Directed Attention

The term directed attention means the avoidance of distraction by irrelevant stimuli while seeking out and focusing on stimuli that are momentarily important. It is affected by both voluntary and reflex mechanisms. An example of voluntary control of directed attention familiar to students is ignoring distracting events in a busy library while studying there.

An example of reflexly directed attention occurs with the presentation of a novel stimulus to a relaxed subject showing an alpha EEG pattern. This causes the EEG to shift to the beta rhythm. If the stimulus has meaning for the individual, behavioral changes also occur. The person stops what he or she is doing and looks around, listening intently and orienting toward the stimulus source. This behavior is called the orienting response. If the person is concentrating hard and is not distracted by the novel stimulus, the orienting response does not occur. It is also possible to focus attention on a particular stimulus without making any behavioral response.

For attention to be directed only toward stimuli that are meaningful, the nervous system must have the means to evaluate the importance of incoming sensory information. Thus, even before we focus attention on an object in our sensory world and become aware of it, a certain amount of processing has already occurred. This so-called preattentive processing serves to direct our attention toward the part of the sensory world that is of particular interest and prepares the brain's perceptual processes as we direct our attention to a particular object or situation.

If a stimulus is repeated but is found to be irrelevant, the behavioral response to the stimulus progressively decreases, a process known as habituation. For example, when a loud bell is sounded for the first time,

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems it may evoke an orienting response because the person might be frightened by or curious about the novel stimulus. After several ringings, however, the individual makes progressively less response and eventually may ignore the bell altogether. An extraneous stimulus of another type or the same stimulus at a different intensity can restore the orienting response.

Habituation involves a depression of synaptic transmission in the involved pathway, possibly related to prolonged inactivation of calcium channels in presynaptic axon terminals. Such inactivation results in a decreased calcium influx during depolarization and, hence, a decrease in the amount of neurotransmitter released by a terminal in response to action potentials.

Neural Mechanisms for Directed Attention Directing our attention to an object involves at least three distinct neurological processes. First, our attention must be disengaged from its present focus. Then, attention must be moved to the new focus. And, finally, attention must be engaged at the new focus. Each of these three processes occurs at a separate place in the brain.

One of the areas that plays an important role in orienting and directed attention is in the brainstem, where interaction of various sensory modalities in single cells can be detected experimentally. The receptive fields of the different modalities overlap such that, for example, a visual and auditory input from the same location in space will significantly enhance the firing rate of certain of these so-called multisensory cells, whereas the same type of stimuli originating at different places will have little effect on or even inhibit their response. Thus, weak clues add together to enhance each other's significance so we pay attention to the event, whereas a single small clue can be ignored.

There are also multisensory neurons in association areas of cerebral cortex (Chapter 8). Whereas the brainstem neurons are concerned with the movements associated with paying attention to a specific stimulus, the cortical multisensory neurons are more involved in the perception of the stimulus. Neuroscientists are only beginning to understand how the various areas of the at-tentional system interact.

The locus ceruleus, a nucleus in the brainstem pons, which projects to the parietal cortex and to many other parts of the central nervous system as well, is also implicated in directed attention. The system of fibers leading from the locus ceruleus helps determine which brain area is to gain temporary predominance in the ongoing stream of the conscious experience. Norepinephrine, the transmitter released by these neurons, acts as a neuro-modulator to enhance the signals transmitted by certain sensory inputs so the difference between them and weaker signals is increased. Thus, neurons of the locus ceruleus improve information processing during directed attention.

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Essentials of Human Physiology

Essentials of Human Physiology

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