Operationally, the question of the location of consciousness boils down to whether we can locate neuronal circuitry that must be active for consciousness and that, if destroyed, impairs or abolishes consciousness. To identify the parts of the CNS necessary for consciousness, we will first cut away and discard the parts unnecessary for consciousness. To ensure that you know the gross parts of the CNS, make an enlarged drawing of Fig. 2-1 on a loose sheet of paper. Then use scissors to cut away the parts of your drawing, as described next in the text, and place these parts in a pile.
First excise the entire spinal cord. Next excise the cerebellum. Then excise the medulla and caudal half of the pons: none of these parts is necessary for consciousness. (Actually cut these parts off of your drawing. This seeming busy work will lead to permanent retention.) Complete transection of the neuraxis at any level from the sacral tip of the cord to the midpons spares consciousness, if we artificially maintain respiration and blood pressure. The pile of parts discarded as unnecessary for consciousness now contains the entire spinal cord, medulla, caudal half of the pons, and the cerebellum. Next, however, complete transection of the brainstem in the upper pons will temporarily abolish consciousness, whereas complete midbrain transection will permanently abolish consciousness.
Next, instead of complete transections of the pons and midbrain, let us make partial transections to determine what must remain to support consciousness. To appreciate the partial cuts, review the cross-sectional anatomy of the brainstem by drawing the generalized cross section of the brainstem (Figs. 2-13 and 2-14). Actually cut the parts off of your drawing as called for in the text.
We find that complete transection or even removal of the entire basis of the midbrain or pons bilaterally does not abolish consciousness. If we start with a completely intact nervous system, bilateral destruction of the basis of pons or midbrain spares vertical eye movements but causes complete paralysis of all other volitional movements (Chapter 5). The Pt retains full sensation and full consciousness but can communicate that consciousness by the only available effector mechanism, the vertical eye movements (see the locked-in syndrome). Now snip off the brainstem basis bilaterally and place it on the discard heap.
After transecting the basis, we can insert the knife blade a little deeper to transect the medial and lateral lemnisci. The Pt loses the sensations mediated by these pathways, but consciousness remains. Next transect the tectum. Consciousness remains. Next, core out the cranial nerve motor nuclei. Consciousness remains. Thus, from your cross-sectional drawing of the brainstem, cut away the tectum, lemnisci, and cranial nerve nuclei and add them to the discard heap.
Now transection of the tegmentum bilaterally between the midpons and rostral midbrain abruptly abolishes consciousness. For consciousness, the midbrain and rostral half of the pontine tegmentum must remain intact (Reznick, 1983) and in continuity with the cerebrum and the diencephalon. Except for the rostral half of the pontomesencephalic tegmentum, we have discarded all other parts of the neuraxis caudal to the diencephalon.
From your original drawing, you now have left in your hand a cerebrum in continuity with the diencephalon and pontomesencephalic tegmentum. Now we will determine the role of the remaining parts of the CNS in consciousness by transecting the diencephalon and basal ganglia at successively more rostral levels. We must insert the knife from the bottom of the cerebrum to transect these gray masses bilaterally without damaging the surrounding white matter or cortex. Bilateral transection of the diencephalon permanently and irreversibly abolishes consciousness. As we extend more rostrally into the basal ganglia, the evidence becomes a little less secure because of the lack of pure lesions in human disease; however, early in the history of surgery on the basal ganglia and diencephalon to treat involuntary movement syndromes, neurosurgeons learned not to make bilateral lesions because of the impairment of mentation, consciousness, and speech. Tentatively, we can state that acute bilateral destruction of the globus pallidus and striatum abolishes consciousness—at least, if the lesion extends, as it usually does, a little into the neighboring diencephalon or septal region, or into the neighboring medial hemispheric wall (Freemon, 1971). Thus, we find that bilateral lesions at any level, from the pontomesencephalic tegmentum up through the diencephalon and basal ganglia to the medial hemispheric wall, abolish consciousness (Figs. 12-2 and 12-3).
Notice in Figs. 12-2 and 12-3 that, because the lesions involve structures rostral to the midbrain, in particular the basal ganglia and medial hemispheric wall, they must become increasingly larger than those required in the pontomesencephalic tegmentum and diencephalon.
The next anatomic region consists of the deep white matter surrounding the diencephalon and basal ganglia, which conveys axonal circuits between those neuronal masses and the cortical neurons. If we destroy the deep white matter of one hemisphere, scrape off all of its cerebral cortex, or remove the white matter and cortex by a hemispherectomy, the Pt can retain consciousness. (However, large acute lesions of a hemisphere temporarily impair consciousness. Curiously, left hemisphere lesions impair consciousness about twice as often as right hemisphere lesions [Albert et al., 1978].) Actually trace a hemisphere from Fig. 12-2, cut it out, and add it to the discard heap. The discard heap of the gross parts unnecessary for consciousness thus includes.
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The spinal cord, medulla, caudal pons, cerebellum, entire basis and tectum of the pons and midbrain, the lemnisci and cranial nerve nuclei, and either one of the two cerebral hemispheres.
Although one of the two cerebral hemispheres is dispensable, we cannot dispense with both. We can discard any pair of the frontal, parietal, occipital, and temporal lobes. We profoundly alter personality and sensorimotor functions, but consciousness per se remains. But if we remove too much of the cerebrum bilaterally, scrape the cerebral cortex off both hemispheres, or destroy much or all of the cortex by hypoxia or hypoglycemia, we produce an apallic syndrome, an old term for a condition that is now considered equivalent to a persistent vegetative state (Dalle et al., 1977). If we suck out the deep white matter from both hemispheres, or if the Pt has a severe demyelinating disease, thus disconnecting the cortical shell from the brainstem and diencephalon, the Pt permanently loses consciousness. Such severe, bilateral destructive decorticating or demyelinating lesions stand in direct contrast to the tiny, confined, bilateral pontomesencephalic tegmental lesions that exquisitely and selectively abolish consciousness, with little effect on functions mediated through other pathways. Review Fig. 12-3, which shows the location in the midbrain tegmentum of the smallest, most discrete lesion that will permanently abolish consciousness.