22 Information Storage and Neural Control 



from Dr. Saltzberg whether he wishes to introduce a second 

 term — noise in this physical sense — or whether he would also Hke 

 to consider things wliich are not related to the required signal 

 over a short-time epoch. There is a good example of tliis in the 

 field of EEG analysis. If you repeat an experiment in time, you 

 expect the signal-to-noise ratio to go up as -^/N , but in almost 

 any biological system you will find it goes up by rather more than 

 this simply because our noise is not "noisy," so to speak, in the 

 sense that it is not white. 



Saltzberg: There are many things which people refer to as 

 noise that are quite diff"erent from one another. The different 

 types of noise have considerably different effects on the informa- 

 tion content of systems. For example, there is distortion which, 

 if reversible, does not reduce the information content of a message 

 at all. Although people commonly refer to this type of distortion 

 as noise, it is not noise in the context of information theory. You 

 have mentioned white noise, which is a special type of random 

 noise, and the chscussion on maximum rate of transmission of 

 information in the presence of noise is applicable to this lype of 

 noise. It is important to distinguish between this type of noise 

 and interference, which is sometimes referred to as noise. The 

 basic difference is that random noise is not coherent with any 

 signals to which meaning is assigned, while interference is an 

 undesired signal which is coherent with desired signals of some 

 otlier system. The improved signal-to-noise ratios that you men- 

 tioned for biological systems may have something to do with the 

 ability of biological systems to narrow their noise bandwidths by 

 providing certain kinds of adaptive filtering. 



Blustein: Is this similar to a television signal in which the 

 audio signal is intact and the video is distorted, and yet one can 

 receive and interpret the signal? 



Saltzberg: I am not sure of the analogy. I have to beg off on this. 



Blustein: Does the system have to filter the signals? 



Saltzberg: If the receiver has some a priori knowledge of what 

 it is looking for, then it can do an excellent job of minimizing the 

 effects of noise. One of the simple ways this is accomplished is 

 by means of frequency filters or correlators. If the information 

 signals occupy a narrow bandwidth, then narrowing the accept- 



