Both lesion and functional imaging studies in humans, as well as neurophysiological studies in nonhuman primates, demonstrate the importance of the prefrontal cortex in representing the emotional value of sensory stimuli. Here we investigated single-neuron responses to emotional stimuli in an awake person with normal intellect. Recording from neurons within healthy tissue in ventral sites of the right prefrontal cortex, we found short-latency (120-160 ms) responses selective for aversive visual stimuli.

The inferior frontal gyrus (IFG) of humans is known to play a critical role in speech production. The IFG is a highly convoluted and cytoarchitectonically diverse structure, classically forming 3 subgyri. It is reasonable to speculate that during speaking the IFG, or some portion of it, influences by corticocortical connections the orofacial representational area of primary motor cortex. To test the hypothesis that such corticocortical connections exist, electrical-stimulation tract tracing experiments were performed intraoperatively on 14 human subjects undergoing surgical treatment of medically intractable epilepsy. Bipolar electrical stimulation was applied to sites on the IFG, while the resulting evoked potentials were recorded from orofacial motor cortex, using a multichannel recording array. Stimulation of the IFG evoked polyphasic waveforms on motor cortex of both language-dominant and -nondominant hemispheres. The evoked waveforms had consistent features across subjects. The responses were seen in discrete regions on precentral cortex. Stimulation of motor cortex also evoked responses on portions of IFG. The data provide evidence for a functional connection between the human IFG and orofacial motor cortex.

Lesion and functional imaging studies in humans have shown that the ventral and medial prefrontal cortex is critically involved in the processing of emotional stimuli, but both of these methods have limited spatiotemporal resolution. Conversely, neurophysiological studies of emotion in nonhuman primates typically rely on stimuli that do not require elaborate cognitive processing. To begin bridging this gap, we recorded from a total of 267 neurons in the left and right orbital and anterior cingulate cortices of four patients who had chronically implanted depth electrodes for monitoring epilepsy. Peristimulus activity was recorded to standardized, complex visual scenes depicting neutral, pleasant, or aversive content. Recording locations were verified with postoperative magnetic resonance imaging. Using a conservative, multistep statistical evaluation, we found significant responses in 56 neurons; 16 of these were selective for only one emotion class, most often aversive. The findings suggest sparse and widely distributed processing of emotional value in the prefrontal cortex, with a predominance of responses to aversive stimuli.