# Show Reference: "Development of the Superior Colliculus/Optic Tectum"

Development of the Superior Colliculus/Optic Tectum In Neural Circuit Development and Function in the Brain (2013), pp. 41-59, doi:10.1016/b978-0-12-397267-5.00150-3 by Barry E. Stein, Terrence R. Stanford
@inbook{stein-and-stanford-2013,
abstract = {The superior colliculus ({SC}) and its non-mammalian homologue, the optic tectum ({OT}) are visually-dominant structures in the midbrain. However, they also receive substantial auditory and somatosensory inputs that aid in the task of transforming sensory cues into motor commands. The best known behavioral role of this brain region is the control of orientiation, especially shifts of gaze. The present chapter focuses on the development of the neural properties (primarily of the {SC}) that underlie this function. Of special concern is the overlapping and map-like form of each of the {SC} sensory representations, and the multisensory neurons that integrate information from multiple senses. The sensory topographies in the {SC} are linked to motor outputs and their overlapping organization ensures consistency in the way different sensory cues derived from the same event are located with respect to the motor effectors. Multisensory neurons use this information to enhance the physiological impact of such cross-modal events so that they have a higher likelihood of eliciting an overt orientation response. However, both the topographic organization of these sensory representations and the multisensory integration capabilities of {SC} neurons develop gradually as the brain accumulates experience with such events. This appears to ensure that the physiological properties of these neurons are best adapted to the environment in which they will be used. These maturational issues and their functional impact are discussed in depth. Keywords: Auditory; Multisensory integration; Somatosensory; Topographies; Visual},
author = {Stein, Barry E. and Stanford, Terrence R.},
booktitle = {Neural Circuit Development and Function in the Brain},
doi = {10.1016/b978-0-12-397267-5.00150-3},
isbn = {9780123972675},
keywords = {biology, development, sc},
pages = {41--59},
posted-at = {2013-05-14 13:24:54},
priority = {2},
publisher = {Elsevier},
title = {Development of the Superior {Colliculus/Optic} Tectum},
url = {http://dx.doi.org/10.1016/b978-0-12-397267-5.00150-3},
year = {2013}
}


The SC localizes events.

Optic tectum and superior colliculus are homologues.

The tectum includes both sc (optic tectum) and ic

The SC is multisensory: it reacts to visual, auditory, and somatosensory stimuli. It does not only initiate gaze shifts, but also other motor behaviour.

The SC is involved in the transformation of multisensory signals into motor commands.

The SC maturates fast compared to the cortex; this is important to protect the young animal from threats in early life.

The mammalian SC is divided into seven layers with alternating fibrous and cellular layers.

The superficial layers include layers I-III, while the deep layers are layers IV-VII.

Some authors distinguish a third, intermediate, set of layers (IV,V).

There are ascending projections from the superficial SC to the Thalamus and from there to extrastriate cortex.

There are descending projections from the SC to the parabigeminal nucleus, or nucleus isthmii as it is called in non-mammals.

The deeper levels of SC are the targets of projections from cortex, auditory, somatosensory and motor systems in the brain.

The deeper layers of the SC project strongly to brainstem, spinal cord, especially to those regions involved in moving eyes, ears, head and limbs, and to sensory and motor areas of thalamus.

The superficial SC is visuotopic.

The part of the visual map in the superficial SC corresponding to the center of the visual field has the highest spatial resolution.

Visual receptive fields in the deeper SC are larger than in the superficial SC.

The parts of the sensory map in the deeper SC corresponding to peripheral visual space have better representation than in the visual superficial SC.

Stein offers an operational definition of multisensory integration as

...the process by which stimuli from different senses combine ... to produce a response that differs from those produced by the component stimuli individually.''

Neurons in the deep SC whose activity spikes before a saccade have preferred amplitudes and directions: Each of these neurons spikes strongest before a saccade with these properties and less strongly before different saccades.

Moving the eyes shifts the auditory and somatosensory maps in the SC.

Altricial species are born with poorly developed capabilities for sensory processing.

(Some) SC neurons in the newborn cat are sensitive to tactile stimuli at birth, to auditory stimuli a few days postnatally, and to visual stimuli last.

Visual responsiveness develops in the cat first from top to bottom in the superficial layers, then, after a long pause, from top to bottom in the lower layers.

The basic topography of retinotectal projections is set up by chemical markers. This topography is coarse and is refined through activity-dependent development.

We do not know whether other sensory maps than the visual map in the SC are initially set up through chemical markers, but it is likely.

There's a retinotopic, polysynaptic pathway from the SC through LGN.

Overt visual function occurs only starting 2-3 weeks postnatally in cats.

Some animals are born with deep-SC neurons responsive to more than one modality.

However, these neurons don't integrate according to Stein's single-neuron definition of multisensory integration. This kind of multisensory integration develops with experience with cross-modal stimuli.

Less is known about the motor properties of SC neurons than about the sensory properties.

Electrical stimulation of the cat SC elicits eye and body movements long before auditory or visual stimuli could have that effect.

These movements already follow the topographic organization of the SC at least roughly.

Stein defines multi-sensory integration on the single-neuron level as

a statistically significant difference between the number of impulses evoked by a cross-modal combination of stimuli and the number evoked by the most effective of these stimuli individually.''

Multisensory enhancement and depression are an increased and decreased response of a multisensory neuron to congruent and incongruent stimuli, respectively.

Neurons in the superficial SC are almost exclusively visual in most species.

Deactivation of AES and rLS leads to a complete lack of cross-modal enhancement while leaving intact the ability of multi-sensory SC neurons to respond to uni-sensory input and even to add input from different sensory modalities.

The direction of a saccade is population-coded in the SC.