Long term memory operates using calcium.  One way it does this is to cause the release of glutamate.  Too much glutamate will over-excite the neural system, killing-off neurons, causing strokes, seizures, and cardiac arrest.    Second, calcium entering the synaptic cleft also causes the production of retro-grade neuro-transmitters which 'loop-back' to the pre-synaptic neuron; too much of this and the nervous system goes into a positive feedback loop causing problems with the development of long term memory.

I'd look to see if there is a genetic relationship in Lefties to the calcium cycle in their nervous systems.