Test, optokinetic nystagmus A test for eliciting
OKN. The subject sits in front of a rotating drum covered with uniform black and
white vertical stripes parallel to the axis of rotation (this apparatus is call
and optokinetoscope or optokinetic drum). When the eyes respond
with a slow movement in the direction of the drum lasting about 0.2s, and a fast
phase in the reverse direction of about 0.1s, the OKN has been elicited and this
fact provides evidence of vision. As finer and finer black and white stripes are
used, this reflex response will cease to be elicited for a particular spatial
frequency of the stripes corresponding to the objective visual acuity of
the subject. See acuity; malingering; nystagmus.
Nystagmus A regular, repetitive, involuntary movement of the eye whose
direction, amplitude and frequency are variable. Nystagmus can be induced,
acquired or congenital. (In a very small percentage of people it can even be
induced voluntarily.) These eye movements characteristically appear as one of
two types: one in which there is a slow and fast phase and the nystagmus is
conventionally defined by the direction of the fast phase.
Such nystagmus is called a jerk nystagmus. A jerk nystagmus is usually
due to a motor defect that may be induced by drug intoxication (upbeat
nystagmus in which the fast phase is in the upward direction); associated
with a lesion of the central nervous system or the vertibular nerve or nuclei (central
nystagmus and vestibular nystagmus); or to disease or injury to the
labyrinth (labyrinth nystagmus); or to multiple sclerosis. Jerk nystagmus
can also be induced physiologically, as for example optokinetic nystagmus
(abbreviated: OKN) or train nystagmus which occurs when watching
objects which traverse the visual field rapidly, or as a result of thermal
stimulation of the labyrinth of the inner ear by cold or hot water (caloric
nystagmus or Barany's nystagmus), or when the eyes of a
fatigued person are turned into an extreme position of gaze (end-point
nystagmus), or when a person who had been spinning round is stopped (vestibular
nystagmus). The other type is a nystagmus which is characterized by
movements of equal velocity in each direction and this is called a pendular
nystagmus. A pendular nystagmus usually occurs as a result of poor central
vision (sensory deprivation nystagmus) as in bilateral chorloretinitis,
total color blindness, albinism, congenital cataract, corneal scarring,
amblyopia (amblyopic nystagmus) or in coal miners after many years of
working in the dark (miner's nystagmus). In some cases there is a mixture
of the two main type (mixed nystagmus). The movements of the eyes are
usually the same in both eyes (associated nystagmus) but in some
cases they may be unrelated (dissociated nystagmus) or show
symmetry of amplitude and type of movement in the two eyes but in opposite
directions (disjunctive nystagmus). There are also cases of unknown
origin (idiopathic nystagmus). See ataxia, hereditary spinal; disease.
Wernicke's monochromat: optokinetic; oscillopia; prisms, yoke; reflex,
vestibuloocular; syndrome. Down's; syndrome, nystagmus blockage; test,
optokinetic nystagmus.
Optokinetic Drum
Optokinetic
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