STIMULI DURING DELAYED REACTION IN DOGS 281 



No attempts at longer delays were made as no further object 

 seemed to justify their continuance. During all these trials 

 of the "Two Light" type, the usual method of avoiding rhythmic 

 succession of trials was used, as well as the system of throwing 

 the food to the dog after a correct reaction had been made. 

 This latter method avoided the possibility of the use of the 

 olfactory stimulus as a cue to reaction. The time gaps already 

 bridged were too long for any sensory after image to have been 

 retained and hence diversion stimuli would have no lasting 

 effects on the cues by which the dog retained the association. 

 No attempt was made to find out definitely what those cues 

 were. The above results show that the power of discrimination 

 of two objects, fairly well separated (six feet), was within the 

 untrained capacity of the dog, and that the responses given 

 after the delay periods were the result of training. This training 

 enabled the retention of these powers of discrimination, necessa- 

 rily through some process, presumably memory. The negative 

 results gained by diverting stimuli during the delay period, 

 show that mere physical clues or sensory after images could 

 not suffice as cue retainers, but that there must be a definite 

 mental process involved, that may be called "Memory Associ- 

 ation." Since an association of stimulus and food compartment 

 was necessary, and since correct reaction demanded that such 

 an association be retained by some mental process, the "C" 

 type of problems having shown the absence of the physical 

 cues, therefore, as psychologists agree that sensory after images 

 could not bridge a gap nearly as wide as the five minute one 

 that this dog has done, the process must be one that involves 

 the memory. 



" Four Light Experiments." 



In the middle of February, 1914, it was suggested that the 

 problem be made more difficult by increasing the number of 

 compartments, the cues to which must be remembered, rather 

 than by the continued increasing of the length of delay periods 

 over which the association cues must bridge. So to increase 

 the number of compartments, No. 4 was added, but not on the 

 same ground level as the other three. The food was obtained 

 only by going up to the compartment on a board, eight feet 

 long and having a 30 degree pitch. The light stimulus was however 



