The topography of the world is mapped onto the brain in overlapping sense-specific fields of information. Some senses, particularly sight and touch, are represented by several maps, each with a slightly different emphasis and a slightly different location within the brain. In the case of vision, for example, there are at least five major map-systems, some devoted to object shape, others to color or object movement, still others to facial features and the imperatives of memory (7). Each of these maps is topographically related to the position of images on the retina; therefore, the world is apprehended not directly but only in its relation to the body. Similarly, the topographic map for touch is not a map of the world but a map of the skin surface; on the surface of the brain, the parts of the body are represented in proportion to their relative sensitivity and not in relation to their real size. Hence, the cortical representation of the thumb, lips, face and tongue covers a much larger territory than does the representation of the chest, back or legs.
The brain, just as a computer, is highly sensitive to the input of its peripheral devices, those that scan or otherwise act upon the external world. And it is fortunate in having peripherals with unusually good technical specifications, such as sensors that function at tolerances of a single molecule. Surprisingly, the entire central nervous system, composed of 1012 cells, each of which is potentially connected to 104 other cells, receives only five million incoming cables to carry all the information from the outside world, the five senses, and the internal milieu of the body (6). What is not widely appreciated is that each optic nerve alone contains one million fibers; the visual system represents about forty percent of all incoming signals to the brain! The primacy of vision is easily demonstrated in any bedside examination of a patient with partial loss of function in another map-making sensory system. Patients who have trouble with touch and proprioception (position sense) in their legs may manage to stand somewhat steadily, even with their feet close together, when they have their eyes open but topple over when their eyes are shut. Similarly, patients with problems in the cerebellum, the brain’s guided missile computer, can hit targets with their fingers and arms outstretched so long as their eyes are open. If visual compensation is removed, however, patients shoot past targets as if they had no muscle sense or joint memory. One way to produce topographical disorientation, therefore, is to shut down or impair a major sensory input into the central nervous system.
Let us now suppose that all the incoming cables are functioning well but that the information received confounds the expectations of the brain. To a certain degree, this is the problem of the nomad who wanders from place to place beholding world topographies that have no correspondence with his or her prior experience. The extreme of this situation occurs in individuals who develop the ability to see or hear after many years of sensory deprivation. Why is this so? Because the world imprints itself upon the structure of the brain during the early developmental period of young animals. For example, within the visual cortex of cats, columns of cells are tuned to lines that cross retinal receptive fields at particular angles. Kittens can be raised with specially made goggles so that one eye always sees vertical lines and the other only sees horizontal lines whenever the animal is in a lighted space (2). If you then put a microelectrode in the brain of the adult cat and test the vertically raised eye with horizontal stimuli, you find that there are no horizontal cells in the appropriate part of the brain. Similar results are obtained when the horizontally trained eye is challenged with a vertical line; there are no vertical cells in the brain’s visual cortex. This is an example of how life experience can interact with the genetic blueprint to help determine the pattern of synaptic connections. Significantly, human infants with congenital cataracts usually have their lens deformed in a dominant axis. EEG studies have shown that they are less responsive to visual stimuli in that particular axis and optical correction does not improve the response of the brain.
The primary sensory cortex of the brain, where the tactile topographic map resides, is chiefly responsible for the analysis of spatial data and the successful projection of the opposite arm and leg into the opposite half of visual space. Because language function resides in the left hemisphere in almost all human brains (that is, in all people who are right handed and at least two-thirds of left-handers), it is easier to study spatial deficits after damage to the right parietal lobe. Nevertheless, spatial deficits after left parietal injuries can be studied in patients who are still able to communicate by speaking or writing. Left parietal lobe lesions produce Gerstmann’s syndrome in which patients are unable to do calculations, cannot recognize right from left, have trouble reading and cannot recognize their own fingers as part of their body. Contrary to popular belief, language is the only sharply determined difference between the two sides of the brain – and no one knows to what purpose or how language develops in the left brain. But damage to the right parietal lobe does cause patients to ignore the left side of their bodies and impairs their ability to judge distances on that side. As a consequence, patients frequently report bumping into the left side of door frames and usually obey commands with the right side of the body even when the command has been directed to the left leg or left arm. Similarly, they tend to ignore stimuli presented in the left half of visual space, and do not ‘see’ the left half of clock faces or signs. Most dramatically, the ability to draw and analyze simple line figures greatly deteriorates.
