Visuo-tactile cross-modal associations in cortical somatosensory cells.
Recent studies show that cells in the somatosensory cortex are involved in the short-term retention of tactile information. In addition, some somatosensory cells appear to retain visual information that has been associated with the touch of an object. The presence of such cells suggests that nontactile stimuli associated with touch have access to cortical neuron networks engaged in the haptic sense. Thus, we inferred that somatosensory cells would respond to behaviorally associated visual and tactile stimuli. To test this assumption, single units were recorded from the anterior parietal cortex (Brodmann's areas 3a, 3b, 1, and 2) of monkeys performing a visuo-haptic delay task, which required the memorization of a visual cue for a tactile choice. Most cells responding to that cue responded also to the corresponding object presented for tactile choice. Significant correlations were observed in some cells between their differential reactions to tactile objects and their differential reactions to the associated visual cues. Some cells were recorded in both the cross-modal task and a haptic unimodal task, where the animal had to retain a tactile cue for a tactile choice. In most of these cells, correlations were observed between stimulus-related firing in corresponding cue periods of the two tasks. These findings suggest that cells in somatosensory cortex are the components of neuronal networks representing tactile information. Associated visual stimuli may activate such networks through visuo-haptic associations established by behavioral training.