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).
Superior colliculi
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).
