reversed conductibility of the sensory pathways

   The name of a classic explanatory model for the mediation of "perceptive hallucinations, i.e. hallucinations which are 'projected outwards' and which therefore appear embedded in sense impressions from the external environment. During the late 19th century, perceptive hallucinations were thought to arise from memories or other endogenously generated images which are first shunted back to the senses via efferent nerve fibres, and then forward via afferent ones to the cerebral sensory cortex, along with perceptual information from the senses. The German chemist Ludwig Staudenmaier (18651933) argues that the sense organs may thus facilitate the 'outward projection' or 'physical projection' of perceptual material into extracor-poreal space. The Italian psychiatrist Eugenio Tanzi (1856-1934) may well have been the first to conceptualize this mechanism, referring to it as 'retrograde expansion' or 'reversed conductibil-ity'. As he wrote in 1909, "It is sufficient to suppose, in order to give a satisfactory explanation of all varieties of hallucination, that... the homolateral and contralateral paths that pass from the centres of sensation to the centres of representation acquire the power, which they never possess in physiological conditions, of allowing impulses to travel in the reverse direction. We may consider that an hallucination takes origin as an idea or symbol, or as a more or less conscious fragment of an idea in the associative area, but that, instead of forming associations with other ideas, or of projecting itself externally in movement, it flows back, either along the same homolateral and contralateral fibres by which it came, or in some other way yet to be determined, to the sen-sorial centres from which it proceeded when it was of the nature of a sensation. Thus it becomes what it originally was - namely, a sensation; but it is a sensation of a pathological character on account of its unusual origin." Tanzi sought to support this concept by an appeal to contemporary neuroanatomy, stating that "a few descending fibres occur with constant regularity in all tracts of ascending projection. They are to be found between the visual cortex and the external geniculate body, between the external genic-ulate body and the retina, between the olfactory centres in the cortex and the olfactory bulb, and in the secondary and tertiary acoustic paths. It is not, therefore, out of the question that such fibres should exist also at a higher level than the senso-rial centres." Tanzi's model may be classified as a "centrifugal model of hallucinatory activity.

   References

   Schorsch, G. (1934). Zur Theorie der Halluzinationen. Leipzig: Verlag von Johann Ambrosius Barth.

   Tanzi, E. (1909). A text-book of mental diseases. Translated by Ford Robertson, W., Mackenzie, T.C. London: Rebman Limited.