How local is the local field potential?

Laminar patterns of auditory responses of LFP, CSD and MUA in the auditory cortexResponses to the BF tone in one example A1 site. Line plots (A) show LFP responses recorded at 23 depths using a linear array multielectrode with 100 μm intercontact spacing (schematic on left). In center (B) is the color plots of the laminar LFP profile shown in A; with negative deflection colored red and positive deflections colored blue. (C) depicts the CSD profile derived by the second derivative approximation of the field potential profile in A and B; red depicts extracellular current sinks (associated with net local inward transmembrane current flow) and blue depicts extracellular current sources (associated with net local outward transmembrane current flow). Selected MUA responses from channels 2, 6, 10, 15, and 19 are superimposed on the CSD plot. Vertical thin lines in all columns indicate stimulus onset. In this example, the peak of MUA at channel 15 corresponded to the peak negativity of LFP and current sink (CSD) at the response onset in Layer 4, and the responses from this location were used for analyzing lateral spread of signals. The asterisk indicates a superficial sink that produced N50.

Time courses of responses to tonesTone-evoked MUA, CSD and LFP responses at two example sites (A and B). MUA, CSD and LFP responses are colored blue, green and red, respectively. All time-bases extend from −30 to 170 ms relative to the onset of 100 ms duration tones, whose onsets and offsets are indicated by vertical dotted lines. Tone frequencies are indicated on the top row (kHz).

Normalized tuning curves for MUA, CSD and LFP(A and B) tuning curves of two exemplars shown in Figure 2. Colors of curves correspond to MUA (blue), CSD (green) and LFP (red). (C) Summary of tuning curves across all penetration sites. Line and dotted line traces represent the median and its 95% confidence intervals of all recording sites (n=130). The MUA, CSD and LFP tuning curves of the individual sites were shifted on the frequency axis to align the best frequencies of MUA tuning curves on zero. (D) Boxplot showing the median and the first and third quartiles of BWMUA (blue), BWCSD (green) and BWLFP (red).

Comparisons of LFPobs and LFPcal(A) Laminar-temporal profiles (−30 – 170 ms) of LFPobs, CSD, LFPcal responses, and disparity of signs between LFPobs and LFPcal (from left to right) responses to tones of frequencies from 0.35 kHz (top) to 32 kHz (bottom) of one example recording site. In the first to third columns, red and blue correspond to negative and positive polarities, respectively. In the forth column, red indicates positions of unequal polarity (Δsign) between laminar-temporal profiles of LFPobs and LFPcal responses. White vertical lines indicate the onset and offset of tones. (B) Tuning curves (blue: MUA, green: CSD, red: LFPobs) of the example site shown in A, with SXCorr (black) overlain over tone frequencies (see text for details). (C) Summary of SXCorr curves across penetration sites. Line and dotted line curves represent the median and its 95% confidence intervals estimated from all penetration sites (n=130).

Spread of auditory LFP responses perpendicular to the auditory cortex(A) Profile of auditory LFP responses to 100 ms BBN stimulus measured during an electrode penetration from the dura down through the depth of auditory cortex (schematic of the penetration depicted on a brain coronal section at the left; inset shows that approximate position and angle of the section). Gray vertical line indicates stimulus onset. Black vertical line indicates the timing 24 ms post onset of stimuli, used to derive the amplitude profiles. Inset shows expanded view of top one third depths, and same applies to two other columns (B and C). (B) The distribution of the amplitude of LFP (red) and CSD (blue) signals at 24 ms for the example shown in A. The origin of the vertical axis is set to the depth of inversion of polarity within the auditory cortex. (C) Distribution of median of normalized amplitudes at 24 ms (n=105). Normalization was done with respect to the mean absolute amplitudes of all depths for each penetration site. For each track, recording depths were rounded to depths with intervals of 0.5 mm and mean amplitudes of multiple depths rounded to each step were used. Split lines show 95 % confidence intervals of median. On the left, a coronal section of Nissl stained brain is shown to illustrate the electrode track. (LS, lateral sulcus; STS, superior temporal sulcus; 3B; somatosensory area 3B; S2, secondary somatosensory cortex; RI, retroinsular cortex; STP, superior temporal polysensory area)

Spread of frequency bands of LFP responses(A) The median amplitude distributions of band-limited signals of auditory LFP responses to BBN (n =105). Panels from left to right show amplitudes of frequency bands (FB1: 1~2.9 Hz, FB2: 3~8.8 Hz, FB3: 9.1~26.7 Hz, FB4: 27.7~81 Hz, FB5: 83.9~256 Hz) at the time of 24 ms post stimulus onset. Amplitudes were normalized with respect to the mean absolute amplitudes of all depths for each penetration site. Asterisks indicate depths where median normalized amplitudes did not significantly differ from zero according to 95 % confidence intervals (bootstrap). Gray crosses label depths where phases of signals were random between penetration sites. Other format conventions are the same as Figure 5C. (B) The median power distributions of band-limited signals of auditory LFP responses. Panels from left to right show increments of power at 24 ms post onset of stimuli from the prestimulus baseline for 5 FB at distances relative to the depth of inversion. In all panels, dashed lines indicate the 95% confidence intervals of median amplitude distributions.

Tuning curves after subtraction of the mean for CSD and LFP(A) Summary of tuning curves of MUA (blue), CSD (green) and LFP (red) across all experiments. Line and dotted line traces represent the median and its 95% confidence intervals (bootstrap, n=130). Tuning curves were normalized to their peaks after subtracting their mean values. MUA tuning curves are the same as those shown in Fig. 1C. (B) Boxplot showing the median and the first and third quartiles (n=130) of BWMUA (blue), BWCSD (green) and BWLFP (red). BWCSD and BWLFP were derived after subtraction of mean values.