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The Pulfrich effect (named after Carl Pulfrich) is a well described visual stereoillusion observed when a swinging pendulum bob is viewed through a neutral density filter in front of one eye.1 Although the bob is moving in a frontal plane, the path seems elliptical. The effect arises from the fact that dimming a stimulus with a neutral density filter slows signal conduction velocity between the eye and the cortex. The visual cortex interprets this as a false depth or disparity cue as the object appears in a different location in the two eyes.2
Suppose an object happened to be moving from left to right, at a constant distance from the observer. Then, no matter what the (fixed) distance to which the eyes happened to be converged, any given instant, the dual images of the object, reversed by the optics of the eyes, would be moving right to left with some particular retinal disparity proper to the real distance between the object and observer.
Now, if a filter happened to be before the left eye (the Pulfrich effect), the response in the right eye effectively would be advanced somewhat. The advance would change the percept so that, for judgement of depth, the image in the right eye were advanced farther to the left than otherwise it would have been. This change in disparity would be the same as the change caused by removing the object to a distance farther away from the observer. Thus, by the geometry, placing a filter before the left eye would be expected to increase the binocularly perceived distance of an object moving left to right, and this is just what is observed. The magnitude of the change in disparity caused by a filter is approximately proportional to the speed of the moving object.
The Pulfrich effect can be demonstrated spontaneously (ie, without a neutral density filter) in patients with multiple sclerosis because of the delayed optic nerve conduction seen after unilateral optic neuritis in multiple sclerosis.3 The detrimental impact of the Pulfrich effect on sporting performance with the use of a neutral density filter has previously been described.4 The Pulfrich phenomenon is normally seen in patients with clinical signs of optic neuropathy, such as an afferent papillary defect or colour vision abnormalities. Here, we report the case of a patient who presented with the solitary complaint of reduced sporting performance without any other associated symptoms or clinical signs.
A 33‐year‐old man presented to the accident and emergency department with problems playing squash. Over a 1 month period, he had noticed difficulty hitting the squash ball. If the ball was hit to his right side he returned it with ease. However, if it was placed to his left side he missed it consistently. He had no other visual symptoms and had no significant past medical history.
On examination, his visual acuity was 6/4 in both eyes. He had no relative afferent pupil defect, no manifest or latent deviation. His ocular range of movement was normal with no evidence of internuclear ophthalmoplegia. He showed normal coordination and cerebellar function. Colour vision (100 Hue Test, total error score right eye=20, left eye=36) and visual fields were also normal. At this point a Pulfrich‐like phenomenon was suspected. A pattern electroretinogram (ERG) and pattern visual evoked potential (VEP) were therefore performed.
The pattern ERG was normal but the pattern VEP showed an abnormal P100 between 126–132.5 ms in the right eye with a normal P100 in the left eye (fig 11)) indicative of conduction delay within the right optic nerve. An MRI brain scan was then performed. FLAIR image sequence revealed two discrete lesions in the deep white matter on the right hemisphere and one lesion on the right anterior visual pathway. The findings were consistent with demyelination. Referral to the neurologist was arranged to decide on the need for further treatment.
In order to improve this patient's symptoms it was decided that reversal of the Pulfrich effect could be beneficial. VEP measurements were repeated using neutral density filters of incremental values placed in front of the normal eye. The VEP was brought to approximate the abnormal eye (fig 11).). With increasing neutral density filters, the VEP amplitude was almost unaltered but the latency progressively increased. This filter was then prescribed in a pair of squash goggles. Further review showed an improvement in his sporting performance.
There was a 41 ms delay between the VEP of this man's eyes. The delay would result in the ball being apparently in a different spatial location for each eye. At 20 m/s this would correspond to a spatial difference of 82 cm. For fast moving ball games it is therefore easy to appreciate why unilateral delays in optic nerve conduction can create significant hand–eye coordination problems which would not be apparent for stationary targets. The increased difficulty in hitting the ball on the left side with a right sided pathology may relate to the fact that on this side the ball would be misjudged such that it would be appear to be further away than it was and so would be played late. On the right side, conversely, it would appear closer and hence would be played earlier but with a greater chance of connecting with the ball.
In this patient the only symptoms related to reduced sporting ability. Other tests performed in the clinic were all normal. Specialised electrophysiological testing and MRI imaging were required to determine an organic basis for his unusual presenting complaint. This case highlights how a simple theoretical concept can be applied to a clinical situation and the need to consider Pulfrich‐like symptoms as a possible indicator of demyelinating disease even in the presence of otherwise normal clinical examination.
Competing interests: None.