Oscillations in spike trains

Examples of oscillatory ring The spiking activity of SUs was oscillatory in many cases.

The oscillation frequencies observed were most often in the range of theta frequencies (4-12 Hz), which includes the frequencies of whisker motion (6-12 Hz), or slightly above, i.e., in the lower beta range. The pattern of activity around interactions indicated that at least for some units, these oscillations were associated with whisking. One example RS from L5B showed very regular activity with a peak frequency of 8.7 Hz (Fig. 3.20A-C). This example cell increased its ring rate from 4.60 Hz during baseline time to 7.26 Hz during interactions (Fig.

3.20D), which was signicant (P<0.001, permutation test). There was, however, a peculiar and seemingly contradictory pattern of activity change around interaction onsets: the activity did not increase around the onset of interactions, i.e., rst whisker overlap (Fig. 3.20A; P

= 0.915, n = 38, signed-rank test), and then decreased highly signicantly after head touch (Fig. 3.20B; P <0.001, n = 32, signed-rank test). Correspondingly, ring increased at the end of interactions, which was again signicant (data not shown; P = 0.034, n = 38, signed-rank test). The apparent contradiction between an overall response increase in interactions, as compared to baseline, and the response decrease at actual interaction start and end, was explained by a particularly high ring rate immediately before and after interactions. Thus, the ring rate in the 500 ms before whisker overlap start was 8.84 Hz (Fig. 3.20D), which was a highly signicant increase compared to the ring rate during baseline time (P<0.001, permutation test). It was also higher than the interacton ring rate, although this dierence was not signicant (P = 0.180, n = 38, signed-rank test).

Oscillations were, however, present at dierent frequencies, and the units showed dierent patterns of activity around interaction onset. Fig. 3.21 presents further examples of oscil-lations in spike trains. The cell shown in Fig. 3.21A-C, a L5B RS, had its peak oscillatory frequency at 18.2 Hz (Fig. 3.21B). Similar to the unit shown in 3.20, it also decreased activity after head touch (Fig. 3.21A), and red at higher rates immediately before interaction onset than during interactions (Fig. 3.21C), a signicant decrease from 13.26 Hz during the 'pre-time' to 10.23 Hz in interactions (P < 0.001, signed-rank test). However, this unit's activity was lower during interactions than even in the baseline time (11.43 Hz), and this decrease was also signicant (P = 0.003, permutation test).

A third example unit, also a RS shown in Fig. 3.21D-F, had an oscillatory frequency peak at 15.4 Hz (Fig. 3.21E), and thus between the two aforementioned units. While the frequency of oscillations was much higher than for the example cell in Fig. 3.20, the pattern of rate change around interactions observed here was similar. This unit also decreased activity around the time of head touch (Fig. 3.21D; P = 0.003, n = 40, signed-rank test), and showed an overall ring rate ranking of 'pre-time' (10.40 Hz) > interaction time (9.74 Hz) > baseline (7.40 Hz).

−5000 0 500

Time to first whisker overlap [ms] Time to head touch [ms]

Interaction #

Figure 3.20: Example unit showing strong oscillatory activity. A, The ring of this unit was very regular (raster plot of spikes in single interactions at the bottom), and activity did not change after rst whisker overlap (response histogram at the top). B, After head touch, the ring rate of this unit strongly decreased. C, The ISI histogram shows a peak at 8.7 Hz.

Note that the abscissa scaling is dierent than in Fig. 3.19B,D,F. D, The ring rate during interactions ('in') was higher than during baseline time ('base'), but lower than during the 500 ms preceding rst whisker overlap ('pre'). Error bars are standard errors of the mean.

Comparison of regular-spiker and fast-spiker oscillatory activity The three example units shown in Fig. 3.20 and Fig. 3.21 were all RS, and also on the population level it was observed that RS had a much stronger tendency to re regularly in a wide range of frequencies (Fig. 3.22). As a measure of oscillation strength, the normalized oscillatory power was calculated by dividing the maximum power value within a window of 3 Hz width by the mean oscillatory power over all analyzed frequencies (3-49 Hz). The sliding windows were set such that window centers were between 4.5 and 47.5 Hz, and these windows were shifted in steps of 1 Hz. The normalized oscillatory power was signicantly higher for RS than FS in the range of 10.5 to 27.5 Hz, and the signicance level for each comparison was below 0.001 in the range of 12.5 to 25.5 Hz (for RS n = 250, for FS n = 81, U test).

In the lowest frequency range analyzed (4.5-6.5 Hz), the pattern was reversed, and FS showed higher oscillatory power than RS (Fig. 3.23). This indicates that the eects ob-served here are not artifacts of a generally higher ring rate of FS, in conjunction with the normalization procedure, as discussed in 4.3.7.

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Interaction #Interaction #Firing rate [Hz]Firing rate [Hz] # of spikes# of spikes

A

Figure 3.21: Units showed diverse oscillatory frequencies and dierent patterns of rate change during interactions and the time immediately preceding them. A, Response histogram of an example unit (top) and raster plot of spikes in single interactions (bottom). Similar to the unit shown in 3.20, activity decreased after head touch. B, ISI histogram for the unit shown in A.

Peak oscillatory frequency was 18.2 Hz. C, Firing rates during dierent behavioral phases for the unit shown in A. 'In' indicates 'in interactions', 'pre' indicates 'in the 500 ms period before rst whisker overlap'. This unit was inhibited during interactions as compared to baseline.

Error bars indicate standard errors of the mean. D-F, Same as A-C for an example unit with peak oscillatory frequency of 15.4 Hz. As for the unit shown in 3.20, activity was increased during interactions, but highest immediately before their start.

10 15 20 25 30 35 40 45 0.9

0.95 1 1.05 1.1 1.15 1.2 1.25

Frequency [Hz]

Normalized oscillatory power

Figure 3.22: Comparison of the power of oscillations in the spike trains of RS and FS at dierent frequencies. RS are marked in green and FS in violet. Error margins are standard errors of the mean. Asterisks indicate signicance values, with the asterisks corresponding to one comparison being grouped below each other.

4.5 5 5.5 6 6.5 7

1 1.5 2 2.5

Frequency [Hz]

Normalized oscillatory power

Figure 3.23: Comparison of the power of oscillations in the spike trains of RS and FS for the lowest frequencies analyzed. RS are marked in green and FS in violet. Error margins are standard errors of the mean. Asterisks indicate signicance values, with the asterisks corresponding to one comparison being grouped below each other.

3.3.3 Overall modulation of the neuronal population during social interactions