What is one way the narrator suggests that a psychologist might operationally define greater cognitive skills when testing the effects of violin lessons on children

The answer is: increased attention span

Learning  instruments (such as violin, drums, or piano) would condition people to remember proper notes and deliver them on proper timing.In the long run, learning to play instrument would help to increase our's memory, attention span, and out hand-eye coordination.

Carpetbagger- hope this helps