Organized into 7 alternating fibrous and cellular layers. These are further categorized into superficial and deep layers:
-- Stratum Zonale (SZ)
-- Stratum Griseum Superficiale (SGS)
-- Stratum Optican (SO)
-- Stratum Griseum Intermedium (SGI)
-- Stratum Album Intermedium (SAI)
-- Stratum Griseum Profundum (SGP)
-- Stratum Album Profundum (SAP)
Sensory Responses -- Superior Colliculus neurons respond to visual, auditory and somatosensory stimuli but do not respond well to specific features of these stimuli.
* Cells in superficial layers respond to visual stimuli:
-- Response insensitive to shape and size of stimulus.
-- Respond to both moving and non moving stimuli, although moving stimuli produce more vigorous responses
-- Non-color opponent
-- Respond more vigorously to stimuli that are the target of a saccade (!)
* Cells in the deep layers respond to multimodal sensory inputs:
-- Visuomotor cells that discharge in association with saccades
-- Bi- and tri-modal cells that respond to visual and somatosensory and/or auditory stimuli
-- Cells responsive only to somatosensory or auditory stimuli
Topographical Maps -- Many researchers have claimed that there is an alignment of auditory, somatosensory, visual and motor maps in the superior colliculus, although the functional significance of this has received little experimental attention. Although these correspondences could be artifactual, Sparks gives two arguments against this:
* Many cells in deeper superior colliculus layers are multimodal. This implies that signals from each independent map are converging.
* Responses of superior colliculus cells to one type of sensory stimulus can be drastically altered by other types of stimuli.
* In addition, the finding that auditory maps can shift in response to eye position.
* Microstimulation produces conjugate, contralateral saccades
-- Amplitude and direction of the saccade induced depends on the site of stimulation within superior colliculus
-- Motor map aligned with visual topography
* Chronic recording reveals movement fields
--Neurons in intermediate and deep superior colliculus discharge maximally prior to saccades with particular amplitudes and directions.
-- Exhibit temporal gradient.
* Animals with superior colliculus lesions can still generate saccades to visual targets with an increase in latency.
-- This suggests that longer latency pathways are being used to control these saccades.
* Recent muscimol (a GABA agonist) studies that the superior colliculus is involved in calculation of saccade amplitude and direction.
-- Saccades were severely hypometric and had high latency.
Foveation Hypothesis - The foveation hypothesis states that the role of the superior colliculus is to code the location of a visual target relative to the fovea and to intitiate a saccade that will produce foveal acquisition of the target.
-- Retinal error represented by site of visually triggered activity in the retinotopic superficial layers of the superior colliculus.
-- Visually triggered discharges are assumed to activate neurons in the deep layers that discharge before saccades.
-- Psychophysical experiments demonstrate that visual targets are perceptually localized in non retinocentric coordinates.
-- Model depends on presumed connections between superficial and deep layers, which are just now beginning to be understood (see Doubell, 2003 for a recent study on the connections in ferret).
-- Timing considerations suggest that the connections from superficial to deep layers (if they exist) are neither direct nor simple.
-- Model requires that superior colliculus is organized in retinotopic coordinates, this does not allow for saccade direction from other modalities that code in other coordinate systems.
Attention Hypothesis - This model is two part: 1) Superior Colliculus is involved in shifting attention to a specific spatial location, and 2) the Superior Colliculus is not involved in specifying the exact location of a visual target or in specifying the precise metrics for a saccade.
* Shift of attention:
-- Based on finding that response of superficial layer neurons is enhanced when visual subject is the target of a saccade.
* Is superior colliculus involved in specifying precise saccade metrics?
-- Based on large receptive and movement fields...but is it possible to code for metrics using coarse individual coding and population codes?
-- Results of lesion experiments showed that superior colliculus is not necessary for saccades ... but more recent lesion data contradicts this.
-- There is some evidence that superior colliculus neuronal activity is not tightly coupled with saccade onset, but these data are contradicted by burst cells that fire prosaccadically.
Motor Error Hypothesis - Since signals from several sensory modalities converge in the deeper layers of the superior colliculus and this site also contains cells with motor properties, it is natural to propose that the superior colliculus is involved in translating sensory signals into motor commands. The hypothesis is that the superior colliculus codes for saccades in motor-error coordinates (position of eyes in the orbit relative to the target position), i.e. that all the sensory maps are organized in these coordinates.
* Collicular neurons discharge before saccades of a particular direction and amplitude regardless of eye position in orbit. This suggests that these neurons encode a motor error signal.
* Cells that look to be coding motor error were found in the intermediate layer of the superior colliculus.
* Auditory neurons in the superior colliculus responded as if in motor-error coordinates.
* Cells that burst before visually initiated saccades also burst before auditorily initiated saccades, suggesting that both visual and auditory signals have been converted to motor error coordinates.
* Left and right hemispheres of superior colliculus are in competition.
* Superior colliculus is responsible for rapid orienting behavior.
Connections are consistent with SC being involved in transforming signals from several sensory systems into motor commands for guiding orienting responses of eyes and head.
Afferent Connections to Superficial Layers:
The superficial layers receive inputs related almost exclusively to vision. Retinotopic map of the contralateral visual field formed by the termination of retinal and cortical axons:
-- Non color opponent retinal ganglion cells
-- Striate Cortex:
* Layers V and VI of Primary Visual Cortex. (Rhesus)
* Six visual cortical regions in owl monkey (areas 17, 18, middle temporal, dorsomedial and medial areas and the posterior parietal region).
-- Extrastriate Cortex:
* Layer V of areas 18, 19, FEF, and pre-motor cortex.
Superficial Layer Efferents:
Efferents from the superficial layers of the SC project to numerous areas involved in vision.
-- Parabigeminal nucleus (ipselateral)
-- Inferior pulvinar nucleus
-- Magnocellular dorsomedial nucleus of thalamus
-- Dorsal and ventral LGN
Afferent Connections to the Deep Layers:
Deep areas receive inputs from several sensory modalities, visual, auditory, and somatosensory, inputs from motor areas and projections from areas neither purely motor nor sensory. These come from both cortical and noncortical sources.
-- Layer V of FEF
-- Prefrontal association cortex
-- Inferior bank of intraparietal sulcus
-- Layer V of temporal lobe and occipital lobe
Subcortical Inputs: (mostly known in cat)
-- Substancia Nigra. (Basal Ganglia)
Deep Layer Efferents:
Ascending efferents from the deep layers project to areas involved in producing saccadic eye movements.
-- Dorsal thalamus ~> FEF and inferior parietal lobule.
-- Rostral interstitial nucleus of the medial longitudinal fasciculus.
-- Oculomotor complex
-- Magnocellular division of the ventral anterior nucleus
-- Paralamellar region of the mediodorsal nucleus
-- Parafascicular nucleus
Descending efferents from the deep layers project to brain stem areas that have direct connections to motor neurons in neck
Projections thought to be involved in controlling eye movements
-- Nucleus reticularis tegmenti-pontis (NRTP)
-- Nucleus reticularis pontis oralis
-- Cell groups central and medial to the abducens nucleus
-- Medial accessory nucleus of the inferior olivary complex
The level of interconnectivity within the Superior Colliculus is still (as of this review) unresolved.