What is Information Theory? 23 



ance band of the system by employing filters will reduce the 

 amount of random noise, since random noise occupies all parts 

 of the spectrum. The object is to use spectrum space for the signal 

 information, not the noise. 



Shapiro: Are there actually two kinds of information involved 

 here, only one of which is treated in this way? Perhaps I should not 

 use the word information for the other kind, but a priori knowledge 

 about the nature of the system which the receiver may have, 

 whether it is a person or a machine, must be important. For 

 example, if the machine or the person knows that nothing is 

 coming over this channel except when some kind of an event 

 occurs, then when a lot of noise, i.e., a lot of signals, comes over, 

 this will be interpreted as meaning a lot of activity in the trans- 

 mitter. On the other hand, if the receiver knows from past experi- 

 ence that this system generates its own noise, then when a lot of 

 noise comes over the channel, the receiver says it does not know 

 what is going on. Only when a clear individual signal comes over 

 will it be interpreted as information. I think that there is another 

 set of values, which I suspect is what you mean by filter theory. 



Saltzberg: I believe your comment relates to filtering in the 

 time domain. It is possible to design a system which simply stops 

 processing information when the signal is too badly corrupted by 

 noise. If cues, which are essentially a priori information, are avail- 

 able, then it is possible to use various types of time filtering. When 

 you do not have any a priori timing cues for determining when 

 you ought to process information, then frequency filtering is sug- 

 gested, providing you know something about the spectral region 

 which the signals occupy. 



Peter Kellaway (Houston, Texas): It would help if you could 

 tell us something about your ow^n results. I understand you are 

 interested in analyzing the EEG. What sort of information have 

 you obtained by applying information theory and technique to 

 this type of analysis, and what type of information do you hope 

 to obtain? 



Saltzberg: I can comment on some of the analysis of EEG 

 which was conducted in an attempt to establish how much infor- 

 mation is processed by one technic^ue of EEG analysis as compared 

 with the amount of information which is processed using another 



