Also referred to as the OPTIC TECTUM, the superior colliculus is part of the VISUAL SYSTEM. It is composed of distinct layers of neurons and fibres: (from dorsal to ventral) zonal, superficial grey, optic, intermediate grey, intermediate white, deep grey, deep white. RETINOTOPIC visual input comes from the RETINA: slow-conducting fibres terminate in the upper part of the superficial grey layer while faster-conducting fibres terminate in the lower parts of this layer, and in the optic layer. Inputs also arrive from the THALAMUS and ZONA INCERTA, cholinergic neurons in the PEDUNCULOPONTINE TEGMENTAL NUCLEUS and PARABIGEMINAL NUCLEUS, and there is a substantial input from the SUBSTANTIA NIGRA pars compacta (the NIGROTECTAL PATHWAY) that uses GABA and tonically inhibits the superior colliculus. Outputs of the colliculus are directed to a variety of motor sites in the BRAINSTEM and to the thalamus (see GENICULOSTRIATE AND TECTOPULVINAR VISUAL PATHWAYS).
The superior colliculus is evolutionarily old. It has important functions for amphibia: the frog superior colliculus operates as a BUG DETECTOR, the sight of an insect moving across the visual field triggering a reflexive tongue extension for prey capture.
In more advanced VERTEBRATES, the superior colliculus retains an important aspect of this process: it has direct visual input and direct output to motor systems, enabling it to activate behaviour very rapidly in response to stimulation. Indeed, in the mammalian superior colliculus, while there are no bug detectors, there are neurons whose firing is graded according to the salience of the visual stimuli being processed. It has been argued that what the superior colliculus does is provide a first-pass analysis of visual input and activate orienting movements, approach or avoidance behaviour as required, and there is evidence to suggest that these processes are served by distinct anatomical systems (see Redgrave et al., 1993). There is also evidence that the superior colliculus is involved in NOCICEPTION, serving a similar function of altering behaviour promptly in response to stimulation that requires immediate attention.
Redgrave P., Westby G.W.M. & Dean P. (1993) Functional architecture of rodent superior colliculus—relevance of multiple output channels. Progress in Brain Research 95: 69–77.
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