Neuronal activity of human caudate nucleus and prefrontal cortex in cognitive tasks

Lesions of the caudate nucleus and prefrontal cortex may display similar cognitive deficits. Recent advances in cognitive neuroscience have clarified the functional role of prefrontal cortical areas in certain cognitive operations involved in simple language tasks. We have addressed the role of the caudate nucleus in tasks of lexical decision, semantic categorization, recognition memory, reading aloud and object naming by recording neuronal activity in patients with depth electrodes. During visual processing of words, caudate cells exhibited excitatory responses related to both semantic and phonological-articulatory encoding with non-overlapping time courses. The firing rate of the cells was increased when the semantic processing was required. This occurred within 400-600 ms after the stimulus onset, or within the first 200-300 ms of the delay period. The increased firing within 1000-1200 ms after the stimulus onset was related to the phonological processing. These responses turned out to be strikingly similar to those in Broca's area. Both reading aloud and explicit memory retrieval tasks elicited a sustained inhibition of firing of the same cells with a greater onset latency. Chronometric comparison of prefrontal, temporo-parietal and caudate activities in similar tasks relates the time course of these activations to the fronto-caudate anatomical loops and helps further understanding of the anatomy and circuitry involved in human cognition.