306 Information Storage and Neural Control 



seems irrelevant. Channel capacity may be lower than the capacity 

 of the components. When the behavior is organized the central 

 decision represents only a small part of the original input in- 

 formation. 



OVERVIEW OF RESEARCH ON SUBSYSTEM FUNCTIONS 



For an input to cross the boundary into the system its energy 

 must be great enough to cause the external transducer to fire. 

 The signal-to-noise ratio also must be sufficiently high. Other 

 environmental conditions which may influence the permeability 

 of a boundary to an input are competing signals, and the simi- 

 larity of the background to the signal — for example, a white 

 stimulus on a white ground may not be detected. McCulloch gave 

 an example of this in the experiment he mentioned in his paper 

 on detecting a signal against monaural and binaural background 

 noise. 



Classical psychophysics in its study of the threshold has tended 

 to ignore some of the important aspects of signal detectability 

 or to assume, sometimes incorrectly, that these other things are 

 held constant. Swets, Tanner and Birdsall (4) have pointed out 

 that this classical concept of the threshold is unreasonable because 

 it ignores the control which is exerted by sensory and psychological 

 variables. That is, it neglects the participation of subsystems other 

 than the boundary. 



The characteristics of human sense organs as input transducers 

 or internal transducers may be specified just as the characteristics 

 of electronic transducers: by transfer function, band width, phase 

 shift, and signal-to-noise ratio. In these respects various sensory 

 subsystems or modalities perform quite differently. 



The transfer function of a transducer refers to its ratio of output 

 to input. In the visual system this is the relationship between 

 intensity of light and reported brightness. In the auditory system 

 it is the relationship between intensity of sound and reported 

 loudness. Some engineers in designing apparatus for man-machine 

 systems have mistakenly assumed that the cu'-ve of perceived in- 

 tensity rises linearly with the increase in strength of the stimulus. 

 As Stevens has pointed out, the subjective intensity increases as a 



