THE TIME TAKEN UP BY CEREBRAL OPERATIONS. 237 others correspond to those where the motion of the speech-organs was directly registered. The length of the reaction-time depends on conditions which can be classified as belonging, partly to the sense-stimulus, partly to the reacting subject. It was my object in the experiments here under consideration rather to eliminate these sources of variation than to investigate them. I used therefore the same sense-stimuli and the same subjects. The only varying conditions were the changing states of the subject due chiefly to different degrees of attention, fatigue and practice. It seemed desirable thoroughly to investigate these owing (1) to the light they throw on the nature of the cerebral operation, and (2) to the necessity of knowing what influence they exert on the lengths of the pro- cesses investigated, before we can judge of the accuracy of our results. I can best postpone the full consideration of this sub- ject until the end of the paper, but it will be of advantage before going further to consider the relation of attention to the length of the reaction-time. It has always been assumed that the length of the reaction varies greatly with different degrees of attention, and this is a natural supposition, when it is believed that the time is mostly taken up by the processes of perceiving and willing. If however the reaction is automatic, the changes not penetrating into the cortex of the cerebrum, then the time would not be greatly dependent on the concentration of the attention during the reaction. The reaction would however be delayed if the con- ditions were such as to make it difficult for the subject to hold the path of communication and motor centre in a state of readi- ness. The simplest way of distracting the attention is to cause a noise while the reactions are being made. I let three metronomes beat and ring rapidly. The results of these experiments are given in Table V. for both light and sound. TABLE V. LIGHT. SOUND. B c B C R R' R R' R R' R R' 2. IV 149 159 152 146 155 150 159 152 148 155 162 146 144 162 168 159 147 142 161 170 122 124 126 132 119 120 127 124 131 119 121 120 128 137 125 118 119 127 138 124 3 4 5 A 152 153 156 156 125 124 126 125 AV 8 5 10 6 10 6 10 6
