Tree shrews (Tupaia belangeri) are small diurnal mammals
capable of quick and agile navigation. Electroretinographic and
behavioral studies have indicated that tree shrews possess very good
temporal vision, but the neuronal mechanisms underlying that temporal
vision are not well understood. We used single-unit extracellular
recording techniques to characterize the temporal response properties
of individual retinal ganglion cell axons recorded from the optic
tract. A prominent characteristic of most cells was their sustained or
transient nature in responding to the flashing spot. Temporal
modulation sensitivity functions were obtained using a Gaussian spot
that was temporally modulated at different frequencies (2–60 Hz).
Sustained cells respond linearly to contrast. They showed an average
peak frequency of 6.9 Hz, a high-frequency cutoff at 31.3 Hz, and
low-pass filtering. Transient cells showed nonlinear response to
contrast. They had a peak frequency of 19.3 Hz, a high-frequency cutoff
at about 47.6 Hz, band-pass filtering, and higher overall sensitivity
than sustained cells. The responses of transient cells also showed a
phase advance of about 88 deg whereas the phase advance for sustained
cells was about 43 deg. Comparison with behavioral temporal modulation
sensitivity results suggested that transient retinal ganglion cells may
underlie detection for a wide range of temporal frequencies, with
sustained ganglion cells possibly mediating detection below 4 Hz. These
data suggest that two well-separated temporal channels exist at the
retinal ganglion cell level in the tree shrew retina, with the
transient channel playing a major role in temporal vision.
Photophobia in migraine: A symptom cluster?
