Coherence Potentials: Loss-Less, All-or-None Network Events in the Cortex
Figure 1
Increasing nLFP amplitude is associated with a sigmoidal transition to a regime of widespread waveform similarity.
(A) The nLFP is defined as the complete negative excursion (thick line) from baseline or average activity (dotted line). Significant nLFPs are identified if they cross a negative threshold of n times the standard deviation (−n SD) estimated from the total time course of the recording. Amplitude and time of the nLFP correspond to the negative peak. (B) Schematic of nLFP triggered comparisons to time-aligned and best-match (up to ±n ms shifted in time) periods at each other electrode (left). As controls, nLFP triggered periods were compared to random periods of similar duration (middle). A priori similarities of LFP segments were estimated from random, time-aligned segments, whose duration distribution was matched to that of nLFP segments (right; see also D). Color code extends to (D) and (E). (C) Waveforms were compared by calculating the correlation R of the time series. Examples of simultaneous waveforms at two sites A and B with R = 0.5, 0.65, and 0.8. (D) Mean duration ±SD of the nLFP as a function of nLFP threshold (M1left; monkey A; average over all four arrays in monkey B; average of n = 6 cultures). (E) Distributions of correlation R (average across all electrodes) between nLFP triggered and time-aligned (black), nLFP triggered and random position (red), and random and time-aligned (green) periods as described in (B) shows a large fraction of correlated sites for nLFP triggers with threshold −4 SD in vivo (left; M1left; monkey A) and −9 SD in vitro (right; single culture). (F) The average fraction of highly correlated sites across events (i.e., R≥0.8, shaded area in Figure 1E) increases sigmoidally with nLFP amplitude beyond ∼1 SD (broken line; sigmoidal fit, R>0.99 all cases) and was significantly different from all controls (random, duration- and amplitude-matched; p<10−4 all cases). Best-match comparisons (shown here up to a maximum shift of ±10 ms) demonstrate higher correlations overall (open arrow: best-match comparison artificially increases the likelihood of finding highly correlated, short segments at low threshold within ±10 ms). Left: M1left; monkey A. Middle: average over all four arrays in monkey B. Right: average of n = 6 cultures.