Functional activation in motor cortex reflects the direction and the degree of handedness

Abstract

Handedness is the clearest example of behavioral lateralization in humans. It is not known whether the obvious asymmetry manifested by hand preference is associated with similar asymmetry in brain activation during movement. We examined the functional activation in cortical motor areas during movement of the dominant and nondominant hand in groups of right-handed and left-handed subjects and found that use of the dominant hand was associated with a greater volume of activation in the contralateral motor cortex. Furthermore, there was a separate relation between the degree of handedness and the extent of functional lateralization in the motor cortex. The patterns of functional activation associated with the direction and degree of handedness suggest that these aspects are independent and are coded separately in the brain.

Figure 1

Effects of hand (a), handedness (b), and their interaction (c) on the volume of contralateral functional activation in the motor cortex during movement of the hand. Only the effect of hand reached statistical significance (P = 0.027, indicated by the ∗).

Figure 2

(a) Ranked correlation between ipsilateral activation in the motor cortex and absolute laterality quotient (Abs. LQ) during use of the dominant hand in RH and LH subjects (r = −0.601, P = 0.03). (b) Ranked correlation between the lateralization index (LI) in the motor cortex and absolute laterality quotient (Abs. LQ) during use of the dominant hand in RH and LH subjects (r = 0.694, P = 0.008). In a and b, the absolute value of the laterality quotient scores was taken, and the subjects were ranked according to the strength of handedness (irrespective of hand preference). The ranking on both axes is from lowest to highest.

Figure 3

Functional activation in the motor cortex during movement of the left hand in two LH subjects with different LQ. Subject with LQ of −47 has LI of 0.69, and subject with LQ of −100 has LI of 0.94. The lateralization index is greater with the larger LQ. In these images, the anterior portion of the brain is on the top, and the right hemisphere is on the right.