In the lizard, Ctenophorus ornatus, anatomical
studies have revealed that optic axons regenerate to visual
centers within 2 months of nerve crush but that, from the
outset, the regenerated projections lack topographic order
(Beazley et al., 1997; Dunlop et al., 1997b).
Here we assess the functional topography of the regenerated
retinotectal projections by electrophysiological recording
of extracellular multiunit responses to visual stimulation
and by observing the lizards' ability to capture live
prey. At the completion of the electrophysiology, DiI was
applied locally to the retina and the topography of the
tectal projection later assessed. Electrophysiology revealed
that, at 2–4.2 months, responses were weak and habituated
readily; no retinotopic order was detected. Between 4.5–6
months, responses were more reliable and the majority of
lizards displayed a crude retinotopic order, especially
in the ventro-temporal to dorso-nasal retinal axis. Although
responses were variable between 6–9 months, they
tended to be more reliable again thereafter. However, from
6–18 months, the projection consistently lacked topography
with many retinal regions projecting to each tectal locus.
Lizards, including those with electrophysiological evidence
of crude retinotopy, were consistently unable to capture
live prey using the experimental eye. Labelling with DiI
confirmed the absence of anatomical retinotopy throughout.
Taken together, the electrophysiological and anatomical
data indicate that retinotopically appropriate axon terminals
(or parts thereof) are transiently active whilst inappropriately
located ones are silent. Presumably in lizard map-making
cues fade with time and/or the mechanisms are lacking to
stabilize and refine the ephemeral map. Moreover, the transient
retinotopy is insufficient for useful visual function.