The traditional approach to human lateral preference considers handedness the dominant form of laterality that determines the sidedness of a person’s foot, eye and ear preference. This explanation makes sense for foot preference since, like handedness, the movements of the right and left foot (and hand) are controlled by brain centers in the hemisphere opposite to the limb. The same side of the brain controls both hand and foot actions…the left hemisphere guides the right foot and hand and the right hemisphere does the same for the left foot and hand. One of my previous blog posts contained data confirming that a person’s preferred hand and foot often are aligned on the same side of the body. This is the case for both left- and right-handers.¹
The neural wiring from eye to brain is more complex. Optic nerve fibers from one eye travel to both the right and the left hemispheres. This diagram illustrates the circuitry that carries visual information from the eyes to the brain. Nerve fibers from the half of the eye closest to the nose cross to the visual centers in the opposite hemisphere while fibers from the other half, closest to the outside of the head, travel to visual centers on the same side of the head as the originating eye. For example, the optic nerve fibers from the nasal side of the right eye cross to the visual centers in the left hemisphere. The remaining outside fibers connect to visual centers in the right hemisphere. The area under the brain where the fibers cross is called the optic chiasm after the Greek letter χ or X. The diagram shows the X configuration of the optic nerve fiber crossing point.
The neural anatomy of eye-to-brain connections does not necessarily predict the alignment of the preferred hand and the preferred eye on the same side of the body. However, a theory proposed in the early 20th century offered a behavioral explanation for why the two preferences should be on the same side. This theory argued that eye dominance is critical for overcoming double vision when performing eye-hand coordinated movements.² For this reason, eye preference is the dominant form of lateral preference, not handedness. The side of the preferred eye determines the side of the preferred hand which then enables accurate eye-hand coordination.
The data on eye-hand side congruence presents a slightly different pattern than the one proposed by this theory. As predicted, the majority of right-handers (66%) and left-handers (57%) display side congruent (right/right or left/left) eye-hand preference. However, there are substantial numbers of people with crossed preference where the preferred hand and the preferred eye are on opposite sides of the body. Left-eye preference occurs for 34% of right-handers while 43% of left-handers show right-eye preference.
It is not unusual to find people who write and throw with opposite hands. There are left-handed writers who are right-handed throwers and the reverse occurs for right-handed writers. If side congruence of eye and hand is important for coordinated aiming movements, then the preferred eye should align with the throwing rather than the writing hand among those who show different writing and throwing preferences. I conducted a study with colleagues to explore this idea. We found 47% of those who threw with the right hand and wrote with the left were left-eyed while 55% of individuals who threw with the left hand and wrote with the right were left-eyed. The higher rate of left-eyedness among the left throwing hand preference group supports a possible relationship between coordinated aiming movements and side congruence of eye and hand.
Use this questionnaire to assess your side congruence for writing/drawing and throwing as well as your side congruence for hand and eye preference (Lateral preference questionnaire). If you write/draw and throw with opposite hands, does your eye preference side with your throwing rather than your writing/drawing hand?
¹See my previous blog post “The game is afoot” March 31, 2017.
²See my previous blog post “A tale of two eyes”, April 12, 2017, for the background information on the role of eye preference in eliminating double vision.