Talk:Accommodation (vertebrate eye)

Simplicity is better than confusion

I have taught this material to medical stduents for 30 years and I think this article will only confuse the lay person who most needs a simple, clear explanation of accommodation. Put simply, in humans, contraction of the ciliary muscle REDUCES tension on the margins of the lens allowing it to relax into a more biconvex shape changing the focal length of the lens allowing the image of the close-up object to be in focus at the level of the neural retina. —Preceding unsigned comment added by 128.143.61.114 (talk) 21:13, 26 October 2007 (UTC) Accommodation includes a number of factors, of which relaxation into a more biconcave shape of the lens is one - but only one. Other factors include anterior movement of the lens (resulting in a shallowing of the anterior chamber), contraction (miosis) of the pupil, convergence of the eyes, and several more factors. Signed: Spencer P. Thornton, M.D. Clinical Professor of Ophthalmology University of Tennessee Health Science Center —Preceding unsigned comment added by Spencer P. Thornton (talk • contribs) 01:32, 17 July 2008 (UTC)

Nov.7, 2014 -- As a physicist teaching some optics to BIMS majors, I wanted to teach more (than in the physics text) about accommodation by the human eye. So I found my way to this article. Unfortunately, it does not get to the point the way the "Accommodation reflex" article does, with its simple figure showing how the lens changes shape during accommodation. Moreover, the discussion of mechanism, especially of the work of Schachar, is inscrutable, largely because of its technicality and length. — Preceding unsigned comment added by 74.192.30.64 (talk) 14:34, 7 November 2014 (UTC)

Recreation of this article

Accommodation (eye) previously redirected to Accommodation reflex. There is sufficient evidence to indicate that this article deserves to stand-alone, or even be the recipient of the redirect. First of all, given that various google searchs of "accommodation" with "lens" or "eye" reveal hits in the neighborhood of 1,200,000 to 4,250,000 whereas "accommodation reflex" gets a bit over 9,000, common usage dictates "notability" for it having its own article. Secondly, accommodation can be a voluntary process and not a reflex action. AED 08:39, 7 December 2005 (UTC)

Different theories

What is the degree of support of the various theories in current mainstream ophthamology? AxelBoldt 21:08, 17 November 2006 (UTC)

Helmholtz's theory is almost universally accepted, but Schachar's theory appears to be generating some serious interest. -AED 21:20, 17 November 2006 (UTC)

Thanks! I think it would be good to add that information to the article; it helps the reader a lot. Cheers, AxelBoldt 04:01, 18 November 2006 (UTC)

do we dont get more information about theories?

I am a lay contributor, but not only does th Schachar's theory seem barking from a mechanical point of view, the attributed reference is in fact a rebuttal of the theory. If we are going to include a reference shouldn't it be to Schirars original paper? Also having tracked down various references two of which quote that Schachar's theories have no support in the profession outside his immediate group, then wouldn't it be more appropriate to leave in but play down this section:

"Ronald Schachar has proposed (contrary to Helmholtz's theory) that the ciliary muscle actually pulls on the crystalline lens' equator in an outwardly radial manner, and the consequent changes in lens shape increase the lens power. [ref to Schachar at al. (1993) (Annals of Ophthalmology, 25 (1) 5-9)] and a derived "Scleral Expansion Band" surgical technique for the reversal of presbyopia. However, both the theory and the efficacy of the procedure has gained little support. [reference 4]"

TerryE 17:30, 2 June 2007 (UTC)

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A footnote to this issue I see that Ronaldaaron (which I suspect is the given names of RA Schachar) has expanded the Schachar section, which is entirely acceptable because the published and reviewed theories have a right to be presented. However, what really worries me about these edits is that RA has also modified the Helmoltz entry to undermine its validity, even though this is the theory accepted by most ophthalmologists. To me this is crossing the bias line, and is counter to the Wikipedia guidelines. What I find mildly irritating me that in focusing on this dispute, the contributors are focusing on the area of conflict and forgetting to explain how accommodation actually works!

What I suggest is that the article should first explain how accommodation works and then present the alternative theories for how the ciliary muscle act together with zonular tension to vary the curvature of the lens. The pro RAS editors should focus on clarifying the Schachar theory (adding some paragraphing would help readability), and not undermine the counter argument. TerryE 00:19, 23 July 2007 (UTC)

[reeditted TerryE 02:00, 24 July 2007 (UTC)]

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The obvious bias on the part of the pro RAS editors leads me to worry about the info's creditability. Are there non-Schachar affiliated papers supporting his hypothesis? Also, did the numerous "in press" sources ever get published?

