VISUAL-STIMULI INDUCE WAVES OF ELECTRICAL-ACTIVITY IN TURTLE CORTEX

The computations involved in the processing of a visual scene invariab ly involve the interactions among neurons throughout all of visual cor tex. One hypothesis is that the timing of neuronal activity, as well a s the amplitude of activity, provides a means to encode features of ob jects. The experimental data from studies on cat [Gray, C. M., Konig, P., Engel, A. K. & Singer, W. (1989) Nature (London) 338, 334-337] sup port a view in which only synchronous (no phase lags) activity carries information about the visual scene. In contrast, theoretical studies suggest, on the one hand, the utility of multiple phases within a popu lation of neurons as a means to encode independent visual features and , on the other hand, the likely existence of timing differences solely on the basis of network dynamics. Here we use widefield imaging in co njunction with voltage-sensitive dyes to record electrical activity fr om the virtually intact, unanesthetized turtle brain. Our data consist of single-trial measurements. We analyze our data in the frequency do main to isolate coherent events that lie in different frequency bands. Low frequency oscillations (<5 Hz) are seen in both ongoing activity and activity induced by visual stimuli. These oscillations propagate p arallel to the afferent input. Higher frequency activity, with spectra l peaks near 10 and 20 Hz, is seen solely in response to stimulation. This activity consists of plane waves and spiral-like waves, as well a s more complex patterns. The plane waves have an average phase gradien t of approximate to pi/2 radians/mm and propagate orthogonally to the low frequency waves. Our results show that large-scale differences in neuronal timing are present and persistent during visual processing.