the original video data) to check their fit and then recompute the paths using 
new parameters. Often several iterations of this process are required to obtain 
an optimum combination of parameters. Since the parameters depend largely 
upon the magnification, the density of organisms and the way in which they 
move, the same “optimum set” of parameters (retained in memory for the 
user’s convenience) are generally used to compute paths for all replicates of an 
experiment. 
Proper selection of pathfinding parameters can significantly reduce the 
number of erroneous paths but cannot be relied upon to eliminate all errors. 
Consequently, we have developed programs which allow the user to interatively 
detect and correct mistakes within path files. Path editing programs (including 
CHOZ, EDIT, MERG and JOIN) allow the user to preform the following basic 
operations: (1) delete a path; (2) truncate a path; (3) cut a path into two 
smaller paths; and (4) join two paths (assuming they do not overlap in time). 
Once a valid collection of paths has been obtained, the user may proceed 
directly to the extraction of time series of behavioral variables (as discussed 
below) from the path files. Before doing so, however, there are several 
additional procedures which the user may choose to apply to the path data. 
Since the use of these procedures (and the order in which they are applied) is 
dependent upon the overall design of the experiment, we will first illustrate 
them by means of a specific example. 
Wilson video taped the behavior of Daphnia pulex (a small freshwater 
crustacean, commonly known as a “water flea”) in Talbot Waterman’s 
laboratory at Yale University. 12 animals were observed from below swimming 
against a brightly and uniformly illuminated background. A variable 
polarizer/depolarizer was interposed between the chamber containing the 
animals and the light source. The tape consisted of 23 separate video records. 
Before each recording the polarizing filter was rotated to a randomly chosen 
angle and, also in random sequence, the device was adjusted so as to polarize or 
depolarize the illumination. Each record began with the image of a strip of 
plastic attached to the filter in order to indicate the angle of the filter and the 
magnification of the image. The plastic strip was then removed and the animals 
were observed swimming under constant conditions. In all, 13 records were 
obtained under polarized light and 10 records were obtained under unpolarized 
light. Each recording lasted two minutes. 
The video tape was analyzed with the aid of the ECLIPSE at Narragansett, 
R.I. For each record the LIVE INPUT operator was used to determine the 
angle of the filter with respect to the Bugsystem reference frame (which is 
fixed with respect to the raster scan of the video signal). As discussed above, 
the appropriate spatial scale factor (approximately 0.40 mm/Bugwatcher unit) 
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