|COMMON FACTORS IN MENTAL HEALTH AND ILLNESS|
I AM asked to deal in these remarks with variations in different human traits which are produced by pathological conditions. For example, although in health John's eyesight is sometimes better or worse than at others, yet John's eyesight is so consistently better than James's that we speak of John as having better eyesight than James. But John's eyesight might become much worse as the result of a central lesion, and, if it remained fairly stationary at its new level, John would have much worse eyesight than James as the result of the pathological condition, and a new individual difference would be produced. The psychoses of which we shall speak, however, do not act in altogether this way. The differences associated with them are not sufficiently stable at any one level to make it just to say, e. g., that a general paralytic has on the average one half the memory capacity of the normal. We can therefore speak of the kind, direction and limits of such changes, but not of their amount as representative of any clinical group.
It is one of the gentle ironies of scientific history that the concept of individual differences should have originated with one of its least significant functions. One must needs be the assistant of a pre-Galtonian astronomer to suffer for his simple reaction time. This fact, together with the necessary technical complications, has not encouraged the accumulation of pathological data on the "personal equation." The most important determinations are those of Diefendorf and Dodge, on the reaction time of the eye-movements. They found lengthened time in all the psychoses tested, slightest in the manic-depressive excitements and in dementia præcox, most marked, as would be expected, in manic-depressive depression. The angular velocity of eye-movements was found by these authors to be somewhat more rapid than normal in dementia præcox, general paralysis, and slightly also in manic-depressive excitement, while the slowest movements were seen in the depressions and in epilepsy. The generally quick movements of manic cases and the slowness of depressed ones are a clinical commonplace.
The rapidity with which some small movements can be repeated has a special neurological meaning, and many observations have been made with the psychoses. The rate is probably a little faster than the normal