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 dyes1-6 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 invertebrate1,2,7 or vertebrate neuronal preparations8-10 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.