25 : 3/ Information Theory and Biology 469 



If we listen to a friend talking in a noisy room, most of the information 

 reaching our ears is consciously or unconsciously blocked from our 

 conscious mind ; that is, we listen only to his speech and not to the back- 

 ground noise. 



Even with the utmost concentration under ideal conditions, it is 

 rarely possible to use all of the 200 bits/second reaching the brain. For 

 short pure tones, most people can detect not more than one correct 

 choice of six possibilities. Hence, we should write 



H f = 2.5 bits/tone 



of useful information. (Some people with perfect pitch can detect 8 bits 

 per piano tone. They use information of more than one frequency and 

 of the relative intensities of the different harmonics. They must hear 

 the tone longer than 0.1 second.) 



Again, for sound-pressure-level choices, people on the average can 

 choose only one of six possibilities. As above, one may recompute // L 

 to give 



H L = 2.5 bits/impulse 



Adding another 2.5 bits for quality of a complex tone, we find that 7.5 

 bits can be detected each 0.1 second. The rate of receipt of useful 

 information by the brain is 



H" = 75 bits/second 



Interpretation of the information received from words is more com- 

 plicated. The best estimates give 



^word — 50 bits/second 



of useful information at the brain. 



In terms of the language of computers, the rate is limited by the 

 ability to read information into the brain. The sound waves in the ear, 

 and even the impulses in the eighth cranial nerve, carry far more infor- 

 mation than can be read into the brain. 



B. Vision 



A similar analysis can be carried out for the visual system. Analogous to 

 sound information, most light information is destroyed by the eye. 

 Vision is very different from hearing in that information may be received 

 and transmitted to the brain at a much greater rate. 



In the eye, there are about 10 7 receptor units, each feeding into a 

 separate ganglion cell. The receptor units are made up of individual 

 receptors, that is, rods and cones. Except in the most sensitive region 

 of the eye, the fovea centralis, several receptors combine to form one 



