Abstract
We investigated interhemispheric interactions between bilateral somatosensory cortices through activity-related hemodynamic signals and their dependence on stimulus time lag. We measured optical intrinsic signals (586 and 610 nm) in rat somatosensory cortex evoked by electrical pulses to the contralateral hindpaw (test stimuli, TS) while delivering electrical pulses to the ipsilateral hindpaw or homotopic cortical regions of the contralateral hemisphere (conditioning stimuli, CS) under α-chloralose anesthesia. The responses to CS-TS were normalized by the responses to TS without CS. RESULTS: (1) Optical responses to either hindpaw CS or TS were observed not only in the contralateral somatosensory cortex but in homotopic regions of the ipsilateral hemisphere with the magnitude of approximately 1/10 of contralateral responses. The cortical CS elicited optical signals bilaterally in the same manner. (2) Hindpaw CS-TS responses were 92 ± 12, 78 ± 23, 56 ± 27, 83 ± 8 and 107 ± 9% at 80, 60, 40, 20 and 0 ms lag time, respectively. The optical responses were significantly suppressed at 40 ms and slightly augmented at 0 ms. (3) Cortical CS-TS responses were highly suppressed at 20 ms and still suppressed at 0 ms. DISCUSSION: The results probably reflect the manner of interhemispheric neural interactions. The earlier suppression in the cortical CS-TS paradigm than in the hindpaw CS-TS paradigm may be explained by neural transmission time from the hindpaw to the contralateral cortex, suggesting interactions via interhemispheric callosal connections. [Jpn J Physiol 55 Suppl:S165 (2005)]