In a sense, memory is a “nomadic” function within the brain since it does not reside in any specific region but is widely distributed throughout the central nervous system. There are probably as many different types of memory as there are human activities and capabilities. Athletic and instrumental performances depend on a kind of muscle memory for complex movement that is brought about through repeated activation of both motor and sensory cortex in combination with centers deep below the surface. There is evidence to show that a program or “tape” plays through the sensory cortex and the deeper centers (i.e., the basal ganglia) to give directional commands to the motor cortex prior to the initiation of visible movement. The recognition of faces depends on a type of visual memory, whereas the recognition of sounds depends on a type of auditory memory. As Proust knew, entire visual scenes can be powerfully evoked through olfactory stimuli. This may be because the olfactory system is directly connected to the most primitive regions of the brain (i.e., the limbic system)—that is, those that are involved in the linkage of emotion and memory. Electrical activity in areas of the temporal lobe, such as the hippocampus and the strengthening of synaptic connections (i.e. memory) throughout the brain are greatly dependent on repetition and rehearsal. This is the opposite of creativity which is based in part on the recognition of unique neuronal events or combinations, a process that Legendy and I have quantified in a probabilistic function we termed “surprise” (4). As is well known from elementary information theory, the information content (i.e., significance) of an event is inversely related to the log of its probability of occurrence (6). When the calculation is based on logarithms to the base 2, the significance or rarity of an event is directly expressed in “bits.” The brain, therefore, utilizes learning paradigms at the extremes of statistical likelihood; the repetition of high probability events brings about performance skills and most memories, while the occurrence of low probability events results in harnessed attention and creativity.
Disorientation may result from being placed in an unfamiliar location, one that does not square with previously learned and memorized maps of the environment. The distress involved in Meena Alexander’s geographic dislocations can only be heightened by linguistic and cultural differences. Older hospitalized patients “sundown” in the evening when familiar visual cues are removed and anxiety is increased. Recent data on spatial disorientation among patients with Alzheimer’s disease and strokes implicate damage to the right parietal lobe (1,5). Significantly, in Alzheimer’s disease, the tendency for patients to wander and get lost appears to be a combination of right inferior parietal lobe dysfunction (i.e., disease in the visuospatial ‘center’) and problems with memory (1). However, patients with only long-term memory problems do not demonstrate spatial disorientation. Furthermore, patients with Parkinson’s disease also appear to suffer from a spatial disorientation that is global in nature. Impairments occur in both tactile and visual space despite the fact that the lesions in Parkinson’s disease are located in very different places from those observed in common stroke syndromes and Alzheimer’s dementia (3).
In summary, there is considerable clinical and metabolic evidence that spatial disorientation in humans occurs in specific types of brain disorders and that the disorientation is worse when compounded by a problem with memory. In a sense then, wandering patients suffer from a type of “nomadic memory.” How deficits in speech output, speech reception, reading and handwriting (i.e., the language functions) interact with disorders of memory and visuospatial orientation is beyond the scope of this essay. Because all elements of language are cortically based and located in the left hemisphere, a human being who experiences spatial disorientation, memory loss, and language dysfunction is a person who is suffering from global and widely dispersed disease of both hemispheres of the brain. At a certain point, it is not possible to know what this type of diseased is brain capable of processing, and outsiders lose the ability to interrogate it. Wandering lost in the world is replaced by endless wandering within the brain itself. Like the meaningless back and forth shuttling of the tape drive in early computers, the soul becomes ‘hung up’ in process and dies.
1-Henderson, VW, Mack, W & Williams, BW: “Spatial disorientation in Alzheimer’s disease.” Archives of Neurology 46:391-394, 1989.
2-Hirsch, HVB & Spinelli, DN: “Modification of the distribution of receptive field orientation in cats by selective visual exposure during development.” Experimental Brain Research 12:509-527, 1971.
3-Hovestadt, A, de Jong, GJ & Meerwaldt, JD: “Spatial disorientation as an early symptom of Parkinson’s disease.” Neurology 37:485-487, 1987.
4-Legendy, CR & Salcman, M: “Bursts and recurrences of bursts in the spike trains of spontaneously active striate cortex neurons.” Journal of Neurophysiology 53:926-939, 1985.
5-Meerwaldt JD & Van Harskamp, F: “Spatial disorientation in right hemisphere infarction.” Journal of Neurology, Neurosurgery and Psychiatry 45:586-590, 1982.
6-Salcman, M: “Probability and the brain.” Neurosurgery 4:75-82, 1979.
7-Zeki, S: Inner Vision, An Exploration of Art and the Brain. (Oxford University Press, 1999).