The neurogenesis of P1 and N1: A concurrent EEG/LFP study

Abstract

It is generally recognised that event related potentials (ERPs) of electroencephalogram (EEG) primarily reflect summed post-synaptic activity of the local pyramidal neural population(s). However, it is still not understood how the positive and negative deflections (e.g. P1, N1 etc) observed in ERP recordings are related to the underlying excitatory and inhibitory post-synaptic activity. We investigated the neurogenesis of P1 and N1 in ERPs by pharmacologically manipulating inhibitory post-synaptic activity in the somatosensory cortex of rodent, and concurrently recording EEG and local field potentials (LFPs). We found that the P1 wave in the ERP and LFP of the supragranular layers is determined solely by the excitatory post-synaptic activity of the local pyramidal neural population, as is the initial segment of the N1 wave across cortical depth. The later part of the N1 wave was modulated by inhibitory post-synaptic activity, with its peak and the pulse width increasing as inhibition was reduced. These findings suggest that the temporal delay of inhibition with respect to excitation observed in intracellular recordings is also reflected in extracellular field potentials (FPs), resulting in a temporal window during which only excitatory post-synaptic activity and leak channel activity are recorded in the ERP and evoked LFP time series. Based on these findings, we provide clarification on the interpretation of P1 and N1 in terms of the excitatory and inhibitory post-synaptic activities of the local pyramidal neural population(s).

Keywords: Local field potentials (LFPs); N1; P1; bicuculline; electroencephalogram (EEG); event related potentials (ERPs); excitation; inhibition; rat; somatosensory cortex; somatosensory evoked potentials (SEP); whisker barrel cortex.

Fig. 1

Set-up for concurrent LFP and EEG recordings. A burr hole was created over S1BF. A spider electrode was placed above the skull and secured using collodion adhesive. Non-conductive mineral oil was placed within the burr hole, and conductive EEG gel was placed between spider electrode and the skull around the burr hole.

Fig. 2

Comparison of evoked LFP between pre- (control) and post-saline injection (saline). Mean evoked LFP responses from the supragranular layers (A, left panel) and granular layer (B, left panel). Saline responses (grey broken) almost overlapped control responses (black solid). Shadows indicate the standard error of control and saline responses across subjects. The triangle indicates stimulus onset. ‘P1’ and ‘N1’ illustrate the first positive and negative peaks of the supragranular LFP, whereas only N1 exists in the granular LFP. The initial slope of the granular LFP is illustrated on the left panel of B. Bar plots (right panels) show parameter comparisons between control and saline conditions normalised to the mean of the control condition across animals, and error bars indicate standard error. Parameters were compared using the two-tailed Student's paired t-test (* p<0.05). Amp: amplitude; FWHM: full width at half maximum; Plat: peak latency.

Fig. 3

BMI Effect on the laminar LFP and CSD. (A) Progression of BMI effect on the laminar profile of evoked LFP. (B) Corresponding CSD calculated using the inverse Current Source Density (spline iCSD) analysis (see methodology); sources are indicated by red and yellow, whereas sinks are blue. For A and B: x-axis: time from the stimulus onset to 40 ms; y-axis, cortical depth 100 μm–1600 μm (top to bottom) from the pia mater. Each subplot shows an aligned average (n=12) of evoked LFP (A) or CSD (B) within 125 s period, with the starting time of the epoch period indicated at the top left of each subplot. The 10 images spanned 1250 s (~21 min), and contained pre-, during, and post-injection phases of BMI (bottom of B). (C,D). Progression of BMI effect on the temporal dynamics of the supragranular LFP (C) and granular LFP (D). Each curve represents an evoked LFP within an epoch averaged across animals. Data before BMI injection was labelled ‘blk 1’ (black). Post-injection, data were plotted from epoch 4 (red) through to epoch 11 (light blue). (E,F). Comparison of evoked LFP under pre- and post-BMI conditions. Mean LFP pre- (black solid) and post- (grey broken) BMI conditions are superimposed for the supragranular (E, left panel) and granular (F, left panel) layers. Shadows indicate the standard error of responses across subjects. The initial period post stimulus (<8 ms) is indicated in E and F by a vertical broken line. For symbols and abbreviation, refer to Fig. 2 legend. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

BMI effect on the concurrently recorded evoked LFP and ERP (A) Mean ERP (solid) and supragranular LFP (broken) from concurrent recordings are superimposed, with the shadow indicating standard error across subjects. (B) Progression of BMI effect on temporal dynamics of the ERP. Each curve represents an evoked LFP within an epoch averaged across animals. Data before BMI injection was labelled ‘blk 1’ (black). Post-injection, data were plotted from epoch 4 (red) through to epoch 11 (light blue). (C,D,E) Mean ERP from skull EEG (C, left panel), supragranular LFP (D, left panel), and granular LFP (E, left panel) were concurrently recorded. LFPs in control condition (black solid) and the BMI condition (grey broken) were superimposed. The initial period post stimulus (<8 ms) is indicated in C, D and E by a vertical broken line. Shadows indicate standard error. For symbols and abbreviation, refer to Fig. 2 legend. (F) Examples of raw granular LFP data from 3 subjects spanning 25 min including all trials pre, during and post-BMI injection. The insets show a zoomed-in period (7 s) of the LFP pre- and post-drug injection. (G) Normalised MUA (obtained via wavelet coefficients with centre frequencies 400:2000 Hz) from the control period meaned across all animals. Superimposed over the MUA are normalised plots of the LFP taken from the supragranular, granular and infragranular layers. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

Example of disassociation between ERP and granular LFP (n=1). Temporal dynamics of the (A) ERP, (B) supragranular LFP and (C) granular LFP are displayed. In each subplot, a single curve represents an evoked LFP within an epoch averaged across animals. Data before BMI injection was labelled ‘blk 1’ (black). Post-injection, data were plotted from epoch 4 (red) through to epoch 11 (light blue). In this case, BMI injection produced a large effect on the temporal dynamics of the granular LFP, but little effect on the supragranular LFP and ERP. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6

Comparison of FP recordings at different stimulus intensities (n=4). Temporal dynamics of the (A) ERP, (B) supragranular, (C) granular and (D) infragranular evoked LFP averaged across animals are displayed at stimulus intensities 0.8 mA (black) and 1.6 mA (grey).

Fig. 7

Power spectral analysis of the LFP pre- and post-BMI injection (n=9). (A) The evoked LFP responses of the supragranular layer pre- (solid line) and post- (broken line) BMI injection are shown in the left panel. Their spectrograms are shown in the middle and right panels respectively. (B) Same as (A) but for the granular layer.