Three-dimensional eye position during static roll and pitch in humans

Area of Science:

• Vestibular Neuroscience
• Oculomotor Control
• Human Physiology

Background:

• The otolith organs are crucial for sensing gravity and linear acceleration.
• Understanding the vestibulo-ocular reflex (VOR) in humans is vital for diagnosing balance disorders.
• Previous studies in monkeys showed gravity influences eye torsion, but human data is limited.

Purpose of the Study:

• To investigate the influence of static head position on three-dimensional (3D) eye position in humans.
• To quantify the torsional eye movements in response to roll and pitch head tilts.
• To compare human responses to previously observed responses in monkeys.

Main Methods:

• Measured 3D eye position using scleral coils in human subjects.
• Subjects were positioned in various head/body tilts (roll and pitch) for 35 seconds.
• Employed methods to minimize torsion artifacts from eyelids and contact lenses.
• Analyzed 3D eye positions using planar fits (Listing's plane) to assess torsional changes.

Main Results:

• Human eyes counterrolled during roll tilts, independent of horizontal and vertical eye position, with a maximum torsion of 4.9 degrees.
• Eye counterroll gain decreased by 50% from upright to ear-down positions, indicating non-proportionality to otolith shear force.
• Pitching forward caused minor torsional changes, dependent on gaze direction, and less pronounced than in monkeys.
• Pitching backward resulted in minimal torsional changes, contrasting with monkey responses.

Conclusions:

• Humans exhibit gravity-dependent eye torsion (counterroll) during roll head tilts, similar to monkeys.
• The human torsional response to gravity is not strictly proportional to otolith stimulation and shows reduced gain at greater roll angles.
• Human torsional responses during pitch are less pronounced and differ from monkey responses, particularly when pitching backward.