ABSTRACT
It is not possible here to discuss in detail the work of the physiologists, Jasper, Magoun and many others, on the 'reticular formation of the brain'. Full discussions are given by Samuels (1959) and Berlyne (1960). However, to summarize briefly: it has been shown that this cell net-work within the brain stem and thalamic region appears to have two main functions with regard to perception. The brain stem reticular formation sends nerve impulses to the cortex producing general arousal from sleep and passivity to sensory stimulation, and these effects appear in high frequency outbursts ('evoked potentials') in the EEG. The thalamic reticular formation gives rise to a more persistent and localized response, sometimes called the 'orienting reflex', in which attention is directed towards particular types of stimulation. Now clearly the degree of attention to environmental stimulation must depend in the first place on
7 general arousal, according as to whether the individual is alert, quiescent, drowsy or asleep. Direct stimulation of the reticular formation in monkeys produced the same pattern of fast discharge in the cortex, together with arousal and wakefulness, as resulted from afferent stimulation (Segundo et al., 1955). But also perception of particular significant aspects of the environment is quicker and more accurate if the thalamic reticular formation operates to reinforce sensory impulses related to stimulation by these aspects. Thus Fuster (1958) showed that direct electrical stimulation of the reticular formation in monkeys improved their speed and accuracy in discriminating between two shapes. Not only are the principal perceptual responses facilitated and discrimination enhanced; in addition irrelevant impulses may be suppressed. Thus Hernandez-Peon et al. (1956) exposed an unaesthetized cat to a regular series of clicks, recording its auditory responses through electrodes inserted in the cochlear nucleus. But these responses ceased altogether when the cat was shown a mouse or could smell fish. Similar inhibitory processes have been demonstrated in man (Jouvet, 1957). Thus it was argued that response to a relatively unimportant type of stimulation was repressed at an early point in the afferent nerve path to the brain by the reticular formation which gave prior entry to the cortex to more significant types of stimulation.
