Visual System II: The Visual Pathways And Subcortical Visual Areas
The retina conveys its information from the ganglion cells to a number of different sites, including:
• Several Cortical Areas, Via The Lateral Geniculate Nucleus (LGN) Of The Thalamus To The Primary Visual Cortex (V1 Or Brodmann’s Area 17). Other Cortical Areas (Known Collectively As The Extrastri- Ate Areas) Receive Information From The LGN As Well As The PulviNar Region Of The Thalamus (See Chapter 26);
• The Hypothalamus;
• The Midbrain.
The projection from the retina to V1 maintains its retinotopic organization, such that a lesion along the course of the pathway produces a predictable visual field defect. Lesions in front of the optic chiasm typically produce uniocular field defects, while lesions of the chiasm (e.g. from pituitary tumours) cause a bitemporal hemianopia. Lesions behind the chiasm typically produce similar field defects in both eyes, e.g. a homonymous hemianopia or quadrantanopia.
Lateral geniculate nucleus
• The LGN consists of six layers in primates, with each layer receiving an input from either the ipsilateral or contralateral eye.
• The inner two with their large neurones form the magnocellular laminae while the remaining four layers constitute the parvocellular laminae. The morphological distinction between the neurones in these two laminae is also evident electrophysiologically.
• The parvocellular neurones display chromatic or colour sensitivity and sensitivity to high spatial frequency (detail) with sustained responses to visual stimuli. In contrast, the magnocellular neu- rones show no colour selectivity, respond best to low spatial frequencies and often have a transient response on being stimulated.
• Thus, the magnocellular layer neurones have similar properties to the Y ganglion cells and the parvocellular neurones to the X ganglion cells, a similarity that is reflected in the retinogeniculate projection of these two classes of ganglion cells. The X ganglion cells and the parvocellular laminae neurones are responsible for the detection of colour and form (or Pattern) and constitute the P channel, while the M channel of the Y ganglion cells and the magnocellular laminae of the LGN are responsible primarily for motion detection (or Movement).
• The LGN mainly projects to V1, where the afferent fibres synapse in layer IV, and to a lesser extent layer VI, with the M and P channels having different synaptic targets within these laminae. In addition, there is a projection from cells that lie between the laminae of the LGN (intralaminar part of the LGN) directly to layers II and III of V1 (see Chapter 26).
The superior colliculus in the midbrain is a multilayered structure, wherein the superficial layers are involved in mapping the visual field and the deep layers with complex sensory integration involv- ing visual, auditory and somatosensory stimuli. The intermediate layers are involved in saccadic eye movements and receive connections from the occipitoparietal cortex, the frontal eye fields and the substantia nigra (see Chapter 56). The saccadic eye movements are mapped in the superior colliculus to the visual field representation. So stimulation in this structure will cause a saccadic eye movement that brings the point of fixation to that point in the visual field that is represented in the more superficial layers of this structure. In the superior colliculus all the different sensorimotor representations lie in register. In other words, a vertical descent through this structure encounters, in the following order:
• Neurones that respond to visual stimuli in a given part of the visual field;
• Neurones that cause saccadic eye movements that bring the fovea to bear onto that same part of the visual scene;
• Auditory and somatosensory neurones that are maximally acti- vated by sounds that originate from that part of the visual environ- ment and by areas of skin that would most likely be activated by a physical contact with an object located in that part of the extrap- ersonal space. This latter feature accounts for the fact that in the superior colliculus the somatosensory representation is primarily skewed towards the nose and face.
Thus, the superior colliculus not only codes for saccades, but tends to code specifically for those saccades that are triggered by stimuli of immediate behavioural significance as well as having a more widespread function in orienting responses. This role for the superior colliculus is reflected in its efferent connections to a number of brainstem structures as well as the spinal cord (tectos- pinal tract). Clinically, damage is rarely confined to this structure, but when it is, there is a profound loss of saccadic eye movements with neglect.
Pretectal structures and the pupillary response to light
There is a projection from the optic tract to the pretectal nuclei of the midbrain which in turn projects bilaterally to the Edinger– Westphal nucleus, which provides the parasympathetic input to the pupil allowing it to constrict.
• Light shone in one eye will cause constriction of both pupils (direct and consensual response).
• Damage to one of the optic nerves will cause a reduced direct and consensual response but that same eye will constrict normally to light shone in the unaffected eye, producing a relative afferent pupillary defect.
Suprachiasmatic nucleus of the hypothalamus
This nucleus receives a direct retinal input and is important in the generation and coordination of circadian rhythms (see Chapter 11).