Purpose: We wished to compare the eye movements seen in two Belgian sheepdo
gs whose crossing retinal fibers from one eye were interrupted at the optic
chiasm (hemichiasma) with those seen in dogs lacking any crossing fibers (
achiasma). In the latter condition, congenital nystagmus (CN), see-saw nyst
agmus (SSN), and strabismus result; also, unyoked or uniocular eye movement
s (saccades and nystagmus) are possible. Methods: Eye movements during fixa
tion were measured using infrared reflection. Data were digitized either at
250 Hz with X-bit resolution or 200 or 400 Hz with 16-bit resolution. Resu
lts: One dog behaved normally, indicating good stereopsis, and had no nysta
gmus. However, unyoked and uniocular saccades were recorded and the number
of fibers from the good eye were close to normal. The other dog mimicked th
e behavior of dogs with achiasma, including CN and SSN and there was a redu
ced number of fibers from the good eye and an increased number of fibers to
the ipsilateral lateral geniculate. Conclusions: Although bilateral redire
ction of retinal fibers that would normally cross may be strongly associate
d with the CN and SSN seen in achiasmatic canines and humans, unilateral re
direction is not. The preservation of one 'binocular' representation of the
central visual field seems to be sufficient to calibrate both horizontal a
nd vertical ocular motor subsystems, thereby preventing the development of
CN and SSN; hemichiasma may not result in strabismus. Dogs with either achi
asma or hemichiasma make uniocular saccades. The discovery of canine achias
ma led to the identification of its human counterpart and canine hemichiasm
a now raises the possibility of a similar syndrome in humans. Human hemichi
asma may mimic achiasma, with CN and SSN, or, unlike achiasma, it might not
be associated with CN, SSN, or strabismus, preserving stereopsis. Thus, it
might easily go undetected in the human population. However, human hemichi
asma may be differentiable from achiasma by imaging the chiasm, or possibly
, using VEP.