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Haptically evoked activation of visual cortex

Simon Lacey and Krish Sathian


Introduction

Thanks to the acceleration of research into multisensory processing in recent years, the idea that the brain is organized around parallel processing of separate sensory inputs is giving way to the concept of a ‘metamodal’ brain [1] organized around particular tasks in a modality-independent fashion. For example, it is now apparent that many cortical areas previously thought to be specialized for processing specific aspects of visual input are also activated during analogous haptic or tactile tasks. In this chapter, we review the circumstances in which haptic or tactile tasks are associated with visual cortical activity and what this means for our internal representation of the external world, i.e., how information about objects and events in the external world is stored in memory. By ‘haptic’, we mean tasks involving active motor exploration of stimuli, whereas ‘tactile’ refers here to tasks in which stimuli are applied to the passive finger or other skin surface.

Visual cortical involvement in touch Gratings and dot-patterns

The first indication that tactile input could activate extrastriate visual cortex came from our positron emission tomographic (PET) study [2]. In this study, participants had to discriminate the orientation of gratings presented to the immobilized right index fingerpad. In a control task, participants discriminated the width of the grooves on the gratings. A contrast between these tasks revealed activity that was selective for tactile discrimination of grating orientation in the left extrastriate visual cortex, at a focus close to the parieto-occipital fissure. This same focus is known to be active during both visual grating orientation discrimination [3] and spatial imagery [4]. We therefore concluded that activation of this focus during tactile discrimination of grating orientation reflected spatial processes that are common to vision and touch [2]. This study suggested, for the first time, that task-selective visual cortex responds to a tactile analog of the relevant visual task and that visual cortical activity might be independent of the sensory modality in which a task is presented.
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