UC San Diego

Lateralization is a part of virtually every function we think makes us human, yet there is no integrated neurophysiological explanation of the development of lateralization and interhemispheric integration. In this thesis I propose how development, lateralization in visual processing, and interhemispheric connectivity are all intertwined. I begin with evidence from neurocomputational modeling that lateralization in visual processing can be accounted for by a difference in the length of long-range lateral connections brought on by typical human developmental processes. The model can explain processing asymmetries for low vs. high spatial frequencies, local vs. global stimuli, as well as the right hemisphere lateralization of faces. Next, I show modeling evidence against the prevailing hypothesis that lateralization and interhemispheric communication are both functions of brain size. Previous papers have argued that lateralization is related to hemispheric independence which increases with brain size, due to both longer latency and proportionally fewer interhemispheric connections in larger brains. I examine interhemispheric connectivity across species using a new allometric analysis of existing data and examine latency effects through a re-analysis of neural network modeling data. Both results suggest that inter- hemispheric communication is robust across brain sizes. Along the way, I examine the neurophysiological development of long-range connections. I use neural network modeling to show that developmental changes in the physiology of axons may bias learning towards more local intrahemispheric circuits early in development, with long distance interhemispheric circuits becoming more prominent as connections mature. These modeling results are broadly consistent with the develop- mental trajectory of both interhemispheric communication and lateralization. I conclude by attempting to integrate these results into existing theories of developmental and adult visual lateralization. In many domains, communication is the key to the development of specialization. I hope this work can refocus our efforts to understand how interhemispheric communication affects the development of each lateralized function in the brain and treat any relationships between hemispheric independence and lateralization as an unexpected special case worth investigating further