Introduction

In this chapter, we review the psychophysical and neurophysiological basis of tactile selective attention. As in other sensory systems, attention plays a major role in the way that sensory inputs are processed and perceived. The importance of attention in touch can easily be demonstrated by switching one's focus of attention to different locations on the body. For example, if you switch your focus of attention to your foot, you immediately become conscious of sensations arising from receptors in your foot that were non-existent a moment earlier. This simple observation demonstrates the power of selective attention and emphasizes two important aspects of sensory processing. First, it shows that we have a limited information processing capacity and that, under normal circumstances, we ignore most of the sensory inputs that impinge on our bodies. Second, it shows that attention is under cognitive control and, like a lens, focuses our mental efforts onto specific sensory inputs at selected body locations.

Defining attention has been difficult. While each of us has our own subjective sense of what attention is, scarcely anyone is able to express exactly what is meant by the term attention. The difficulty lies in the fact that attention is a property of our internal mental states and, as such, it is a property that is unique to the mind; and, nobody has a definition of what constitutes the mind. However, if one assumes that the mind and brain are the same then one definition of attention is that it is a neural mechanism that allows observers to direct their mental efforts onto specific objects or events. In this definition, the specific objects and events can be practically anything, including external stimuli such as visual scenes, sounds, or locations on the body surface, or internal mental states such as stored memories.

There have been numerous studies that have attempted to characterize attention. In these studies, differences in human sensory performance and their underlying neural processes are monitored while selective attention is switched between different sensory modalities, different locations on the body, and different aspects of the sensory stimulus. These studies show that attention is rapidly engaged and withdrawn from particular stimuli, and when engaged, stimuli that are under the attentional focus are perceived more rapidly and accurately.

There are two main approaches that have been used to characterize attention in the somatosensory system. Psychophysical studies on humans demonstrate that the amount of attention needed to process different kinds of tactile stimuli differ and that some tactile stimuli are processed pre-attentively. These stimuli are processed by "bottom-up," or ascending, mechanisms that are thought to require minimal attentional resources. Bottom-up processing mechanisms are important because they capture and draw our attention to stimuli such as a pinprick on the skin that require immediate attention. There is also a selective component of attention that allows us to focus or concentrate our efforts onto a specific stimulus at a specific location on the body. These "top-down," or descending, mechanisms provide us with the ability to selectively filter out and suppress irrelevant information and enhance the central representations of sensory stimuli that are immediately relevant. Some of the issues that are important here are to determine the capacity of selective attention, the degree that attention can be focused to particular body sites and modalities, and the effect that selective attention has on information processing.

The second approach to understanding attention is to study the effects directly on the responses of neurons in the nervous system. There are two ways that this is done. In the first, animals are trained to perform specific behavioral tasks that require them to switch their focus of attention (or cognitive efforts) between different stimuli presented within or between sensory systems. The assumption is then made that changes in neural activity that occur with changes in the animal behavior is a reflection of the attentional effort required to perform the task. In most of these animal studies, animals are trained in tactile tasks that require them to switch their focus of attention back and forth between a tactile task at a specific location on the body and a control task that diverts the animal's focus of attention to either a different body site, or to an auditory or visual stimulus. This paradigm controls for differences in arousal since the animals are continuously performing a behavioral task. The other way that is now commonly used to study attention is to perform imaging studies directly on human subjects. In this approach, humans are asked to perform a variety of tactile tasks and the effects of attention are assessed by examining the change in activation of different cortical regions. This method has the advantage of allowing one to simultaneously study many areas of the nervous system involved in attention. However, the approach is limited because it only provides information about which anatomical locations are affected by attention.

Here, we describe psychophysical studies on humans, neurophysiological studies of monkeys, and imaging studies of humans performing various selective attention tasks.

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