REFLEX ACTION 505 



take the first as being associated with pain, the second with the fine, discriminating, 

 higher receptors of touch, heat, and cold, and the last as the Pacinian and similar 

 receptors, connected with the muscular sense and pressure. 



It will be clear that the method of conditioned reflexes presents great 

 opportunities of testing the delicacy of the appreciation of external forces. The 

 dog has been found, in this way, to be able to distinguish small differences of the 

 pitch of musical notes. 



For example, a note of 100 vibrations per second has been made into a conditioned stimulus 

 by presentation along with food, so that on hearing this note by itself, secretion results. But 

 no effect is produced by a note of 104 vibrations, nor by one of 96. A similar conditioned 

 reflex to electrical stimulation of a spot on the skin ceases to appear when the electrodes 

 are moved 1 cm. away. 



This differentiation is found to be brought about by inhibition, that is, by 

 exclusion of all parts of the analyser with the exception of a limited region. 



The co-operation of inhibition in the formation of conditioned reflexes may be 

 seen thus : an electrical current sufficiently strong to give signs of pain when 

 applied to the skin, is made the signal of a conditioned reflex when applied to a 

 particular spot. It is found now that it gives signs of pain no longer, but, if 

 moved 1 cm. away, there is no secretion but there are signs of pain. In practice, 

 of course, it is necessary to shave the spot and mark it for future accurate 

 localisation. 



We may here pause for a moment to note the difference between the spinal and the higher 

 centres in regard to nociceptive reflexes. We saw that these reflexes are prepotent in the 

 former case, but the conditioned reflex in the cae quoted above has obtained the mastery 

 over one. Nocuous skin stimuli can even be made into a conditioned stimulus for the feeding 

 reflex, but not when applied over bone, nor when acid in the mouth is the stimulus for the 

 unconditioned reflex instead of food. So that nocuous stimuli are more difficult to deal with, 

 even by the higher centres, and the mastery over them is only a qualified one. 



A spot which has been made the signal for a conditioned reflex, by presentation 

 of food along with the stimulation of the spot, is called an " active " spot. It is 

 necessary to distinguish between an "inactive" and an "indifferent" spot. The 

 latter name is given to spots which have not been used as signals for any particular 

 purpose ; while an " inactive " spot is one which has been made, by stimulation, 

 the signal for non-presentation of food ; that is, as it were, a reflex for "no food," 

 and is associated with absence of salivary secretion. 



The following experiment presents several points of interest (Pavlov, 1912, 

 pp. 330-331). Along a series of spots on the hind leg, there were arranged five 

 devices for producing mechanical stimulation of the skin. The stimulus was 

 equal in all. The upper four were made " active," that is, were accompanied by 

 secretion of saliva. The lowest one was made "inactive," that is, whenever it 

 was stimulated, no food was presented. Suppose, for the sake of example, that 

 stimulation of each of the upper four for thirty seconds is accompanied by the 

 production of 10 drops of saliva, while that of the fifth gives none. Stimulate the 

 inactive spot, and then, thirty seconds later, stimulation of any one of the upper 

 four spots will be found ineffective. At one minute interval, activity begins to 

 return and in order from above down. Thus, the following numbers of drops were 

 obtained, 5, 3, 1, 0. After two minutes, 10, 8, 5, 2. After three to four minutes, 

 10, 10, 10, 4 ; and complete return to normal after five to six minutes. Inhibition, 

 therefore, spreads over a wide area of the analyser and disappears from the more 

 distant parts first. 



Those conditioned reflexes in which the time element enters are also very 

 instructive. Suppose that the sound of a bell is not at once accompanied by the 

 presentation of food, but only after two minutes' interval ; and that this succession 

 is repeated until the conditioned reflex is formed. It is then found that the 

 sound of the bell is not at once followed by secretion, but only after the interval 

 of two minutes has elapsed. Now, during the time between the stimulation and 

 the reflex, it is clear that something must be going on in the centres, but that its 

 manifestation is inhibited. This can be shown by the application of some external 

 indifferent stimulus during the interval, when the saliva immediately appears. It 

 is to be understood that this indifferent agent is not one that produces secretion 



