Page:Mind (Old Series) Volume 11.djvu/378

 IV. EESEAECH. THE TIME TAKEN UP BY CEREBRAL OPERATIONS.* By JAMES McKEEN CATTELL, Ph. D., Assistant in the Psychological Laboratory, University of Leipsic. III. The Perception-Time. We have found the simple reaction-time on daylight for B and C to be about 150,?, and I have given my reasons for assuming that a perception-time is not included in this interval. The per- ception-time can be defined as the interval between sensation and perception (or between indefinite and definite perception, apper- ception), that is, the time passing after the impression has reached consciousness before it is distinguished. The impression is per- haps in the back- ground of consciousness when it reaches the optic thalami ; before it is in the centre of consciousness it must probably travel to the cortex of the cerebrum and excite there changes corresponding to its nature. The method used by Wundt 2 to determine this time is to let the subject react as quickly as possible in one series of experiments, and in a second series not to react until he has distinguished the impression, the difference of the times in the two series giving the perception- time for the impression. I have not been able myself to get results by this method ; I apparently either distinguished the impression and made the motion simultaneously, or if I tried to avoid this by waiting until I had formed a distinct impression before I began to make the motion, I added to the simple re- action, not only a perception, but also a volition. The method for determining the perception-time suggested by Bonders 3 and since used by a number of others, is to let the motion depend on the nature of the stimulus. It has been thought by Bonders, v. Kries and Auerbach and others, that if the subject reacts on one of two impressions and makes no motion when the other occurs, only a perception has been added to the simple reaction. This is however not the case, it being necessary after the impres- sion has been distinguished to decide between making a motion and not making it. This question, which has been much dis- cussed, becomes quite simple if we consider the cerebral operations that probably take place. I assume that the changes do not penetrate into the cortex at all when a simple reaction is made. 1 Continued from MIND 42, pp. 220-42. - Physiol. Psych., ii., 247 ff. ; Phil. Studien., i., 25 if. 3 De Jaager, De physiologische Tijd Bij psychische Processen, Utrecht, 1865 ; Bonders, Archivf. Anat. u. Physiol., 1868.