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要旨:
We are investigating how auditory and visual information are integrated in the neocortex by recording single units and local field potential (LFP) activity with multiple electrodes in the lateral belt (LB) region of auditory cortex. Rhesus monkeys performed a task where they were required to maintain their fixations within the video frame for the duration of conspecific vocalization stimuli. The stimuli were presented as Face+Voice, Voice alone Face alone conditions. Our initial study demonstrated that raw LFP power reflects integration of face/voice signals in the form of both enhancement and suppression. While this LFP modulation was evident in the majority of LB cortical sites (89), parallel modulation in the analog multiunit firing rates was not as consistently apparent (40 of sites). This suggests that the integrative effects are masked in multiunit activity. This may be because LFP modulations affect the firing reliability of particular cells--not necessarily through firing rate, but via the probability that they will fire for a given ensemble pattern of synaptic activity. In the present study, an analysis of 18 vocalization-sensitive single units revealed that 5 show some form of multisensory integration via firing rate (~28). Although preliminary, this proportion of multisensory LB neurons is roughly equivalent to that reported for the superior temporal sulcus (STS). The notion that multisensory processes in LB auditory cortex may be reflected in the cooperation of neural signals receives support from our LFP analyses. LFP coherence measured across 27 pairs of LB cortical sites revealed modulations in the gamma band (40-80 Hz) and lower frequencies (5-30 Hz) that were greater for bimodal versus unimodal conditions. In a subset of experiments, we recorded STS in parallel with LB. Coherence between LB and STS (20 pairs) showed the greatest strength in the bimodal condition for low frequencies, but in the gamma range both the Face+Voice and Face alone conditions showed an equivalent increase in gamma band activity relative to the Voice alone condition. Our data suggest that firing rates may be less relevant to the integration of information than the temporal structure of neural activity and its effects on small neural networks.
