Rivers[311], who seems to have been the first to appreciate fully the genuine and practical importance of thoroughly controlling the psychological factors that are likely to play a role in such experiments, concludes that "caffein increases the capacity for both muscular and mental work, this stimulating action persisting for a considerable time after the substance has been taken without there being any evidence, with moderate doses, of reaction leading to diminished capacity for work, the substance thus really diminishing and not merely obscuring the effects of fatigue." EFFECT OF CAFFEIN ON MENTAL AND MOTOR PROCESSES Schematic Summary of All Results St.=Stimulation. 0=No effect. Ret.=Retardation. PRIMARY EFFECT Small Doses | Medium Doses | | Large Doses | | | Secondary Reaction | | | | Action Time Hrs. | | | | | Duration | | | | | in Hrs. Process Tests | | | | | | Motor speed 1. Tapping St. St. St. None .75-1.5 2-4 Coordination 2. Three-hole St. 0 Ret. None 1-1.5 3-4 3. Typewriting (a) Speed St. 0 Ret. None Results show (b) Errors Fewer for all None only in total doses days' work Association 4. Color-naming St. St. St. None 2-2.5 3-4 5. Opposites St. St. St. None 2.5-3 Next day 6. Calculation St. St. St. None 2.5 Next day Choice 7. Discrimination reaction time Ret. 0 St. None 2-4 Next day 8. Cancellation Ret. ? St. None 3-5 No data 9. S-W illusion 0 0 0 General 10. Steadiness ? Unsteadiness None 1-3 3-4 11. Sleep quality Individual differences 12. Sleep quantity depending on body weight 2 ? 13. General health and conditions of administration