Central nervous system amplification

Abstract

Ability to record a sizable somatosensory evoked potential (SEP) in the absence of a recordable sensory nerve action potential (SNAP) suggests a normally occurring central nervous system amplifying process. Increments in SEP and SNAP amplitude with increasing stimulus strength between threshold and 2.5 times threshold (maximum) were compared. A threshold (40% of maximum stimulus) and 50% maximum stimulus, amplification measured 2.3 +/- 0.7 and 2.0 +/- 0.6, respectively. In 21 MS patients, the SEP at threshold stimulation was absent in 15, but normal in 5 of these at maximum stimulation. It is postulated that normal central amplification is markedly attenuated in MS, and this may be a sensitive indicator of early disease.

AAEM minimonograph 19: Somatosensory evoked potentials

“…However, complex facilitatory and occlusive interactions leading to ''central gain'' can prevent amplitude reduction in the SEP, masking axonal or neuronal loss. 35,89 SEP amplitude increases in the elderly, 24 and ''giant'' potentials typify hereditary myoclonic epilepsy. 101 SEPs evoked by dermatomal-as opposed to nerve-stimulation are much more variable in amplitude.…”

“…The inability to record peripheral SNAPs in such cases must reflect pathologic dispersal of impulses, in which context the preservation of cortically generated SEPs probably results from reorganization and synchronization of afferent volleys at different synaptic levels. 25,31,35,89 If this is the case, the responses of a few normally conducting surviving axons must account for the misleadingly normal values for conduction velocity sometimes obtained by the SEP technique. 89 Focal nerve lesions can be diagnosed by scalprecorded SEPs, 23 but this is only helpful when the lesion is too proximal to be recognized by conventional techniques, such as in meralgia paresthetica.…”

“…43 However, when a mixed nerve is stimulated, both group Ia muscle afferents and cutaneous group II afferents contribute to the resulting SEP. 55 Its amplitude is almost maximal when the peripheral nerve action potential is only 50% of its maximum. 35 This translates into a requisite stimulus intensity of about twice sensory threshold. It is also possible to elicit SEPs using a variety of mechanical stimuli.…”

Selective temporal shift in the somatosensory evoked potential produced by chronic stimulation of the human index finger

“…The amplitude of the nerve volley was subsequently re-checked. Previous studies have documented this relation in detail (Gandevia et al 1982;Gandevia and Burke 1984; see also Eisen et al 1982).…”

Volitional Monocular Lilliputian Visual Hallucinations and Synesthesia

“…It is a common observation in clinical practice that patients with episodic painful or epileptic disorders have an inherent permissive ability to ‘prime’ or ‘cue’ their episodes, either consciously or unconsciously. In multiple sclerosis, amplification of abnormal signals arising from the areas of demyelination perhaps may be ‘gated ad lib‘ in some exceptional cases, as the one detailed here [6]. The discovery of Bumgartner and Epstein [7]that normal individuals can voluntarily alter their visual evoked potentials without shifting their eyes supports this inherent quality of the human brain to freely modify ongoing electrophysiologic activities.…”