Intracellular potentials were recorded from 164 cells in cat precruciate cortex, and their responses were examined during the phasic and tonic phases of EEG arousal. 2. According to the initial responses corresponding to phasic EEG arousal, these cells were classified into 71 E (excitation), 49 I (inhibition), 38 DF (disfacilitation), and 6 DI (disinhibition) cells. 3. The late responses corresponding to tonic EEG arousal varied among these cells. Thirty-two of the 164 cells were unresponsive. Of the remaining cells, 102 showed excitation, 16 inhibition, 12 disfacilitation, and 2 disinhibition. These cells were retermed +E, +I, +DF, and +DI cells, respectively. 4. +E cells were located at all depths through laminae I to VI, but the majority were found in laminae IIIb-VI. +I cells were between lamina I and the lower half of lamina III (IIIb), and +DF and +DI cells were between lamina I and the upper half of lamina III (IIIa). From these characteristic layer distributions it is postulated that an upward cascade transmission from deep to superficial layers occurs during tonic EEG arousal. 5. Nine types of combinations of the initial and late responses were found: E, I, DF or DI with +E, E or I with +I, E or I with +DF, and I with +DI. To explain these various activity patterns, an 'arousal' circuit model was proposed by combining the downward and upward cascade transmission patterns. The elementary structure of cortical arousal was thus formulated, and discussed from the development aspect of behavior.
