294 REPORTS ON THE STATE OF SCIENCE, ETC. 



all the time. It seemed, then, that the basis of the judgment ' flickering ' 

 or ' steady ' was given by each eye independently, and was not subsequent 

 to central fusion of the two fields. 



Vernon (1934) found the critical flicker frequency to be affected by 

 conditions which caused binocular rivalry. 



There remains the third group of factors— those which affect the sense 

 organ itself, by changing either the nature of the stimulus or the sensitivity 

 of the sense organ. 



It was found that the absolute threshold for touch and the differential 

 threshold for tactile pressure (Grindley, 1936, i and 2) and for light in- 

 tensity (Drew, 1936) were raised if the increase was made more slowly ; 

 no rise was found for tactile pain, however. Experiments by Rawdon- 

 Smith (1935) showed indirectly that the differential loudness threshold is 

 higher for slow than for rapid change. Thus a sound which was made 

 alternately to increase slowly and decrease rapidly appeared to become 

 steadily less loud, as the slower increases were not noticed. Direct proof 

 of the same effect is given by Sturdy 's work, at present in progress here. 



The interpretation of these results is uncertain, and the mechanism may 

 not be peripheral. Explanations might be proposed in terms of a trace- 

 theory or of adaptation ; at least it seemed desirable to consider these factors 

 here, since they are more specific and more closely connected with the 

 stimulus than those considered previously. 



The peripheral aspects of auditory adaptation — the action of the tensor 

 tympani and stapedius — have been investigated by Hallpike and Rawdon- 

 Smith (1934, i). The masking of one sound by another is a further 

 example of the interaction of stimuli, and was studied by Lane and Wegel 

 (1929) in the Bell Telephone Laboratories. 



In visual perception, the presence of black masses or surrounds, or of 

 glare spots, can affect the absolute and differential thresholds, critical flicker 

 frequency, and acuity, in neighbouring areas, as shown by Lythgoe (1935), 

 Vernon (1934) and Stiles (1929). Though similar in its effects to the 

 ' masking ' of one sound by another, the mechanism is probably different. 

 Unlike hearing, visual perception is multi-dimensional, so that simul- 

 taneously presented stimuli are not necessarily superimposed in sensation. 

 It appears more likely that interaction, and contrast effects between different 

 stimuH presented together, are mainly due to the power of these stimuli 

 to change the state of adaptation of the retina in neighbouring areas (Lythgoe 

 and Tansley, 1929). This conclusion is supported by the results of Craik 

 (1938), showing a close similarity between the effects of previous dark or 

 bright adaptation upon intensity discrimination or acuity (unpubHsheci) and 

 those of dark surrounds or glare spots as studied by Lythgoe and Stiles. 

 There is evidence of other interactive processes in the retina ; Adrian has 

 facilitated neural summation by strychnine (1928). But whether or not 

 all cases of mutual influence between two simultaneous visual stimuli can 

 be explained in terms of adaptation, spatially regarded, it is certain that 

 previous adaptation of the eye to an illumination different from that at 

 which its brightness discrimination is measured, can cause a marked 

 deterioration in such discrimination (Rawdon-Smith and Mellone, 1935, 

 unpublished ; Craik, 1938). The regularity of these effects, their restric- 

 tion to the stimulated eye, and the failure of attempts to disinhibit them, 

 suggest a retinal origin. 



Throughout these experiments on hearing and vision, an endeavour has 

 been made to find the main processes at work, to attribute them to their 

 correct ' levels,' peripheral or central, and to correlate those events which 

 appear to be peripheral with anatomical and electrophysiological findings. 



