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Show Journal of Neuro- Ophthalmology 17( 4): 240- 242, 1997. © 1997 Lippincott- Raven Publishers, Philadelphia The Pulfrich Effect and its Relationship to Retinal Illumination Michael S. Vaphiades, D. o., and Eric Eggenberger, D. o. Summary: The Pulfrich stereo- illusion effect occurs in patients with monocular anterior visual system disease when viewing a pendulum. It also can be induced by placing a monocular neutral- density filter in a normal subject. We studied 10 normal subjects to determine if the Pulfrich effect could be induced by monocular illumination in addition to monocular neutral- density filtration. Eighteen of 20 eyes observed the Pulfrich effect with monocular illumination, but the rotation of the ellipse was opposite in direction to that of the neutral- density filter. Our data suggests that the Pulfrich effect results from intereye luminance disparity. Key Words: Pulfrich stereo- illusion- Monocular illumination- Intereye luminance disparity. The clinical concept of the Pulfrich effect is a binocular stereo- illusion that occurs when viewing a pendulum swinging perpendicular to the visual axis under certain conditions. It has been reported in macular disease ( 1) as well as in optic neuropathies ( 2,3) ( spontaneous Pulfrich effect), and has been postulated to depend on intereye disparity of retinal illumination ( 1). It also has been reported in normal subjects by placing a neutral- density filter ( NDF) over one eye ( provoked Pulfrich effect) ( 2, 4- 8). The pendulum is perceived as a counterclockwise ( CCW) rotation ellipse if the right eye is affected and clockwise ( CW) ellipse if the left eye is involved ( Fig. 1) ( 2,4,6- 9). A better understanding of this phenomenon is important not only from an academic standpoint, but also from a clinical perspective. Physicians need only to have a pen at their disposal ( the pendulum) to reproduce the Pulfrich effect at the bedside in patients with anterior visual system disease ( 3). Although the relative afferent pupillary defect remains the single most useful clinical test for optic neuropathy, the Pulfrich effect is a quick simple bedside test useful in confirming pathology in patients with subtle optic neuropathies, especially in those with nonreactive pupils. Manuscript received December 5, 1996; accepted May 15, 1997. From the Department of Ophthalmology, University of Arkansas for Medical Sciences, Little Rock, Arkansas ( M. S. V.), and the Neuro- Visual Unit, Michigan State University, East Lansing, Michigan ( E. E.). Address correspondence and reprint requests to Dr. Michael S. Vaphiades, Harvey & Bernice Jones Eye Institute, Department of Ophthalmology, University of Arkansas for Medical Sciences, 4301 West Markham, Mail Slot 523, Little Rock, AR 72205- 7199, U. S. A. Our objective was to determine if the Pulfrich effect could be produced by monocular illumination, as well as placement of a NDF over one eye, creating an intereye luminance disparity and thereby provide more insight into the nature of this phenomenon. METHODS We studied 24 eyes in 12 subjects ( eight women and four men) composed of 10 visually asymptomatic participants with unremarkable past medical histories, one subject with congenital dyschromatopsia, and another subject with a left Horner's syndrome from a previous carotid artery dissection ( 0.5 mm anisocoria in light). Each eye was assessed with and without a 0.6 log unit NDF when testing visual acuity using a standard Snellen chart, color vision using Ishihara pseudo- isochromatic plates, and a Stereo- Optical Co. three- dimensional vec-tograph of nine circles. The pendulum apparatus was a flat steel bar 100 cm in length and 2 cm in width. A 20/ 200 " E " was affixed to the bottom of the bar. The pendulum moved through 80 degrees of arc centered at 12 o'clock. The subjects stood 253 cm in front of the apparatus. At its lowest point, the pendulum swung 114 cm from the floor. The subjects observed the pendulum swinging at right angles to the viewing axis under five viewing conditions: monocular 0.6 log unit NDF in the right and left eye, monocular illumination in the right and left eye using a Welch Allen illuminator, and without illumination or NDF. The filter was held 15 mm in front of the appropriate eye by the subject. The illuminator was held 15 mm from the eye by the examiner, angled at 30 degrees from visual axis, and it followed the eye as it moved to maintain the brightest illumination through the pupil at all times as the eye tracked the pendulum. There were no obvious entopic phenomena noted by the patients. Additionally, the pupil in the left eye of three subjects ( 1,3, and 7) was dilated with one drop of phenylephrine hydrochloride 2.5% and tropicamide 1% before viewing the pendulum with and without monocular illumination. Two subjects ( 2 and 6) were given pilocarpine hydrochloride in the left eye before viewing the pendulum with and without monocular illumination. 