Eyes and Brain
Seeing is a complex task that involves the eyes and brain. The term vision refers to the interactive processes between the eye and brain that results in seeing, but vision is not synonymous with sight.1 Consider the flow of a common set of visual experiences. We walk down a street, step up and down curbs, maneuver around objects or people and adjust our pace, while visually monitoring our position. Moments later we get into our car, drive through traffic and judge where we are relative to other vehicles while anticipating the flow of traffic. The conversion of light entering the eyes into ideas in the brain is a complex task far beyond the abilities of the world's most powerful computers.
The neural mechanisms of visual perception offer rich insight into how the brain handles such computationally complex situations. Vision involves distilling foreground from background, recognizing objects presented in a wide range of orientations, and accurately interpreting spatial cues. The seeing of objects involves many sources of information beyond those meeting the eye when we look at an object. It generally involves knowledge of the object derived from previous experience. Visual perception begins as soon as the eye focuses light onto the retina. Visual information is transferred from the retina to many other areas in the brain where it is connected to previous experience in order for understanding and meaning to occur.
The retina is a thin sheet of brain tissue in the eyes. It is the place where the brain first encounters light.2 Signals travel back-and-forth between the eyes and the rest of the brain. Visual processing involves several major sub-cortical centers plus a mosaic of dozens of distinct areas in the cerebral cortex.10 It is currently acknowledged that vision is the result of parallel, distributed processing in multiple areas and through multiple pathways.11 For example, information gathered by the retina about color is processed in a different area of the brain than information about movement.4
As another example, egocentric direction describes the perceived location of an object compared to our body. This is derived from a combination of oculocentric direction (where our eyes are aimed), position of the eyes in the head, and the head's position relative to the body. The brain uses a reference point midway between the two eyes, known as the ego center to compute egocentric direction.12 Binocular vision is an intricate organization of biologic and psychologic components.7 enabling the brain to engage a whole body experience.
The majority of nerve cells from the retina project to the visual cortex. However, at least ten percent of the nerve cells take a different pathway8 stimulating areas of the brain stem dedicated to functions that seem remote to vision, when vision is narrowly defined.9 The existence of extensive sensory motor pathways supports a broader conceptualization of vision, integrating functions such as balance and visual-auditory localization.13 "As surely as the old system (for explaining vision) considered that the problem of knowledge and understanding could be separated from the problem of seeing, so the present one will find it increasingly difficult to draw a dividing line between the two."3
Advances in imaging technology allow for a better understanding of the structure of the visual system. For example, high resolution MRIs are a tool allowing optometry to better understand specialized centers of the brain involved with a variety of pathways leading to important functions of daily living. Examples include, cortical centers specialized for language processing, attentional pathways, eye movements and vergence centers. Our knowledge of these centers allows optometrists to better understand neuroplasticity for improved treatment and outcomes. 15
Many aspects of the optometric examination probe the eye-brain collaboration. Consider the complexity of the evaluation of visual fields. A patient is instructed to simultaneously maintain steady central fixation, attend to central and peripheral stimuli, discriminate threshold stimuli, and demonstrate awareness with an appropriate motor response.
Similarly, the clinical assessment of color vision requires more than the discrimination of colors. Every color test has multiple perceptual components. For example, color vision plates require recognition of form and the emergence of figure from background. Color cap tests (Farnsworth) are predicated on good sequencing abilities and subtle discriminatory skills.
The process of binocular vision is a reflection of complex interactions within the eye-brain continuum. For neural binocular summation to occur, inputs from both eyes to the brain must be synchronized in both space and time.5 Alignment of the eyes is maintained via ongoing collaboration of eyes and brain. Binocular dysfunctions such as suppression and anomalous correspondence demonstrate cortical adaptations in the eye-brain function to minimize visual confusion and maintain some level of visual performance.6
It is evident that, beyond eye health, optometrists evaluate a wide variety of visual abilities. These include visual-spatial orientation skills, visual analysis skills (including auditory-visual integration, visual discrimination, visual figure-ground perception, visual closure, visual memory, and visualization), visual motor integration skills, and visual-verbal integration skills.14
Information from neuroimaging and insights from cognitive neuroscience demand a significant reformulation of the understanding of vision. Vision occurs neither in the eyes nor in the brain, but emerges from the collaboration of the eyes and the rest of the brain. Vision is a pervasive aspect of our existence which permeates all of our activities and is, in fact a whole body experience.17 We depend on vision, more than on any other sense, to perceive the world of objects and events beyond our bodies and to guide our actions and adaptive behaviors.18 Vision develops and, due to neural plasticity, can be enhanced.19 Optometry is the discipline dedicated to the care of all aspects of the visual process.
This publication was formulated by the American Optometric Association's Binocular Vision Working Group. The following individuals are acknowledged for their contributions: