1950's and 1960's. Now, these too are found wanting. The original assumptions of a unitary, non- specific process based on activation of the brain stem reticular formation have been shown to be misleading. Current work in neurobiology has demonstrated evidence of discrete neurotransmitter systems having quite specific information processing functions, and central roles in the regulation of behaviour. Even the venerable curvilinear relationship between motivation and per­ formance (the Yerkes-Dodson law) has been shown to be, at best, an unhelpful oversimplification. On a different front psychophysiologists have found complex patterns in the response of different bodily systems to external stressors and to task demands.