

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