36 Aubrey Lewis 



involutional process in the thalamus (consisting chiefly of 

 nerve-cell degeneration and death in tlie centro-median 

 nucleus) is the same in normal old men witli unimpaired 

 intellect as in senile dements. This does not preclude a, 

 relation between the normal mental deterioration of old age 

 and the plaques, neurofibrillary thickening and other changes 

 seen in ageing brains, but it makes it dubious, and certainly 

 does not help in discriminating normal from pathological 

 mental changes. 



It has similarly been impossible so far to link the kind and 

 degree of involutional mental changes to either cerebral 

 arterio-venous oxygen difference or to the electro-encephelo- 

 gram or to endocrine findings. Pincus and his colleagues at 

 Worcester have given us data about limited stress responses 

 which show no falling-ofP with age in healthy men, and I hope 

 Dr. Freeman will further illuminate this matter in his paper. 

 Up to now, however, purely psychological methods of 

 assessing what is normal have been unchecked by reference 

 to the material basis or accompaniments of the functions in 

 question. 



Most of the detailed information about normal psychological 

 ageing has regard to cognition. Intelligence, perception, 

 memory, mental efficiency and speed have been measured, 

 and the results indicate a decline, it is true, in intellectual 

 powers but one that is not uniform, nor certain, nor predict- 

 able. On the analogy of physical functions this is what one 

 would expect: allometric growth, and allometric decline: 

 in some ripe old age, and in others premature senility: no 

 tokens of rapid decay to come, no guarantee against it. There 

 is, however, very little exact information about how any 

 individual ages mentally: the data we have are derived from 

 analysing the scores obtained by sample populations contain- 

 ing more or less adequate numbers of people of different age 

 groups. This method, which has been so fruitful in determin- 

 ing the mental growth of children, has led to rather doubtful 

 conclusions when apphed to ageing adults. Samples have 

 been small or biased: and it has not been safe to assume that 



