1. We recorded abducens neurons, identified by electrical stimulation
as internuclear neurons or motoneurons, in awake rabbits. The relation
ship of firing rate to eye movement was determined from responses duri
ng stable fixations, sinusoidal rotation in the light (0.05-0.8 Hz), a
nd triangular optokinetic stimulation at 0.1 Hz. 2. All abducens neuro
ns were excited during temporal movement of the ipsilateral eye. Tempo
ral and nasal saccades were associated with bursts or pauses, respecti
vely, in the firing rate. 3. Motoneurons and internuclear neurons are
qualitatively indistinguishable. There was no significant quantitative
difference between the phase and sensitivity of the two groups for 0.
2-Hz sinusoidal rotation in the light. 4. On the basis of the response
to stable eye positions, we determined static eye position sensitivit
y of the abducens neuron pool to be 8.2 +/- 2.5 (SD) spikes s(-1)/degr
ees, with a static hysteresis of 8.9 spikes/s (1.14 +/- 0.37 degrees).
5. We determined apparent eye position sensitivity (k) and apparent e
ye velocity sensitivity (r) from the responses to sinusoidal rotation
in the light. k increases and r decreases with stimulus frequency, whi
ch indicates that the simplest transfer function mediating conversion
of abducens nucleus (VI) firing rate to eye position (E) has two poles
and one zero. 6. The VI --> E relationship has an ''amplitude nonline
arity,'' manifest as a tendency for k, r, and firing rate phase lead t
o decrease as eye movement amplitude increases at a fixed frequency. O
n a percentage basis, phase is less affected than are the sensitivitie
s. The nonlinearity becomes less pronounced for stimulus amplitudes >2
.5 degrees, and consequently a linear model of the VI --> E transforma
tion remains useful, provided that consideration is restricted to the
appropriate range of stimulus/response amplitudes. 7. We determined ti
me constants of the linear two-pole, one-zero transfer function from t
he variation of r/k versus stimulus frequency. The pole time constants
were T-1 = 3.4 s and T-2 = 0.28 s, and the zero time constant (T-z) =
1.6 s. The magnitude of T-z was corroborated by measuring the time co
nstant of the exponential decay in firing rate after step changes in e
ye position. This transient method yielded a T-z of 1.1 s. 8. The time
constants of the VI --> E transfer function are roughly 10 times larg
er than those reported for the rhesus macaque. The difference is attri
butable to the reported 10-fold lower stiffness of the rabbit oculomot
or plant, which may in turn relate to rabbit's postulated lower degree
of activation of extraocular muscles at any given position. The subst
antial differences in time constants have important implications for c
omparing signaling properties of ocular motoneurons and premotor neuro
ns in different species.