Assignment of units to layers

Figure 3.4: Units classied as FS had on average higher ring rates than putative RS. All units ring above 25 Hz were FS. Note that the FS population also included cells with very low ring rates.

positive correlation including all units recorded independently of displacement and was close to signicance (P = 0.058, Pearson's correlation). It can be speculated that this is explained by properties of tissue adhesion to the tetrode, such that the tissue remained attached to the tetrode for small displacements, and then relaxed slowly, while the tetrode was immediately repositioned relative to the tissue for larger displacements.

3.1.4 Assignment of units to layers

In 60% of all RS neurons, the layer of the recording site could be determined. There was a bias towards layer 5B (L5B), while particularly L6 was underrepresented. Also, no L1 units were recorded at all. These patterns were probably due to both the time course of tetrode placement and the high ring rates of layer 5B cells (see 4.3.2). The distribution across layers for RS, as shown in Fig. 3.6A, was as follows: L1 no units, L2/3 23, L4 25, L5A -32, L5B - 65, L6 - 6. The recording sites as veried by histology were consistent with the RFs described above, insofar as most of the sites were localized in the postero-medial barrel subeld, which corresponds to the macrovibrissae. As Fig. 3.6A also shows, the number of FS recorded was substantially lower than the number of RS, and the number of MUs was between the two. However, it should be kept in mind that these numbers refer only to those units, to which a layer could be assigned based on histology. The overall dataset consisted of a much higher number of units (e.g. 242 RS overall vs. 151 RS with layer assignment), and

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Instability (firing rate drift)

Tetrode movement before recording [mm]

Figure 3.5: Recording instability as a function of tetrode displacement before the start of recording. Positive tetrode movement indicates lowering the tetrode. Units marked in green were recorded without any tetrode displacement before the recording. Red circles indicate medians. Three stability values from instances where the tetrode was elevated before recording have been omitted from the gure, but entered statistics, for which the absolute values of displacements were used.

The contribution of dierent layers to the dataset with veried histology did not dier strongly between the three types of units recorded (Fig. 3.6B), with the exception of L6, which contributed only 4% of RS, but 16% of FS. There were, however, considerable dierences between the contributions of dierent layers to the datasets recorded from male and female rats (Fig. 3.6C). The data shown here and in Fig. 3.6D refer to RS only, as only RS have been used for the analysis of sex-specicity of BC activity in section 3.3.12. Thus, within the RS with veried layer, the male dataset (n = 90) was heavily biased towards L5B (62%), while L5B contributed only 15% in recordings from females (n = 61). Layer 5A was also slightly more represented in males, while in females the contributions from L2/3 and L4 RS were dominating. In contrast, when the RS were parsed by the estrus state of the subject female, contributions of dierent layers were roughly comparable (Fig. 3.6D), although L2/3 had a higher contribution in estrus (45%) than in non-estrus females (24%). However, the numbers on layer contribution as a function of estrus state should be taken very cautiously, as from the 32 RS recorded from females in estrus, only 11 had a veried recording layer, such that even the L2/3 contribution consisted of only ve units. The corresponding dataset recorded in non-estrus females included 37 RS.

As histology was not available for all animals and all tetrodes, it is interesting to ask whether the laminar location of a neuron could be reliably determined from the recording depth, as deduced from the number of tetrode turns. For this analysis, recording depth was calculated relative to the rst location where spikes were observed. As Fig. 3.7 shows, there is the expected very prominent correlation of recording depth with cortical layer (R = 0.568, P < 0.0001, Pearson's correlation). However, it is also obvious that such an assignment would

0.9

Number of units Fraction of units

Figure 3.6: Contributions of the units recorded in dierent layers to the overall dataset with layer assignment. A, Absolute numbers of units from dierent layers for RS, FS, and MUs.

No L1 units were recorded. B, Normalized display of the data in A, showing the relative contributions of the dierent layers for each type of unit. C, Relative contributions of dierent layers to the RS recorded in males and females, respectively. D, Relative contributions of dierent layers to the RS recorded from females in estrus and non-estrus, respectively.

be very imprecise. For a discussion of how this could be improved and made useful, see 4.1.5.

Histology also showed that a very small group of units (n = 4) was inadvertently recorded in the striatum. An example of a striatal interaction-related response and a short discussion of this observation is given in an appendix (section 6).

0.5 1 1.5 2 2.5 L2/3

L4

L5A

L5B

L6

Layer

Recording depth [mm]

Figure 3.7: Correlation of layers as determined by histology with the depth of the recording location, based on tetrode positioning. A depth of 0 mm corresponded to the rst location where spiking activity was observed. Within the units from L4, those recorded in an unusually lateroposterior location constituted a large fraction (17/41). As these were also recorded at larger depths, they are separately plotted in light blue, and were not included for the calculation of the median and quartiles of the corresponding box.