The Superior Colliculus
The superior colliculus in the mammalian mid-brain is responsible for
the control of goal directed
orientation responses towards novel sensory stimuli. On anatomical
grounds the SC is divisible in
to seven alternating cell and fibre layers from dorsal to ventral. The
three superficial laminae
(SSC) are the striatum zonalae (SZ) stratum griseum superficiale (SGS)
and stratum opticum. Sensory neurones in these layers are exclusively
responsive to visual stimulation. Generally the
visual responses of individual neurones are binocular, direction
selective, and have discrete
retinotopically organized spatial receptive fields. Whilst afferents
from the retina and cortex
innervate the whole SGS the distribution of inputs from these areas is
not even. Indeed, the
stratum griseum superficiale can be further subdivided into three
sub-layers, SGS1-3 in terms of
the distribution of inputs. SGS1 (25um-120um from the surface) derives
much of its input from
W type retinal ganglion cells. SGS2 occupies approximately 300um and
much of the input to this
region is from areas 17, 18, and 19 of the visual cortex. Y type
retinal ganglion cells synapse in
SGS3.
The deep layers of the
mammalian superior colliculus (DSC) are responsible for the generation
of
appropriate orientation responses of the eyes, head, pinnae and
whiskers towards novel sensory
stimuli. To this end, visual, auditory and somatosensory space are
represented in mutually aligned
neuronal arrays in the deep SC. The spatial representations of the
individual modalities are formed
from the same population of neurones, many of which are influenced by
stimuli of more than one
sensory modality.
There is strong evidence to suggest that multi-modal sensory
integration takes place in the DSC. In general, if the stimuli are
spatially and temporally coincident then the response is facilitatory
but
if there are disparities in either time or space, the response is
depressed. Behaviourally this
mechanism is useful since, stimuli with similar temporal and spatial
characteristics are likely to
arise from the same event whereas stimuli with spatial or temporal
disparities are likely to be
unrelated.
This page was written by Tom
Salt, and is part of the Neurotransmitters
in Sensory Systems Home Page.
