Attention: Physiological

Often, when we're interested in an object, we move our eyes toward it, allowing us to see it more clearly. However, it has long been known that even in the absence of overt movements, perception of an object in the periphery can be improved by covertly shifting attention to it. Covert attention refers to our ability to internally modulate sensory processing of a selected stimulus. Attention can improve perception of the attended object, even though the input to the system (the physical stimulus reaching the retina, for example) is unchanged.

When we shift attention to an object, it may appear clearer, more distinct, or more intense, whereas ignored objects may seem less distracting or even fade from awareness. These perceptual effects are caused by changes in the responses of sensory neurons. Thus, one important issue in the physiology of attention is to understand the effects of attention on sensory processing. A second major issue is to understand how the brain controls which particular objects are attended. This is referred to as the neural control of attention.

Effects of attention on sensory neurons have been observed in all the major sensory systems. Most studies have found that attention increases the gain of sensory neurons encoding the attended object or feature, effectively amplifying signals from the attended stimulus. The effects of attention on sensory neurons become progressively [Page 98]stronger in higher-level brain areas compared with areas closer to the sensory input. Attention has been studied most extensively in the visual system, and effects of attention on visual neurons can be observed as early as the lateral geniculate nucleus (LGN), an area that receives input directly from the retina.

The control of attention has been studied more extensively for spatial attention than for feature attention. Spatial attention refers to the allocation of attention to specific objects or locations in the environment, whereas feature attention refers to the allocation of attention to particular sensory features (for example, the color red) regardless of location. Behavioral studies have shown that spatial attention can be attracted to objects or locations, either by virtue of their physical salience (for example, a bright flash of lightning tends to draw attention) or their behavioral relevance (for example, when driving, one can intentionally ignore a distracting billboard along the side of the road to attend to the cars ahead).

Physiological research suggests that spatial attention is controlled by a network of brain areas that includes the frontal and parietal cortex, as well as subcortical areas. Neurons in most of these areas are organized into maps that represent locations in the environment. The level of neural activity at a given location in the map is thought to represent the physical salience and behavioral relevance of stimuli at the corresponding location in the environment. The cells on the map having the greatest activity correspond to the most important location in the environment, and the greatest amount of attention is directed to this location. The assumption is that these maps influence sensory processing by modulating the activity of sensory neurons. Indeed, feedback connections linking attention control areas to sensory areas are known to exist and could serve as a conduit for these control signals to modulate sensory neurons. This entry describes the effects of attention on sensory neuron responses and neural control of attention.

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