Pulfrich testing is a method to detect the Pulfrich phenomenon/effect, a disorder involving the perception of movement in depth. It is described as a binocular visual illusion where a swinging pendulum follows an elliptical pathway with one eye covered using a light-decreasing filter. Usually, an individual perceives an object traveling along an illusory 3D path due to an illumination inconsistency in the eyes. The individual experiences an illusory clockwise rotation if the filter is on the left eye. On the converse, the experience is anticlockwise if the filter is put over the right eye. The phenomenon was first described by Carl Pulfrich, a German stereoscopy expert and physicist, and occurs in unilateral or asymmetric optic neuropathy cases.
The Pulfrich effect usually occurs in individuals with neurological or ocular conditions where the retinal illumination and visual pathway are affected. It results from a delay in signal processing from one eye leading to targets moving in variance to reality. Other causes include optic nerve inflammation caused by multiple sclerosis, nerve trauma, and vascular accidents. The Pulfrich effect can be induced when light is reduced using a pinhole or by darkening one eye using a tinted lens in front of only one eye.
The Pulfrich phenomenon affects vision where an individual has trouble with tasks that involve judgment of moving objects like driving, crossing the road, or playing some types of sports. The Pulfrich effect is treated using a tinted contact lens worn on the better eye or using uniocular tinted spectacles.
The Pulfrich test is designed to measure and graphically document the effect while considering the magnitude of the rotation perceived by the patient at various densities of the filter.
Before the Procedure
The eye doctor takes an ocular history and asks questions to identify the symptoms related to the Pulfrich effect. Such questions include whether the patient experienced difficulties in performing daily tasks like pouring fluids, handling objects, writing, watching 3D films, sports, using escalators, and so on.
The patient will be assessed for best-corrected visual acuity (BCVA), pupil reaction, visual field, and color vision. Other tests performed include optical coherence tomography, pattern visual evoked potential (VEP), and a pattern electroretinogram (ERG). The test should last between 10 to 45 minutes.
The patient wears an appropriate refractive error. One way to detect the Pulfrich effect's presence is to swing a weighted pendulum against a black-white pattern background or a 300 cm by 90 cm background picture and viewed from 3 meters. The pendulum can be a monkey face, plain black ball, etc. Those who report seeing a rotational or circular pendulum's movement during the patients' habitual visual state instead of a veridical linear movement are said to be Pulfrich-positive.
For corrective purposes, the Pulfrich-positive patient also undergoes further testing using filters to determine the best correction. The doctor shows the patient an assortment of neutral density (ND) filters in front of the eye with better vision. The eye doctor determines the most effective filter based on the density that offers a linear movement. The patient can report the least amount of rotational or circular motion, which is also acceptable. The patient is also requested to walk about with his/her trial spectacles to check if there is an impact on auto locomotion. The doctor will give a prescription for refractive error and dispense uniocular tinted spectacles in case of symptom reduction.
Another way to do a Pulfrich test is to use a personal computer strong enough to generate a pendulum stimulus. The computer should be installed with a programming technique capable of dynamically computing the target or blob position. The programming technique should erase an old blob and recreate a new blob. The doctor sets the appropriate illuminance and room lighting. The patient binocularly views the swinging pendulum through an ND filter placed over one eye with a clear filter over the other eye. The patient describes whether the pendulum rotates in a clockwise, neutral, or anticlockwise manner.
The swinging pen test (SPT) is another option where the doctor oscillates a pen by hand.|
Some patients may not perceive pendulum rotation regardless of how strong the density filters are. Others can perceive pendulum rotation, which may or may not be reversed.
Risks & Complications
The doctor needs to ask relevant questions before performing the Pulfrich test for the proper identification of the phenomenon. Besides, the assessor must identify appropriate pendulum brightness, which can affect the outcome. Research indicates that brightly lit, highly reflective objects are best for demonstrating the Pulfrich phenomenon. Care also ought to be exercised with the velocity of the stimulus. Symptoms may not be shown if the pendulum does not move at a great enough speed.