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Dynamics of the brain at global and microscopic scales: Neural networks and the EEG
- J. J. Wright, D. T. J. Liley
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- Journal:
- Behavioral and Brain Sciences / Volume 19 / Issue 2 / June 1996
- Published online by Cambridge University Press:
- 04 February 2010, pp. 285-295
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- Article
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There is some complementarity of models for the origin of the electroencephalogram (EEG) and neural network models for information storage in brainlike systems. From the EEG models of Freeman, of Nunez, and of the authors' group we argue that the wavelike processes revealed in the EEG exhibit linear and near-equilibrium dynamics at macroscopic scale, despite extremely nonlinear – probably chaotic – dynamics at microscopic scale. Simulations of cortical neuronal interactions at global and microscopic scales are then presented. The simulations depend on anatomical and physiological estimates of synaptic densities, coupling symmetries, synaptic gain, dendritic time constants, and axonal delays. It is shown that the frequency content, wave velocities, frequency/wavenumber spectra and response to cortical activation of the electrocorticogram (ECoG) can be reproduced by a “lumped” simulation treating small cortical areas as single-function units. The corresponding cellular neural network simulation has properties that include those of attractor neural networks proposed by Amit and by Parisi. Within the simulations at both scales, sharp transitions occur between low and high cell firing rates. These transitions may form a basis for neural interactions across scale. To maintain overall cortical dynamics in the normal low firing-rate range, interactions between the cortex and the subcortical systems are required to prevent runaway global excitation. Thus, the interaction of cortex and subcortex via corticostriatal and related pathways may partly regulate global dynamics by a principle analogous to adiabatic control of artificial neural networks.
Cortical mapping of gamma oscillations in areas V1 and V4 of the macaque monkey
- GABRIEL ROLS, CATHERINE TALLON-BAUDRY, PASCAL GIRARD, OLIVIER BERTRAND, JEAN BULLIER
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- Journal:
- Visual Neuroscience / Volume 18 / Issue 4 / July 2001
- Published online by Cambridge University Press:
- 11 January 2002, pp. 527-540
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To characterize the temporal and spatial parameters of gamma activity evoked by visual stimuli in areas V1 and V4 of the monkey cortex, we recorded the electrocorticogram (ECoG) with an implanted array of 28 and 31 subdural electrodes placed over the surface of the operculum in two anesthetized monkeys. This intermediate level of recordings should help to bridge the gap between multiunit and scalp recordings. Both averaged and single-trial responses to small flashed stimuli, for which we varied the retinotopic position, the luminance and the color, were analyzed in the time-frequency domain using a wavelet-based decomposition of the signal. Large gamma oscillations (40–55 Hz), not phase locked to stimulus onset, were observed during the whole stimulus presentation, whereas visual evoked potentials (VEPs) were present mainly at stimulus onset and offset. Cortical mapping showed that both activities were restricted in spatial extent and followed the retinotopic organization of area V1 on the operculum, thus strongly suggesting they were generated in the underlying cortex. Oscillatory burst detection in single trials showed that one to two bursts lasting from 100 ms to 500 ms occurred in the first 500 ms following stimulus onset, and that bursts occurring during the subsequent phases of the response had a smaller amplitude and duration. Finally, we showed that gamma activity was stronger with higher luminances and for red than for green, yellow, or white stimuli of same luminance. In one animal we recorded gamma activity over area V4. This was of lower magnitude than the activity recorded over V1 and was delayed by 40 ms with respect to the beginning of gamma activity in V1, in contrast with the VEPs that were delayed by 20 ms only. Both gamma oscillations and early VEP followed the retinotopic organization of V4 over the prelunate gyrus. The results show that gamma oscillations are dependent upon the same parameters as the VEPs (retinotopic position, luminance, and color). However, the differences in the time course of VEPs and gamma activity (transient vs. sustained) suggests that these two responses may reflect different cell populations, different networks, or different firing modes.