Cognitive functions are related to our ability to build an internal representation of the world, the conceptual representation system, based on a large-scale neuronal network. This system is connected with more circumscribed and lateralized operational systems that allow us to translate thoughts into words (spoken, written or gestures), images, numbers or other symbols, to store and retrieve information when necessary and to make decisions or act upon them. Most of these operational abilities are subserved by distributed networks with areas of regional specialization, organized according to their specific processing capacities.

The pattern of cognitive/behavioral impairment observed after ischemic stroke is relatively stereotyped, since it follows the distribution of the vascular territories. However, in the hyperacute stage symptoms are likely to be amplified by additional regions of ischemic penumbra, mass effects and diaschisis (impairment of intact regions that are functionally connected with the damaged area), and, in the chronic stage, functional reorganization and brain plasticity mechanisms make neuroanatomical correlations loose and less predictable.

In hemorrhagic lesions, vasculitis, and cerebral venous thrombosis the pattern of cognitive defects is less stereotyped due to the variability of lesion localization, size and number, or particular pathogenic mechanisms that may cause diffuse impairment.

In this chapter we will present the most common cognitive and neurobehavioral deficits secondary to stroke, according to symptom presentation.

Language disorders

Language disorders, or aphasia, occur following perisylvian lesions (middle cerebral artery territory) of the left hemisphere and have a marked impact on the individual quality of life, autonomy and the ability to return to work or previous activities.

Introduction

Although the brain is a symmetrical organ, its functional architecture is rather asymmetric. Beyond a certain degree of functional hierarchy, there is a lateralized distribution of cerebral functions, each hemisphere being dominant for a specific type of information processing. This functional asymmetry is evident in the association and multimodal cortices and extends to the subcortical networks.

The mechanisms of hemispheric specialization are not completely understood. Yet, there is converging evidence that the phenomenon of cerebral dominance is based on hemispheric differences in cortical cytoarchitecture and inter-area connectivity that create neural networks with specific styles of information processing (Hustler and Galuske, 2003). Innate computational differences between the two hemispheres are likely to be enhanced or modulated by experience, function acquisition (Gaillard et al., 2003; Castro-Caldas et al., 1998), timing of brain maturation, as the development of the right hemisphere precedes the development of the left (Chiron et al., 1997), and by the degree of expertise achieved in specific functions (Tatsuno and Sakai, 2005; Pascual-Leone et al., 1994). Neurochemical asymmetries may also explain some aspects of behavior (Mohr et al., 2003).

The left cerebral hemisphere is characterized by a discreet organization of functions and a rather sequential, linear, and analytic processing style. This is probably responsible for its ability to extract and process perceptual detail (Delis et al., 1988) and for its advantage in the temporal resolution of events (Zatorre & Belin, 2001), such as the rapidly changing speech sounds.