240 PULFRICH EFFECT 241 FIG. 1. Subject with monocular NDF over the left eye with the Pulfrich effect perceived in a clockwise ellipse. RESULTS Of the 10 normal subjects ( Table 1), all had visual acuity with correction of 20/ 20 or better in both eyes unchanged by the 0.6 log unit NDF. Visual acuity was diminished one line to 20/ 20 in the right eye from 20/ 15 in the right eye in subject 6. All subjects perceived at least 12 of 14 color plates, except for subject 10, who had diminished color by 2.5 plates in the right eye and 3.5 plates in the left eye. In two subjects, the NDF diminished the score on stereoscopic plates. All normal subjects reported the pendulum moving in an appropriately directed ellipse with the NDF in place ( CW for the left eye filter, CCW for the right eye filter). With monocular illumination, 18 of 20 eyes demonstrated the ellipse move opposite to that created by the filter ( CW ellipse with monocular illumination in the right eye and CCW with monocular illumination in the left eye). Subject 10 did not report the Pulfrich effect with monocular illumination ( Table 1). All the subjects who noted the stereo-illusion effect with monocular illumination noted a smaller ellipse than when the NDF was used. The three subjects whose left pupil was pharmacologically dilated ( without illumination or NDF) noted an ellipse, but smaller than with a NDF or monocular illumination. With monocular illumination in the dilated eye, the subjects reported an ellipse ( CCW) similar in arc size to that seen with NDF but in the opposite direction. The two subjects receiving pilocarpine in the left eye reported a clockwise directed ellipse. A larger CW ellipse was reported with simultaneous illumination in the right eye, and no ellipse was noted with monocular illumination in the left eye. The subject with the Horner's could not appreciate the Pulfrich effect with monocular illumination in the affected eye, but did report the effect with monocular illumination in the contralateral eye. The subject with the congenital dyschromatopsia appreciated the Pulfrich effect in the expected direction with both filter and illumination. DISCUSSION In Germany in the 1920s, Carl Pulfrich designed stereoscopic instruments for various uses, including naval gunnery and astronomy. The ' ' Pulfrich phenomena'' is a term originated in 1920 by the astronomer Max Wolf using one of these instruments. Ironically, Pulfrich developed traumatic blindness in the left eye in the early TABLE 1. Subjects 1 2 3 4 5 6 7 8 9 10 11 Color 14 plates 14 OU 14 OU 12.5 OD 12 OS 14 OU 12.5 OU 14 OU 13.5 OD 14 OS 14 OU 13 OU 11.5 OD 10.5 OS 14 OU ( Homers OS) 12 ( Congenital 6 0U Visual OD 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 20 20/ 20 20/ 15 dyschromatopsia) acuity OS 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 Visual acuity with 0.6 filter OD 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 20 20/ 15 20/ 15 20/ 15 20/ 20 20/ 25 20/ 15 OS 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 15 20/ 20 20/ 15 Stereo of 9 9 9 9 9 6 9 9 9 9 9 9 9 Stereo 0.6 filter ODof 9 9 8 0 9 6 9 9 9 9 9 5 9 Pendulum STR STR STR STR STR STR STR STR STR STR STR STR Pendulum 0.6 filter OD OS ccw/ cw ccw/ cw ccw/ cw ccw/ cw ccw/ cw ccw/ cw ccw/ cw ccw/ cw ccw/ cw ccw/ cw ccw/ cw ccw/ cw Pendulum LT OD OS cw/ ccw cw/ ccw cw/ ccw cw/ ccw cw/ ccw cw/ ccw cw/ ccw cw/ ccw cw/ ccw STR/ STR CW/ STR CW/ CCW OD, right eye; OS, left eye, OU, both eyes; CCW, counterclockwise; CW, clockwise; STR, straight. J Neuro- Ophthalmol, Vol. 17, No. 4, 1997 242 M. S. VAPHIADES AND E. EGGENBERGER 1900s and could never appreciate the phenomena bearing his name ( 5). We studied 10 normal subjects, all of whom observed the Pulfrich