Spherical Lenses and Prisms Lead to Postural Instability in Both Dyslexic and Non Dyslexic Adolescents

Abstract

There is controversy as to whether dyslexic children present systematic postural deficiency. Clinicians use a combination of ophthalmic prisms and proprioceptive soles to improve postural performances. This study examines the effects of convergent prisms and spherical lenses on posture. Fourteen dyslexics (13–17 years-old) and 11 non dyslexics (13–16 years-old) participated in the study. Quiet stance posturography was performed with the TechnoConcept device while subjects fixated a target at eye-level from a distance of 1_m. Four conditions were run: normal viewing; viewing the target with spherical lenses of −1 diopter (ACCOM1) over each eye; viewing with −3 diopters over each eye (ACCOM3); viewing with a convergent prism of 8 diopters per eye. Relative to normal viewing, the −1 lenses increased the surface of body sway significantly whereas the −3 diopter lenses only resulted in a significant increase of antero-posterior body sway. Thus, adolescents would appear to cope more effectively with stronger conflicts rather than subtle ones. The prism condition resulted in a significant increase in both the surface and the antero-posterior body sway. Importantly, all of these effects were similar for the two groups. Wavelet analysis (time frequency domain) revealed high spectral power of antero-posterior sway for the prism condition in both groups. In the ACCOM3 condition, the spectral power of antero-posterior sway decreased for non dyslexics but increased for dyslexics suggesting that dyslexics encounter more difficulty with accommodation. The cancelling time for medium range frequency (believed to be controlled by the cerebellum), was shorter in dyslexics, suggesting fewer instances of optimal control. We conclude that dyslexics achieve similar postural performances albeit less efficiently. Prisms and lenses destabilize posture for all teenagers. Thus, contrary to adults, adolescents do not seem to use efferent, proprioceptive ocular motor signals to improve their posture, at least not immediately when confronted to convergence accommodation conflict.

Temporal Co-Variation between Eye Lens Accommodation and Trapezius Muscle Activity during a Dynamic Near-Far Visual Task

“…In line with the above suggestion, and since the visual system has little tolerance to errors in convergence [ 29 ], a relevant question would be if incongruence affects not only instability [ 28 ], but also may increase muscular activity. In our study, the negative trial lenses put conflicting demands on the visual system during the Near-Far task.…”

“…One suggestion arising from those results was that oculomotor signals and gaze related activity in the neck muscles are involved in postural stability [ 27 ]. A more recent study from the same group investigated the effect of spherical and prisms lenses’ on postural stability [ 28 ]. They found that prism lenses induced the highest degree of body sway (destabilisation), but interestingly, they also found increased body sway with spherical minus lenses, and the body sway was most pronounced with the -1 D lens, compared with the -3 D lens.…”

“…Both the prism lenses and spherical minus lenses created a conflict (incongruence) between convergence and accommodation, and it appeared that the subjects were able to handle the conflict more effectively with the stronger conflict (-3 D lenses) than with a weaker one (-1 D lenses). The authors suggested that accommodation and accommodative vergence induced by lenses could modify activity in the neck muscles involved in postural control [ 28 ].…”

Visual, musculoskeletal and balance symptoms in individuals with visual impairment

“…Uncorrected or improperly corrected refractive errors have been identified as risk factors for visual, musculoskeletal and balance symptoms in people with normal vision who perform prolonged near activities during leisure or at work . Additionally, the use of optical magnification has been observed to affect neck and head posture, which may increase visual strain during continuous near work and result in neck or scapular area symptoms …”

Dyslexia and the Integration of Sensory Cues into Motor Action

“…Several studies have showed that dyslexic children display different eye behavior (Kapoula & Bucci, 2007;Kapoula, Gaertner, & Matheron, 2012) while performing, for example, postural tasks. Employing the light touch paradigm to examine the use of sensory cues to postural control in dyslexic children (Viana et al, 2013) also allowed us to observe that dyslexic children, in using cues from the fingertip touching the bar, need to applied a slight higher amount of force than non-dyslexic children (Viana et al, 2013).…”