Structure of Human Visual System and Visual Neurological Pathways

Structure of Human Visual System and Visual Neurological Pathways
The brain cortex has at least 30 specialized areas (Hyvarien, 2005) that handle specific parts of visual information. In our practice, we lack information about the specific exact lesion sites, but we do need to know the localized lesion areas that interfere with visual functions of CVI people. Human relies on vision for daily functioning, especially critical for young children who are in active developing stage of eye-hand functions. Human’s visual function highly depends on both ocular structures and neurological structures in our brain. These structures can be simply divided into Anterior Visual Pathway and Posterior Visual Pathway.


Geniculostriate Pathway
Visual images are received by Eyeballs and are converted into visual information in retina. The visual information is thus transmitted through Anterior Visual Pathway (Including Eyeball, Optic nerve, Optic Chiasm and Optic tract) to Lateral Geniculate Nucleus (LGN). Lateral Geniculate Nucleus, located in Thalamus, (Pitto & Kupers, 2005) is the landmark which differentiates Anterior Visual Pathway and Posterior Visual Pathway. The visual information is thus transferred through Optic Radiation to Primary Visual Cortex (striate cortex, V1) (Fig 2), located at the back of the brain, for primary visual processing.
Dorsal stream and Ventral stream
To serve higher visual functions (e.g. visually guided movement; integrating attention, recognition and route finding (Dutton, et al, 2003)), the Geniculostriate Pathway branches into the Dorsal stream and Ventral stream so as to associate with other lobes in the brain. The Dorsal stream connects between occipital lobes and parietal lobes to process the whole visual scene and carry out visually guided movement. (Dutton, et al, 2003) The Ventral stream connects between occipital lobes between occipital lobes and temporal lobe and process visual recognition (e.g. face recognition, line, angle and shape perception) and visual memory. (Dutton, et al, 2003; 2004)

Choose of element in visual scene (Dutton, et al, 2003)
“Choose of element” involves both dorsal and ventral stream. In daily life, to handle massive information surrounded us; our posterior parietal lobes are responsible for processing the overall visual scene and allow us to attend to the element of scene at one time. Posterior parietal lobes, together cooperate with temporal lobes (serve recognition) and the frontal lobes (serve attention to areas of interest and motor planning), human is able to integrate attention, visually guided movement (eye-hand coordinated movement), recognition and route finding. (Dutton, et al, 2003)

Tectal Pathway or Extrageniculate Pathway
There is another parallel visual pathway (Hyvarinen, 2005), which is called Tectal Pathway (Hyvarinen, 2005) or Extrageniculate Pathway (Ro & Rafal, 2006) serves for visual functions including perception of movement, visually guided movement, eye saccade (Hoyt, 2003; Ro & Rafal, 2006) and less effective color processing (Ro & Rafal, 2006). This pathway branches before Lateral Geniculate Nucleus (LGN) and brings visual information to Superior Colliculus (SC) (serves for rapid eye movements)   and  Pulvinar (PU).

Fig. 3 Tectal pathway (Hyvarinen, 2005)

Finally, visual information is transmitted to the cortical areas in parietal and occipital without the analysis of V1. (Hyvarienen, 2005; Ro & Rafal, 2006) (Fig. 3) As a result, people with cortical visual impairment still have good eye-hand co-ordination, detection of movement (Hoyt, 2003) and less effective color processing. (Hoyt, et al, 2003; Ro & Rafal, 2006) Although this parallel pathway together serves for visual function, CVI clients still have other significant deficits in visual recognitions including visual object agnosia, (Ogden, 1993) prosopagnosia (Ogden, 1993; Dutton, 2003) and Topographic agnosia. (Dutton, 2003) Since most of the lesion sites in CVI clients are Posterior Visual Pathway, neurological structures beyond LGN, (Watson, et al, 2007; Malkowicz, et al, 2006) Visual rehabilitation begins with the visual stimulation that promotes connections between synapse in human brain. (Powell, 1996) Neural-based stimulation that will eventually strengthen the neural pathway that are so critical for vision. (Powell, 1996) Therefore, training of these retinotectal and / or geniculoextrastriate pathways may be advantageous in restoration of visual function after primary visual cortex damage (Ro & Rafal, 2006).<Tang, W. (2008). Visual Rehabilitation for People with Cortical Visual Impairment. In Progress.>


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