I have a client who asked me a visual question. In a rural setting they had a potential new employee return from their physical with the comment from the MD stating “depth perception issue”. The potential employee will be drivng a forklift as part of their duties. Knowing all of the visual ergomasters out there I told him I would ask for feedback. What are the potential problems, how can they be overcome and are there specifc tests an optometrist/opthamologist can perform to identify the problem and how to correct it. I would assume glasses would do the trick. Can they withold hiring until this is corrected. Any thoughts, suggestions or other ideas would be helpful. Thanks ergowebers!
True depth perception (called “stereopsis”) requires the use of two eyes seeing approximately equally. The “depth” effect comes from the fact that the line of sight between them is from a different angle (that is, the eyes are separated by a certain distance). This ability is easily measured (by us eye docs).
However, one-eyed people can also perceive “depth” but it’s by using monocular clues to do so. Some of these are using size estimations, shadowing, illumination differentials, etc. You’d be amazed at how many one-eyed drivers there are out there! Once this process is learned by the brain, it appears “normal” to the person and they perform relatively well. I had a patient who lost an eye and within 4 months, he was driving dune buggies in the desert again!
Essentially, a routine eye exam (either optometrist or ophthalmologist) should be able to give you the information you need to qualify this person for driving.
Let me know if you need more info.
“… comment from the MD stating “depth perception issue”. As you indicated in your post the MD failed to provide you with the information you need to take action, and it appears you let them get away with it. Now, you’re jumping through hoops to resolve a “created problem” where the MD should had given you the solution from the diagnosis rendered. It’s like the doc-in-a-box medical “testing” services… these guys will give you results, but they don’t or won’t give you a plan to solve the problem – they push the problem off on someone else; it’s irresponsible and unethical in my opinion. This appears a continuing major issue: the health care community “feedback” to ergonomists or employers is often incomplete, confused, or of no use whatsoever. The medical community “feedback” accountability must be raised to a higher standard (this would make a great masters degree thesis for anyone looking to do one, if not already buried somewhere). Another example, “no heavy lifting.” What does this really mean? It’s incomplete! If you’re getting these kinds of statements back from the medical community and its ancilliary support functions, you need to go right back to them and get their proposed solutions. Better yet, establish a policy with your medical provider that these kinds of statements are unacceptable and future statements will include details and solutions. You should not be forced to operate in a “knowledge vacuum,” don’t accept it. Ergonomists have let this “stuff” go on for far too long. A diagnosis without a proposed solution is incomplete. But, you have work to do too, you must ensure that the medical provider has a detailed and complete job desciption of what the employee does at work. Include pictures, if necessary. This will help ensure that you and your medical provider are working as a team to keep that employee functioning and productive, rather than the employee having an “excuse” why they can’t do the task. Back to the original post… “depth perception issues” while using forklifts, is similar to depth perception problems for pilots. Airlines won’t hire pilots that have depth perception issues… would you fly on a plane with a pilot that can’t judge heights, for example? I won’t. I would not let a person with depth perception issues operate a forklift, corrected or not, due to the high risk for accident. It’s a hazard. “Corrected” or “controlled” hazards are not eliminated hazards; and this particular forklift scenario is going to bite – eventually. The hazard is still there lurking in the background waiting for “complacency” to show it’s ugly presence.
Binocular vision is only one of about ten ways that we perceive depth. It is helpful for things that are very close, eg 3 feet away, and virtually useless for things 30 feet or more away. The average forklift job is say 6 feet from eye to product, and that certainly would benefit from binocular vision, and you could expect either slower performance or more product damage from a monocular forklift driver.
Monocular car drivers have many more accidents than binocular car drivers, something that I don’t think has been adequately explained as most accidents are caused when the cars are much more than 30 feet apart (the collision being simply the detail at the end!) Perhaps it’s something to do with lack of peripheral vision on the blind side.
I also think that is that people don’t become monocular just by chance. Something happens, such as an accident or illness, and those things can have widespread effects. For instance is the monocular person perfect in the “good” eye or is it perhaps not very good at all? Do they have a brain injury? A sore neck from the accident, and then be unable to turn the head properly?
In summary don’t put monocular people on work that involves precise location judgments; that requires intact peripheral vision; or that involves fast moving objects close to body. For instance, forget baseball!
Depth perception is something that is learned; thus it can be “re-learned” if lost. The factor is time! The longer one depends on monocular clues to distance and depth, the better they become at doing it.
You bring up some good points regarding the other “good” eye and also physical factors that might affect one’s viewing ability on a forklift. Peripheral vision is limited for monocular viewers but unless they lose the vision completely on the poor eye, often times the peripheral vision is intact (even if central vision is lost).
Again, the doc did a disservice in not following up but most general practitioners have no idea of what to do. They leave this to us professionals- but in this case it needs to be a vision professional. I recommend a full eye exam to find the problem and get a reasonable solution.
