Most sources discussing the development of optokinetic nystagmus (OKN) address the well-recognized early nasotemporal asymmetry in physiologically induced nystagmus that tends to persist in, and may even be a driving force for the development of infantile esotropia. My purpose here is not to discuss strabismus, but a developmental pattern I’ve noticed with OKN in children who struggle with reading readiness skills from a visual standpoint.
fMRI studies have demonstrated small field OKN responses, as elicited clinically with the pediatric OKN drum pictured above, reflect activity from both cortical and subcortical structures. The cortical structures involved are the occipito-temporal cortex, posterior parietal cortex, precentral, and posterior median frontal gyrus, the anterior and the posterior insula, the prefrontal cortex, and the medial part of the superior frontal gyrus. Subcortical structures shown to be activated during OKN are the caudate nucleus, the putamen, the globus pallidus, and the paramedian thalamus.
When probing OKN clinically in this context, I am looking for yoking and symmetry of the two eyes in following the rotation of the drum clockwise (CW), counter-clockwise (CCW), and switching midline crossing reflexively. The routine I follow in order to engage attention is to have the child point to an animal on the top row (for example, the Lion) and then follow it on the merry-go-round as I rotate it CW, and touch it when it’s in front of them again. On the second row I’ll use the giraffe and rotate CCW. On the bottom row I’ll use the tiger, and without telling the child I’ll switch direction from CW to CCW without telling them in advance, watching that they immediately shift direction of their OKN pattern reflexively.
But what I want to share with you is something that I’ve observed, that many young children with reading readiness issues appear to do, and is evidenced in the video below. It’s not that the child has any difficulty with the basic reflex tracking patterns noted above but that the eyes will flick upward while tracking laterally, interspersed with the horizontal OKN direction.
To date I can only share this with you as a trend that I’ve observed. I’ve seen nothing about it in the literature, and haven’t done a study on it, but these interspersed vertical drifts tend to decrease as the child’s visual scanning patterns mature. Again, it is not attentional in nature, since these children are always accurate in touch the animal of interest when it appears again in front of them as directed. I believe it reflects difficulty in keeping the horizontal and vertical scanpaths distinct, which is a developmental issue. Whether this is reflected in the anatomical pathways noted above is an open question.