OCULAR MOTOR ABNORMALITIES IN ACHIASMATIC MUTANT BELGIAN SHEEPDOGS - UNYOKED EYE-MOVEMENTS IN A MAMMAL

We studied the eye movements of several members of a family of Belgian sheepdogs that includes achiasmatic mutants. Our aim was to identify the types of nystagmus and other ocular motor abnormalities exhibited by the mutants. We also recorded from several unaffected heterozygous carriers of the genetic mutation and from a normal Irish Setter. Mutant dogs exhibited nystagmus waveforms that were occasionally similar to those of humans with congenital nystagmus (CN). Foveating and braking saccades and foveation periods were seen in some waveforms. More common were pendular oscillations of both eyes that were essentially independent in amplitude and phase. At some times there was a pendular nystagmus with a 180 deg phase shift between the movements of the eyes. Similar to the nystagmus, saccades were often unyoked or uniocular. The eye movements of unaffected relatives did not reveal any saccadic instabilities. However, small saccadic intrusions could have been masked by quantization artifacts. Individual dogs from this family provide an animal model of the ocular motor consequences of the disturbed visual input caused by the absence of an optic chiasm and a novel model of CN. Despite any other ocular motor abnormalities present, the CN may be studied in isolation just as in humans it is studied when strabismus and other types of nystagmus are present. Further studies of ocular motor development and function in achiasmatic dogs have the potential to reveal both the organization of the control systems for each extraocular muscle and the role of yoking of the agonist muscles of the two eyes.