Anatomical traces of learning in the ICX of the barn owl.
(A) Digital image drawings of labeled axons in 40-μm
thick, horizontal sections through the optic lobe of a normal juvenile
(Left) and a prism-reared adult (Right).
To visualize the pattern of axonal projection from the ICC to the ICX,
the anterograde tracer, biocytin, was injected into the ICC at a site
representing 20 μs ITD contralateral ear leading. The location of the
injection site in the ICC is indicated as a dark circle. Labeled axons
in the ICX are shown as thin lines. In a normal juvenile, the axonal
projection field in the ICX is spatially restricted, symmetric, and
centered around the rostral-caudal level of the injection site. In
contrast, in a prism-reared adult expressing a rostral map shift, there
is a dramatic expansion of the projection field in the rostral portion
of the ICX. The direction of this axonal expansion is adaptive for the
direction of prismatic displacement. Thus, these axons represent the
learned projection field. (B) Composite spatial
distributions of axons for normal juveniles (empty gray bars) and
prism-reared adults (solid black bars). To quantify the spatial
distribution for each case, the ICX was subdivided into measurement
zones oriented orthogonal to the rostral-caudal axis. Each measurement
zone corresponded to 5% of the rostral-caudal extent of the ICX. Total
axonal length in each measurement zone was determined by computer
analysis of the digital image drawings. Histograms were aligned
relative to the rostral-caudal level of the injection site. Composite
distributions were obtained by averaging the individual cases. In
prism-reared adults with rostral map shifts (n =
4), the axonal density on the rostral flank of the distribution is
significantly greater than in normal juveniles (n =
7; ANOVA, P < 0.0001), indicating that remodeling
occurs by a net elaboration of axons. In contrast, the axonal density
within the normal projection field, located at the rostral-caudal level
of the injection site, is not significantly changed by prism
experience. Data are from W.M.D., D. Feldman, and E.I.K., unpublished
data.