Historically, experts have considered the retina as a sensory system, feeding information into the brain’s visual cortex. However, research has now demonstrated that the retina is a bidirectional neural interface that is an actual part of the central nervous system (CNS) (Vaney 1999). Since retinal signals are processed by many regions of the brain—not just the visual cortex—the implication is that retinal stimulation can affect other physical, physiological, and psychological processes, such as motor control, biochemical activity, and cognitive abilities.

 

During the past decades, researchers have discovered a retinal cell type that responds to luminance (external light) levels. These photosensitive cells combine the external luminance information with signals obtained through eyesight and not only send this combined feedforward information to the brain but concurrently receive feedback signals from the body (Chen et al 2011, 2013, Schmidt et al 2011).

 

The mixture of feedforward and feedback signaling enables the retina to be used as a two- way, noninvasive portal for influencing and monitoring body functions and thought processes, largely beneath the level of consciousness. Because of the retina’s critical role in brain function, therapeutic eyeglasses—an important tool in neuro-optometric rehabilitation—may be used to modify processing in a range of physical and mental health disorders. These individualized lenses can change the dynamic relationship between the mind’s visual inputs and the body’s internal responses by altering spatial and temporal distribution of light on the retina. The novel use of light to affect the nervous system has already been successfully applied to a range of dis- orders, including jaundice (Tayman et al 2010), jet lag (Parry 2002), seasonal affective disorder (Lavoie et al 2009), brain injury (Naeser et al 2011, Sinclair et al 2014), and spinal cord injury (Alilain et al 2008, Alilain and Silver 2009). Neuro-optometry also uses light to modulate brain and body functions.

 

This chapter presents both the theoretical framework and empirical evidence to support the use of customized eyeglasses for altering brain function. The underlying premise is that there exists a hierarchy of separate, yet interdependent, cortical and subcortical pathways, which are linked to various visual systems. The main emphasis of this discussion is on the retina’s complex connections with systems other than the conscious eyesight. Those subconscious and unconscious systems can be altered by changes in the amount, frequency, intensity, or direction of incoming light to the eye. 

 

The following section overviews retinal function, at cortical and subcortical levels, with examples of how the mind and body adapt to environmental changes. Section 3 discusses retinal structure. Section 4 introduces the concept of neuromodulation, and its effects on behavior and processing. Neuromodulation is described here as the process of achieving balance between mental and physical functions at both conscious and nonconscious levels. Section 5 describes the impact that eyeglasses can have on the nervous system.