Mechanisms of Hallucinations

Cerebral mechanisms of perception--where sensory events are really experienced--are somewhat removed from the scene of the action. When did you last say to someone, "I saw it with my own eyes"? In so doing, you were inviting the inference that it existed "out there" somewhere "in the real world." If, instead, you had said, "I saw it with my own brain," your statement, although closer to the truth, would have lost much of its intended force. That is because it would no longer demand the concession that you had perceived a tangible object "out there." Given this opening, the listener might even have had the temerity to suggest, "Couldn't it have been a figment of your imagination?"

Although the statement "I saw it with my own brain" may have an odd ring, it serves to remind us that the cerebral mechanisms of perception where sensory events are really experienced are at some remove from the scene of the action. Perceptions are transformations of environmental stimuli. Perception has sometimes been referred to as "sensory reasoning"--as a way of acknowledging the fact that the final product owes more to cognitive interpretation and embellishment than most people think.[31] Perceptions are transformations of environmental stimuli encoded in networks of active brain cells. Quite early in the process, inputs from the brain's memory, emotional, and cognitive systems are inserted into the perceptual stream. As a result, the exact placement of the boundary between raw sensation and the cognitive processes responsible for constructing our global awareness of the world is somewhat arbitrary.

The 19th-century psychologist Sir Francis Galton emphasized the continuity of all forms of visualization, whether stimulus-driven or memory-driven.[32] Neurologically speaking, sensations, mental imagery, dreams, daydreams, and hallucinations are cut from the same cloth. Perceptions are cognitive constructions, assembled from raw sensory data that are seamlessly combined with images from memory that "flesh out" all conscious awareness. The relative preponderance of external (sensory) versus internal (memory) inputs to the cerebral mechanisms that assemble our conscious model of reality is constantly shifting. Siegel[33] emphasized that there is competition between external and internal inputs for access to this central awareness system and that hampering any of the contenders invites additional assaults from its rivals. Marks[26] and Loverock and Modigliani[34] reviewed studies supporting the conclusion that the same cerebral mechanisms serve perception and imagery. Schatzman[35] reinforced this conclusion with electroencephalographic data from people who could produce extremely vivid hallucinations at will. Auditory hallucinations that are correlated with activity in brain areas that mediate perceptions of real sounds have been demonstrated with EEG recordings[28] and, more recently, with PET scans.[36,37]

Because functionally equivalent states of the central awareness system can arise from either memory or sensory sources, it is possible for dreams, perceptual memories, fantasies, and hallucinations to become indistinguishable from real events. Hallucinations result whenever internal events trigger a pattern of brain activity equivalent to that normally generated when sense organs respond to a publicly observable event. Thus, if the brain's awareness mechanisms were to be flooded by neural discharges from memory banks, the experience could feel just as real as if it had been engendered by actual events "out there."

How we ordinarily distinguish reality from vivid mental imagery or daydreams and why this ability should occasionally break down are central to an understanding of hallucinations.[28] The ability of the brain to create illusory experiences that pass for reality can be seen as a cost we incur in return for the ability to think and remember in complex images (ie, to conjure up believable tableaux in our mind's eye). In order for imagery to be useful in solving problems and mentally testing out prospective actions in a "dry run," we require neural systems capable of creating a model of the world with considerable veracity inside our heads. That a representational system of this sophistication should occasionally fool us into thinking it is real ought to surprise no one.