The Pulfrich phenomenon

IntroductionWhat is the Pulfrich Phenomenon?

The Pulfrich Phenomenon is a three dimensional illusion where a moving object is seen to move in an anomalous pathway when viewed binocularly with a light attenuating filter placed in front of one eye. This creates the illusion that an object oscillating on a frontal plane appears to follow an elliptical pathway. The most widely accepted theory for this illusion is that reducing the brightness of a stimulus to the retina of one eye causes relatively slower excitation of the photoreceptors ( and therefore a delayed processing of the image relative to the other eye. (1) This creates a latency difference between the two eyes and as a result, information for two different scenes from each retina arrives at the visual cortex at the same time. Information from one eye will arrive at the cortex milliseconds later than the other. A geometric disparity is registered cortically from the input of the two eyes (7) and this leads to the incorrect perception of the movement and relative positions of objects observed by the individual. Many ocular or neurological disorders that affect the visual pathway can produce a delay in signal transmission to the brain, creating a geometric disparity. The illusions created by this geometric disparity produce troublesome symptoms for individuals with a condition and is why many patients experience problems in everyday life as a result of the phenomenon.(5)

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Who first discovered the Phenomenon?

Carl Pulfrich, a physicist with the Zeiss Company, described the phenomenon in 1922. He was not able to observe the effect himself however due to a childhood injury, which left him blind in one eye by 1905. The phenomenon was first noted earlier however by astronomers using stereo-comparators to search for planets.(3)

Demonstrating the phenomenon

The Pulfrich phenomenon can be observed in a normal individual by binocularly viewing a swinging pendulum bob whilst holding a neutral density filter in front of one eye. The pendulum bob or target used to elicit the phenomenon may take the form of a mechanical or computerized target. The neutral density filter lowers the retinal illumination of the eye it covers, and this creates the difference in signal transmission time between the two eyes. This delay created by a reduction in retinal illumination is supported by several psychophysical and electro-physical investigations.(1)((8,9)) The geometric disparity created by the visual latency difference between the eyes is interpreted by the brain as depth by the binocular disparity detectors.(2) This is why the pendulum bob is perceived to oscillate in an elliptical motion instead of moving along a frontal plane. The perceived direction of the pendulum’s swing is dependent on which eye the filter is covering; the pendulum appears to rotate clockwise when the filter is placed in front of the left eye and anti-clockwise when it is in front of the right eye.

Provoked and Spontaneous Pulfrich Phenomenon’s

When the Pulfrich phenomenon is induced in a normal individual i.e. using neutral density filters, the phenomenon is sometimes called the provoked Pulfrich effect. Since a delay can occur in individuals with ocular or neurological disorders that affect the visual pathway, the Pulfrich effect can occur spontaneously. Such diseases which may result in a spontaneous Pulfrich effect include optic neuritis, an inflammation of the optic nerve which can cause swelling and destruction of the myelin sheath covering the optic nerve. Since this myelin sheath helps conduct nerve impulses along its axons faster than non-myelinated axons, its destruction will lead to a slower transmission to the occipital cortex, where the information is processed. Optic neuropathies with bilaterally asymmetrical demyelination are thought to be the most common causes of the Pulfrich Phenomenon and the visual symptoms it is associated with. (website) Systemic conditions such as Multiple Sclerosis, a demyelinating disease were the fatty myelin sheaths around axons are damaged (Miller DH, Leary SM (October 2007). “Primary-progressive multiple sclerosis”), can lead to many individuals experiencing the Pulfrich Phenomenon. The Pulfrich Phenomenon has also been reported in many other diseases such as unilateral cataract or asymmetrical bilateral cataract. A cataract may reduce retinal illumination or produce retinal blur thus leading to a visual latency in the visual pathway of the affected eye causing the phenomenon in the individual. Anisometropic amblyopia, strabismus, corneal opacity, anisocoria, unilateral mydriasis, Central serous retinopathy, and age-related macular degeneration have all been accountable to produce the Pulfrich effect to some extent in an individual. Retinal blur has also been thought to be cause of the effect, most noticeable in smaller targets. Sokol and Moskowitz (cat(13)) showed that a 12 ms increase in the visual latency resulted from a 3 dioptre refractive error.()

A spontaneous Pulfrich phenomenon is not uncommon and has been found quite frequently in studies with patients with ocular and neurological disorders. It was found in 24 out of 93 patients with recovered optic neuritis, 16 out of 29 patients with uncomplicated unilateral cataract and three out of 70 patients with various strabismus problems.(3)

