Title: Short-term and long-term memory deficits in handedness learning in mice with absent corpus callosum and reduced hippocampal commissure Short title: Handedness learning in mice with absent CC/HC

The corpus callosum (CC) and hippocampal commissure (HC) are major interhemispheric connections whose role in brain function and behaviors is fascinating and contentious. Paw preference of laboratory mice is a genetically regulated, adaptive behavior, continuously shaped by training and learning. We studied variation with training in paw-preference in mice of the 9XCA/WahBid ('9XCA') recombinant inbred strain, selected for complete absence of the CC and severely reduced HC. We measured sequences of paw choices in 9XCA mice in two training sessions in unbiased test chambers, separated by one-week. We compared them with sequences of paw choices in model non-learner mice that have random unbiased paw choices and with those of C57BL/6JBid ('C57BL/6J') mice that have normal interhemispheric connections and learn a paw preference. Positive autocorrelation between successive paw choices during each session and change in paw-preference bias between sessions indicate that 9XCA mice have weak, but not null, learning skills. We tested the effect of the forebrain commissural defect on paw-preference learning with the independent BTBR T+ tf/J ('BTBR') mouse strain that has a genetically identical, non-complementing commissural trait. BTBR has weak short-term and long-term memory skills, identical to 9XCA. The results provide strong evidence that CC and HC contribute in memory function and formation of paw-preference biases.