Postural Skews and Stereo Views – Part 1


We’re at an interesting multidimensional crossroad in the development of stereoscopic 3D technologies, and adaptive abilities in what is commonly referred to as “human visual factors”.  It is the classic interface challenge, in which developers of technologies must anticipate the extent to which the average user or observer brings the visual tools to the table to enable them to have productive, enjoyable, immersive experiences.  Much of our panel discussion on the topic at this year’s annual meeting of the College of Optometrists in Vision Development in Orlando will address these factors, and you can get a brief glimpse at the topics on pages 23-24 of the program (see here).

Beginning today, and for the next few days, I’ll give you a teaser to some of the issues that will be discussed during Thursday’s panel.  What’s wrong with this picture?

Cyclodisparity

 

It’s a slide from the Video Quality Measurement Tool 3D Project.  The Project takes an in-depth look at stereoscopic  3D films and critiques them based on various visual components, in this instance an unintentionally induced vertical disparity.  As you watch the switch between left and right eye views, you’ll notice that there’s actually a diagonal displacement.  During the presentation on Thursday I’ll be noting how we clinically detect and deal with postural skews translating into cyclovertical imbalances.

cyclovertical

In this instance, a 2D representation of 3D projection done via cheiroscopic tracing, the image is not only vertically displaced exhibits a very obvious torque or cyclorotation.  One can also detect this in space by looking closely at the tip from horizontal on red Maddox rod testing.

Red Maddox RodPart of the challenge as I see it is to determine the extent to which postural skews and adaptive head postures are purposeful and, more importantly, will be challenged by sustained viewing of stereoscopic 3D formats.  Brief periods of S3D viewing may not unmask inherent binocular challenges.  But the longer the view is expected to sustain depth demands that exacerbate binocular problems and/or postural adaptations to maintain binocular vision, the more likely symptoms and performance problems are to occur.   While comfort criteria for binocular viewing has classically been related to horizontal fusional ranges including blur/break relationships and various criteria, comparatively little attention has been paid to cyclofusion.  In considering cylinder power and axes, as well as prism vectors, these factors become more relevant.

Disparity

To probe the limits of your cyclo-fusion abilities, first use your convergence abilities to obtain a third image in the center between the original two.  For you non-optometrists, that means obtaining the feeling of crossing your eyes inward while looking at the white space between the two views of the forest.  You’ll notice that the depth takes a bit of time to grow.  There’s actually a bit of perceptual learning for depth going on in your brain while you accomplish this.  Now, tilt your head slightly toward your left shoulder and see roughly how far you can go before the S3D image in the center  separates or loses depth.  After you’ve come back to your starting point to re-capture the S3D image in the center, see how far you can tilt your head toward your right shoulder before losing it.  Is there symmetry?  If not, why?

7 thoughts on “Postural Skews and Stereo Views – Part 1

      • Ah-ha! As it should be! There is relatively little tolerance in the most brains for induced cyclovertical imbalance. Hence the challenge for stereographers to be precise in calculating and implementing a good stereo 3D match of R & L eyes in production. On the flip side, for individuals with natural cyclovertical imbalances, sustained viewing of S3D that is comfortable for most people will be taxing for them. Symptomatic viewing therefore means a BV problem until proven otherwise.

  1. I see. So it is important to remain level-headed. 😉

    Nice post, Len.

    A few things come to mind.
    1. Patients with a cyclofusion disorder can learn to build ranges with a troposcope. It may be the only binocular instrument that incorporates rotational ranges.

    2. Cyclodeviations may have some chicken-and-egg effect. Postural tilts and skews may create cyclodeviations and preferences of tilt, just as likely as cyclodeviations are to invite postural tilts and skews as a solution to the binocular problem. To that end, chiropractic care may be a valuable co-management for patients who present with cyclodeviations/phorias.

    3. There is a little-known technique which requires looking at a thick, black vertical line through -2 DC x 180 OU, and tilting the lenses about the vertical axis (flipper handle down and up). This can help a person build binocular cyclorotary ranges with a flat fusion target. Adding red/blue glasses for lustre can offer additional biofeedback during this procedure.

    4. Recovering from a break in cyclofusion is particularly challenging, as it requires more oculomotor control than a vertical break, and far more oculomotor control than a horizontal break. However, a top-down consciousness for one’s visual world can help the viewer to “keep it together” and fill in the gaps. This can be tested with the above example: cross-fuse the target, tilt the head. One time, try to just observe when it breaks. The next time, try to tune in to the image, really engage, and see if that alters the range of tilt, the perception of depth (while the image is tenuous), or both. The time-course of the tilting has a similar impact, altering one’s ability to sustain or recover.

    5. Keep in mind that this cross-fused example activates the obliques in abduction and changes the potential cyclorotary range, compared with a natural binocular target or a vectogram.

    6. I had a case a long time ago, a patient with an ABI and a 25 degree cyclotropia. We were amazed at her ability to fuse, once we facilitated it in-office. (This was at SUNY.) My investigation brought up Dove Prism as a mode for tilting images with a lens. Putting that into glasses would have been highly impractical. But I am curious if you’ve ever made use of it as a compensatory tool for cyclodeviation.

    That’s all for me for now. Thanks for the engaging blog- glad I took the time to reflect on it.

    Warmly,
    Samantha

    • Samantha, you raises some interesting points and suggestions. These need to be discussed fully. We all function in a spatial world. Working with the peripheral world decreased or eliminated requires transfer training to the real world. This process is of greater length. We should concentrate on procedures that utilize central and peripheral vision at the same time.

    • Thanks for the detailed comments and observations, Samantha, and I’ll respond in reverse order. Interesting you mention Dove prisms. As I recall, the inventor of the concept was Adelbert Ames Jr., who speaheaded the Dartmouth Eye Institute – famous for promoting awareness of aniseikonia to the ophthalmic community. I read that Ames had a huge anatomical cyclovertical imbalance, and invented the Dove prism for his own use. He actually wore a spectacle Rx incoprorating it manufactured by AO. But no – I haven’t used it for patients. There are two ways we train cyclovertical imbalances in our office – either by torquing the target or by having the patient change head position. When the head is maintained in habitual compensatory position, the cyclofusional ranges are built by torquing the target. We’ve found that using targets with a high degree of S3D helps somewhat. I generally prefer out of instrument to the troposcope, and you can torque ECs of various sizes pretty readily to achieve this. When the target is stationary but the head is tilted of course we’re activating VOR in addition to the counter-roll doll’s eye — that may help or hinder. Brock String can be useful, as it gives patient good feedback on cyclovertical if they are good observers — the two strings will look like wings of a plane that are tilted. With all of this, we use covere/uncover to condition fusion reflex with small cyclorotation first. We have used loose prism torqued as a vector between horizonal and vertical. I haven’t tried the cylindrical lens apporoach you describe, but makes sense.

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