Head posture and deprivation of visual stimuli.

INTRODUCTION Natural head position (NHP) has been defined as the cranial position that is attained when individuals stand with their visual axes in the true horizontal plane. The visual axis is considered to be a key component in establishing, and maintaining NHP so that, in the absence of visual stimuli, gravitation and muscular proprioception chiefly control head position. SUBJECTS AND METHODS In this study, the natural head positions of a group of normal sighted subjects were measured in a well-lighted room. The findings were compared to similar observations made in the same subjects when they were placed in a completely dark environment. Measurements of cranio-cervical angulation were made on adults (N = 30, males = 17, females = 13) by means of an electronic inclinometer. Sitting Head Position (SIHP), Standing Head Position (STHP), and Orthoposition (OP) were determined for each subject in a well-lighted room, and subsequently, in a completely dark room. RESULTS The data indicate that there were statistically significant differences between Standing Head Position, and Orthoposition, measured in the light and in the dark. There were no statistically significant differences between Sitting Head Position measured in the light and in the dark. CONCLUSION The results indicate that craniocervical angulation is more extended in the dark than it is in the light. This finding suggests that head position responds to visual stimuli, and that when this sensory input is not present, there is a tendency for subjects to extend their heads.