The centrifugal control of afferent transmission in central sensory stations, long suspected by the perceptive neuroanatomists Cajal,5 Brouwer,3 and Wallenberg,42 is now firmly established as the result of numerous experiments carried out during the last six years.Early as well as recent anatomical studies,2,3'5.6'33,34,4n,41 *42 have shown in the central nervous system the presence of efferent or centrifugal fibers projecting down to sensory synapses as low as those of the spinal cord, of the spinal fifth sensory nucleus, of the dorsal column nuclei, of the cochlear nucleus, of the retina, and of the olfactory bulb. It has also been demonstrated in acute experiments that electrical stimulation of these fibers elicits inhibitory" ,32~1n or facilitatoryll~ZfiJIJo interactions with the ascending afferent impulses in all the above mentioned sensory centers (see the reviews on this subject by Hagbarth13 and by Hern&ndez-Pe6nlG).Furthermore, some descending influences that control synaptic afferent transmission act tonically during wakefulness. It has been shown that synchronized afferent volleys recorded from the lowest levels of the somatic, auditory, and olfactory central pathways are enhanced by anesthetics and by brain stem lesions, which impair c o n s c i o u~n e s s .~~~~~~~~~~~ These experimental observations are consistent with older observations in which larger cortical sensoryevoked potentials were recorded during deep rather than during light anesthesia.* Therefore the conclusion is warranted that the reticular activity related to wakefulness normally regulates the passage of a great number of sensory impulses to the central nervous system. Undoubtedly this new neurophysiological concept can account, at least in part, for the well-known psychological phenomenon concerning the constantly limited span of perception.Since perception is invariably associated with the process of attention, it occurred to one of us (R.H.-P.) that sensory impulses generated by stimuli outside of the span of attention would be prevented from entering the brain by descending inhibitory influences such as those mentioned above. Correlated behavioral and electrophysiological studies carried out in unanesthetized cats with electrodes permanently implanted in the lowest subcortical sensory relays have substantiated this hypothesis. By recording synchronized afferent volleys evoked by brief sensory stimuli, HernSndez-Pebn and his co-workers17 have demonstrated that the corresponding evoked potentials are reduced or abolished when the animal attends to stimuli more significant than the test stimulus. This phenomenon has been observed in the dorsal cochlear nucleus,28 in the axons of the retinal ganglion cells along the optic tract,22,233J7 in the spinal fifth sensory nucleus,7 in the lateral columns of the spinal cord: and in the olfactory bulb.I9 A similar reduction of sensory-evoked potentials at those lower stations of the mentioned sensory pathways has also been observed by Hernindez-PeQ and his co-~orkers'~ consecutive to monotonou...
