Chaotic and phase-locked responses of the somatosensory cortex to a periodic medial lemniscus stimulation in the anesthetized rat

Field potential responses of the somatosensory cortex to a periodic medial lemniscus (ML) fiber stimulation were investigated in anesthetized rats. Since the field potential responses of the cortex depend on the depth of anesthesia, two criteria were introduced to control the depth of anesthesia. One criterion is that spindle oscillations are caused by a single shock to ML fibers, and the other is that the dominant frequency of spontaneous field potential rhythm is in the frequency range of the delta wave. Phase-lockings and chaotic responses occurred depending on stimulus parameters under the above conditions. Trajectories of the chaotic responses in the two-dimensional phase space (V, dV/dt) reconstruct strange attractors, and stroboscopic cross-sections of each attractor show stretching and folding process. Each one-dimensional strobomap of the chaotic responses is a noninvertible function with an unstable fixed point. This is undoubted evidence for chaotic responses of the somatosensory cortex in vivo. Power spectra well characterize the periodic and the chaotic responses. Positive Lyapunov exponents and low, noninteger correlation dimensions of the chaotic responses are consistent with the above evidence. Consequently, four kinds of chaotic responses were classified. A periodic-chaotic transition sequence was observed at a relatively low stimulus current when the frequency of the stimulation was varied. A cascade of period-doubling bifurcations was also observed on a route from the region of the 1:1 phase-locking to one of the regions of the chaotic responses.