Understanding of neural adaptation may lead to better vision correction

The image-enhancement software in our occipital cortex is key to better vision correction.

The software in the occipital cortex of the human brain is more sophisticated than the best image-enhancing computer, according to an optics expert. Understanding that software and the process of neural adaptation is key to setting the proper goals for vision-correcting surgery, he said.

“Refractive surgeons and other clinicians often forget that the visual system is comprised of an optical system, which is the cornea, the crystalline lens and the media, and a sensory system that begins at the retinal photoreceptors and goes all the way back to the occipital cortex,” said Jack T. Holladay, MD, MSEE, FACS. “More and more work in these areas is allowing us to determine exactly how the visual system functions.”

The sensory system begins with the retina, then proceeds through the optic nerve and radiations, which relay visual information back to the occipital cortex, he said. Research suggests that visual information is first received at Brodmann’s area 17, then compared with stored images in Brodmann’s 18, and a final image is produced in Brodmann’s 19.

“We must think of the occipital cortex like an image-enhancing computer that is more sophisticated than any program or software or computer that’s available today,” Dr. Holladay said. “And before we settle on specific treatments to reduce higher-order aberrations, we must understand better the human computer’s image enhancements to make sure that our goal optically is what’s best for the entire visual system.”

Two phases of adaptation

Neural adaptation is the process that takes place as the brain adapts to changes in the visual information being supplied by the eye’s optical system, Dr. Holladay said. There are two types of neural adaptation: a quick phase that can occur over a few seconds or minutes and a longer phase of adjustment that can take several months to a year.

Jack T. Holladay

The short phase of neural adaptation can be illustrated by the figures accompanying this article. In the first set of images, the picture of the little girl is of equal contrast on the left and the right. Fixating on the green dot in the center, the images look equal. Now look at second set of figures, in which one side is blurred and the other is at higher contrast. After about 10 seconds of staring at the green dot on the second set of images, when you look back at the original figures, the image on the right looks blurred.

“It takes almost 10 seconds for them to come back to where they look the same,” he explained. “This is our computer adapting to the blur, knowing that the eyes should see the same image when they are looking at the same object. Our computer is always trying to get rid of ‘noise’ when it’s not present in both eyes, to help us see the best quality image we can see.”

The longer phase of neural adaptation has been demonstrated in trials of aspheric and multifocal IOLs, he said.

“In several multifocal lens studies that have been done for Food and Drug Administration approval, the patient satisfaction level is much better at the end of 1 year than it is a few weeks after the implantation, even though there has been no optical change during this period,” Dr. Holladay said. “That is, complaints of halos, glare and dysphotopsia are common in the early postop period, within a few weeks of the operation, and yet by the end of the year, 99% of patients with a multifocal IOL, which clearly reduces contrast sensitivity and provides halos and glare, are extremely happy with their vision and have adapted to the induced aberrations.”

The same process of long-term neural adaptation can be seen in patients who become accustomed over the course of several months to aberrations induced by excimer laser surgery, he said.

In a study by Steven S. Schallhorn, MD, and others, at 1 and 3 months after excimer surgery there was a correlation between the quality of the image on the patient’s retina and subjective visual complaints, Dr. Holladay said.

“But by 6 months there was no correlation between subjective complaints and induced optical aberrations shown by wavefront or topography,” he said. “By 6 months the subjective complaints became no longer statistically significantly related to the image quality, even though there was no change in the optics between 3 and 6 months. This study is the reason that some people are arguing that visual complaints are not related to aberrations, they are related to neural adaptation, and the success of a patient’s outcomes continues to improve because they have adapted to aberrations. We don’t really want aberrations in the eye, but the patients can do well despite them.”

Short Phase of neural adaptation

In this set of images, the picture of the little girl is of equal contrast on the left and the right. Fixating on the green dot in the center, the images look equal.

In this set of images, one side is blurred and the other is at higher contrast. Stare at the green dot for about 10 seconds, and then look back at the first set of images. When you do so, the image on the right looks blurred. “This is our computer adapting to the blur, knowing that the eyes should see the same image when they are looking at the same object,” Dr. Holladay said.

Souce: Holladay JT

The best vision

Recent work by Pablo Artal, PhD, and colleagues in Spain has shed interesting light on the process of neural adaptation, Dr. Holladay noted.

He said Dr. Artal held a contest at his university, offering a $300 award for the person with the best visual acuity. Anyone with visual acuity of 20/20 could enter the contest. More than 300 students participated, and among those three people with 20/9 vision were identified. (The best vision ever recorded was 20/8, Dr. Holladay said.)

Dr. Artal performed two studies with these subjects. He began by measuring their wavefront aberrometry and plotting each person’s point spread function (PSF) vs. their visual acuity.

“The PSF should look like a point,” Dr. Holladay said, “and if it looks like a cobweb or a triangle, that’s what the patient actually sees when they look at a star or a single LED, rather than a point.”

He continued, “The remarkable thing is, those people with the best PSF were not the ones that had the 20/9 visual acuity. In fact, people with cobwebs as their PSF were the ones that had the 20/9 visual acuity. Those with the single point PSF were downstream, with visual acuities between 20/16 and 20/20. There was no correlation between performance by wavefront aberrometry and the best visual acuity.”

Dr. Holladay said the lesson of this study is that it was not the subjects with the best optics in their visual systems who had the 20/9 vision, it was the people with the best computer software in their sensory systems filtering out the optical aberrations.

