HEMISPHERIC DISTRIBUTION OF MEMORY TRACES

It is well-established from work on mammals that when input can be limited to one cerebral hemisphere, testing the other subsequently reveals that it also has access to memory traces created in relation to that lateralized input. What is unclear is whether this aptness derives from the trace having been imparted to both hemispheres, or whether it resides in the one and is called forth by the other. The situation is even less clear when, as is normally the case, input is likely to be distributed concurrently, but unevenly, to both hemispheres. The evidence reviewed herein suggests that there are many answers. Bilateral engrams can often be assumed, and in a few instances can be demonstrated. Unilateral engrams are also formed, as a consequence of lateralized input or manipulation, maturational age, “focussed” attention, or perhaps in many other instances not readily amenable to analysis. In mammals the unprivileged hemisphere has access via components of the forebrain commissures to unilateral engrams held by its partner; but fish and birds in some instances lack such ability. Tetanization of the hippocampal system can be used to reveal some of these effects; and data from split-brain macaques suggest that brainstem processes participate bilaterally in mnemonic processing.