stereo- illusion effect with a 0.6 log unit NDF placed in front of either eye. Nine of 10 also observed the Pulfrich effect with monocular illumination with the ellipse reported in the direction opposite that created by the monocular placement of an NDF. The Pulfrich effect also occurred after pharmacologic anisocoria. This effect was enhanced ( larger ellipse) by monocular illumination of the larger pupil ( enhancing intereye luminance difference) and nullified with illumination of the miotic eye after monocular pilocarpine use ( equalization in intereye luminance). When the pupil was pharmacologically dilated or constricted with monocular illumination of the larger pupil, a larger ellipse was noted. In eyes without pharmacologic intervention, a smaller ellipse was reported with monocular illumination than with NDF. This paradox may be explained by the miotic effect of monocular illumination creating a relatively smaller intereye luminance difference than NDF. However, when the pupil was pharmacologically dilated or constricted with monocular illumination of the larger pupil, a larger ellipse was noted. These data suggest that the Pulfrich effect relies on an intereye luminance disparity created by filtering or illuminating one eye. This physiologic mechanism probably lies in the latency difference of the two images perceived by the brain. This theory is supported by electrophysiologic studies. Pattern visual evoked potential ( P- VEP) data suggest that a decrease in retinal illumination increases P100 latency ( 1,10,11). Pattern VEP data also suggest that the spontaneous Pulfrich effect is influenced by ocular diseases other than optic neuropathies ( which probably influence the amount of light ultimately reaching the brain via the visual pathways) ( 1). Many factors come into play when viewing a moving pendulum. Contrast, color, edges, and stereopsis all influence cortical processing of visual information ( 12). The anatomy of the visual system of the macaque monkey is similar in many ways to that of humans ( 13). At least 32 distinct cortical areas are associated with processing visual information ( 13). The visual system therefore acts like a coding device with multichannel input, processing information from the retina via these parallel channels ( 10). The two major streams of information are transmitted by the parvocellular and magnocellular pathways, both of which originate within the retina ( 13). There are no separate pathways for visual activity, color, and luminance, but the parvocellular and magnocellular pathways are biased toward specific functions. The parvocellular pathway is predominately biased toward visual activity, spatial resolution, color, and fine stereopsis ( 13,14). The magnocellular pathway is more sensitive for luminance and motion perception ( 13,14). The Pulfrich effect and the pathways by which it is relayed to the primary visual cortex and visual association areas is subserved by portions of each major pathway. Our data suggests that pathways biased toward luminance ( magnocellular) exert a greater influence on the Pulfrich effect than those for color or visual acuity. It is well known that visual acuity, color, and retinal luminance are affected in patients with an optic neuropathy. Only intereye luminance disparity need be present to induce the Pulfrich effect. This can be accomplished by either reducing monocular illumination by the placement of a NDF or constricting one pupil, or increasing monocular illumination by dilating a pupil or by direct illumination of one eye. The effect is consistent in that the direction of the ellipse is the same in each instance ( CW if the right eye receives more illumination or if the left eye receives less illumination). Acknowledgment: This research was performed at Michigan State University with Institutional Review Board for Experiments on Humans approval. We thank Wendy Robinson for her assistance with this project. REFERENCES 1. Hofeldt AJ, Hofeldt GT, Weiss MJ. Pulfrich psychophysical stereo- illusion and visual evoked potentials in epiretinal membrane of the macula. J Neuro- ophthalmol 1996; 16: 36- 40. 2. Heron G, Dutton G, The Pulfrich phenomenon and its alleviation with a neutral density filter. Br J Ophthalmol 1989; 73: 1004- 8. 3. Mojon DS, Rosier KM, Oetliker H, Koerner F. 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