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Colloquial use of "theory". How can there be competing theories? Surely they are competing hypotheses?

I have added a critical reference (Atchison). Strasburger (talk) 07:26, 30 September 2018 (UTC)

The Content of the Synopsis

Ronaldaaron (22:02, 12 July 2007) replaced the previous intro synopsis with a complete rewrite. The previous synopsis was short and specific to the main theme of this article. The new synopsis:

• removes discussion of the cornea in accommodation mechansim
• removes most references to the related Wikipedia pages
• shifts the content of the paragraph to discussion of presbyopia which is already an existing article
• is neutral to the Helmholtz / Schachar camps.

All in all I feel that the new intro is not an improvement. If we want to introduce the age dynamics of accommodation then this should be moved to a small linking section. TerryE 02:32, 24 July 2007 (UTC)

How about this:

Accommodation is the process by which the eye varies its optical power to maintain the image of an object in focus at the retina as the object varies in distance from the eye. The optical power of the eye is a result of refraction as light travels within the body of eye, with the interface between the air and the curved cornea accounting for some 45 diopters of this in the human eye, the lens and its interfaces some 20 diopters when focusing on a far object ^{[ref to Hecht, Eugene, Optics, 2nd Ed, Addison Wesley, 1987]} . Whilst the cornea is relatively rigid and therefore its power is fixed, the lens is flexible and varies in geometry due to contraction of the surrounding ciliary muscle. This ability to flex the lens is at a maximum in an infant who can add perhaps 20 diopters of optical power enabling objects less than 10 cms from the nose to be brought into focus. This declines steadily with age so that the healthy eye will typically have about 1 diopter accommodation remaining by the age of 70 years. This condition is known as presbyopia.

Given that this article is underthreat of a COI embargo I have asked for a stay until the weekend. If no one has any comments / changes, then I will swap in this intro on Sunday. At the same time I will backout AR's changes to the Helmholtz section and reinstate the pro Helmholtz references here. TerryE 19:16, 26 July 2007 (UTC)

I don't know what you imagine when you say COI "embargo". All I said at Wikipedia:Help desk#Undeleting a Deleted Article was that I might add {{COI}}. This places a template message at the top of the article to warn readers that there may be problems with the content and it can be discussed on the talk page. It does not prevent editing, and the template can be removed when the problem has been resolved. Wikipedia has many templates to tag problematic articles and it's often done. PrimeHunter 23:49, 26 July 2007 (UTC)

OK not CIO "embargo"; how about "CIO warning flag" ? TerryE 01:54, 27 July 2007 (UTC)

Yes, warning flag is a better term for it. Wikipedians usually just say "tag" about templates like this and others in Category:Neutrality templates and Category:Dispute templates. PrimeHunter 10:39, 27 July 2007 (UTC)

Hi, I did my doctoral dissertation on accommodation (I'm in a different area now), and this article doesn't reflect the subject properly. It violates both the conflict-of-interest and no-original-research rules. Essentially the article has become boilerplate from Dr. Schachar's papers. Dr. Schachar is a legitimate researcher, but his theory is by no means mainstream. I think the introduction proposed above by TerryE is a good placeholder, but that the article be rewritten to follow the standard wikipedia format, i.e. Function of Accommodation; Mechanisms of accommodation (There are at least 6 different ways to solve the focusing problem -- change lens shape, change lens distance, change retina distance, change cornea shape, etc -- and there are examples of all of them in the animal kingdom. It's fascinating.) History of accommodation research (in which Dr. Schachar will get an appropriate mention, along with Scheiner, Donders, Young, Helmholtz, Crane, Charman, etc); Neurological Aspects of Accommodation (E-W nucleus, AC/A, etc); Disorders of Accommodation (including presbyopia). Cfneveu 18:59, 25 September 2007 (UTC)

I've modified the introduction suggested above to be less anthropocentric:

Accommodation is the process by which the eye varies its optical power to maintain the image of an object in focus at the retina as the object varies in distance from the eye. There are various ways this is accomplished in vertebrate eyes:

• Changing the shape of the lens
• Changing the position of the lens relative to the retina
• Changing the axial length of the eyeball.
• Changing the shape of the cornea

Humans accommodate by changing the shape of the crystalline lens. The optical power of the eye is a result of refraction, with the interface between the air and the curved cornea accounting for some 45 diopters in the human eye, and the lens some 15 diopters when focusing on a far object ^{[ref to Hecht, Eugene, Optics, 2nd Ed, Addison Wesley, 1987]} . In humans the shape of the cornea does not change with accommodation, but the lens is flexible and varies in geometry due to contraction of the surrounding ciliary muscle. This ability to flex the lens is at a maximum in an infant who can add perhaps 20 diopters of optical power enabling objects 5 cms from the nose to be brought into focus^{citation needed}. This declines steadily with age so that the healthy eye will typically have about 1 diopter accommodation remaining by the age of 70 years. This condition is known as presbyopia.

