Narrow Results

A discrete model of a neural network of excitatory and inhibitory neurons is presented which yields oscillations of its global activity. Different types of dynamics occur depending on the selection of parameters: oscillating population activity as well as randomly fluctuating but mainly constant activity. For certain sets of parameters the model also shows temporary transitions from apparently random to periodic behavior in one run, similar to an epileptic seizure.

Interactions by mutual excitation in neural populations in human and animal brains create a mesoscopic order parameter that is recorded in brain waves (electroencephalogram, EEG). Spatially and spectrally distributed oscillations are imposed on the background activity by inhibitory feedback in the gamma range (30–80 Hz). Beats recur at theta rates (3–7 Hz), at which the order parameter transiently approaches zero and microscopic activity becomes disordered. After these null spikes, the order parameter resurges and initiates a frame bearing a mesoscopic spatial pattern of gamma amplitude modulation that governs the microscopic activity, and that is correlated with behavior. The brain waves also reveal a spatial pattern of phase modulation in the form of a cone. Using the formalism of the dissipative many-body model of brain, we describe the null spike as a singularity, the following amplitude pattern as a ground state, and the phase cone as the manifestation of a stabilizing vortex.

We experimentally study stochastic resonance (SR) in the human brain through a noise effect for entrainment dynamics of the alpha (α) wave. The measurement has been carried out under the following conditions in order to obtain clear evidence of the SR phenomenon in the central nervous system. The periodic and noisy stimuli are respectively applied to the right and the left eyes of the subject independently. When only periodic and constant (but weak enough) stimulus is applied to the right eye, it does not induce any global entrainment of α-oscillators to the stimulus frequency. In this situation increasing the amplitude of the noise stimulus (frequency bandwidth from 15 to 60 Hz), harmonic entrainment occurs for a certain range of the noise amplitude and shows a sharp peak in the power spectrum of brain waves. Plotting the amplitude of the peak against the noise amplitude, a bell-type shape is seen, i.e. the maximum signal amplitude at an optimum noise. It clearly indicates the SR phenomenon. Taking account of our experimental conditions and set-up, the present SR phenomenon occurs not on the retina but on the visual processing area in the central nervous systems beyond the optic chiasma.