A not so ‘primitive’ reflex?

A study published this month in The Journal of Neuroscience questions the assumption that the Vestibular Ocular Reflex (VOR) is a primitive reflex, with its neural substrate solely located in brain stem areas we share with invertebrates.

The group, led by Qadeer Arshad at Imperial College, London, found an interaction between handedness and the VOR.  This is a reflex (normally attributed to low-level processes) which stabilizes images on the retina by inducing a compensatory eye movement, which balances head movements. Executed by the extraocular muscles, this has been assumed to involve ‘low-level circuits’ with interactions with the vestibular system.

This new study casts doubt on this assumption because differences due to handedness in the VOR suggest the involvement of higher cortical areas (i.e. those greater developed in higher vertebrates and phylogenically more recent). Handedness refers to the dominance of one hand (or both hands in ambidextrous individuals) in performing manual tasks and it is thought to be reflecting in different patterns of cortical lateralization (i.e. higher brain regions than are thought to be involved in the VOR).  In the study, the researchers measured the time taken for subjects’ eyes to adjust when seated in a seat rotating at a speed of one revolution per 4 seconds.  They were then shown bistable visual stimuli – optical illusions which are ambiguously interpretable and subjective interpretation of them changes between 2 images. Examples include the Necker cube, a simple wire diagram which flips projection direction every several seconds. This flipping is attributed to higher regions and decision-making processes.

Arshad et al. unexpectedly found an interaction between handedness and VOR with these stimuli. Subjects were unable to stabilize gaze-holding of these stimuli in a handedness dependent way. Left handed individuals had significant difficulty in following leftward rotation and right handed individuals with rightward rotation.