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Upward Lens Movement Not Explained by Eye Movement
Proper Controls Are Required for Accommodative Experiments
Author Response: Proper Controls Are Required for Accommodative Experiments
Movement of the Lens during Accomodation
Author Response: Movement of the Lens during Accommodation
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Robert H. Marmer
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marmermd{at}aol.com Robert H. Marmer
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Glasser et al.,1 in response to my question,2 attribute their observation of upward movement of the young rhesus lens to "A slight upward eye movement" during zonular relaxation induced by electrically stimulated maximal accommodation. As referenced by the authors,3 Grossman4 measured the change in equatorial lens diameter of an aniridic patient during accommodation. He also observed an upward movement of the lens during maximum accommodation (see Grossman's Fig. 4B). There are three possibilities for the apparent upward movement of the lens as a result of eye movement during maximal accommodation when the zonules are relaxed:
As readily seen in the authors'3 Figure 1B, and Grossman's4 Figure 4B, there is a space between the ciliary body and the inferior edge of the lens during maximum accommodation. Therefore, their observations were a consequence of perspective distortion, or their observations demonstrate that equatorial zonular tension was increased during accommodation. Robert H. Marmer References 1. Glasser A, Wendt M, Ostrin L. Author response: movement of the lens during accommodation (letter). Invest Ophthalmol Vis Sci [serial online]. Available at
http://www.iovs.org/cgi/eletters/47/1/278#312. Accessed on September 22, 2006. |
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Ronald A. Schachar
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ron{at}2ras.com Ronald A. Schachar
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Using photorefractometry and goniovideography in four iridectomized eyes from two monkeys, Glasser et al.1 attempted to assess the change in central lenticular refraction and equatorial lens diameter in response to electrical stimulation of the Edinger Westphal nucleus (EW). Their observations occurred during the presence of spurious extraocular eye movements before and during EW stimulation. The authors state, "The analysis shown here demonstrates that measurements of lens diameter are unaffected by the extent of eye movements observed."1 The authors did not undertake the control studies required to support this statement. Such studies require the incorporation of non-varying positional references in their images. They must show that these control eyes, before and during random wandering eye movements, do not demonstrate any apparent change in lens diameter. Then, they must evaluate the non-random condition, which specifically obtains to EW induced stimulation of the extraocular eye movements. During EW stimulation, the eye responds in a non-random pattern, with linear and rotational movement defined by the character of the electrical stimulus. The authors need to provide referenced positional controls for the non-random condition, where extraocular movements are driven by the EW induced stimulus. They must prove that non-random extraocular eye movements are not responsible for their observed systematic changes in lens diameter. The photorefractive images of the same monkey eye before and during EW stimulation demonstrate the positional instability of the EW stimulated eye relative to the recording camera (Figs. 1A and 1B from the authors' paper). Superposition of the images from Figures 1A and 1B demonstrates that the meridional distance between the white crosses2,3 before EW stimulation is greater than the distance between these crosses during EW stimulation (Fig. 2).
The decrease in distance between these marks is comparable to the authors' observed decrease in lens equatorial diameter. Thus, the apparent change in lens equatorial diameter between these images can result from the optical minification associated with movement of the whole eye relative to the camera in response to EW stimulation. This observation is repeatable due to the non-random nature of the eye movements in response to the EW stimulation and does not demonstrate any actual change in the lens diameter. The goniovideographic images (Figs. 1C and 1D of the authors' paper, which are reproduced here in Figures 3B and 3B) reveal that the width of the upper left edge of the gonioscopic lens decreased more than 10% in response to EW stimulation.