“The computer software is evidently the limiting factor in our vision, not the optical performance and the aberrations present in our eyes,” he said. “This is a big shift in our concept of how the visual system works, and it has practical implications on what we should be doing.”

Dr. Artal performed a second experiment using the same subjects, he said.

“He took these people, the ones who saw extremely well, and rotated their aberrations optically in 30° increments from the orientation of their original aberrations. He found that when you rotated the person’s aberrations by just 30°, they dropped to about 80% to 60% of their original performance on both contrast sensitivity and visual acuity compared to their performance with their own error and their own neural adaptation.”

What this means, Dr. Holladay said, is that the neural computer in these subjects has adapted over the years to filter out the noise of their own particular optical systems, to enhance the contrast of the image using the software of the occipital cortex to provide a better image than is provided optically.

“It’s like those crime movies where they take a blurry image and enhance it, and all of a sudden the face jumps out and you can recognize who it is. Well, the brain actually does that, and it makes a much better image than we provide from our retinas because of the computer. The refinement and the adjustment that has been made over the years is specifically designed to improve the quality of images for the input that it’s getting out of our own optical systems. When you change the input, all of a sudden the computer doesn’t work correctly anymore.”

NeuroVision training

Another recent study that bears on the issue of neural adaptation was presented at the Refractive Surgery Subspecialty Day before the American Academy of Ophthalmology meeting this year, he said.

The study, presented by Donald T.H. Tan, FRCS, evaluated the NeuroVision NVC system, which employs a Gabor patch for visual processing exercises to sharpen visual acuity.

“Dr. Tran has been able to show that, in adults with amblyopia and 20/200 vision to start with, they’ve been able to improve the contrast sensitivity and visual acuity to almost normal levels,” Dr. Holladay said. “This is remarkable because most of us have felt that if you miss the imprinting period when you’re a young child, the chances of reversing amblyopia or training the brain to have better stereopsis or contrast sensitivity was no longer possible. And these absolutes turn out not to be true.”

In a related phenomenon, Dr. Holladay noted, it has been shown that in people with small refractive errors, on the order of –0.25 D or –0.5 D, those who never wear glasses have better contrast sensitivity and visual acuity than those with similar errors who wear glasses and are tested without them.

“You hear this from low myope all the time,” he said. “They say, ‘I don’t want to wear my glasses because after I wear them, when I take them off I see a lot worse than I do if I never wear them at all.’ We thought that that was just a comparison the patient was making. But in fact it turns out that it’s true. Patients who are –1 D who don’t wear glasses have much better unaided visual acuity than a person who wears glasses all the time because the brain deals with that optical blur. It begins to enhance the quality of the image, and people who are –1 D who should be seeing 20/40 are seeing 20/25. And if they wear glasses all the time they can only see 20/40 because their computer is not enhancing the image because it’s not seeing the blur all the time.”

Practical implications

What do these findings mean, practically, for the refractive surgeon and clinician? Dr. Holladay said there are three points to consider.

First, if each person’s neural computer has become attuned to the small higher-order aberrations in his or her own eyes, is it wise to try to correct their higher-order aberrations during refractive surgery?

“It appears that if we change the aberrations we actually make worse visual performance, so maybe we should rethink what our goal is,” he said. “Maybe we should specify that we want to eliminate the lower-order aberrations, sphere and cylinder and maybe spherical aberration, but leave the higher-order aberrations as is. Only time will tell what’s the best from that standpoint.”

Second, neural adaptation seems to explain the improvement over time of patients’ unwanted visual phenomena with multifocal lenses or suboptimal excimer laser treatments

“Clearly, this is the long-term phase of neural adaptation kicking in, not only reducing the patient’s symptoms, but actually improving the optical performance of the eye by software enhancements in the brain,” he said.

Third, for patients with small refractive errors, either after refractive surgery or in unoperated eyes, “when they say they function better without glasses, we must listen to these patients because the studies begin to show now that it is true, that the person who doesn’t wear correction has a software program that is enhancing his image with blur, and if he doesn’t wear his glasses his computer is going to do a better job,” Dr. Holladay said.

“This doesn’t mean that what we’ve been doing in the past isn’t correct, but it does mean that we have to look at the computer system back there to see the consequences,” he said. “If we have a patient who’s had refractive surgery and is –0.5 D in both eyes, we should weigh the risk/benefit ratio of relifting the flaps, the epithelial ingrowth risks and the other things that can happen, to correct that 0.5 D. If the patient were to stay with that small amount of error, he’d end up with a little better near vision and still probably have 20/25 at distance, won’t need an additional surgery and won’t need to wear glasses.”

For Your Information:

• Jack T. Holladay, MD, MSEE, FACS, can be reached at Holladay LASIK Institute, Bellaire Triangle Building, 6802 Mapleridge, Suite 200, Bellaire, TX 77401; 713-668-7337; 713-668-7336; e-mail: docholladay@docholladay.com.

References:

• Schallhorn SC, Kaupp SE, et al. Pupil size and quality of vision after LASIK. Ophthalmology. 2003;110(8):1606-1614.
• Artal P, Chen L, et al. Neural compensation for the eye’s optical aberrations. J Vis. 2004;4(4):281-287.
• Artal P, Chen L, et al. Adaptive optics for vision: the eye’s adaptation to point spread function. J Refract Surg. 2003;19(5):S585-587.
• Tan DTH. What is still lacking in refractive surgery is the role of neuroprocessing. Paper presented at: ISRS/AAO Refractive Surgery Subspecialty Day; October 14, 2005; Chicago.

• Tim Donald, OSN Copy Chief, is writing the QOV series.