Aug 1, 2008

Neurological Explanations (Part 2)

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Neural transmission delay

Another class of neurological interpretations assumes that the deja vu experience results from a momentary delay in neuronal transmission from the perceptual organ to the higher order processing centers of the brain. In one version, there is a slight increase in the normal time taken to transmit the message, due to a synaptic dysfunction, and this slight slowing in the routine processing time (several milliseconds) is misinterpreted as representing that the information is old (Grasset, 1904). Anjel (1896) noted that the fatigued state often accompanying deja vu may underlie this neural slowing, temporarily elongating the time between sensation and perception. The logic behind this position is vague and unspecified, and it seems at odds with research on perceptual fluency. More specifically, individuals interpret faster (easier) processing of information as indicating that the item has been experienced before (Jacoby, 1988; Jacoby et al., 1988). Thus, slower processing should imply that the information is new, not old.

Another version of this transmission delay interpretation involves two neuronal pathways rather than one and has more logical explanatory appeal than the single-pathway version. In the visual system, sensory information traverses multiple pathways between the sensory organ and the higher cortical centers (Goodale & Milner, 1992; Milner & Goodale, 1995; Schneider, 1969). In most instances, the information is first received cortically from the primary and then from the secondary pathway (Weizkrantz, 1986). When the normally brief difference in processing time between the two tracks becomes lengthened, the usually seamless integration of the two messages into a single perception becomes disrupted and is experienced as two separate messages (Comfort, 1977). The brain interprets the second version, through the slowed secondary track, as a separate perceptual experience, and thus the inappropriate feeling of oldness derives from the match with the first input processed moments earlier.

Another version of this dual-pathway speculation assumes that the primary perceptual pathway goes directly to the dominant hemisphere while the secondary pathway routes first through the nondominant and then to the dominant hemisphere (Ephron, 1963; Humphrey, 1923). When the delay of information from the nondominant to the dominant hemisphere is slightly extended, a deja vu results (Maudsley, 1889; Osborn, 1884; Weinand et al., 1994). A simpler version of this position is that the two hemispheres normally receive the same information simultaneously, but a slight delay of information to one hemisphere results in a déjà vu (Humphrey, 1923; Wigan, 1844). Weinand et al. (1994) and Ephron (1963) proposed that the preseizure electrical disturbance confined to one hemisphere may cause the slight temporal delay in the secondary pathway, resulting in deja vu. Ephron also speculated that a lesion in the nondominant hemisphere could occasionally slow information transmission from the nondominant to the dominant hemispheres, an interpretation he suggested could be tested during neurosurgical procedures through local anesthesia of the corpus callosum.

A final neurological interpretation of deja vu, also involving two perceptual pathways, assumes that the primary rather than the secondary pathway is delayed, causing the information from the secondary pathway to arrive slightly before that received from the primary pathway. Information from the primary pathway is routinely interpreted by the individual as the initial perception, so when this information arrives after the secondary pathway information it feels familiar because a “memory” match already exists, only milliseconds old (Comfort, 1977; Ephron, 1963).