fig. 8.1. - Visual Optic pathway
The blue pathway: The nonvisual optic pathway (fig. 8.2), for its part, is mostly influenced by the color blue, to which the ipRGC ganglion cells driving it are most sensitive. It is clearly involved in the physiological reactions to color, especially in the blue to green part of the spectrum. Because of the relatively slow reaction time of the ipRGCs, chromotherapeutic effects on this system will usually require several minutes of exposure at comparatively high intensities. However, these generalities are increasingly qualified as our knowledge of neurology improves.
The retinotectal pathway: A third network is the retinotectal (subcortical) pathway that connects the retina directly with the superior colliculus (SC), the small midbrain structures that play a role in the sensory motor control of the eyes, somewhat like an internal gyroscope. The SC is also involved in unconsciously perceived emotional stimuli and is linked to other structures of the brain implicated in this function, such as the pulvinar and even the amygdala, which is connected to the eyes by more than one pathway. The retinotectal pathway originates in the ganglion cells, which are joined to the cones of the retina and are therefore sensitive to color.
The accessory optic tract: This fourth network connects the retina directly with the brainstem. Light is thus involved in the multiple physiological functions of the brain stem, including regulation of the heart rate and breathing, and managing our level of alertness and the quality of sleep. We have known since the 1960s that this optic pathway is color sensitive, each color activating distinct sets of neurons.