cortical probing and hallucinations
   The term cortical probing refers to an experimental method in which cerebral cortical areas are electrically stimulated with the aid of unipolar silver electrodes in order to determine their physiological function. The American physician Roberts Bartholow (1831-1904) has been credited with initiating this type of research shortly before 1874. Arguably the most celebrated work in this domain is that of the Canadian neurosurgeon Wilder Graves Penfield (1891-1976). From the 1930s through the 1950s, Penfield and his group performed open temporal lobe explorations on 520 individuals with uncontrollable epileptic seizures. These individuals were given a local anaesthetic injected into the scalp, so that they remained conscious during the probing experiment, and thus were capable of verbalizing their experiences. Upon the probing of distinct sensory cortical areas (mostly temporal), 40 of these individuals reported vivid hallucinations. According to Penfield, many of these hallucinations seemed to be re-enactments of previously memorized events. He gives the example of a young South African patient, who "lying on the operating table exclaimed, when he realized what was happening, that it was astonishing to realize that he was laughing with his cousins on a farm in South Africa, while he was also fully conscious of being in the operating room in Montreal." Throughout his work Penfield refers to hallucinations evoked by cortical probing either as *experiential phenomena, experiential responses, experiential hallucinations, memory flashbacks, psychical illusions, * psychical hallucinations, or * flashbacks. As he considered the sensory cerebral cortex responsible for receiving and storing sensory input signals, he conjectured that these hallucinations might well be re-perceived memory traces of prior perceptual experiences (i.e. *reperceptive hallucinations). He was impressed by their varied, acute, and detailed nature, and hypothesized that the human memory data base may well contain a literal record of its total conscious experience. He used the terms ganglionic record, neuronal record, and memory cortex to refer to this data base, which he located tentatively within or just beneath the cerebral sensory cortex. In later years, Penfield felt compelled to adjust this view. For example, he moderated his ideas on the alleged completeness of the gan-glionic record, on the grounds that certain modes of experience were conspicuously absent from the test persons' reports (such as eating and tasting food, sexual arousal and performance, the execution of skilled procedures, speaking, resolving to do this or that, and memories of pain, suffering, and weeping). Moreover, he realized that memories might well be altered by dream activity after their initial recording, and, ironically, by prior instances of reperception. Therefore, he concluded that reperceptions could hardly be exact copies of prior sensory experiences. However, he remained convinced that they derived from memorized sense impressions. Long before Penfield's time, the concept behind this physiological model had been conceived by the British physician John Ferriar (1761-1815), who suggested that apparitions might spring from recollections of familiar images. The German psychiatrist Karl Ludwig Kahlbaum (1828-1899) gave this process the name * reperception. He dubbed the resulting percepts * reperceptive hallucinations, so as to distinguish them from what he called *perceptive hallucinations. Criticisms of Penfield's cortical hypothesis derive mainly from studies with stereotacti-cally implanted depth electrodes conducted from the 1970s onwards. On the basis of these studies it has been suggested that reperception can only occur when subcortical as well as cortical centres are activated. As the Swiss-Canadian neurologists Pierre Gloor (1923-2003) et al. state categorically, "Unless limbic structures are activated, either in the course of a spontaneous seizure or through artificial electrical stimulation, experiential phenomena do not occur." Gloor's view concerning the involvement of limbic structures is in keeping with the now dominant long-term potentia-tion (LTP) model ofsynaptic transmission, which links memories primarily to alterations in the synaptic transmission of hippocampal neuronal circuits. As a result, Penfield's concept of a 'gan-glionic record' within or beneath the cerebral sensory cortex is generally regarded as superseded today. In his final, retrospective work The Mystery Of The Mind Penfield indicates that around 1958 he himself had also come to the conclusion that "the record is not in the cortex". From that time onwards, he discarded the terms ganglionic record, neuronal record, and memory cortex, and introduced the term interpretive cortex to denote the temporal cortical areas that upon stimulation would mediate experiential phenomena. As he mused accordingly, "Stimulation of the interpretive cortex activates a record located at a distance from that cortex, in a secondary center of gray matter. Putting this together with other evidence makes it altogether likely that the activated gray matter is in the diencephalon (higher brain stem)."
   References
   Ferriar, J. (1813). An essay towards a theory of apparitions. London: Cadell and Davies.
   Gloor, P., Olivier, A., Quesney, L.F., Andermann, F., Horowitz, S. (1982). The role of the lim-bic system in experiential phenomena of temporal lobe epilepsy. Annals ofNeurology, 12, 129-144.
   Penfield, W. (1975). The mystery of the mind. A critical study ofconsciousness and the human brain. Princeton, NJ: Princeton University Press.
   Penfield, W., Perot, P. (1963). The brain's record of auditory and visual experience: A final summary and discussion. Brain, 86, 595-696.

Dictionary of Hallucinations. . 2010.

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