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 somatose nsory cells appear to retain visual information that has been associated wi th the touch of an object. The presence of such cells suggests that nontact ile stimuli associated with touch have access to cortical neuron networks e ngaged in the haptic sense. Thus, we inferred that somatosensory cells woul d respond to behaviorally associated visual and tactile stimuli. To test th is assumption, single units were recorded from the anterior parietal cortex (Brodmann's areas 3a, 3b, 1, and 2) of monkeys performing a visuo-haptic d elay task, which required the memorization of a visual cue for a tactile ch oice. Most cells responding to that cue responded also to the corresponding object presented for tactile choice. Significant correlations were observe d in some cells between their differential reactions to tactile objects and their differential reactions to the associated visual cues. Some cells wer e recorded in both the cross-modal task and a haptic unimodal task, where t he animal had to retain a tactile cue for a tactile choice. In most of thes e cells, correlations were observed between stimulus-related firing in corr esponding cue periods of the two tasks. These findings suggest that cells i n 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.