PROPERTIES DESIRABLE IN TRANSIENT COMMUNICATION SYSTEMS 



253 



since the number of signs to which he must 

 respond are few, and do not require complex 

 discrimination. 



Demand on the Senders and Receivers 



Here we face the basic psychological prob- 

 lems of communication: the control with 

 certainty of the response of the receiver. 

 Many of the difficulties found in current 

 communication systems stem from the de- 

 mands placed upon the receiver by the com- 

 plexity of the messages transmitted, and by 

 the speed with which they must be responded 

 to. With highly selected and trained per- 

 sonnel, the problem is a serious one; as 

 selection and training procedures are less 

 effective, the less effective the complex com- 

 munication system and the more difficult 

 the problem. The span of apprehension and 

 the immediate memory span of the receiver 

 must both be carefully considered. 



The span of apprehension has been per- 

 haps most effectively defined and studied in 

 the field of vision. Essentially, it is the 

 number of discrete stimuli which, presented 

 simultaneously for a brief period, can be 

 accurately responded to. Many recent in- 

 vestigations have clarified considerably the 

 controlling variables, which prove to be the 

 length of time through which the stimuli are 

 presented to the subject, and secondly (for 

 stimuli of very short duration), the bright- 

 ness of the stimulating field. The number 

 of stimuli which can be simultaneously ac- 

 curately responded to is sharply limited for 

 very short exposures; it is approximately 

 four at 1^0 second (4). This process of im- 

 mediate discrimination of numerosity has 

 been termed "subitizing" by Reese (12), 

 When the time increases up to tens of sec- 

 onds, the number of stimuli correctly re- 

 sponded to increases regularly, as the subject 

 beings to count, whether by single stimuli, 

 or by groups of them (12). Such discrimi- 

 nation of number is a special case of the 

 fundamental case of how complex a stimulus 

 field the organism can respond to appropri- 

 ately and accurately in a brief time. It may 

 be presumed that the same variables, time 



and intensity, play much the same roles 

 when stimulus dimensions other than nu- 

 merosity are concerned in the discrimina- 

 tion. The equivalent problem in hearing 

 has not been as well defined, although there 

 is unquestionably a problem of discrimina- 

 tion (i.e., comprehension) for rapidly de- 

 livered speech. On the other hand, it is well 

 established that where discrimination is diffi- 

 cult, whether because of speed or lack of 

 clarity of delivery, or because of background 

 noise at or above the loudness of speech, the 

 receiver will respond inappropriately (7, 10). 

 The response to the message becomes inde- 

 pendent of the actual message and depends 

 upon the receiver's dominating motivation 

 at the time of misapprehension and on his 

 own working vocabulary; unfamiliar words 

 in the message are responded to as familiar 

 words which sound somewhat like them. 



The second span with which we must deal 

 in communication is the immediate memory 

 span. This is defined as the maximum 

 length of a series of stimuli which can be 

 repeated correctly by the receiver after one 

 presentation of the series. Thus, the num- 

 ber of integers in the longest series which the 

 individual can repeat correctly immediately 

 after he has heard it defines the immediate 

 auditory memory span for integers. Two 

 different such "spans" may be identified in 

 vision : the maximum size of a set of different 

 stimuli which the subject can report cor- 

 rectly after having seen the variegated set 

 when it is presented either (a) simultane- 

 ously, or (b) successively. The latter is ob- 

 viously closer to the auditory memory span, 

 and the former to the visual span of appre- 

 hension. 



The immediate auditory memory span is 

 one of the performances which define the 

 scientific concept of intelligence. Systems 

 which place undue demands on these psy- 

 chological functions of the receiver may be 

 expected to produce unsatisfactory and un- 

 reliable communication. "The men don't 

 understand what is said," and, "They forget 

 what they heard," are not uncommon com- 

 plaints about voice systems. When voice 



