VARLITIONS IN VISUAL STIMULI 



257 



Temporal Patterning 



Temporal patterning of the visual stimulus 

 is an excellent basis for visual communica- 

 tion. It is, indeed, the technique employed 

 in all blinker communications. However, 

 a second type of temporal patterning also 

 suggests itself : the simple flickering, at three 

 or four rates, of the stimulus. Several flash- 

 rates selected, as with contrast and hue, for 

 accuracy and speed of discrimination could 

 readily serve as the signs for an equivalent 

 number of significates. Such flashing lights 

 have the property, not shared by radio- 

 coded temporal patterns, of continuing un- 

 changing until the message is acknowledged, 

 and the appropriate action begun. 



Temporal patterning in blinker com- 

 munication is a special problem. It will 

 be suggested later that it may be possible 

 to increase the rates at which such patterns 

 can be received. 



Although the duration of a single flash 

 could serve as the basis of a discrimination, 

 it should perhaps be ruled out of communica- 

 tions, since a single flash might be missed, 

 by blinking of the receiver, or by his faulty 

 orientation. If the flash is repeated reg- 

 ularly in order to avoid this problem, dura- 

 tion reduces to an equivalent of flash rate. 



Size 



Size of the stimulus is another basis for 

 discrimination. Since the size of the retinal 

 image is a function of both the size of the 

 stimulus and the distance of the receiver 

 from it, size does not recommend itself for 

 communication over variable distances, but 

 can be relied upon if the receiver remains 

 at a relatively constant position with re- 

 spect to the stimulating display. Thus, size 

 may be employed in interior communication 

 with assurance. 



Sliape 



The shape of a stimulating patch of light — 

 whether it is a circle, square, triangle, or a 

 particular letter — is a convenient and ob- 

 vious variable property of a stimulus for 



use in communication. The limiting values 

 of visual acuity reduce the usefulness of 

 pattern for exterior communication, al- 

 though it is employed in the signal flag. 



Position 



The stimulating light may appear in any 

 of several designated positions. This var- 

 iable is often inconvenient for exterior com- 

 munication. 



Movement 



The single stimulus light may be made 

 to move back and forth in easily discrimin- 

 able directions, either actually or by use of 

 the phi phenomenon. Again, this variable 

 does not lend itself to exterior communica- 

 tion. 



So far, we have dealt with eight vari- 

 ables, seven of which can easily be em- 

 ployed as a means of communication. If 

 only four values of each of these variables 

 are taken, we have 4^ different combinations 

 which define modifications of a single spot, 

 each of which may serve as a symbol in 

 interior communications. There is, then, 

 ample opportunity, using only one spatially- 

 continuous visual stimulus, to communicate 

 a large variety of information if the receiver 

 is at a fixed distance from it, and views it 

 against a constant illuminated background. 

 Such a system could be most flexible. Since 

 the number of different messages need never 

 be nearly so great (it will be determined by 

 the traffic and content analysis of the com- 

 munication system), the number of such 

 symbols actually used would be a small 

 fraction of the theoretical total, and the 

 amount of training required to learn to 

 send and to respond appropriately to such 

 symbols would be within practical limits. 



Numerosity 



The number of stimuli may be varied. 

 The studies of Reese (12) suggest that the 