This minification is due to the change in angular perspective resulting from movement of the eye in response to EW stimulation. Without proper controls to reference the position of the eye itself, induced distortion confound the authors' measurements. The authors reference other published reports of the lens equatorial diameter decreasing during in vivo accommodation. However, each of these examples, like that of the authors' present study, lacked positional references to control for perspective distortions.4-8 To determine what changes in the equatorial lens diameter occur during accommodation, high resolution imaging techniques incorporating eye tracking, triangulation, image registration, and proper controls are essential. We await studies that employ these basic requisites. Ronald A. Schachar References 1. Glasser A, Wendt M, Ostrin L. Accommodative changes in lens diameter in rhesus monkeys. Invest Ophthalmol Vis Sci. 2006;47:278-286. |
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Adrian Glasser
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aglasser{at}uh.edu Adrian Glasser
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We are writing to respond to the letter from Drs. Schachar and Kamangar regarding our recent study "Accommodative Changes in Lens Diameter in Rhesus Monkeys."1 Drs. Schachar and Kamangar appear to believe that we measured lens diameter from the photorefraction images; however we did not. Photorefraction was used to measure the accommodative refractive changes. The equatorial edge of the monkey lens is not visible in these photorefraction images, and we did not make, analyze or report lens diameter measurements from these images. The photorefraction images (Figs. 1A, 1B) are from a video camera at 0.3 m from the eye. The gonioscopy images (Figs. 1C, 1D), from which lens diameter measurements were made, are from a slit-lamp microscope in front of the eye. The magnification of the photorefraction images and the gonioscopy images are neither similar nor comparable. Their assumptions and assertions regarding lens diameter changes determined from these images are therefore incorrect. Drs. Schachar and Kamangar appear to believe that the width of the upper edge of the gonioscopic lens is visible in our images (Figs. 1B, 1D); however it is not. This is a view of the inside of the gonioscopy lens. It is not clear even to us what their arrows point to (perhaps internal reflections inside the lens), but it is absolutely not, as they assert, "the edge" of the gonioscopy lens. Drs. Schachar and Kamangar suggest that, based on their analysis, the edge of the gonioscopic lens decreases by 10%. In the paper we state that the "gonioscopy lens was clamped in front of the eye with holder" and therefore the gonioscopy lens does not move. Their assumptions and assertions regarding movements of the gonioscopy lens are therefore incorrect. Drs. Schachar and Kamangar assert that our analysis requires non-varying positional references. However, they themselves have attempted an analysis of our images without knowing what the structures are, without clearly understanding how the experiments were performed and without any positional references. The analysis we show (Fig. 4) clearly demonstrates that the eye movements that are occurring (which in some cases are a secondary consequence of EW stimulation and are larger than the eye movements that occur with EW stimulated accommodation) do not result in any change in lens diameter. The results of our study clearly show that lens diameter decreases during accommodation in all cases and by a similar magnitude regardless of the presence or absence of eye movements. Adrian Glasser Reference 1. Glasser A, Wendt M, Ostrin L. Accommodative changes in lens diameter in rhesus monkeys. Invest Ophthalmol Vis Sci. 2006;47:278-286. |
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Robert H. Marmer
Send letter to journal:
marmermd{at}aol.com Robert H. Marmer
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In their recent paper, Glasser et al.1 create accommodation via electrical stimulation of the Edinger-Westphal nucleus and then measure the changes in the equatorial rhesus monkey lens diameter. They demonstrate in Figures 1C and 1D that the lens diameter decreases and that the whole lens moves upward during 13.5 diopters of accommodation. Since it has been documented that during this amplitude of accommodation the rhesus monkey zonules relax to the point of folding,2 and since the monkey's head was vertically oriented, how do the authors explain the upward movement of the lens? Robert H. Marmer Atlanta, Georgia References 1. Glasser A, Wendt M, Ostrin L. Accommodative changes in lens diameter in rhesus monkeys. Invest Ophthalmol Vis Sci. 2006;47:278-286. |
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Adrian Glasser
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aglasser{at}uh.edu Adrian Glasser
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We are writing to respond to Dr. Marmer's letter regarding our recent study "Accommodative Changes in Lens Diameter in Rhesus Monkeys."1 It is not just the lens that is moving up, but the whole eye. As is stated in the legend to Figure 1, "A slight upward eye movement and a clear decrease in [lens] diameter can be seen." Adrian Glasser College of Optometry, University of Houston, Texas Reference 1. Glasser A, Wendt M, Ostrin L. Accommodative changes in lens diameter in rhesus monkeys. Invest Ophthalmol Vis Sci. 2006;47:278-286. |
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