SUPPOSE YOU WERE BLIND 



Starting point, or even whether it was there at all. His 

 task was to walk along the hallway, say when he first 

 thought he was approaching the screen, and then walk 

 up as close as he could without striking it. 



Some of the subjects, both blind and blindfolded, 

 could judge accurately the presence or absence of the 

 screen at several feet and then move in untU their faces 

 were within a few inches before deciding that any further 

 approach would bring them into contact with it. The 

 phenomenon of obstacle detection was thus brought into 

 the laboratory in a manner which allowed it to be studied 

 repeatedly under reasonably constant conditions. This 

 step is often a crucial one in attacking scientific problems 

 of this sort. Elusive and unpredictable events are very 

 much more difl&cult to study than those which can be 

 repeated under known conditions. Only in the latter case 

 is it fairly easy to vary the factors that seem likely to be 

 important and then observe the results. EarUer studies 

 of obstacle detection by the bUnd had been plagued with 

 great variability in the performance of the subjects. That 

 mainly is why they had not led to clear and decisive 

 answers. Yet Supa, Cotzin, and Dallenbach built their 

 experimental design on the extensive, if inconclusive, 

 experience of earUer experimenters. Without this back- 

 ground they would probably not have been able to devise 

 such decisive experiments. 



Once they had arranged conditions where blind or 

 bUndfolded people were regularly detecting a standard- 

 ized test obstacle, the next step was the theoretically ob- 

 vious but nevertheless rather difficult one of eUminating 

 one possible channel of sensory communication at a 

 time, while leaving the subject with free use of the others. 

 One leading theory was that the skin supplied some kind 

 of sensation of touch or pressure when obstacles were 

 nearby; another was that sound played a major role. The 



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