Depth perception is more than a “learned” activity. It is an actual brain wired activity based on activities incurred during a critical period during development. Refer to past studies where stereo vision was blocked during critical development phases….subjects even with two intact working eyes could not overcome this time factor. Subjects can accommodate to their new environment but not without limitations as discussed by earlier posters. Here in Kansas we recently incurred a forklift death caused by an inattentive drive running into an inattentive pedestrian. Lack of peripheral vision would have not prevented this accident for attentive drivers. Warning sensors such as those on vehicles for backing up or cameras may be the answer…of course these are not cheap.
You are correct about the “brain wired activity based on activities incurred during the critical period…”, and that is what “learning” is! Thus, we agree that depth perception is a learned skill. The term “two intact eyes” is pretty vague because eyesight and vision are not absolutes- there are levels of seeing and levels of blindness. For example, someone seeing 20/40 does not see the same way that someone else with 20/400 eyesight might.
I’m not sure what correlation you see in the example of inattention causing a fatal accident, since depth perception and stereopsis have little or nothing to do with attention to a task. True, a lack of peripheral vision might not have prevented this accident but unless we know the particulars of their visual abilities, we are always just guessing about how well people see.
Keeney, et al., state, “nationwide, monocularly impaired individuals have seven times more accidents than the general population with which they were compared.” He recommends monocularly impaired drivers be denied class 1 licenses, (commercial driver license for transport of people), and that they be warned by by their doctors regarding increased risk of accident with driving.”
Learned consciously or by synapses change based on experience not based on controlled behaviors, there is a difference. The critical period is also a factor to consider. The brain is not capable of rewiring to compensate after this period. I also agree that I do not have the whole story in the forklift case; however, I do work in an area where numerous types of transports are used with a flurry of pedestrian traffic inside an assembly plant. The adaptation of moving the head around constantly to detect motion and widen the focus area is not a sufficient adaptation. During lift operation with focus on the item being lifted the head is not allowed to focus to adapt for the loss of peripheral vision. Thus the need for additional warning devices or controls would be needed. As far as 20/20 vs 20/40 in one eye the brain has to regulate both images and selects what to occlude and what to fill in (this is not a conscious decision). There are still two images coming into the compensator. I think the keys words in all the posts are the fact that those with any type of vision loss, the impaired have to be retrained to be able to survive their surroundings. This at no time fully substitutes what was lost.
While I don’t have the hands-on experience that you do with transports and I agree with several of your statements, all I’m saying is that we may not have the whole story. “Vision loss” can be a whole host of conditions, ranging from mild amblyopia (lazy eye) to total loss of the eyeball itself. Each of those situations will present differently from an adaption standpoint. And we will use a combination of head and eye movements to compenstate for loss of the field of vision, which may or may not be adequate to operate certain types of machinery. Again, testing of the individual should be a priority.
The concept of critical period has long standing medical support but has also been shown to be overridden by visual therapies. This is not to say that someone born with no vision in one eye can be trained to see out of it, but the amblyopia of long standing can often times be corrected with the proper program.
Thus, we have to consider what “monocular” really means on a case-by-case basis. Yes, visual fields will be reduced and other adaptations made. This absolutely will call for additional controls and warning systems. And yes, a retraining period is an absolute must.
So it looks like we agree on several issues here….and especially that the original statement from the referring doctor is just the start of the process and by no means the end of it.
No argument here…of course just two years ago I was told by the driver’s license office, at the age of 36, that I needed to go to the eye doc before they gave me a renewal license. It turns out that after ten years of taking eye exams in the military and being up for 20 hours, it was not just tired eyes. I have a curvature issue on both eyes that makes some little letters fuzzy. The doc signed me off and I only wear glasses if I’m tired to take care of the minor fuzzies. Brings up the old human factors question as to whether the vision test is not an adequate test. It really should check for ability to track what is going around you…not the fine letters on the bottom of the parking sign.
Now we’re onto a different subject altogether! This “curvature issue” with your eyes was likely uncorrected astigmatism. This is not the same as the “vision loss” that we were discussing.
Uncorrected astigmatism can very often be missed (especially in “mass” eye exams like the military) and DMV tests don’t even qualify. This amount of astigmatism might have easily gone undetected because in your earlier years, you could compensate adequately. As your eyes age, they can accommodate less and any refractive error (like astigmatism) becomes more obvious.
So, now we need to discuss what “vision tests” are. As you noted, it’s MUCH more complicated than just reading letters on a chart. I liken that to going to your physician for your annual physical; the doc weighs you and states that you weigh about the right amount…..so you must be healthy!! Yeah, right- that doesn’t happen! They take a SERIES of tests and compile them into a profile of your general health. Likewise, eye docs take a SERIES of tests to see how well your eyes perform under various circumstances (at least that’s the way it SHOULD work….).j
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