J. C. Prechtl, Lawrence B. Cohen, B. Pesaran, P. P. Mitra, and D. Kleinfeld
Marine Biology Laboratory, Woods Hole
Department of Physics and Scripps Institution of Oceanography, UCSD
Department of Cellular and Molecular Physiology, Yale University School of Medicine
Bell Laboratories, Lucent Technologies
The computations involved in the processing of a visual scene invariably involve the interactions among many neurons throughout all of visual cortex. One hypothesis is that the timing of neuronal activity, as well as the amplitude of activity, provides a means to link features of objects that are part of the visual field. The experimental data from studies with cat [Gray, Konig, Engel, and Singer, Nature 338:334, 1989] support 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 population of neurons as a means to encode independent visual features and, on the other hand, the likely existence of timing differences on the basis of network dynamics.
We used widefield imaging in conjunction with voltage sensitive dyes to record electrical activity from the virtually intact, unanesthetized turtle brain. Our data consists of single-trial measurements demonstrating complex, non-repeating, coherent activity. We analyzed our data in the frequency domain 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 parallel to the afferent input. Higher frequency activity, with spectral 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 as more complex patterns. When further sorted by SVD analysis, the largest component between 5 and 30 Hz are plane waves have an average phase gradient of radians/mm and which propagate orthogonally to the low frequency waves. Our results show that in the turtle cortex there is large scale coherent activity together with large-scale differences in neuronal timing during normal cortical function. These patterns of activity are not predicted either by the known anatomy or by the physical features of the visual stimuli.