MEMORY 



685 



latent image is now perceivable in the field of dark vision. 

 When this point has been reached, the hand is suddenly 

 withdrawn from its position over the closed right eye. The 

 light in the room now percolates through the semi-translucent 

 eye-lid, and suddenly gives a moderate diffuse stimulus to 

 the retina. Under these circumstances, the latent image is 

 revived, as a negative that is to say, as a very dark filament 

 against a brighter background. Thus the essential condition 

 for reviving the latent impression of stimulus would seem to 

 be the subjecting of the unequally impressed tissue to diffuse 

 stimulation. The revival of the image as positive or negative 

 will then be a question of whether the stimulus have been 

 moderate or intense. 



I have already shown, by actual experiment on nervous 

 tissues themselves, that the differential excitability induced 

 as an after-effect of moderate stimulus (memory-impression) 

 will give rise, on diffuse stimulation, to one kind of response, 

 and the after-effect induced by strong stimulus to the reverse 

 (cf. figs. 311, 312). In the former case, the moderately 

 stimulated area, on diffuse re-stimulation exhibits induced 

 galvanometric negativity, as compared with the indifferent 

 contact, this being the sign of its relatively greater excitation. 

 In the second case, the sign of response is reversed, the 

 over-stimulated area, on re- stimulation, becoming galvano- 

 metrically positive. 



The revival of memory-images is thus seen to be due to 

 differential response, evoked by diffuse stimulus, in an organ 

 rendered anisotropic, by the unequal impressions which it 

 contains of previous stimulation. A similar differential effect 

 under diffuse stimulation has been seen in plagiotropic stems. 

 Here the upper surface has a deep impression or memory of 

 over-stimulating sunlight, and on diffuse stimulation this 

 upper surface becomes galvanometrically positive, a respon- 

 sive current flowing from below to above. It will thus be 

 seen that there is a continuity between the impressions made 

 on the sensitive neurile elements, and the physiological 

 anisotropy induced by the differential action of past stimulus. 



