Optical imaging of neural activity in transient ischemia and plasticity in cortex

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The spatio-temporal pattern of sound-evoked neural activity in the mammalian auditory cortex was studied by optical recording with aid of voltage-sensitive dye (RH 795). This method was applied to observation of transient ischemic changes in the cortical activity. Changes in light intensity at 620 nm induced sounds at various frequency and pressure levels were recorded with a 12 x 12 array of photodiodes. The amplitude of the responses were displayed as sequential two-dimensional images. Tonotopical organization was found in the multi-fields of the auditory cortex. However, frequency representation through the functional organization was transient. The focal activity that began in the primary field propagated dorsally along the iso-frequency bands. In the rats auditory cortex, the optical signals induced by sounds were disappeared by 1 min after induction of ischemia. During the 30 min of ischemia, no components of the responses were observed. After recirculation the responses from 144 sites appeared again. However the pattern of the responses after the transient ischemia was significantly different from the control. Especially the later component did not recover to the preischemic level. Although functional organization appeared, but vulnerable regions were found within the organization where the optical responses ded not return to the control Our results suggest that the functional changes in the auditory cortex precede the post-ischemic neuronal death in those cerebral regions. In guinea pigs, there were some cases which showed an increase in the magnitude of optical responses from the cortex by transient ischemia. The optical recording method has the advantage of real-time imaging of two-dimensional cortical activity and will be used widely in the future.