The Noise Ratio and Cognitive Load

JOV Cover March 2020

The impetus for this piece comes from an open access article in the March 2020 issue of Journal of Vision titled: Factors limiting sensitivity to binocular disparity in human vision: Evidence from a noise-masking approach.  One of the co-authors is Robert Hess, and thinking about him reminds me that Bob Sanet, Pilar Vergara, and I were scheduled to do a joint presentation at the COVD meeting in Canada this month on Amblyopia which was derailed by you-know-what.  One of the researchers whose work I was going to spend considerable time discussing was Robert Hess, an individual with an Optometry background who is Professor and Director of Research in the Department of Ophthalmology at McGill University.  But Hess isn’t a one trick pony — his research extends into many areas of vision, and you’ll enjoy checking out his lab’s home page.

Screen Shot 2020-04-03 at 9.28.17 AM

In the article above, Hess and colleagues refer to noise which, for our purposes, I’ll define as perturbation in vision that influences visual input or processing. If the source of the noise is the target or the environment we’ll call it external.  If it is within the individual’s visual system we’ll call it internal.  Examples of external noise include perturbation through stimulus changes in luminance, contrast, form, motion, and contour.  Sources of internal noise include changes in the ocular-motor, accommodation, vergence, and executive function systems.

Describing the influence of noise on stereo sensitivity, Hess and colleagues write:

“In noise-masking experiments, thresholds are typically unaffected by low levels of external noise. Once the external noise exceeds some critical value, however, thresholds increase linearly with its standard deviation. The transition point is the subject’s “equivalent internal noise” for performing the task. When the external noise is much smaller than the equivalent internal noise then performance is limited by the internal noise. When the external noise is much greater than the internal noise then performance is limited by that external noise.”


So let’s take the S’s to represent the various sources of noise in the external stimulus.  Now let’s take the Z’s to represent the tendency for you to fall asleep while reading this.  (Just wanted to see if you were paying attention!)  Seriously, let’s take the Z’s to represent the various sources of internal noise.  We know from studies in adaptive optics that aberrations can be purposeful, so it is safe to assume that some degree of noise is desirable (much as a certain degree of stress is purposeful).  Let’s assume that the idealized noise ratio of S/Z = 1.0.  Anything less that 1.0 would indicate that internal noise is predominating over external noise, and is more likely to degrade visual processing.


We know that through optometric vision therapy we can reduce internal noise, most readily through lenses, prisms, disparity processing and so forth, restoring the ratio closer to 1.0.  Hess and colleagues note that the visual system can compensate for noisier input by adjusting the efficiency of processing, but their analysis was conducted within the framework of the linear amplifier model. They chose this approach because … “it provides a simple method for analyzing noise-masking data, which appear to agree with the behavior expected according to that model.  It is almost certain that a full account of stereo sensitivity will require a model that is significantly more complex than the one we apply here.”

Cognitive Load Graphic

A more complex model, and one that would add to the non-linear amplifications that exist in the visual system, may reside in the cognitive elements of visual processing. Uploading or downloading the cognitive load during optometric vision therapy modulates noise ratio.  This may be one reason why scientists working in artificial intelligence have been so challenged to design visual systems that perform well in natural scene environments.  I and many others in our field have suggested that the extent to which we successfully incorporate cognitive loading is a significant factor in generalizing and transferring learning (optical as well as perceptual) to the patient’s activities and demands of daily living.


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