Real-time optical imaging of naturally evoked electrical activity in intact frog brain

A major obstacle to understanding the function and development of the vertebrate brain is the difficulty in monitoring dynamic patterns of electrical activity in the millesecond time domain; this has limited investigations of such phenomena as modular organization of functional units, seizure activities and spreading depression. The use of voltage-sensitive dyes^1-6 and the recent development of the use of an array of photodiodes⁷ has provided a unique technique for monitoring the dynamic patterns of electrical activity in real time from a variety of invertebrate^1,2,7 or vertebrate neuronal preparations^8-10 including the rat cortex ¹¹. In the present study, this technique has been used to investigate the intact optic tectum of the frog. We demonstrate that optical measurements can be used for real-time imaging of spatio-temporal patterns of neuronal responses and for identification of functional units evoked by natural visual stimuli. We report also the structure of the new voltage-sensitive probe that facilitates the in vivo applications of this technique.