48 Information Storage and Neural Control 



CUearly, the type of grammar rules will determine both how 

 much we have written down at any time and the maximum 

 capacity required of the scratch pad. Since rules may be used 

 recursively, i.e., we permit rules such as S = S + and + S, we 

 might generate grammatical sentences which exceed the capacity 

 of any given scratch pad. This is not such a danger in algebraic 

 notation, which is not generally used as a spoken language except 

 for short expressions, but it could be critical in spoken English. 

 Since human span of attention is quite limited — and we have 

 some pretty consistent evidence as to what this limit is — English 

 has evolved rules of grammar which spare our mental scratch pads. 

 For example, we can see that elaborate phrases which occur at 

 the beginning of a sentence must be expanded while keeping in 

 mind the structure of that which is to follow. Grammarians ad- 

 monish us not to use such "top-heavy" sentences. It is not sur- 

 prising to find that we have been provided with alternate ways of 

 modifying nouns, and that these ways allow us to postpone some 

 of the modifiers until we have gotten rid of the object of modi- 

 fication. Discontinuous constituents are such mechanisms. 



The same argument accounts for the existence of the passive 

 voice when the active is just as accurate. If the subject of a sentence 

 is greatly elaborated, we can postpone it until later by making it 

 the predicate of a sentence in the passive voice. Note how the 

 following sentence, used as an exaniple by Yngve and taken from 

 a U. S. patent, organizes the information so that one need not 

 expand the middle while keeping in mind other features: "The 

 said rocker lever is operated by means of a pair of opposed fingers 

 which extend from a pitman that is oscillated by nieans of a crank 

 stud which extends eccentrically from a shaft that is rotatably 

 mounted in a bracket and has a worm gear thereon that is driven 

 by a worm pinion which is mounted upon the drive shaft of the 

 motor." The same sentence can be expressed in the active voice, 

 but this requires a memory which is beyond the capabilities of 

 most of us. The sentence is ungrammatical for that reason, accord- 

 ing to Yngve's model: "A pair of opposed fingers (that extend 

 from a pitman (which a crank stud (that extends eccentrically 

 from a shaft (which is rotatably mounted in a bracket and which 

 a worm gear (that a worm pinion (which is mounted upon the 



