An analog to digital converter transforms the data from the circular potentiometer measuring the angular position of an arbitrary point of reference on the cylindrical panorama (arena position) into an sequential array of consecutive data points with possible values ranging from -2048 to 2047 (sampling frequency 20Hz). The zero value corresponds to an arena position at which all quadrant borders are at a 45° angle with respect to the fly's longitudinal axis. These raw data are stored in the computer memory (position trace).
Avoidance is assessed as the preference of a fly to keep one pattern orientation in the frontal position rather than the other. From the position trace the preference index is calculated as (p2-pl)/(p2+pl), with p2 being the number of data points corresponding to a position of the arena at which the pattern orientation not associated with heat was kept in the frontal quadrant of the visual field and p1 denoting the remaining data points. A group of flies are said to have learned if their preference index at t2 (the last two periods) differs significantly from that of the respective control group.
The mean duration of periods of staying in one quadrant (dwelling times) can also be calculated from the position trace by dividing p1 and p2 by the number of stays in the respective sector.
The ability to keep optomotor balance with one pattern directly in front of the fly (i.e. to fly straight towards the pattern) is assessed as the time the fly kept the patterns in the frontal octant of its visual field compared to the time the quadrant borders were in this position. In order to calculate a measure for fixation, the absolute values of the position trace data array are transformed with modulo 1024 to yield values ranging from 0 to 1024. From the resulting array (where now the two extreme values represent the centers of two adjacent patterns) the fixation index is calculated as (f1-f2)/(f1+f2), with f2 being the number of data points n fulfilling 256<n<768 and f1 being the remaining data points.
As a measure for the activity of the fly during the experiment, the number of quadrant changes and the total amount of arena rotation are calculated from the position trace. Adding up the events where data points corresponding to one pattern orientation in the frontal position are followed by points corresponding to the adjacent pattern yields the number of quadrant changes. The amount of arena rotation is given by the sum of the distances between consecutive data points in degrees.