One does not see with the eyes; one sees with the brain, which has dozens of different systems for analyzing the input from the eyes. In the primary visual cortex, located in the occipital lobes, at the back of the brain, there are point-to-point mappings of the retina onto the cortex, and it is here that light, shape, orientation, and location in the visual field are represented. Impulses from the eyes take a circuitous route to the cerebral cortex, some of them crossing to the opposite side of the brain as they do so, so that the left half of the visual field of each eye goes to the right occipital cortex, and vice versa. If, therefore, one occipital lobe is damaged (as by a stroke, for example), there will be blindness or impaired vision in the opposite half of the visual field—a hemianopia.
Besides the impairment or loss of vision to one side, there may be positive symptoms, too—hallucinations in the blind or purblind area. About 10 percent of patients with sudden hemianopia get such hallucinations—and immediately recognize them to
In contrast to the relatively brief and stereotyped hallucinations of migraine or epilepsy, the hallucinations of hemianopia may continue for days or weeks on end; and, far from being fixed or uniform in format, they tend to be ever changing. Here, one might envisage not a small knot of irritable cells discharging paroxysmally, as in an attack of migraine or epilepsy, but a large area of the brain—whole fields of neurons—in a state of chronic hyperactivity, out of control and misbehaving because of the lessening of forces that normally control or organize them. The mechanism here thus resembles that of Charles Bonnet syndrome.
While such notions were implicit in Hughlings Jackson’s vision of the nervous system as having hierarchically ordered levels (the higher levels controlling the lower ones, and lower ones
starting to behave independently, even anarchically, if released from control by damage at the higher levels), the idea of “release” hallucinations was made explicit by L. Jolyon West
in his 1962 book
Ellen O. was a young woman who came to see me in 2006, about a year after surgery for a vascular malformation in her right occipital lobe. The procedure was a fairly simple one, sealing off the swollen vessels of the malformation. As her doctors had warned her, she had some visual problems following the procedure: a blurring of vision to the left side, as well as some agnosia and alexia—difficulties recognizing people and printed words (English words looked like “Dutch,” she said). These difficulties prevented her from driving for six weeks and interfered with her reading and enjoyment of television, but they were transient. She also had visual seizures in the first weeks after surgery. These took the form of simple visual hallucinations, flashes of light and color to the left that lasted a few seconds. The seizures came several times a day at first but had practically ceased by the time she returned to work. She was not too concerned about them, for her doctors had warned her that she might experience such aftereffects.
What they had not warned her about was that she might develop complex hallucinations later. The first of these, about six weeks after her surgery, was of a huge flower, occupying most of the left half of her vision. This had been stimulated, she thought, by seeing an actual flower in bright, dazzling sunlight; it seemed to burn itself into her brain, and the vision of it persisted in the left half of her visual field, “like an afterimage”—but an afterimage that lasted not for a few seconds but for an entire week. The following weekend, after her brother visited, she saw his face—or, rather, part of his profile, just one eye and cheek—for several days.51
