The analysis of some recording sessions showed effects that match the ones previously observed in LFP analysis (ipsilateral deactivation leads to decrease in activity, contralateral deactivation leads to no change or a slight increase, bilateral deactivation again leads to decreased activity; see Figs. 3.8-3.12; cf. Ebisch (2007); Geider (2008)), but these effects were not apparent in all sessions (see Fig. 3.13). From this level of detail, there is an indication that the activity evoked by the stimulus was similar (see Fig. 3.17), but the initial state (as shown in Fig. 3.15) was not. This observation led to the question about the reason for these deviations. One possibility could be a difference in the initial brain state. Therefore, a way to define this brain state had to be identified. From previous studies, the level of gamma activity
deemed a suitable candidate. Based on this idea, all sessions were grouped based on their initial gamma amplitudes during the warm condition. Subsequent analyses were then performed separately for three groups (Low / Medium / High Initial Gamma Power; see Fig. 3.19, p. 73).
With this, it should be kept in mind that the separation based on gamma activity is only one of many possible options to split up the data. Other possible criteria include LFP power in other frequency bands, or the initial spike rate during the warm condition, which might have led to a different assignment of sessions to the individual groups.
Further, it is visible that especially the Low and High Initial Gamma groups are biased towards individual experiments. This is partly due to the approach that was taken with the separation, because sessions recordings from the same hemisphere were generally kept in the same group. Only extreme outliers were then re-assigned to other groups. Experiment 121007, which is mainly represented in the High Initial Gamma group also had the highest signal-to-noise ratio for the spike signal, so that signal quality also might have had an impact here. Thus, even though the separation into the three groups was statistically justified based on the rank (see Fig. 3.19), a larger sample size could mend these issues.
4.1.3 Spike rates
For multi-unit activity, warm spike rates were at a similar level for all three Gamma groups for all phases of the stimulus (see Fig. 3.22, Table 3.5). As expected, the rates were highest during presentation of a moving stimulus. Interestingly, though at similar levels, the observed rates were highest for the Low Initial Gamma group. For single-unit activity, considerable differences in spike rates were observed between the different Gamma groups (Fig. 3.23, Table 3.6), with the rates being highest for the High Initial Gamma group for all stimulus phases. One thing to consider with these observations is that many trials exhibited very low spike rates of zero or close to zero, while at the same time the spike rates showed very high variability (see standard deviations in Tables 3.5 and 3.6).
During pMS deactivation, again, the spike rates calculated for multi- and single-unit activity showed a slightly different behaviour (see Figs. 3.22-3.23 and Tables 3.7-3.8): for the multi-units, there was a clear decrease in rates for bilateral deactivation in the High Initial Gamma group, which also reflected in the Medium Initial Gamma group (though not as pronounced) and was absent for the Low Initial Gamma group. For the single-units, the Low Initial Gamma group showed a decrease in single-unit rates for bilateral deactivation, but an increase for ipsilateral pMS deactivation. For the Medium and High Initial Gamma groups, a clearer picture emerged, with a decrease in rates for ipsi- and bilateral deactivation, which was especially visible for spontaneous activity and during static stimulation. For the Medium Initial Gamma group, the single-unit rates went back to the level of the warm condition once the moving stimulation started, while for the High Initial Gamma group, the rates never returned to the level of the warm condition, even for the phases of moving stimulation (see Fig. 3.23). The observations for the Medium and High Initial Gamma group fit previous observations for the LFP (Ebisch, 2007; Geider, 2008; Barnes, 2014), i.e. decreased activity during ipsi- and bilateral deactivation, and a similar, or even increased activity level during contralateral deactivation. The deviance of the Low Initial Gamma group could be due to the general impairment of the network for this condition. Activity levels and initial connectivity seem so low for this group that meaningful information processing cannot be attained. Therefore, a further manipulation of the network by deactivation pMS might lead to no or little changes, and could also increase the weight of input signals from other areas, which in turn might have led to the observed slight increase in rates for the ipsilateral deactivation condition.
4.1.4 Tuning properties
In many cases, a striking similarity in the shape of the tuning curve throughout the layers was observed, indicating that the electrode was moved through an orientation column, i.e. perpendicular to the surface. This observation supports the estimations of electrode depths and layers, as this can only be sufficiently accurate when the electrodes are moved vertically (an example for the tunings and estimated laminae is shown in Fig. 3.25).
Overall, the tuning selectivity, i.e. the vector strength for the preferred stimulus orientation and di- rection, respectively, was strongest for the High Initial Gamma group and lowest for the Medium Initial Gamma group. The strength of both orientation and direction selectivity were found to be reduced by pMS deactivation: with ipsilateral, but also contralateral pMS deactivation, a significant decrease in di- rection selectivity was observed for all Gamma groups. During bilateral pMS deactivation, no significant change in direction tuning was observed. Orientation selectivity in the Medium Initial Gamma group decreased for all deactivation conditions, while for the Low Initial Gamma group a significant decrease was only observed during ipsilateral pMS deactivation. For the High Initial Gamma group, no significant changes in orientation selectivity were observed.
The maintenance of direction selectivity during bilateral pMS deactivation is surprising, since the pMS cortex was described as an important contributor to the manifestation of direction selectivity Galuske et al. (2002). On the other hand, in the awake animal, a restitution in behaviour can be observed when pMS is deactivated in both hemispheres (Lomber and Payne, 1996), which could be related to this effect: other areas might take over functions that had previously been covered by the pMS cortex and the equilibrium in activity between the hemispheres might play a crucial role in this context (Payne and Rushmore, 2004).
Interestingly, orientation selectivity slightly increased for ipsi- and contralateral deactivation for the High Initial Gamma group. This could be a hint that the influence of the pMS cortex that might have sent corrective signals before, modulating the sensory signal already at the level of the primary visual cortex, is lacking, and with this, the feedforward impact gets stronger, resulting in a more stimulus-driven system.