Cfneveu 08:16, 1 October 2007 (UTC)

It seems to me after 5 minutes, that this article still needs work. The explanation is too technical and complex; you want something that is *simple* and *clear*. As well, remnants of the COI and other disputes, above, still remain; the majority of footnotes are still the same... KenThomas (talk) 22:10, 5 January 2012 (UTC)

Sub-issue: I'm perplexed by the claims in the lede paragraph that a vast majority of humans cannot focus at 5cm by age 40. While this may be true for a defined percentage of the population, I doubt it is true for 50% at that age-- but to return to a point relevant above, we need verifiability, not speculation and unreferenced claims. KenThomas (talk) 22:10, 5 January 2012 (UTC)

Biased towards Schachar Claim

The section for Schachar seems quite biased, it reads like Schachar defending his own, questionable, hypothesis. Refer to the following papers for some decent numerical studies severely calling into question the model that Schachar based his initial claims upon:

Burd (1999): Mechanics of Accommodation of the Human Eye

Judge (2002): Modelling the Mechanics of Accommodation and Presbyopia

Burd (2002): Numerical Modelling of the Accommodating Lens.

EDIT: Sorry about the 'unsigned' just an undergraduate (not an expert) doing some related reading, seems like pretty clear bias even without expert level knowledge of the topic though. —Preceding unsigned comment added by 58.172.106.11 (talk) 10:41, 17 May 2010 (UTC)

For now I have just flagged the article, but I do think most of the section should be removed. —Preceding unsigned comment added by 58.172.106.11 (talk) 10:37, 17 May 2010 (UTC)

+1, Schachar's content is mostly his own and from my non-expert reading seems to be a very minority view that isn't well cited. Yes, it needs to be covered but in my view this level of detail is WP:UNDUE. PS, 58.172.106.11, remember to add the ~~~~ :-) TerryE (talk) 01:37, 18 May 2010 (UTC)

I have decided to remove some of the content that Schachar added: claims of falsifying the mainstream Helmholtz theory, clear self-promotion, content already covered in the presbyopia article and references to an unapproved and no widely accepted surgical procedure. It's detail is still WP:UNDUE IMHO, but I am not sufficiently expert to précis further. -- TerryE (talk) 00:44, 20 April 2011 (UTC)

This 1994 article [1] and Schachar's book [2] contain accessible descriptions of Schachar's Accommodation Hypothesis. Both include critiques of several papers supporting the Helmholtz Theory. To date, I have been unable to find articles supporting the Helmholtz Theory that explicitly address Schachar's criticisms. Personally, the Schachar Hypothesis seems to do a better job explaining eye strain, though I have yet to find references either pro or con. Rcunning (talk) 02:06, 26 October 2011 (UTC)

I don't dispute your view, but it is just that a POV isn't it? - TerryE (talk) 17:52, 2 October 2012 (UTC)

Schachar's theory has not found any support in Ophthalmology nor Vision Science. All references in that section are from the author, so it has been given undue weight (WP:UNDUE). There are further several papers that effectively disprove that theory. I have removed the entire section to a chapter here in the talk page (below), so as to conserve it for reference. I kept the introductory paragraph, as a pointer to that theory. Strasburger (talk) 17:28, 8 October 2015 (UTC)

Schachar's Theory

(taken from the article namespace in Oct. 2015) Strasburger (talk) 17:28, 8 October 2015 (UTC)

