QUANTITATIVE ESTIMATES OF SENSORY EVENTS 293 



organ. It is important to be familiar with these influences, so as to avoid 

 erroneous or anomalous results in sensory experiments ; they also supply 

 many interesting ways of observing the behaviour of the senses in unusual 

 situations or under slightly abnormal conditions. 



It is principally the last two classes of factors which have been studied 

 in Cambridge ; the results, and their theoretical implications, will be 

 briefly summarised, and reference will be made to some of the relevant 

 work done elsewhere. 



Rawdon-Smith (1934, i) found that the absolute threshold for a pure 

 tone of 1,000 cycles might be raised as much as 30 db. by exposing the ear 

 for some minutes to a similar tone at 100 db. above threshold. A similar 

 rise, though less marked, was found in the ear which had not been stimulated; 

 this could not be explained as direct fatigue by bone conducted or air 

 conducted sound, and appeared therefore to be due to central inhibition. 

 The rise of threshold in the stimulated ear was apparently in great part 

 central, since it could be ' disinhibited ' by unexpectedly turning off the 

 light in the room where the subject was seated. A considerable degree of 

 variability and irregularity in the rise of threshold also suggested its central 

 nature, as did the absence of any known peripheral mechanism which 

 could produce so marked an effect. (The tensor tympani and stapedius 

 muscles were shown in the cat to produce a fall of responses not exceeding 

 6 db. (Rawdon-Smith and Hallpike, 1934, i).) Here, then, there appeared 

 to be two central processes which could affect the auditory threshold — 

 inhibition following exposure to a loud tone, and disinhibition by changing 

 the stimulation of a different sense organ, the eye. A similar rise of the 

 differential threshold, using a loud ' adapting ' tone and a much softer 

 testing one, has been found by Rawdon-Smith and Sturdy (unpublished). 

 There was a tendency for the differential threshold to be lowest when the 

 adapting and testing tones were of equal loudness. Thus, after several 

 minutes' exposure to a tone at 100 db. above threshold, the differential 

 threshold at this intensity was often lower than after a period of silence. 



Zangwill, and after him Jones (unpublished), measured the differential 

 brightness threshold for a small patch within a field, and subsequently 

 gave large numbers of exposures in which the patch might or might not 

 be present ; the subject was required to state whether or not it was there. 

 The number of correct judgments was markedly increased day by day if 

 the subject was informed after each judgment whether he was right or 

 wrong, in comparison with a control group who were not given this 

 information. 



It was found by Gelb and Granit (1923) that the differential brightness 

 threshold was raised inside a figure drawn on a background ; they concluded 

 that the threshold is affected by factors of configuration and organisation 

 in the visual field. As certain simpler effects of contrast and adaptation 

 did not seem to have been fully controlled, some experiments were under- 

 taken in Cambridge by Craik and Zangwill (1938, in press) which showed 

 a similar rise for figures having the same degree of contrast but much less 

 strong ' figural character.' Certain other results, to be mentioned below, 

 suggest that visual brightness discrimination is very largely peripheral, and 

 in this case higher processes might be expected to have little influence on 

 it. It is probable, however, that central factors are at work in visual 

 localisation, after-effect of seen movement, and binocular fusion. An 

 ingenious proof of the peripheral nature of flicker-fusion was given by 

 Sherrington (1906), who showed that the critical flicker frequency is practi- 

 cally the same for both eyes as for one, though the phase relations of the 

 flicker to the two eyes might be so arranged that one or other was illuminated 



