The clinical setting for the occurrence of visual hallucinations varies. The simplest black-and-white moving scintillations are part of migraine. Others, some colored, occur as a seizure aura (see Chap. 15). Often, they are associated with a homonymous hemianopia, as already indicated. Frequently, they are part of a confusional state or delirium (see Chap. 19). Similar phenomena may occur as part of hypnagogic hallucinations in the narcolepsy–cataplexy syndrome. In the “peduncular hallucinosis” of Lhermitte (1932), the hallucinations are purely visual, appear natural in form and color, sometimes in pastels, move about as in an animated cartoon, and are considered by the patient to be unreal, abnormal phenomena (preserved insight). Ischemia in the territories of the posterior cerebral arteries is the usual cause. Lhermitte used the term peduncle to represent the midbrain as the source of the hallucinations was ischemia in the high mesencephalon, creating images that may be akin to those experienced in dreaming. The hallucinations as mentioned are purely visual; if hallucinations are polymodal, the lesion is always in the occipitotemporal parts of the cerebrum.
A special syndrome of ophthalmopathic hallucinations occurs in persons with reduced vision, as discussed in Chap. 12. A similar phenomenon in elderly patients with partially impaired vision has been called the Charles Bonnet syndrome, following his description of visual hallucinations in a “sane” person. The topic of senile hallucinosis has been reviewed by Gold and Rabin, and 60 such patients with Bonnet syndrome were reported in detail by Teunisse and colleagues. The latter authors found that 11 percent of older persons with reduced vision experienced these phenomena at one time or another.
It is usually the case that the lesions responsible for visual hallucinations are situated in the occipital lobe or posterior part of the temporal lobe and that elementary hallucinations have their origin in the occipital cortex, and complex ones in the temporal cortex. However, the opposite may pertain; in some cases, formed hallucinations are related to lesions of the occipital lobe and unformed ones to lesions of the temporal lobe, according to Weinberger and Grant. Also, as emphasized by these authors, lesions that give rise to visual hallucinations, simple or elaborate, need not be confined to central nervous system structures but may be caused by lesions at every level of the neurooptic apparatus (retina, optic nerve, chiasm, etc.).
The Visual Agnosias (See Also Lesions of the Parietal Lobe and Temporal Lobe)
Several syndromes involving visual dysfunction are due to lesions that span the occipital lobe and either the adjacent temporal or parietal lobes. They have been divided conceptually and anatomically into a dorsal and a ventral stream of information processing, the former running from the occipital to the parietal lobe and the latter from the occipital to the temporal lobe. Those of the temporal lobe include visual object agnosia, prosopagnosia, alexia, and color agnosia. In this way, the ventral stream may be considered to represent the “what” of visual processing to identify objects. The parietal-occipital, or dorsal stream syndromes are visual simultanagnosia, Balint syndrome and topographagnosia, that reflect disorders of “where” in visual behavior as described by Levine and colleagues.