REACTION TIME. 685 



Reaction Time. This time is known as " reaction time," and is distinctly longer than the 

 simple reflex time required for a reflex act. It can be measured by causing the person experi- 

 mented on to indicate by means of an electrical signal the moment when the stimulus} is 

 applied. The reaction time consists of the following events : (1) The duration of perception, 

 i.e., when we become conscious of the impression ; (2) the duration of the time required to 

 direct the attention to the impression, i.e., the duration of apperception ; and (3) the duration of 

 the voluntary impulse, together with (4) the time required for conducting the impulse in the 

 afferent nerves to the centre, and (5) the time for the impulse to travel outwards in the motor 

 nerves. If the signal be made with the hand, then the reaction time for the impression of 

 sound is 0-136 to 0*167 second ; for taste, 0*15 to 0*23 ; touch, 0-133 to 0*201 second (Horsch, 

 v. Vintschgau and Hbnigschmied); for olfactory impressions, which, of course, depend upon 

 many conditions (the phase of respiration, current of air), "2 to *5 second. Intense stimula- 

 tion, increased attention, practice, expectation, and knowledge of the kind of stimulus to be 

 applied, all diminish the time. Tactile impressions are most rapidly perceived when they are 

 applied to the most sensitive parts (v. Vintschgau). The time is increased with very strong 

 stimuli, and when objects difficult to be distinguished are applied (v. Helmholtz and Baxt). 

 The time required to direct the attention to a number consisting of 1 to 3 figures, Tigerstedt 

 and Bergquist found to be 0*015 to 0*035 second. Alcohol and the anaesthetics alter the time ; 

 according to their degree of action they shorten or lengthen it (Kraplin). In order that two 

 shocks applied after each other be distinguished as two distinct impressions, a certain interval 

 must elapse between the two shocks for the ear, 0*002 to 0*0075 second ; for the eye, 0*044 to 

 0*47 second ; for the finger, 0*277 second. 



[The Dilemma. When a person is experimented on, and he is not told whether the right or 

 left side is to be stimulated, or what coloured disc may be presented to the eye, then the time 

 to respond correctly is longer.] 



[Drugs and other conditions affect the reaction time. Ether and chloroform lengthen it, 

 while alcohol does the same, but the person imagines he really reacts quicker. Noises also 

 lengthen it.] 



In sleep and waking, we observe the periodicity of the active and passive conditions of the 

 brain. During sleep, there is diminished excitability of the whole nervous system, which is 

 only partly due to the fatigue of afferent nerves, but is largely due to the condition of the 

 central nervous system. During sleep, we require to apply strong stimuli to produce reflex acts. 

 In the deepest sleep the psychical or mental processes seem to be completely in abeyance, so 

 that a person asleep might be compared to an animal with its cerebral hemispheres removed. 

 Towards the approach of the period when a person is about to waken, psychical activity 

 may manifest itself in the form of dreams, which differ, however, from normal mental 

 processes. They consist either of impressions, where there is no objective cause (hallucinations), 

 or of voluntary impulses which are not executed, or trains of thought where the reasoning and 

 judging powers are disturbed. Often, especially near the time of waking, the actual stimuli 

 may so act as to give rise to impressions which become mixed with the thoughts of a dream. 

 The diminished activity of the heart ( 70, 3, c), the respiration ( 127, 4), the gastric and 

 intestinal movements ( 213, 4), the formation of heat ( 216, 4), and the secretions, point to 

 a diminished excitability of the corresponding nerve-centres, and the diminished reflex excita- 

 bility to a corresponding condition of the spinal cord. The pupils are contracted during sleep, 

 the deeper the latter is; so that in the deepest sleep they do not become contracted on the 

 application of light. The pupils dilate when sensory or auditory stimuli are applied, and the 

 lighter the sleep, the more is this the case ; they are widest at the moment of awaking (Plotke). 

 [Hughlings Jackson finds that the retina is more anaemic than in the waking state.] During 

 sleep, there seems to be a condition of increased action of certain sphincter muscles those for 

 contracting the pupil and closing the eyelids (Eosenbach). The soundness of the sleep may be 

 determined by the intensity of the sound required to waken a person. Kohlschiitter found that 

 at first sleep deepens very quickly, then more slowly, and the maximum is reached after one 

 hour (according to Monninghoff and Priesbergen after If hour) ; it then rapidly lightens, until 

 several hours before waking it is very light. External or internal stimuli may suddenly diminish 

 the depth of the sleep, but this may be followed again by deep sleep. The deeper the sleep, 

 the longer it lasts. [Durham asserts that the brain is anaemic, that the arteries and veins of 

 the pia mater are contracted during sleep and the brain smaller ; but is this cause or effect ?] 



The cause of sleep is the using up of the potential energy, especially in the central nervous 

 system, which renders a restitution of energy necessary. Perhaps the accumulation of the de- 

 composition-products of the nervous activity may also act as producers of sleep (? lactates 

 Preyer). Sleep cannot be kept up for above a certain time, nor can it be interrupted voluntarily. 

 Many narcotics rapidly produce sleep. [The "diastolic phase of cerebral activity," as sleep has 

 been called, is largely dependent on the- absence of stimuli. We must suppose that there are 

 two factors, one central, represented by the excitability of the cerebrum, which will vary under 

 different conditions, and. the other external, represented by the impulses reaching the cerebrum 

 through the different sense-organs. We know that a tendency to sleep is favoured by removal 

 of external stimuli, by shutting the eyes, retiring to a quiet place, &c. The external sensory 