• Schachar—Ronald Schachar has proposed an alternative theory which indicates that focus by the human lens is associated with increased tension on the lens via the equatorial zonules; that when the ciliary muscle contracts, equatorial zonular tension is increased, causing the central surfaces of the crystalline lens to steepen, the central thickness of the lens to increase (anterior-posterior diameter), and the peripheral surfaces of the lens to flatten. While the tension on equatorial zonules is increased during accommodation, the anterior and posterior zonules are simultaneously relaxing. As a consequence of the changes in lens shape during human in vivo accommodation, the central optical power of the lens increases and spherical aberration of the lens shifts in the negative direction. Because of the increased equatorial zonular tension on the lens during accommodation, the stress on the lens capsule is increased and the lens remains stable and unaffected by gravity. The same shape changes that occur to the crystalline lens during accommodation are observed when equatorial tension is applied to any encapsulated biconvex object that encloses a minimally compressible material (volume change less than approximately 3%) and has an elliptical profile with an aspect ratio ≤ 0.6 (minor axis/major axis ratio). Equatorial tension is very efficient when applied to biconvex objects that have a profile with an aspect ratio ≤ 0.6. Minimal equatorial tension and only a small increase in equatorial diameter causes a large increase in central curvature. This explains why the aspect ratio of a vertebrate crystalline lens can be used to predict the qualitative amplitude of accommodation of the vertebrate eye. Vertebrates that have lenses with aspect ratios ≤ 0.6 have high amplitudes of accommodation; e.g., primates and falcons, while those vertebrates with lenticular aspect ratios > 0.6 have low amplitudes of accommodation; e.g. owls and antelopes. The decline in the amplitude of accommodation eventually results in the clinical manifestation of presbyopia As the equatorial diameter of the lens continuously increases over life, baseline zonular tension simultaneously declines. This results in a reduction in baseline ciliary muscle length that is associated with both lens growth and increasing age. Since the ciliary muscle, like all muscles, has a length-tension relationship, the maximum force the ciliary muscle can apply decreases, as its length shortens with increasing age. This is the etiology of the age-related decline in accommodative amplitude that results in presbyopia.

References

No Mention of Nervous System

The article would be greatly improved by a (possibly small) section describing the Visceromotor Nerves that innervate the Ciliary Body and effect contraction/relaxation. While the theoretical explanations are highly valuable, the well understood Nervous aspects of Accommodation should be included (e.g. The parasympathetic nervous system innervates the ciliary body via the short ciliary fibers that arise from the ciliary ganglion and are derived from parasympathetic neurons traveling with Cranial Nerve III.) --TwinSteel (talk) 03:23, 8 October 2010 (UTC)

This article is important

I'm a layperson with respect to the scientific understanding of accommodation but wanted to note that I believe the article is of great importance (addressing the categorization of Unknown Importance - Biology). The reason is that accommodation is one of the few remaining problems to be solved in creating practical head-mounted displays. When these devices become mainstream they will have a huge impact on our society.

I believe the problem is typically solved by using a Fresnel lens in the display. Strasburger (talk) 16:30, 8 October 2015 (UTC)

Proper role of convergence

Convergence -- this does not help focus the image on the retina as stated in the article, it helps to center the image over the macula and fovea, the center (densest part) of the retina. Accomodation is what focuses the image on the retina. Convergence controls the position of the image, accomodation the focus. -- I am a practicing board certified ophthalmologist in the USA. Relopez3 (talk) 03:11, 21 September 2011 (UTC)

Proof: Covering one eye eliminates convergence, but does not prevent adaptation. Rcunning (talk) 01:33, 26 October 2011 (UTC)

My problem with this page is the diagram of the eye optics. The eye lens is only a small part of the focussing system of the eye; most of the bending of the light entering the eye takes place at the cornea. Most eye diagrams (including the ones associated with this article) incorrectly show the light not being bent at the cornea, but being bent at the two surfaces of the lens. The refractive indices of the aqueous humour, the vitreous humour, and the eye lens itself are quite close one to another, and so the bending of light introduced by the lens is comparatively small. There is a huge difference between the refractive index of the cornea and of the surrounding air, and so that surface has the potential to introduce large changes in the direction of a ray of light.58.173.194.6 (talk) 22:04, 4 December 2012 (UTC)

You are correct, there are three main points where the rays are bent, and none of those is the one shown. However, there is a simplified model of the eye's optics known as the simplified Gullstrand eye model. In that model the three points are collapsed into one for simplicity. I forgot, though, where that point is. Strasburger (talk) 12:47, 15 January 2017 (UTC)

Accommodation in non-human animals

This article seems to focus on accommodation in the human eye. However, other animals have different strategies for accommodation. For instance, some fish accommodate by adjusting the position of the lens relative to the retina, rather than changing the lens shape. Animals with compound eyes do not accommodate at all, as far as I know. The article should probably touch on this sort of thing somewhere. 138.16.21.199 (talk) 20:59, 12 December 2012 (UTC)

Does changing focal distance from infinity to 7 cm imply a 13 diopter change in focal power?

