Eric Knudsen, Stanford University
In:- The Neuroscience of Attention – Ed. George R. Mangun
Oxford University Press (2012)
Summary and review of the above chapter
Spatial attention allows organisms to concentrate information processing on a limited spatial region. This concentration or control can be top-down or bottom-up. Top-down control is related to working memory and frontoparietal areas tuning sensory neurons to attend to a particular location, while bottom up derives from the nature of an external object in a particular location. Forebrain and midbrain systems normally act together in controlling attention, but a particularly strong or sudden stimulus can cede control to the midbrain, which is capable of faster reactions than the forebrain. Specialised circuits in the midbrain appear to relate to selective attention.
Salience filters are viewed as being located at an early stage of the attention process. These filters discriminate the biological importance of stimuli, including those related to motion, sudden change, stimuli learnt as important or unusual ‘pop-out’ features. The stimuli are encoded by increases in neural activity at early stages of the sensory pathways.
With top-down or voluntary attention, working memory and related areas of the brain control the responsiveness of sensory neurons to particular stimuli or particular locations, and inhibits neurons that might attend to other stimuli. In this way, stimuli are selected for processing by the working memory, which is limited to dealing with only a small number of items.
Shifts of gaze are also important in directing attention. Two different structures are involved here. In the forebrain, the frontal eye field (FEF) directs gaze, while the superior colliculus, which has neurons responding to the relative importance of stimuli, can also shift the gaze. The shifts controlled by the FEF are voluntary shifts, whereas the superior colliculus is related to sudden involuntary shifts of attention. Studies have demonstrated gamma synchrony between the FEF and V4 in the visual cortex. The FEF is also thought to be involved in planning motor actions. In the forebrain, the FEF and the parietal cortex both encode the relative importance of stimuli, but studies suggest that the FEF is downstream of the actual process of selecting the direction of attention. This process of direction is more relelated to the dorsolateral prefrontal, which controls working memory, and also to parts of the parietal.
The midbrain system has an adaptive advantage in quick responses to sudden and potentially dangerous changes in the environment, while the forebrain parietal cortex system relates to the more deliberative, slower acting working memory. Both forebrain and midbrain systems have similar processing, although the midbrain processes a more restricted set of information.