Response modulations of single units during interactions

As this was the rst study to examine responses of BC neurons during social touch, the rst step was to observe responses on the level of individual units. In many units strong response increases during social touch were observed. The response increase of a representative RS from L5B is shown in Fig. 3.16A,B. This unit, for which the corresponding histological recording location is shown in Fig. 2.4, increased its ring rate from 1.03 Hz outside interactions (i.e., baseline) to 3.73 Hz during interactions (P < 0.001, permutation test). The response increase around interaction onset was also highly signicant (P = 0.002, signed-rank test), when the ring rates in the 500 ms before and after the start of each interaction were compared. As shown in Fig. 3.16C, interactions were associated with decreasing whisking amplitudes and lower regularity.

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Figure 3.16: Response increase of a representative SU, a RS from L5B, at interaction onset.

A, Raster plot of spikes in single interactions. Red line indicates the time of rst whisker overlap. B, PSTH of responses shown in A. C, Whisker motion and nose-to-nose distance in an interaction (underlaid gray in A). Times of spikes are marked by black vertical lines (both solid and dashed). Note the decrease in amplitude and regularity after interaction start, in particular for the subject rat.

Responses typically started to appear after rst whisker overlap, but for many units they peaked or plateaued only when the heads of the two rats touched for the rst time (Fig.

3.17A, top left, and Fig. 3.17C, top left). At the end of interactions, response changes were often sharper (Fig. 3.17A, top right, and Fig. 3.17C, top right), in line with the observation that the time between the rst whisker overlap and rst head touch was much longer than the time between last head touch and last whisker overlap. In other words, rats approached each other slowly, but typically separated rapidly.

Observed responses in the neuronal population were very diverse, as can be seen in Fig.

3.17 and 3.19. Examples of strong response increases were observed in both RS (Fig. 3.16, Fig. 3.17C) and FS (Fig. 3.17A). The example FS increased its ring rate from 25.3 Hz at baseline to 37.1 Hz during interactions (P < 0.001, permutation test). Interestingly, the ring rate in the 500 ms before interaction start was much lower than baseline (17.0 Hz), and this was also highly signicant (P < 0.001, permutation test). It has not been established, whether this lower ring rate before interactions was due to interaction-related inhibition, or was simply an eect of high ring rates during other periods of the baseline time, e.g. when the walls were touched. This was very improbable to happen within 500 ms before interaction onset, and could thus produce the appearance of inhibition.

While responses were easier to detect in highly active units, examples of units which were very sparsely activated by social touch were also observed. Thus, the unit shown in Fig.

3.17B, a L5B RS, had a baseline ring rate of only 0.12 Hz, which increased to 0.26 Hz during interactions. This response increase, as well as the increase at rst head touch and decrease at interaction end, were signicant (P < 0.03 for all three comparisons). The sparsely responding unit in Fig. 3.17B was recorded in the same session and at the same location as the one shown in Fig. 3.17C. This unit was also a RS which increased its response signicantly (P < 0.001 for the three aforementioned comparisons). This serves to show that recordings from low-ring units could be achieved even in the presence of much more active units. The separation of the unit in Fig. 3.17B from the one in Fig. 3.17C, as well as others from this recording, is established with high probability, as the L-ratio of the former was 0.007. This placed it in the top eighth of all units by separation.

As shown above, there were strong responses of many units associated with social touch.

However, it could be asked whether comparably strong responses were observed during the touch of other stimuli. This question has been investigated in depth for objects (see 3.3.9) and stued rats (see 3.3.10), while a quantication of wall and oor touch events, which did occur during baseline time, was not attempted. However, as Fig. 3.18 shows for an example unit, it was observed repeatedly that interactions were associated with peaks of activity which exceeded those dispersed over the baseline time. For the particular unit in Fig. 3.18 (same as shown in Fig. 3.56A,B), the 12 highest peaks were all associated with interactions, and

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Figure 3.17: Example responses of SUs aligned to the time of rst head touch (left column) and interaction end (right column). A, FS with a particularly strong response during interactions.

The response sharply increases at the begin of head touch (response histogram in top left, raster plot of spikes in single interactions in bottom left). At interaction end, the response decreases even more sharply (right), indicating that in the end of interactions the head touch and whisker overlap events were much closer than at interaction start. B, Very sparse response increase of a RS during interactions. C, Another RS recorded in the same session and from the same tetrode as the unit shown in B. Note that the response increases before the head

the peak ring rate during interactions was 14.6 Hz, as opposed to 6.2 Hz during baseline time. Amongst 41 distinctive peaks over 3.5 Hz, the ring rate during those associated with interactions (mean 6.8 Hz, n = 23) was signicantly higher than for those independent of interactions (mean 4.4 Hz; n = 18, P = 0.004, U test).

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Figure 3.18: Moving average of the ring rate of a strongly modulated RS over all ve stimulus rat presentation blocks of a recording session. Times of interactions are marked by green boxes. Note the strong response increases associated with interactions, and the absence of comparable peaks outside of interactions. It can also be seen that interactions were not evenly distributed, but rather occurred in bouts. For calculations of the moving average and the error bars, see 2.7.1. Same unit as shown in Fig. 3.56A,B.

A considerable diversity of responses was observed apart from response strengths and ring rates, as shown in Fig. 3.17. Thus, some units exhibited bursty ring, as displayed in Fig. 3.19A-D. These bursts were much higher in frequency than the oscillations in spike trains described in 3.3.2. Bursty units could be associated with both excitation (Fig. 3.19A) and inhibition (3.19C). Bursts with high intra-burst frequencies of 100 Hz and above were typically observed in FS, as the one shown in Fig. 3.19C,D.

While this study focuses on SUs, it should be noted that MUs often also showed very strong modulations by social touch. Thus, a MU recorded in L5A showed a sharp response increase at head touch (Fig. 3.19E). The ISI histogram (Fig. 3.19F) of this unit shows some of the criteria used for classication of clusters as MUs: there is no clear refractory time, indicated here by numerous spikes in the rst bin of 2 ms width. In addition, the ISI histogram bin counts decreased smoothly, indicating that this was not a bursty FS.

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Figure 3.19: Diversity of neuronal responses in BC associated with social touch. A, Response histogram (top) and raster plot of spikes in single interactions (bottom) for a bursty RS. B, ISI histogram for the unit shown in A. Bursts had frequencies of up to 100 Hz. C, Same as A for a FS which decreased responses after head touch. D, Same as B for the unit shown in C.

This unit red high-frequency bursts with intra-burst frequencies of 80 to 125 Hz. E, Same as A for a MU. Responses increased sharply around the begin of head touch. F, Same as B