I'm curious about the following:

"The young human eye can change focus from distance (infinity) to 7 cm from the eye in 350 milliseconds. This dramatic change in focal power of the eye of approximately 13 diopters (diopter is the reciprocal of focal length in meters) occurs as a consequence of a reduction in zonular tension induced by ciliary muscle contraction."

How is that 13 diopters calculated? Since 7 cm is equal to .07 meters and 1/.07 = 14.3 it seems to me that the change in focal power is just over 14 diopters, and the "13" in the quoted sentence ought to be "14". Is my reasoning correct or am I missing something? John Link (talk) 08:04, 9 February 2013 (UTC)

Reference 3 (Chen et al., 2000, Fig. 5) reports 15 dpt, which converts to 6.7 dpt. I have put these two numbers in. Strasburger (talk) 08:08, 5 May 2015 (UTC)

Simple experiment

You can cause double vision using eyeglasses, broken ones (with temples or earpieces off) would be best for this. Put the glasses before your eyes and look at some object. If you have good vision and the lenses are +2 or +3 dioptres, the object has to be quite near for good focus. Now move one lens up and the other down by, say, 3 mm each. One image will go up and the other down. After a second your vision will align the images back. Now question: Is the adjustment made on "software" or "hardware" level? --Stankot (talk) 10:39, 15 March 2016 (UTC)

Vertebrate eye or human eye, accommodation or focusing?

I'm not sure if anyone is active on this page. If not I'll be going ahead in the near future with a revision that I think is required as it's a C class article. Firstly, for a member of the public, accommodation means a place to stay. "Focusing" of the eye has to also point to this article and if possible the word focus appear in the title. Also it currently seems to be an article only on the human lens with other vertebrates left out. While the human lens is the most studied there is plenty to include from other vertebrates, at least a summary outlining differences among vertebrates and referring within the article to fish, reptile and amphibian mechanisms. Another article on the vertebrate lens is getting quite long and the section on accommodation within it needs expanding. It would be better to expand this shorter article and refer to from that article. If there are no objections I'll go ahead with the ideas outlined here if no one else is currently interested in doing it. Tgru001 (talk) 23:35, 18 June 2023 (UTC)

Editing the article is surely a good idea. Before you start, however, perhaps let’s consider what would be a good scope. The title – until very recently – was “Accommodation (Eye)” (if I remember correctly), until someone noticed that that title was too general since the article mostly just described accommodation in the human eye. Someone commented that many other vertebrate eyes are similar to the human eye and it was decided to narrow it down to the vertebrate eye and change the title accordingly.

It seems to me now that either the scope should be narrowed down further, to eyes that are similar to that of the human, or keep the vertebrate and proceed as you proposed. Strasburger (talk) 09:43, 19 June 2023 (UTC)

My feeling is that the article would not get to long in the short term if it covered vertebrate eyes, at least with the currently accepted firm knowledge base and the amount of time I have. Having an article on human eyes only would end up as inconclusive debate around who has the best model, a large chunk of the current article. The rest of the current article seems to be mainly on defects or diseases in focusing, something which would be better in another article in my opinion. Accommodation is hard enough for the average reader to get your head around without the complexities of diseases that effect the incompletely understood mechanism. While I don't think they contribute much I intend to leave them for someone else to tidy up of move to a new article.

While I don't intend to do it all at once, categorizing vertebrate eye focusing into something like Land- Mammals-human/primate, carnivore, herbivore, birds, reptile Water-fish, whales and Amphibious- some birds and reptiles, would probably cover most of the main vertebrate focusing mechanisms currently understood to some degree without making the article too long. If other editors want to expand any one area, human focusing for example, to the point it makes this article too long they could take that and make a new article on it, referring to that article from this one and visa versa. This article could then be shortened again as needed. Unless I hear other preferences I'll work towards covering vertebrates under Land, Water and Amphibious categories for now, rather than narrowing to human only. I'll assume if someone feels a strong need for a human specific page they can create a new article and pad it out with medical debates and diseases as they see fit. Tgru001 (talk) 05:17, 29 June 2023 (UTC)

Sounds great -- go ahead (from my side). Strasburger (talk) 08:05, 29 June 2023 (UTC)