Recording Methods

Stimulation of any sensory receptor evokes a minute electrical signal (i.e. microvolts) in the appropriate region of the cerebral cortex. Averaging techniques permit recording and analysis of this signal normally lost within the background electrical activity. When sensitive apparatus is triggered to record cortical activity at a specific time after the stimulus, the background electrical 'noise' averages out, i.e. random positive activity subtracts from random negative activity, leaving the signal evoked from the specific stimulus.

Trigger pulse

x-y plotter

Averager

! *H

Alternating 'checkerboard' or flash

Visual evoked potential (VEP) 50ms t +

Checkerboard stimulus

Visual evoked potential (VEP) 50ms t +

Checkerboard stimulus

Vertex (reference)

Occipital (active)

Vertex (reference)

Occipital (active)

Brain stem auditory evoked potential (BAEP)

IV V

IV V

Vertex (active)

Vertex (active)

Mastoid (reference)

A stroboscopic flash diffusely stimulates the retina; alternatively an alternating checkerboard pattern stimulates the macula and produces more consistent results. The evoked visual signal is recorded over the occipital cortex. The first large positive wave (P0 provides a useful point for measuring conduction through the visual pathways.

Uses: Multiple sclerosis detection - 30% with normal ophthalmological examination have abnormal VEP. Peroperative monitoring - pituitary surgery.

Electrical activity evoked in the first 10 milliseconds after a 'click' stimulus provides a wave pattern related to conduction through the auditory pathways in the VIII nerve and nucleus (waves I and II) and in the pons and midbrain (waves III-V). Longer latency potentials (up to 500 ms), recorded from the auditory cortex in response to a 'tone' stimulus, are of less clinical value.

Uses: Hearing assessment - especially in children.

Detection of intrinsic and extrinsic brain stem and cerebellopontine angle lesions, e.g. acoustic tumours.

Peroperative recording during acoustic tumour operations.

Assessment of brain stem function in coma.

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