The Pulfrich phenomenon is an illusion in which an object moving in a plane seems to travel in an ellipse, because the two eyes do not process the image at the same speed.
In a patient with a damaged optic nerve it appears spontaneously.
It explains complaints such as trouble judging the path of a ball or a passing car (see optic neuritis).
The Origin of the Pulfrich Phenomenon
The visual system uses small differences in the timing and position of signals from the two eyes to compute depth.
If one eye sends its signal later than the other, the brain reads the time lag as a disparity in position.
A pendulum swinging from side to side appears to move toward and away from the viewer.
Carl Pulfrich described the effect in 1922, using a dark filter in front of one eye, which delays the signal because dim light is processed more slowly (see the references).
Pulfrich Phenomenon: What Patients Describe
- Difficulty catching or hitting a ball
- Misjudging the distance of cars or people moving across the field
- Feeling that moving objects are also curved or at the wrong depth
- Problems that are worse in dim light, when delays increase
Many patients with recovered optic neuritis have normal acuity but notice this symptom.
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From Choroida — the team behind this siteConditions That Cause It
- Demyelinating optic neuritis, with slowed conduction in the affected nerve (see the references)
- Multiple sclerosis, even without a history of an attack
- Compressive and other optic neuropathies
- Glaucoma and macular disease, in some patients
- Unilateral cataract and amblyopia, from reduced retinal illumination or processing delay
Pulfrich Phenomenon: Bedside Test
Hang a pendulum, such as a ring on a string, or use a small object swung by hand, in a plane from side to side.
Ask the patient to look at it with both eyes open and describe the path, and then place a neutral density filter over one eye.
A patient with a delayed signal in one eye will also see an elliptical path, and the direction of the ellipse shows the affected eye.
Quantifying the Delay
Add filters of increasing density over the faster eye until the path looks straight, which neutralizes the effect, and the strength of the filter gives an estimate of the delay (see the references).
A rough rule is that each 0.3 log unit of filter gives a delay of about 15 milliseconds.
Computer-based versions give a more precise measure.
Pulfrich Phenomenon: Value and Limits
The test is simple, cheap, and sensitive to subtle conduction delay, and it fits with abnormal visual evoked potentials (see visual evoked potentials).
It requires reasonable vision in both eyes and depends on the patient’s judgment, so it is not a diagnostic test.
The results may vary with lighting.
Practical Advice for Patients
Explain that the symptom arises from a timing difference between the eyes, that it is common after optic neuritis, and that it is not a sign of a new attack.
Suggest using extra caution when driving, especially in dim light, and when judging the path of a ball.
Special tinted filters over one eye can neutralize the effect in some patients, and they should be tried before any commitment.
Pulfrich Phenomenon: An Example Case
A 29-year-old woman had optic neuritis in her right eye a year ago, and acuity has returned to 20/20.
She complains that tennis balls “curve” and that she misjudges cars crossing the road, especially at dusk.
The pendulum test shows an elliptical path that is neutralized with a 0.6 log unit filter over the left eye, and visual evoked potentials show a delayed P100 in the right eye, matching the history of optic neuritis.
Practical Steps in the Clinic
- Ask about difficulty with moving objects after any optic neuritis
- Do the test in a dim room with a simple pendulum
- Use filters of increasing strength to measure the delay
- Compare with visual evoked potentials, if available
- Advise safe practices for driving and sports
Counselling
Explain that the symptom does not mean that the nerve is getting worse, and that the brain can adapt over time.
Offer tinted lenses in a trial for patients with severe problems at work or in sports.
Key Points to Remember
- A conduction delay in one eye creates a false depth for moving objects
- The effect is worse in dim light
- It can persist after full recovery of acuity
- It is a clue to a past or active optic nerve problem
Limits of the Test
Results depend on the cooperation of the patient.
The test has no standard scoring, so it is a supplement and not a replacement for visual evoked potentials.
For the latest studies, see the PubMed results for Pulfrich phenomenon.
Document what you see
Two smartphone imaging tools built for everyday clinic use — one for the slit lamp, one for the fundus.
From Choroida — the team behind this siteReferences
- Pulfrich C. Die Stereoskopie im Dienste der isochromen und heterochromen Photometrie. Naturwissenschaften. 1922;10:553-564.
- Rushton D. Use of the Pulfrich pendulum for detecting abnormal delay in the visual pathway in multiple sclerosis. Brain. 1975;98:283-296.
- Standing LG, Dodwell PC, Lang D. Dark adaptation and the Pulfrich effect. Percept Psychophys. 1968;4:118-120.
- American Academy of Ophthalmology. Basic and Clinical Science Course, Section 5: Neuro-Ophthalmology. AAO; current edition.
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