In vertebrates, vestibular sensory inputs from semi-circular canals of the inner ear detect the position and acceleration of the head in space, and include its response to gravity. They project directly to the vestibular nuclei of the upper brain stem (pons). During the course of evolution, longer internal circuits were added forming the flocculonodular lobe or vestibulocerebellum, a layered structure of the primitive component of the cerebellum. These circuits superimposed new neural loops on the vestibulo-locomotor centre-spinal loops which in turn are superimposed on the lower spinal neuromechanical loops, thereby contributing to an enhanced ability to maintain balance, particularly during more challenging movements of terrestrial animals.
Vestibular inputs are also involved in the system of multiple loops which control eye gaze to maintain stability of the visual field during movements of the head (the so-called vestibulo-ocular reflex).
Green stars, vestibulocerebellum; blue stars, spinocerebellum; red stars, cerebrocerebellum.
Most of the small vestibulocerebellum is obscured by the brain stem in this view.
Structures in red are the roots of the cranial nerves.
Image: Ian Gibbins
5.4 Internal loops involved in primordial attention
The more anterior part of the brain stem is called midbrain. The dorsal part of the midbrain is called tectum (‘roof‘ from Latin) where clusters of interneurons form primordial control centres for behaviour. In amphibia, these centres are the highest levels in their nervous system. Their primitive but complex functions include making use of visual and auditory signals to organise behaviour appropriate to an environment. For example, a sudden flash of light activates visual pathways to the tectum elicits coordinated quick turning of eyes and head towards the light. A similar response has been described as ‘blindsight’ in monkeys with their primary visual cortex removed1. These pathways involve approximately 100,000-150,000 nerve fibres that project directly to the superior colliculi (mammalian equivalent of the tectum)2. Similar processes in the inferior colliculi underlie turning towards a sudden noise in the environment.
orange, superior colliculi; pink, inferior colliculi; yellow, thalamus; grey, site of cerebellum removal.
Image: Ian Gibbins
These orienting responses are mediated by internal loops within the brainstem and spinal cord, completely independent from higher brain circuits in the cerebral cortex and may represent a primordial form of attention below the level of awareness.
- L Weiskrantz (1996): Blindsight revisited. Current Opinion in Neurobiology 6, 215-220;
For an easily readable overview of blindsight and related phenomena, see Melvyn Goodale & David Milner (2004): Sight Unseen. Oxford University Press. ↩︎ - A Cowey & P Stoerig (1991): The neurobiology of blindsight. Trends in Neuroscience 14, 140-145. ↩︎