Factors affecting the magnitude of the Phenomenon seen

The magnitude of the Pulfrich Phenomenon seen depends on a number of factors with retinal illumination being one of the most significant. The magnitude of Pulfrich effect seen i.e. the depth of the elliptical trajectory, is larger for increasing disparity of retinal illumination between the two eyes. This may occur in a patient with a spontaneous Pulfrich phenomenon, for example when one eye has a much denser cataract than the fellow eye. It may also occur in cases of uniocular mydriasis when anisocoria could be induced. (2) The eye with the larger pupil receives increased retinal illumination than the other eye with a smaller pupil and therefore the phenomenon may be induced. This may also occur when the Pulfrich phenomenon is provoked, i.e. when a neutral density filter covering the eye is increased in depth. Lit ((2)12) has shown that the magnitude of the Pulfrich effect seen, that is the size of the ellipse, is proportional to the density of the filter used.

The distance from which a pendulum or similar target is viewed, also has an effect on the magnitude of the Pulfrich phenomenon seen. An increase in the magnitude of the phenomenon can be observed as the distance between the observer and target increases. Target velocity also contributes to the magnitude of the Pulfrich phenomenon. As the target velocity increases, so does the magnitude of the Pulfrich effect seen by the observer, leading to much more noticeable illusions and increased visual symptoms.

Symptoms experienced due to the Phenomenon

The symptoms expressed by a patient with the Pulfrich phenomenon to a general practitioner or optometrist can be quite strange. Objects appear to move in incorrect pathways when either the observer or the target is in motion.(3) Objects moving up and down however will not demonstrate this effect or lead to symptoms as a result of misjudged distances. These symptoms experienced lead to difficulties in everyday tasks. Tasks such as driving and various ball game activities like football, which require good perception of moving objects, have been found to be more difficult for patients who have the Pulfrich phenomenon. Even the simplest of tasks such as navigating through doorways or pouring a drink have been found to produce an increased level of difficulty for such patients.

Several studies have revealed that visual impairment leads to increased car accidents and difficulty driving. Visual perception has a key role in the safe operation of a vehicle, helping prevent driving errors and therefore prevent accidents and fatalities. In order to drive safely, a driver must be able to act and react to the changing environment around him/her. This requires the ability to correctly estimate the relative positions of moving and non-moving objects. Some Patients with a spontaneous Pulfrich phenomenon have complained of oncoming cars apparently swerving across the road towards them. This visual perception could lead to a serious accident and therefore must be treated to help prevent any casualties. Individuals who were later diagnosed with pathologies thought to be the cause of a spontaneous Pulfrich Phenomenon have reported the illusion that whilst driving, oncoming cars appear to swerve across the road towards them. Perception of this would lead to the driver making a sharp turn away to escape what he/she thinks is a certain accident. Forty years ago, a traffic accident was reported which occurred as a result of an induced Pulfrich phenomenon due to unilateral pupillary dilation. (reference) Since then, many investigations have been carried out to learn more about the effect a spontaneous Pulfrich phenomenon has on driving. It was found that many individuals had problems with judging distances when driving, especially when performing manoeuvres such as reverse parking into a garage.(references)

Why is it useful for optometrists to be able to diagnose and treat the Pulfrich phenomenon?

Due to troublesome visual symptoms experienced by patients known to have the phenomenon, it is in the interest of both the patient and Optometrist that the Pulfrich effect is diagnosed and treated as soon as possible. Various studies have shown that unilateral mydriasis was found to always produce the phenomenon on subjects.(reference) Because of this, it would be important for the Optometrist to warn the patient in advance of dilation, the visual symptoms and difficulty judging distance that can be expected, and also to warn, if possible, the patient should not drive home. It has also been suggested that, because patients with conditions such as cataract and optic neuropathy often demonstrate the Pulfrich phenomenon, an effective test for the Pulfrich phenomenon might be a useful addition to the tests available to Optometrists in primary care practice. The detection of this phenomenon may aid prompt diagnosis of any underlying pathology therefore the correct management and treatment could be initiated without delay. Recognition of this phenomenon could be particularly helpful in the early diagnosis of Retrobulbar neuritis, the most common form of optic neuritis in adults and frequently associated with multiple sclerosis (Jack J Kanski). In retrobulbar neuritis, the optic disc appearance is normal, at least initially, because the optic nerve head is not involved and so early detection is difficult using direct or indirect Ophthalmoscopy. Testing for the Pulfrich phenomenon is the only way to clinically determine if motion stereopsis is normal or not. Visual Evoked Potentials are not useful in analysing how motion stereopsis is affected, as their signals are travel via different visual pathways. VEP’s are also not available to primary care optometrists. Standard stereo-tests such as the Frisby stereotest commonly used in Optometric Practice also fail to detect any abnormalities in motion stereopsis. (1) Knowledge and understanding of the Pulfrich would help the Optometrist appreciate many of the strange symptoms experienced by an individual with the Pulfrich phenomenon and the many kinds of problems faced in daily life by i.e. sports games and traffic situations.

