Page:Mind (Old Series) Volume 11.djvu/233

 232 J. M. CATTELL : colour, the second light washing away, as it seems, the impression made on the retina by the first light. Under these circumstances a violet light had to work on the retina 12-5<r, if it were to be distinguished. It seems, therefore, probable that the violet light had not been converted into a -nervous impulse within this interval, and if this is the case it would give us a minimum time for this process. The familiar experiment with rotating discs shows that light-impressions of moderate intensity following one another at intervals of 25<r are just fused together. It seems, therefore, that the retina is excited, and begins to resume its normal condition in about 25<r. If this assumption is correct we have the maximum time for the period under consideration. We may be tolerably sure that the time passing before a light is con- verted into a nervous impulse varies with the intensity of the light, and may perhaps assume the time to be 15-20</ for daylight reflected from a white surface. These considerations lead us to suppose that, when a reaction is made on light, only about half the time, that is 75<r, is taken up by the cerebral operations. We naturally ask what happens in the brain after the nervous impulse reaches it. It has generally been assumed that the largest factors of the reaction-time are taken up by the processes of perception and willing. I think however that if these processes are present at all they are very rudimentary. Perception and volition are due, we may assume, to changes in the cortex of the cerebrum, but reflex motions in answer to sense-stimuli, as in contraction of the pupil and in winking, can be made after the cortex has been removed, and an animal in this condition can carry out motions adapted to the nature of the stimulus. If a pigeon from which the cerebral hemispheres have been removed is thrown into the air, it will not only fly, but also avoid obstacles and alight naturally on the ground. It seems to have consequently sensations of light, but apparently no perceptions, either because it does not see colour and form, or because it lacks the intelligence needed to under- stand their meaning. In the same way a reaction such as we are considering can probably be made without need of the cortex, that is, without perception or willing. When a subject has had no practice in making reactions (in which case the reaction-time is usually longer than 150<r) I think the will-time precedes the occurrence of the stimulus. That is, the subject by a voluntary effort, the time taken up by which could be determined, puts the lines of communication between the centre for simple light sen- sations (in the optic thalami probably), and the centre for the co-ordination of motions (in the corpora Btriata, perhaps, connected with the cerebellum), as well as the latter centre, in a state of unstable equilibrium. When therefore a nervous impulse reaches the thalami, it causes brain-changes in two directions ; an im- pulse moves along to the cortex, and calls forth there a percep- tion corresponding to the stimulus, while at the same time an