Does Stimulation of Plantar Mechanoreceptors Alter Visual Spatial Localization?

Objectives: Few studies have previously shown an interaction between feet and gaze in postural control. The aim of the current study is to more specifically examine how vertical heterophoria (VH) varies with stimulation of the mechanoreceptors in the foot sole. A new method is proposed for fast assessment of minimum VH, especially when values are less than 1/2 diopter (pD). Methods: The Maddox rod test was used to assess VH with a new device made of a small point source of light located in the center of a circular box that enables the measurement of minimum VH. The VH assessment was based on a group of 95 adults in upright posture on different subtle plantar stimulations (with and without pins and on foam). Results: Almost half of the subjects exhibited minimum heterophoria with vertical deviations less than 1 pD, most of them (96%) with a VH ≤ 1/2 pD (small heterophoria). For the latter, a subtle plantar foot change induced by pins (1 mm high) or foam (4 mm high) can have different effects on ortho- and heterophoria. Orthophoric subjects became heterophoric with foam and pins, while heterophoric subjects became orthophoric with foam and pins, which can increase or decrease their phorias depending on the foam or pins and the type of HV. Using the new circular box makes it fast and easy to highlight even the smallest of heterophorias. We suggest that clinicians use this device to assess the effects of postural interventions, even though it has not yet proven its validity. The findings of the current study highlight the interrelation between gaze and foot systems, while the importance of the plantar mechanoreceptors is demonstrated.