Time labelling of independently acquired lateralized engrams in rats

Abstract

Functionally hemidecorticated rats were trained with the right hemisphere to go right and 24 hr later with the left hemisphere to go left in a simple T-maze. In 5 free choice (FC) trials performed with intact brain on Day 3 equilibration of the right and left turns (40% FC left) was found when 9 10 avoidance + 30 overtraining trials to the right were followed by 3 3 escapes to the left. Control groups trained only in the first or the second habit chose the left arm in 15% and 73%, respectively, whereas intact rats learning the two tasks sequentially displayed a marked reversal (83% FC left). When rats trained under the above equilibrated choice conditions were subjected with intact brain two hr before the second hemidecorticate training to a strong emotional experience (20 min swimming), the percentage of left choices was significantly increased on Day 3 (70% FC left). Swimming neither interfered with retention of the isolated first engram (20% FC left) nor improved the acquisition of the isolated second engram (65% FC left), but when inserted between two lateralized learning sessions with opposite hemispheres it increased the weight of the second experience to the level found in reversal learning. Similar results were obtained with a water T-maze when using foot shocks (30 min) as reference. A hypothetical model is proposed explaining the absence of relative time labelling of lateralized engrams and the possibility to restore it by the bilaterally recorded reference event, bridging the gap in the hemispheric memory records.