MEMORY IMAGES 167 



Thus in one dog "by persistent training a conditioned 

 reflex has been established to the stimulation with 100 

 oscillations per minute of the metronome. The stimu 

 lation of intermittent sounds of such frequency called 

 forth 6 to 10 drops of saliva every time. The interval 

 between successive oscillations was then modified, the 

 moment of the disappearance of the conditioned salivary 

 reflex indicating the lowest limit of differentiation. With- 

 out going into any details of this most interesting investi- 

 gation or quoting actual data, I will say that the dog 

 could sharply distinguish the shortening of the interval 

 by less than 1/40 to 1/43 of a second. Indeed with the 

 well-developed reflex to the stimulation of 100 beats per 

 minute a change of the rate to either 96 or 104 beats was 

 immediately reacted upon by a marked diminution or even 

 complete cessation of the flow of saliva." 



This example will give an indication how sensitive is 

 this method of measuring the effect of a memory 

 association. 



It is not our purpose to give the details of Pawlow's 

 results they have only been published in Russian and 

 are therefore not accessible to the writer but to show 

 that the influence of an associative memory image is as 

 exactly measurable as, e.g., the direct illumination of the 

 eye; and moreover that what we call a memory image 

 is not a "spiritual" but a physical agency. We there- 

 fore need not be surprised to find that such memory 

 images or "conditioned reflexes" can vary and multiply 

 the number of possible tropistic reactions. 



We have mentioned in the previous chapter that the 

 stereotropism in the mating instinct includes apparently 

 an element of species specificity inasmuch as naturally 

 only males and females of the same species mate. The 



