was also computed. The BUGWATCHER INPUT operator was then used to 
generate one video file (480 frames of data at ten frames/sec. or 48 sec. of 
data) for each record on the tape. Wilson then MASKed the data, saving only 
those points within a centered circular region of Bugspace. The centroids and 
then the paths were separately computed for each masked video file using the 
same set of centroid and pathfinding parameters. Very few pathfinding errors 
resulted from these computations, but these were corrected by editing the 
separate path files. Wilson then employed the PARAMETER operator to 
associate the angle of the fdter with respect to the Bugsystem reference frame 
(an experimental parameter) with each path. This angle was the same for every 
path within a given path file. By repeated use of the MERGE operator, Wilson 
condensed the data to produce two exceedingly large files: one containing all 
paths observed under polarized light (154 paths, 13210 data points) and 
another containing all paths observed under unpolarized light (166 paths, 
13691 data points). These paths were still represented in Cartesian coordinates 
relative to the Bugsystem reference frame. However, orientation with respect 
to the plane of polarization should only be manifest with respect to the 
reference frame of the filter. Using the ROTATE operator, the two frames of 
reference were made to coincide: every path within each merged file was 
rotated in Bugspace (about an axis passing through the center of this space) 
through an angle obtained by negating the appropriate experimental parameter 
for each path. All resultant paths for Daphnia swimming under polarized light 
are PLOTed in Figure 17-1. Many of these paths may be seen to be aligned 
approximately orthogonal to the axis of the filter (i.e., perpendicular to the 
E-vector of the polarized light). 
The preceading analysis illustrates the use of three operators (vis., the 
PARAMETER, MERGE and ROTATE operators) to organize path data prior 
to the computation of behavioral variables. The ROTATE operator was 
implemented to expedite studies of animal orientation. Using this operator, all 
spatial data in a file may be rotated through a constant angle, each path may be 
rotated through a constant angle associated with that path or each point may 
be rotated through an angle associated with a corresponding moment in time. 
The last option enables the investigator to study orientation with respect to a 
moving stimulus (e.g., be MERGED) whenever the respective files may be 
taken to be replicates of the same experiment. Not only does this simplify the 
bookkeeping tasks associated with subsequent analysis, but, in addition, allows 
a set of similar data to be treated as a single sample by statistical operators. The 
fundamental advantage of an operator-based interactive system for the analysis 
of behavioral data is its flexibility: the operators which the user chooses to 
apply to the data-and the order in which they are applied—can be selected to 
correspond to the design of the original experiment. 
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