Attention, Evolutionary Perspectives

The Ubiquity of Selective Attention

In humans, the neuroscience of selective attention emphasizes the contributions of large cortical networks. It is clear that an intact cortex is necessary for efficient selective attention in humans, and diverse forms of cortical damage produce many kinds of selective attention disorders, such as hemispatial neglect, in which patients may not respond to objects in the left side of space (e.g., they may refuse to eat food from the left side of the plate or fail to read words on the left side of the page). Many brain imaging studies show that even apparently simple tasks, like shifting attention from one object to another, activate a large network of cortical areas.

However, the cerebral cortex is a very special structure in evolutionary terms. Within the mammals, the cortex is significantly larger in primates than non-primates, and larger still in humans than other primates. Selective attention in humans, therefore, requires machinery that is, in evolutionary terms, unique and expensive. On this basis, we might expect selective attention to be crude or even absent in other species lacking a well-developed cortex. However, this intuition would be mistaken: Selective attention is far from uniquely human and appears to operate to some degree in even the smallest brains.

Selective attention abilities and costs of various kinds have been documented in many diverse nervous systems, as indicated by the examples in Table 1. Inclusion in the table is based on whether the brain and overt response to a stimulus have been shown to depend on the animal's current state and behavioral goals. This is a broad definition, and the table entries represented are not perfectly comparable. For example, fruit flies are included in the table because they respond more selectively to visual motion signals when they are engaged in flight. Frogs selectively respond to one prey stimulus when there are multiple prey present. Selective processing by blue jays has been documented in a variety of tasks, including costs for switching between targets, and the use of templates in visual search for prey. The distance between human brains and those of other species can be expressed both in time (millions of years since the last common ancestor), and space (ratio of brain volumes). There are always different concerns about when it is appropriate to adjust for body size when comparing brain sizes. Table 1 presents brain sizes unadjusted for body size. The reason is to better indicate just how small some of the brains are that can be engaged in selective attention. What is clear, even from this incomplete sketch, is that attention systems have been in place a long time, they are prevalent throughout the animals, and they can operate, at least to some degree, in the absence of large and evolutionarily expensive machinery like the human cortex.

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