centripetal theory of hallucinatory activity

   The name centripetal theory refers to an explanatory model of hallucinatory activity which is traditionally attributed to the German physiologist and zoologist Johannes Peter Müller (18011858). Conceptually, the centripetal theory constitutes the logical counterpart of the * centrifugal theory of hallucinatory activity. The centripetal theory suggests that the sense organs or the peripheral nervous system must be held responsible for mediating the initial impulse for some types of hallucinatory activity, which is then 'projected upwards' towards the higher sensory or ideational centres of the brain (or to the mind, in a dualist reading), to produce the false impression of a sensory percept. A historical example of this type of hallucination is known as *psychosensorial hallucination. Today a compelling literature indicates that hallucinations can indeed be mediated by aberrant activity of the sensory pathways. These neural structures can be subdivided into the sense organs and the primary sensory pathways. The latter are charged with conducting perceptual information from the sense organs towards the cerebral sensory cortex. Theoretically, hallucinations can be mediated by any component of these trajectories, including the sense organs themselves. Thus instances of *tinnitus have traditionally been associated with a lesion affecting the vestibular organ (although a central origin is equally possible), *phosphenes with retinal disease, *floaters with protein clots inside the vitreous body, and some types of *metamorphopsia with ablatio retinae. Traditionally, however, biomedicine recognizes only a minority of these phenomena as * hallucinations proper. Instead, they tend to be relegated to the classes of *entoptic or otopathic phenomena, or to the class of * automatisms as envisaged by the French alienist Jules Gabriel François Baillarger (1806-1891). Percepts most likely to be acknowledged as hallucinations proper are those which arise from the aberrant activity of groups of neurons in and around the primary sensory pathways. Studies of isolated hallucinations in single sensory modalities indicate that these percepts may arise from focal anatomical lesions and/or partial epileptic seizures. Because of their physiological characteristics, the American ophthalmologist David Glendenning Cogan (1908-1993) designates this class of phenomena as the * irritative form of hallucinatory activity (so as to distinguish it from the * release form of hallucinatory activity). As each of the sensory pathways serves a single sensory modality, focal pathology is thought to result in hallucinations limited to that specific modality. It has traditionally been assumed that the complexity of the resulting phenomena correlates with the function of the brain area involved. Thus lesions in the relative proximity of the sense organs have been associated primarily with the mediation of * simple or even *incomplete hallucinations, such as * ophthalmopathic hallucinations manifesting in one of the hemifields in ocular disease, or * unilateral auditory hallucinations in lesions affecting one of the acoustic nerves. Circumscript lesions within the primary sensory cortex of the occipital lobe are typically associated with simple visual hallucinations such as phosphenes, * fortifications, and other geometric patterns. A well-known example is the *aura that may precede or accompany a migraine attack. In general, it is believed that lesions in and around the primary sensory pathways are capable of evoking little more than unvarying, stereotypical hallucinations. However, this situation can change when damaged tissue starts to recover, or when partial epileptic activity spreads to other anatomical loci.

   References

   Braun, C.M.J., Dumont, M., Duval, J., Hamel-Hébert, I., Godbout, L. (2003).

   Brain modules of hallucination: An analysis of multiple patients with brain lesions. Journal of Psychiatry and Neuroscience, 28, 432-449.

   Müller, J. (1826). Ueber die phantastischen Gesichtserscheinungen. Koblenz: Hölscher.

   Parish, E. (1897). Hallucinations and illusions. A study ofthe fallacies ofperception. London: Walter Scott.