It was mentioned above (1.6.1) that the torque baseline is believed to correspond to optomotor response behavior (optomotor balance), whereas the body saccades (spikes) were mainly employed to adjust flight direction (Heisenberg and Wolf, 1979; Heisenberg and Wolf, 1984; Mayer et al. 1988; Heisenberg and Wolf, 1993). The rotation index was derived to quantify the amount of angular displacement accounted for by detected spikes in relation to the amount of displacement between the spikes (see above, 2.4.3). The rotation index did not reveal enough between group variation in the non-reinforced periods (periods 1, 2, 5, 8 and 9) to reject the null hypothesis that all four groups were samples from the same population. Therefore, the descriptive statistics of all four groups are given in Table 1.
| Confidence | |||||
| N | Mean | -95% | +95% | Median | |
| RotInd1 | 400 | 0.35 | 0.33 | 0.38 | 0.39 |
| RotInd2 | 399 | 0.32 | 0.29 | 0.34 | 0.36 |
| RotInd8 | 396 | 0.25 | 0.22 | 0.28 | 0.30 |
| RotInd9 | 386 | 0.22 | 0.19 | 0.25 | 0.25 |
| Min | Max | Lower 25% | Upper 25% | ||
| RotInd1 | -.67 | 0.82 | 0.23 | 0.53 | |
| RotInd2 | -.70 | 0.79 | 0.17 | 0.51 | |
| RotInd8 | -.85 | 0.86 | 0.07 | 0.44 | |
| RotInd9 | -.75 | 0.84 | 0.05 | 0.42 | |
Table 1: Descriptive statistics of the rotation indices (RotInd) in all flies at t1 (periods 1 and 2) and at t2 (periods 8 and 9).
Considering that only between 12% and 25% of the time of each 2min. period is consumed by spikes, the data presented in table 1 indicate that indeed most flies rotate the arena stepwise, i.e. the flight direction is fairly constant between the sudden turns caused by the spikes. For instance, in the first period more than twice as much arena rotation was caused by the spikes than by behavior in the interspike intervals (rotation index 0.35). The rotation index exhibits dependence only from the mean number of spikes per period (Spearman Rank Order Correlation 0.58, p<0.001 in the first and 0.50, p<0.001 in the last period). This dependence can be exemplified in two flight modes: in oscillating mode, the spikes (if there are any) are hard to detect and much orientation might be carried out by omitting optomotor waggles. In quiet mode, there are very few spikes and some orientation is accomplished by baseline drift (personal observation). Probably the minimal values in table 1 are examples of those flies. Of course, if a fly is producing many spikes, there is not much room for interspike navigation. As the overall spike number decreases during the course of the experiment (see above, 3.2), the decrease of the rotation index is not surprising. For the same reason, some flies are not accounted for later in the experiment: they ceased to produce spikes at all.
It seems that the rotation index is the lower estimate of the degree to which body saccades are used in free flight for the adjustment of flight direction: 1) as was discussed in Heisenberg and Wolf (1979) and in Heisenberg and Wolf (1984) the three flight modes depicted in Fig. 8 may be instrumental artifacts disturbing spike evaluation. 2) Undetected spikes might contribute to 'interspike' orientation. 3) Spiking behavior correlates well with learning (preference indices) in the Drosophila flight simulator (see below, 3.6) and thus seems indeed to be responsible for the adjustment of flight direction.
However, some flies seem to use a behavior for choosing flight direction that is not covered by the spike detector: in about 25% of the flies at t2 (table 1) the amount of summed interspike arena rotation reaches or exceeds the amount of summed arena rotation caused by spikes. Moreover, in studies of the Drosophila mutant strain ebo678 some flies showed orientation behavior without spikes, but by baseline drift (Ilius, 1992; Ilius et al. 1994) as could be observed during this study with wildtype flies in the quiet flight mode. Thus there are apparently two components of orientation behavior. In the majority of flies the dominant component is spiking behavior.
Therefore, the evidence from the rotation index is taken as confirmation of the findings from Heisenberg and Wolf (1979), Heisenberg and Wolf (1984), Mayer et al. (1988) and Heisenberg and Wolf (1993) where torque spikes have been shown to be endogenous motor patterns ('actions' and not responses to external stimuli) that are produced to adjust flight direction, whereas the baseline is assumed to contain the mechanism to establish and maintain optomotor balance in a responsive way and to be of minor importance for choosing flight direction.