The sensory material which provides the essential data for conscious activity is conveyed to the higher functional levels of the brain by impulses which stream up the olfactory tracts, the optic tracts, and the tracts of the brain-stem and spinal cord. With the exception only of the olfactory impulses, all these sensory impulses are filtered through the thalamic region of the brain, or diencephalon, before they can be relayed to the cerebral cortex which forms the anatomical substratum of the more elaborate mental processes. It is an interesting fact that, while the functional localization in the cerebral cortex and the functional localization in regard to the numerous fibre tracts in the brain-stem and spinal cord have been established in quite considerable detail by anatomical, physiological and clinical studies extending over many years, the localization and the connections of the various relay mechanisms in the diencephalon still remain obscure. Since the nature of the sensory material which is delivered to the cerebral cortex depends ultimately on the influences and modifications which may be imposed on the afferent impulses during their passage through the diencephalon, it becomes a matter of extreme importance, from the point of view of the study of the physiology of sensation and of psychological interpretation of sensory experience, that attention should be concentrated on this diencephalic mechanism. The minute anatomy of the diencephalon has recently been worked out in great detail, and it is now the task of the anatomist, physiologist and clinician to discover the functional significance of the numerous cell groups and fibre tracts which have been defined.
Branching Thalamic Afferents Link Action and Perception
