Depth Perception

[Jo]

Depth perception

We can determine the relative distance of objects in two different ways. One uses cues involving only one eye; the second requires two eyes. When something is far from us, we rely on monocular cues, those that require the use of only one eye. For closer objects, we use both monocular cues and binocular cues, those that necessitate both eyes.

The ability to perceive depth seems to exist early in life. Research with infants has revealed that by two months of age, babies can perceive depth. Prior to that, they may be unable to do so in part because of weak eye muscles that do not let them use binocular depth cues.

Monocular depth cues. Psychologists have identified two different kinds of monocular cues. One comes into play when we use the muscles of the eye to change the shape of the eye's lens to focus on an object. We make use of the amount of muscular tension to give feedback about distance. A second kind of monocular cue relates to external visual stimuli. These cues appear in the table below. Artists use these visual cues to make two dimensional paintings appear realistic. These cues may seem obvious to us now, but artistic renderings from earlier than about the sixteenth century often seem distorted because artists had not yet developed all the techniques to capture these visual cues.

Binocular cues. Binocular cues require that we use both eyes. One cue makes use of the fact that when we look at a nearby object with both eyes, we bring our eyes together; the muscle tension associated with looking at close objects gives us information about their distance. The second binocular cue involves retinal disparity. This means that each eye (or, more specifically, the retina of each eye) has a slightly different perspective. The slight difference in appearance of an object in each eye when we gaze at it gives us further information about depth. Children's Viewmasters produce a three-dimensional image that has depth because of a slightly different picture that is delivered to each eye. In the natural world, because of the relatively small distance from one pupil to another (about 2.5 inches or 6.5 centimeters) binocular cues are effective only for objects that are within about 500 yards (455 m) of the viewer. Animals that have eyes on front of the face, like primates, will be able to use binocular depth cues because the two eyes see almost, but not quite, the same scene; on the other hand, animals with eyes on the side of the head, like most birds, will be less able to use binocular cues because the visual fields of the two eyes do not overlap very much and each eye sees different scenes.

Gale Encyclopedia of Psychology, 2nd ed. Gale Group, 2001.
http://www.findarticles.com/cf_dls/g.../article.jhtml

Given the above information, wouldn't new surgical procedures that involve putting bands around the ciliary muscles affect depth perception? I would think the brain would suddenly recieve a mixed message and be thrown for a loop.

How severely does aging affect depth perception at close range? Should more states be testing older drivers for vision impairment if it does have a severe affect?

[Darryl Meister]

Hi Jo,

Good points... I don't know that the bands will have that much effect upon binocular vision, since they involve different muscles. (I am assuming that you are referring to Chakar's (sp?) PMMA bands for producing accommodation.) The bands work on the ciliary process, but convergence for binocular vision is produced by the extra-ocular muscles (i.e., the recti and obliques). I should probably point out that there is a relationship between accommodation (or the stimulation of the ciliary muscle) and convergence, known as the near synkinetic reflex (or near triad). However, binocular vision still seems to function well in presbyopes, who no longer have the same relationship between accommodation and convergence that pre-presbyopes have.

Also, I don't know of any states that have binocular vision requirements for driving, so even if binocular vision has degraded with age there would probably be little motivation to test the elderly for it. I would also venture to say that monocular cues play a bigger role during driving than binocular cues.

Best regards,
Darryl

[Jo]

Thanks Darryl!

From what I've noticed most auto accidents involving the elderly tend to be at close range, this is totally unscientific and just based on personal observance. I was wondering if this was a combination of following too closely combined with an inability to visually tell when the car in front of them was slowing down to stop, due to slowed muscle and joint reaction (being able to move their feet over and quickly hit the brake) or poor concentration.

I have also noticed that artists and folks who do highly detailed work often have a hard time dealing with the changes in size and orientation when they wear glasses for the first time. I imagine the brain has its own way of creating a sense of depth even if these muscles do not work properly.