Frontal units of the monkey coding the associative significance of visual and auditory stimuli

Two monkeys were trained on both visual and auditory association tasks. Single unit activity of the frontal (prefrontal and post-arcuate premotor) cortex was recorded in these monkeys to investigate the convergence of visual and auditory inputs and to examine whether the frontal units are involved in coding the meaning (associative significance) of the stimulus, independent of its modality. A total of 289 units showed changes in firing rate after the cue presentation on the visual and/or auditory tasks and were examined on both modalities of tasks, 175 of them showing differential activity in relation to either the associative significance and/or physical properties of the visual and/or auditory cues. Of the 289 units, 136 (47.0%) were responsive only to the visual cue (76 of them showing cue-related differential activity), 13 units (4.5%) only to the auditory cue (6 of them showing cue-related differential activity) and the remaining 140 units (48.5%) to both modalities of cues (18 of them showing visual, 7 of them showing auditory and 68 showing both modalities of cue-related differential activity). Fifty of the 68 bimodal differential units showed changes in firing in relation to the associative significance of both modalities of cues independent of the cue's physical properties, and are considered to be involved in the crossmodal coding of the associative significance of the stimulus. The proportion of bimodal differential units was higher in the pre- and post-arcuate areas than in the principalis and inferior convexity areas of the frontal cortex. The results indicate that some frontal units participate in the crossmodal coding of the associative significance of the stimulus independent of its physical properties, and most frontal units play different roles depending on the modality of the stimulus.