Lateralization of brain function

A functional asymmetry between cerebral hemispheres, measured for a stated task, contrast or operation. Population-level left dominance does not imply complete right-side inactivity or an identical pattern in every individual.

For nonnegative measurements $L,R$ with $L+R>0$, one convention is $\mathrm{LI}=(L-R)/(L+R)$. The bound $|L-R|\le L+R$ gives $-1\le \mathrm{LI}\le 1$. Multiplying both measurements by the same positive factor leaves the index unchanged. A change from $(L,R)=(20,10)$ to $(10,10)$ changes the index from $1/3$ to $0$ without increasing $R$. Thus a reduced leftward index alone cannot establish right-side recruitment. Baseline, threshold and negative-response conventions must be specified.