The Pulfrich phenomenon is a neuro-ophthalmic phenomenon in which a difference in signal transmission time from the two eyes to the visual cortex causes a moving object in a single plane to be perceived as having depth in three dimensions.
In 1922, stereoscopy expert Carl Pulfrich first described it and named it the “stereo effect.” It later became widely known as the “Pulfrich effect” (Lanska et al., 2015) [1].
Classically reported in demyelinating optic neuritis (typically associated with multiple sclerosis), but also observed in other eye diseases such as unilateral cataract. It has also been applied to media that induce 3D visual effects from 2D images, such as 3D glasses.
QIn what situations is the Pulfrich phenomenon often noticed?
A
It is easily noticed when tracking moving objects, such as in sports or driving. In a classic demonstration, a pendulum swinging left and right appears to move in a circular path. If there is discomfort in perceiving moving objects in daily life, it can be a clue to suspect this phenomenon.
Stereoscopic misperception of moving objects: An object moving in a single plane is perceived as having depth. Typically, it appears as elliptical or circular motion.
Pendular circular motion: A pendulum swinging left and right appears to trace a circle. This is a classic subjective symptom of the Pulfrich phenomenon.
Interference with daily life: It can cause difficulties in situations requiring accurate tracking of moving objects, such as sports or driving.
Clinical Findings (Findings Confirmed by a Physician)
Pendulum test: A pendulum is swung in a single plane to check if it is perceived as circular motion. This is the most classic clinical test.
ND filter test: An ND filter is placed over the unaffected (or less affected) eye to check if the Pulfrich effect is neutralized. It is also used for quantitative assessment. Reducing retinal illuminance in one eye to 1/10 causes a signal delay of approximately 15 milliseconds.
Visual evoked potential (VEP): Visual stimuli are presented to each eye, and evoked potentials in the visual cortex are measured. By quantifying the difference in conduction velocity between the two eyes, the function of the optic nerve pathway can be objectively evaluated. Cases have been reported where abnormal motion perception during sports led to VEP revealing prolonged P100 latency in the right optic nerve, confirming demyelinating changes (O’Doherty et al., 2007) [5].
Mechanism: Autoimmune inflammatory attack damages oligodendrocytes that myelinate the optic nerve. The main role of myelin is to increase the speed of electrical conduction along axons, and unilateral demyelination creates a difference in conduction velocity between the two eyes.
Unilateral Cataract
Causative disease: Unilateral cataract (especially when the contralateral eye is pseudophakic).
Mechanism: Lens opacity reduces retinal illuminance in the affected eye, causing a delay in signal transmission. In a study of 29 patients with unilateral cataract and contralateral pseudophakia, the Pulfrich effect was confirmed using a computer-generated pendulum and was reported to disappear after cataract surgery (Scotcher et al., 1997) [3].
In healthy individuals, the Pulfrich phenomenon can be artificially reproduced by placing an ND filter over the non-affected eye. Reducing retinal illuminance in one eye to 1/10 causes a delay of approximately 15 milliseconds.
QAre there diseases other than multiple sclerosis that cause the Pulfrich phenomenon?
A
It has also been reported in unilateral cataract. Any disease that causes unilateral afferent conduction delay can be a cause. Identifying the underlying disease is important, as treatment strategies differ greatly depending on whether it is a demyelinating disease or cataract.
Diagnosis of the Pulfrich phenomenon involves a combination of clinical and electrophysiological examinations. Identifying the underlying disease is essential for determining the treatment strategy.
The characteristics of the main examination methods are shown below.
Examination method
Main use
Pendulum test
Confirms stereoscopic misperception of motion
ND filter test
Neutralization and quantitative evaluation of effect
Quantification of interocular conduction time difference
Pendulum test: The patient observes a pendulum swinging in a single plane and is asked whether it is perceived as an elliptical or circular motion. This is the most classic and simple clinical test.
ND filter test: An ND filter is placed over the unaffected or less affected eye to see if the Pulfrich effect disappears or reduces. By changing the filter density, quantitative evaluation is also possible.
Visual evoked potential (VEP): Evoked potentials are measured after flash or pattern stimulation to each eye. This allows objective and quantitative assessment of conduction velocity in the optic pathway, and the interocular latency difference can be expressed numerically.
In differential diagnosis, identifying the underlying disease is important. If demyelinating disease is suspected, neuroimaging such as MRI is performed; if unilateral cataract is suspected, slit-lamp microscopy is performed.
When the underlying disease is irreversible (e.g., demyelination after optic neuritis), an ND filter is placed on the unaffected or less affected eye. This intentionally delays signal transmission in the less affected eye to match the slower conduction in the more affected eye, correcting the interocular time difference (Farr et al., 2018) [2].
Worn as a monocular contact lens or tinted glasses.
Long-term studies have reported sustained effects over several years (Heron et al., 2007; cases up to 20 years) [4].
QHow long does an ND filter need to be used?
A
If the underlying disease is irreversible, long-term use is required. Long-term studies have reported sustained effects over several years. If the disease is progressive, filter density may need readjustment, so regular visits are necessary.
In normal binocular vision, retinal signals from both eyes are transmitted to the visual cortex at the same speed, and moving objects are interpreted as being on a single plane.
In the Pulfrich phenomenon, a conduction delay in one visual pathway causes a temporal mismatch between the retinal images of a moving object in the two eyes. During processing in the visual cortex, this time difference is misinterpreted as a spatial depth difference, causing two-dimensional motion to be perceived as three-dimensional.
The main mechanisms involved in the onset are as follows.
Reduced conduction velocity due to demyelination: In multiple sclerosis, autoimmune inflammatory attacks on the central nervous system damage oligodendrocytes. Myelin sheaths are structures that increase the speed of electrical conduction along axons, and demyelination in one optic nerve creates a difference in conduction velocity between the two eyes.
Signal delay due to reduced retinal illuminance: In unilateral cataract, clouding of the lens reduces the amount of light reaching the retina of the affected eye. Reduced retinal illuminance causes a delay in signal latency to the visual cortex, leading to the Pulfrich phenomenon.
Reproduction of the phenomenon with an ND filter: Even in healthy individuals, wearing an ND filter over the unaffected eye produces a similar reduction in illuminance, artificially reproducing the Pulfrich phenomenon. The ND filter therapy utilizes this principle in reverse.
Lanska DJ, Lanska JM, Remler BF. Description and clinical application of the Pulfrich effect. Neurology. 2015;85(9):821-825. PMID: 26033336
Farr J, McGarva E, Nij Bijvank J, et al. The Pulfrich Phenomenon: Practical Implications of the Assessment of Cases and Effectiveness of Treatment. Neuroophthalmology. 2018;42(6):349-355. PMID: 30524488
Heron G, Thompson KJ, Dutton GN. The symptomatic Pulfrich phenomenon can be successfully managed with a coloured lens in front of the good eye—a long-term follow-up study. Eye (Lond). 2007;21(12):1469-1472. PMID: 16763654
O’Doherty M, Flitcroft DI. An unusual presentation of optic neuritis and the Pulfrich phenomenon. J Neurol Neurosurg Psychiatry. 2007;78(8):906-907. PMID: 17635984
Copy the article text and paste it into your preferred AI assistant.
Article copied to clipboard
Open an AI assistant below and paste the copied text into the chat box.
