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 pathfmding 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 filter 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 tlirough 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. 



264 



