Figure 5. Relationship between firing rate and theta phase.

(A). Top row, polar plots of group mean of preferred phase and mean resultant length as a function of instantaneous firing rate (color coded). Instantaneous rate was quantified for each spike in ten increments (1, 2,… or≥10 spikes per 250 msec (two theta cycles), corresponding to 4, 8, 12,…, ≥ 40 Hz; Harris et al., 2002). Second and third rows show preferred phase and mean resultant length for each significantly modulated principal neuron at 4 and ≥ 40 Hz. Fourth row, same but for bursting spikes (interspike interval < 6msec). Each circle is a single neuron. Black lines, group mean. Note the large and progressive shifts of preferred phase in EC2 and CA1 cells with rate increase. In each firing rate category, only neurons with significant theta modulation (at least 50 spikes in a given category and Rayleigh test, p<0.01) are shown. (B) Firing probability of principal neurons from different layers as a function of theta phase (as in Figure 3A and 3E), at different instantaneous frequencies (4 Hz, ≥40 Hz, bursting spikes, ISI<6msec). In each category, all neurons are included regardless of the significance of theta modulation. Note the large shift of active phase of EC2 and CA1 neurons between 4 Hz and ≥40 Hz of activity. Arrows in the bottom row indicate double peaks of bursting activity (ISI<6msec) in EC2 and CA1.