Schematic representation of the 16-electrode array.

AdP

B

C

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lOOms

2.2m m

2.6m m

2.5m m

7

2.6m m

i^ m m

depth relative

to electrode 1

8

_3.2

1.2 mm

shallow er

1mm

deeper

2.4m m

Fig. 5.3.2. Representation of phases across an array of LFP recordings. Each electrode is represented by a circle. The phase of the LFP recorded on that electrode, relative to that from electrode 1 is shown as a 'clock hand'. Electrode 1 therefore shows zero phase (12 o'clock) with increasing phase clockwise. Depths of electrodes relative to electrode 1 are represented by grey rectangles: above the clock for sites deeper than electrode land below the clock for shallower sites. LFP is in phase with electrode 1 (p<0.05) on all sites except electrode 6, which was 4 mm deeper than electrode 1.

4mm below the others in the grid, and its LFP has a 180° phase reversal compared to the field potential recorded at these sites.

This finding broadly agrees with work by Murthy and Fetz (1996a) who found phase reversal between LFP recorded in the deep layers and at <lm m below the cortical surface. In our recordings the phase reversal is observed between sites typically separated by 3-4 mm, where one is either deeper or shallower relative to the pyramidal cell layer. Our knowledge of the exact cortical layer in which the recordings were made is complicated by the possibility that most of our electrodes were located in the rostral bank of the central sulcus, where the layers are perpendicular to the brain surface. We can, however, assume that sites with a difference in depth of 3-4 mm relative to the pyramidal cell layer are unlikely to be in layer V. I therefore conclude that there was a phase reversal between fields from pyramidal cell layer V and both deeper and shallower layers.

5.4 Coherence between cortical cell discharge and LFP.

The main aim of this analysis was to investigate the nature and extent of the synchrony between cortical cells, especially PTNs, and the LFP. The coherence analysis was performed as described in section 5.2, using blocks of cell and LFP data 1.024s long from the hold period. For each cell, the coherence was tested in two frequency bands (see below) and the phase relationship was then quantified for each band. The number of LFPs recorded in each session resulted in the following alteration to the method.

Since, for sessions recorded in monkeys 30 and 33, there were up to 12 LFP signals to perform coherence with, this allowed a reduction in the uncertainty of the phase estimate. For electrodes amongst which the LFP was coherent with zero phase lag (see section 5.3) the phase relationship of cell to LFP was found to be the same (to within the 95% confidence limits on each phase estimate).

Therefore the phase relationship of cell to LFP was averaged over all the coherent LFP sites, using the circular mean (equation (9), above) and combined confidence limit (equation 10, above). LFP recorded at sites that were phase-reversed compared to the layer V sites, and LFP from the same electrode as the cell tested, were not used to form the average phase for each cell. The averaging of phases to produce a mean phase estimate with smaller confidence limits is illustrated in Fig. 5.4.1. The phase of one cell with the LFP at 9 different sites is shown. The mean phase between all sites was 139±2°. For cells which showed the phase plus delay relationship, the regression of phase on frequency was performed on data from one cell-LFP pair only.

The database used was that represented in Table 5.2.1. The majority of cells analysed (206/231) were sampled in 22 sessions in monkey 33. Fewer cells were analysed from monkeys 29 and 30 (15 and 10 respectively). O f the total of 231 neurons sampled, 171 (74%) were PTNs; 26 of these PTNs were further identified as CM cells. The total number of spikes recorded per neuron ranged from 1,672 to 121,440 and neuronal activity was recorded for between 133 and 1022 successful trials. All but three neurons modulated their discharge rate in association with the task and all were active during the hold period.

For the majority of sessions analysed (96%) at least one cell showed significant coherence with the LFP. A typical example of a cell which is coherent with the LFP is shown in Fig. 5.4.2. This is a CM cell (9-2) from the left M l of monkey 33, and is the same cell as that presented in Fig. 5.3.1. Discharge of the neuron was recorded for 62 minutes, during which the monkey successfully performed 470 trials of the precision grip task. Fig. 5.4.2A shows the PSF that this cell produced in the EMG recorded from the AdP muscle, identifying it as a CM cell. The autocorrelogram for the CM cell is shown in Fig. 5.4.2B. A peak is present around 30 ms, indicating a tendency for periodic discharge. Fig. 5.4.2C shows the power spectrum of the spike train; a peak in the range

A

oc

A. Phase relationship between one ceil and LFP recorded at 9 sites is shown. Each electrode position is represented by a circle, within which the 'clock display' shows the phase of cell-LFP coherence at that site. The red lines to either side indicate 95%

confidence limits on phase.

The cell was recorded on electrode 6, and is coherent with LFP at all sites, with the same phase at each (p<0.05). B. Phase averaged over all 9 sites, using

the circular mean, with combined 95% confidence limits in red.

B Mean phase for all sites

O'

270

180°

Rostral

90'

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c