This study is concerned with how information about
the direction of visual motion is encoded by motion-sensitive
neurons. Motion-sensitive neurons are usually studied using
stimuli unchanging in speed and direction over several
seconds. Recently, it has been suggested that neuronal
responses to more naturalistic stimuli cannot be understood
on the basis of experiments with constant-motion stimuli
(de Ruyter van Steveninck et al., 1997). We measured the
variability and information content of spike trains recorded
from directional neurons in the nucleus of the optic tract
(NOT) of the wallaby, Macropus eugenii, in response
to constant and time-varying motion. While the NOT forms
part of the mammalian optokinetic system, we have shown
previously that the responses of its directional neurons
resemble those of insect H1 in many respects (Ibbotson
et al., 1994). We find that directional neurons in the
wallaby NOT respond with lower variability and higher rates
of information transmission to time-varying stimuli than
to constant motion. The difference in response variability
is predicted by an inhomogeneous Poisson model of neuronal
spiking incorporating an absolute refractory period of
2 ms during which no subsequent spike can be fired. Refractoriness
imposes structure on the spike train, reducing variability
(de Ruyter van Steveninck & Bialek, 1988; Berry &
Meister, 1998). A given refractory period has a greater
impact when firing rates are high, as for the responses
of NOT neurons to time-varying stimuli. It is in just these
cases that variability in experimentally observed spike
trains is lowest. Thus, differences in response variability
do not necessarily imply that different models are required
to predict neuronal responses to constant- and time-varying
motion stimuli.