Fellow’s research: How the midbrain helps us pay attention
11 Oct 2019
Dr Sridharan Devarajan, Intermediate Fellow
Indian Institute of Science, Bangalore
Attention, a critical cognitive capacity, is widely known to be controlled by the forebrain. Our research suggests that the superior colliculus, an evolutionarily-conserved midbrain structure, also plays a key role in mediating a particular aspect of attention.
The human brain is continuously bombarded with information. However, not all of this information is relevant for behaviour. The capacity for “selective attention” allows our brain to filter out irrelevant information and retain only information that is relevant for making important decisions.
Attention can operate in two ways: i) by enhancing the visual clarity of the attended information/stimulus (sensitivity) or ii) by focussing on the attended information/stimulus over others (bias). For example, when driving down a busy road, greater sensitivity to the vehicle directly in front, enables us to keep accurate track of its movements, whereas greater bias for the vehicle in front, enables reacting rapidly to sudden movements of that vehicle.
How attention works in the brain remains an active topic of research. Attention's neural correlates have been largely studied in the outer layer of the brain, known as the cerebral cortex. Yet, it is increasingly clear that a “sub-cortical” network, including a key midbrain region called the superior colliculus (SC), plays an important role in attention. The SC is an evolutionarily conserved structure that can be found in all vertebrate species including fish, reptiles, birds, and mammals. In all of these species, the SC is known for its role in mediating eye-movements. Yet, how the SC is involved in attention – vis-a-vis sensitivity versus bias – remains a topic of active debate.
In this study, we address this debate for the first time in humans, by conducting 2 sets of experiments using non-invasive brain imaging in conjunction with model-based analysis of behaviour.
First, we tested 22 human participants on an attention-demanding task and showed that sensitivity and bias are asymmetrically distributed in the visual field: sensitivity is higher on the left visual field, on average, whereas bias is higher on the right.
Next, employing imaging and modelling techniques (Diffusion Magnetic Resonance Imaging (dMRI) and tractography, respectively) in 82 participants, (including the above-mentioned 22 participants), we tracked white-matter fibres that connect the SC with other parts of the cortex. We found that the SC connects strongly and asymmetrically to the “parietal” cortex, a region important for decision-making, in a manner that specifically mirrors the asymmetries in bias. We show, further, that the strength of SC connectivity correlates, with and is predictive of, an individual’s bias but not sensitivity. These results suggest a specific role for the SC’s connectivity in mediating bias, but not sensitivity, during attention.
Surface map of connections of the Superior Colliculus (SC) with the cortex. Colours denote connections estimated with different datasets
Understanding how attention works in the brain and how it controls behaviour can help understand the neural basis of attention disorders such as Attention-Deficit Hyperactivity Disorder (ADHD). In the future, we aim to understand if the strength of the neural connections of the SC with other regions of the cortex can be altered by strategic learning paradigms in novel attention tasks.
Subcortical connectivity correlates selectively with attention’s effects on spatial choice bias. Varsha Sreenivasan and Devarajan Sridharan, PNAS, September 2019
Banner Image: Fibers connecting the Superior Colliculus (SC) with the motor cortex (brown fibers), and the prefrontal cortex with the posterior parietal cortex (purple), overlaid on a brain slice.