The ‘roof’ of the human midbrain

Our ability to control goal-directed behaviour typically involves the selection of an object that is often surrounded by other competing objects: for example, when shopping we must locate and select relevant products whilst ignoring competing ‘distractor’ items. In practice, this process involves making active scanning eye movements, (called saccades), that shift our gaze and attention to locate and examine objects of interest. The brain circuits involved in the control of goal-directed shifts of gaze and attention must integrate both sensory (visual, auditory and even tactile stimuli) and high-level cognitive factors (memory, relevance, motivation, etc.) to reach a final decision about where to direct our gaze and attention for the selection of an object for action.

The superior colliculus (SC) is a small structure located deep in the midbrain that has an important role in the process of translating sensory inputs to control shifts of gaze and attention onto objects of interest. The superior colliculus exists across species (called the optic tectum in non mammalian species) and is clearly visible as a pair of small bumps on either side of the brainstem. Most of what we know about the SC comes from studies of primates and other species such as the owl, cat and ferret. It is often assumed that the functions of the human SC are similar but this has not been demonstrated and may be an over-simplification. 

Figure 1 jpg

The pattern of eye movements and fixations (or scanpath) made when viewing a face for 5 seconds.

Recent neurophysiological work has shown that activity in the SC (and potentially the inferior colliculus, a centre for processing auditory stimuli, located just below it) may not simply reflect the output of higher-level cortical inputs, but may play a more important role in the representation of the relevance and salience of objects for the process of target selection. This opens up the interesting possibility that the SC could show greater complexity in humans than in other animals, reflecting the greater sophistication of high-level information that feeds into the SC from the cortex.Superior colliculus gif

The human superior colliculus (image courtesy of Anatomography, website maintained by Life Science Databases).

Our study will use functional magnetic resonance imaging (fMRI), extensively used to investigate human cortical function but rarely applied to sub-cortical regions, to investigate higher-level cognitive processing, including target selection and decision making for the voluntary selection of a response, reward schedules and integration of visual and tactile signals. The project will inform our understanding of the human SC, whilst also opening the way for fMRI to be applied to other small non-cortical structures that have also largely been ignored.

Professor Robin Walker and Professor Andy Smith
Royal Holloway, University of London

Robin was awarded a Research Project Grant in December 2012.