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 rephcates 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. Twelve 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 hght 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 hght 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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