My Vision on Vision

Abstract: 

Experiences from over 40 years in Vision Rehabilitation have given me some insight into the nature of “Vision” that differs from the traditional approach by clinicians and scientists.

Visual perception is often portrayed as a one-way stream from the object, to its retinal image, to the visual cortex and beyond. The fact that recognizing an object or person activates far more informational content than is contained in the retinal image, implies that they must be an additional source.  Visual perception results when the retinal input is matched to concepts stored in memory.  The result is a Mental Model of the environment.  Our understanding of our interactions with the environment is based on this Mental Model, not just on the retinal image.  The Mental Model is not just a snapshot; it builds over time from successive retinal images.

Visual processing is not just a one-way stream.  The brain is known to contain massive feedback loops that allow parallel processing.  Some of these streams connect to memory banks and to conscious awareness. Large amounts of visual information, however, are processed autonomously without reaching conscious awareness. This is comparable to the autonomous regulation of bodily functions, such as breathing, heart rate, digestion, and many others.  This autonomous part of vision has rarely attracted much attention.  Yet, it is important for normal functioning and for vision rehabilitation, since vision for Orientation and Mobility falls largely in this area.

Awareness of these differences affects how we test and interpret vision tests.  The most important attributes of vision are visual acuity, visual field, and contrast.

Visual acuity is often considered to reflect vision in general.  This is not true.  In fact, letter chart acuity is only one scale on which the ability to see detail can be measured, and detail vision is only one aspect of vision. Letter chart acuity assesses only the area where the letter is projected. Even for a 20/200 letter, this is less than 1 degree.  Most tests, moreover, determine threshold performance, while sustainable performance is more relevant for activities of daily living.

Visual fields are most often measured to determine the topography of retinal changes.  How peripheral vision loss affects mobility is often misunderstood, because this aspect of vision is largely processed autonomously and subconsciously.  Visual field tests are strenuous because they require conscious responses from the sub-conscious autonomous system. A new test of peripheral vision requires only autonomous responses (fixation and saccades). It thus offers a much faster and less strenuous screening test for peripheral vision loss.

Contrast sensitivity is usually measured separately from visual acuity and therefore, often is considered to be an independent parameter. This is not true; contrast vision interacts with both visual acuity and visual field.  This interaction takes place at supra-threshold levels and therefore is missed by threshold measurements. The Mixed Contrast cards measure this interaction and are more demonstrative for the patient. Contrast deficits occur early in both outer-retinal disease (AMD) and inner-retinal disease (glaucoma). More attention should be paid to their potential for early disease detection.