(1) In macaques with the optic chiasm transected, and forebrain commis
sural communication limited to the anterior commissure or the posterio
r 5 mm of the splenium of the corpus callosum, visual patterns viewed
initially by only one eye (hemisphere) are subsequently recognized by
the other with normal accuracy. (2) The efficiency of these commissura
l paths is further indicated by the fact that even when as many as six
''target'' images are presented for memorization to only one hemisphe
re, it makes essentially no difference as to accuracy or latency of pe
rformance which hemisphere is then required to distinguish ''target''
from ''non-target'' images. (3) By electrically tetanizing structures
in one or the other temporal lobe at various times in relation to Visu
al input and/or mnemonic testing it could be shown: (a) that a memory
trace restricted in its formation to a single hemisphere was available
to the other via either forebrain commissure, and (b) that the memory
is formed bilaterally despite unilateral input. (4) When the chiasm i
s split but the commissures are intact, simultaneous presentation of d
isparate images to each hemisphere severely perturbs performance, sugg
esting that the callosal system operates continuously to unify visual
percepts; but when only the anterior commissure is intact, the two hem
ispheres accept incongruent images without perturbation. (5) In the fu
lly ''split-brain'' condition, when one hemisphere cannot access memor
ies held in the other, the accuracy of performance by each hemisphere
is nevertheless burdened by the memory load of its neocortically disco
nnected partner. It can thus be inferred that the brainstem plays a cr
itical, unifying role in this mnemonic process.