Oscillation Phase Is Critical for Optimal Discrimination

<div><p>(A) Two examples of input waveforms, each containing seven stimulus input trains (overlaid) that differed only during the indicated 100° windows (centered around −20° for the peak/falling phase situation (1) and 160° for the rising phase situation (2); the phase was measured relative to the downward midline crossing of the oscillatory drive).</p> <p>(B1) Rasters from an in vitro recording from a CA1 pyramidal neuron. The two stimuli only differ in the phase window indicated in (A). (Middle panels) PSTHs for both stimuli; the gray shading indicates the SD of the PSTH for stimulus 2. Note that while for the falling phase (1) both PSTHs overlap, they are easily separable for stimuli differing in the rising phase (2). (Bottom panels) Difference between the two PSTHs normalized to the combined SD. Red bars indicate where this normalized difference is larger than 1, that is, regions where the two stimuli are discernible based on the PSTHs.</p> <p>(B2) Summary of the in vitro recordings for the PSTH difference (the fraction of time where the normalized difference between the two PSTHs is larger than 1) for 22 stimulus pairs. The PSTH difference for rising phase stimuli is significantly larger than for falling phase stimuli ( <i>p</i> < 0.05). </p> <p>(C1) Stimulus discrimination is plotted under control and oscillation conditions as a function of the phase of the window for simulations in an InF neuron. A sinusoidal fit to the membrane potential is shown as a thin red line for reference. Stimulus windows similar to those shown in (A) are indicated by (1) and (2). To allow for higher temporal resolution the phase window was reduced to 60° and varied in steps of 12° across the cycle. Phase is measured relative to the downward midline crossing of the MPO. Solid bar indicates a period of best discrimination, while the open bar shows the period for the oscillatory case where discrimination is relatively poor.</p> <p>(C2) A sinusoid is fitted to the discrimination–phase plot. “Phase dominance” is defined as the peak-to-trough amplitude of the fitted sinusoid and plotted as a function of the oscillation frequency. As a control, the black line indicates the average phase dominance in the absence of a MPO. The red line is a Gaussian fit on a lin-log scale.</p>