Optokinetic nystagmus in cats with congenital strabismus
K.-P. Hoffmann, C. Distler & C. Markner
Journal of Neurophysiology, 75:1495-1502, 1996
- 1. Eye movements were recorded in seven innately esotropic cats during monocular and binocular horizontal optokinetic
stimulation, using the search coil technique in five cats and electrooculography in two cats. 2. During closed
loop measurements in these strabismic cats, slow phases of optokinetic nystagmus (OKN) were characterized by an
overall reduced gain when compared with normal controls. In addition, response gain to monocular nasotemporal stimulation
was even more reduced than that to temporonasal stimulation, resulting in an increased asymmetry of closed loop
gain. 3. During open loop measurements, eye velocity in strabismic cats was very low at all velocities tested.
4. Differential analysis of the symmetry of OKN revealed that all our strabismic cats had a "good" or
more symmetric and a "poor" or more asymmetric eye. In addition, when analyzed separately at individual
velocities, the symmetry index of the good eye was fairly constant over the velocity range tested. By contrast,
the symmetry index of the poor eye dropped dramatically at higher stimulus velocities. 5. To analyze the relationship
of OKN symmetry and cortical physiology, we calculated the ratio between the percentage of neurons driven by one
eye in the ipsilateral and the contralateral cortical hemisphere. We found a weak correlation between OKN symmetry
and this cortical symmetry index (P < 0.05, analysis of variance). 6. In conclusion, slow eye movements in cats
with congenital esotropia are characterized by extremely low gain, especially at higher stimulus velocities. In
addition, OKN symmetry during monocular stimulation is decreased. Our data suggest that OKN symmetry is weakly
correlated with the proportion of binocular neurons in the visual cortex ipsilateral to the stimulated eye. However,
OKN characteristics seem to reflect to a higher degree the response properties of neurons in the pretectal nucleus
of the optic tract and the dorsal terminal nucleus of the accessory optic system than properties of neurons in
the visual cortex.
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