We also now have a rich opportunity to address how neurons and circuits relate to behaviors such as orienting, escape and freezing, and even more complex behaviors such as attention and decision-making. With the explosion of molecular and genetic methods, new results are appearing and we are well poised to answer long-standing questions regarding neuronal types and circuits across species that will shed light on critical comparative and evolutionary questions. Much of our knowledge of the SC however, remains at the level of layers and regions rather than neuronal cell types and circuits. The SC also plays a role in avoidance behaviors and, more recently revealed, in higher cognitive functions such as visual spatial attention and decision-making ( Basso and May, 2017). Based on extensive fundamental research, we now understand the SC to be a system integral for encoding spatial locations and transforming them into stimulus-directed orienting and approach behaviors. During this time, the SC became and remains one of the most well studied structures in the brain. The SC has been studied for over 100 years ( Ramón y Cajal, 1909, 1995). In this review, we shall use SC as a general term unless specifically referring to non-mammals. The tectum is referred to as the superior colliculus (SC) in mammals, and the optic tectum (OT) in non-mammals. Our knowledge of the anatomy, physiology and function of the tectum stems from work performed in all classes of vertebrates: from the primitive jawless fish, the lamprey, to bony and cartilagenous fish, amphibians, reptiles, birds, and mammals, including primates ( Figure 1). The tectum (roof in Latin) lies on the dorsal surface of the midbrain. Today, modern molecular and genetic methods combined with sophisticated behavioral assessments have the potential to make significant breakthroughs in our understanding of the evolution and conservation of neuronal cell types and circuits in the superior colliculus that give rise to simple and complex behaviors.
Indeed, in addition to collicular outputs targeting brainstem regions controlling movements, the superior colliculus also has ascending projections linking it to forebrain structures including the basal ganglia and amygdala, highlighting the fact that the superior colliculus, with its vast inputs and outputs, can influence processing throughout the neuraxis. Although the evidence for this is strong and compelling, a number of experiments reveal a role for the superior colliculus in behaviors usually associated with the cerebral cortex, such as attention and decision-making. The superior colliculus is a conserved sensorimotor structure that integrates visual and other sensory information to drive reflexive behaviors.
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