The symptoms experienced by a patient with the phenomenon can be quite easily ignored or dismissed if the clinician is unaware of the phenomenon. If optometrists had the appropriate equipment and knowledge to diagnose the Pulfrich phenomenon in everyday practice, they would also be able to treat the phenomenon and its sometimes-disabling effects using simple filtered lenses. This would help reduce or eliminate any of the problems experienced as a result. Patients, that find even the simplest of tasks such as pouring a drink or navigating through doorways, may be enabled to carry out these tasks with ease after treatment. Management and treatment of the phenomenon may also allow patients who retired from driving a vehicle due to the visual difficulties faced with the Pulfrich effect to feel confident in driving again. The unusual symptoms experienced when driving due to the Pulfrich phenomenon, especially the location of moving objects encountered in traffic, could be eliminated with the simple treatment available.

How is the Pulfrich phenomenon treated?

The symptoms experienced by patients with a spontaneous Pulfrich phenomenon can be reduced or eliminated by placing an appropriate neutral density filter or optic tint in front of the unaffected eye. (3) The lens has the effect of reducing retinal illumination of the unaffected eye and therefore creates a delay in signal transmission to the visual cortex, which equals the delay in the affected eye. To choosing a correct filter, the patient is shown a range of filters of varying densities and asked to decide which lens alleviates their symptoms and removes phenomenon. Once the suitable lens filter has been chosen, it may then be worn as spectacles or as a contact lens. Optic tints are preferred to neutral density filters as they are more readily available and cost effective for the patient.(3)The filters used to treat the phenomenon should remain the same without needing to be updated, so long as the underlying pathology remains stationary.()

Other Diagnostic methods

Doctors and other Healthcare professionals have found many different ways of testing for the phenomenon. The most often suggested method is to swing a pendulum in 5 different pathways in front of a patient.

A pendulum is swung in each of the pathways, from A-E. If the right eye is affected, the pathways C and B produce the most marked effect for the patient. The pendulum appears to veer towards the patient as it gets closer on these pathways.

Picture from Charles J. M Diaper, 1997 Surv Ophthalmol

Another technique used by some medical professionals in detecting the Pulfrich Phenomenon is when the examiner instructs the patient to quickly walk past the examiner whilst trying to brush his/her shoulder on either side. If the effect is present the patient’s left eye for example as shown in the diagram below, usually the affected eyes side will result in a large gap between the patient and the examiner. Errors in spatial localization result in the misjudgement of the distance between the patient and examiner. The patient can be seen to make a course correction, which can be seen as a rapid lurch away from the examiner. (6)

The incorrect distance judgement by the patient is a result of an illusion caused by the Pulfrich phenomenon leading the patient to believe the examiner is closer than what he/she actually is.(3) Note that this gap would only occur on the same side of the affected eye. For example if the patient’s right eye were affected on the picture left, a large gap would not be observed.

What my project is about?

In this project we are interested in investigating which tests prove most effective to an Optometrist in detecting the Pulfrich Phenomenon and also what the optimal parameters are for eliciting the Pulfrich effect in visually normal observers. With the ready availability of computerised stimuli for eliciting the Pulfrich effect, this may now be possible in practice. We would like to investigate which of the currently available online versions of the Pulfrich phenomenon stimuli are most successful and sensitive at eliciting the Pulfrich effect. I will also test a range of neutral density filters of varying depths to see which induces the phenomenon best in a group of visually normal adults aged 18-30. I will also be recording the distances at which the subjects were placed in relation to the screen throughout the test to see what effect this may have on detecting the phenomenon on how sensitive the different stimuli were at detecting the Pulfrich phenomenon in the subjects.

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