One hand is better than two: motor extinction of left hand advantage in unilateral neglect

Severe left visual neglect has previously been shown to reduce when left hand movements in left hemispace were performed simultaneously to a series of cancellation tasks (Robertson and North, Neuropsychologia 30, 553-563, 1992). The present study demonstrates in two separate cases that this advantage associated with left hand activation was eliminated by simultaneous right hand activation, whether carried out in left or right hemispace. This suggests the existence of an extinction phenomenon for motor output which affects perceptual functioning, and a possible theoretical basis for this is discussed.