Introduction

Research on impairments of brain development has in the past been characterized by two empirical paradigms that examine developmental plasticity from very different perspectives. The early lesion paradigm, on the one hand, focuses on patients with gross structural brain damage acquired peri- or postnatally and investigates effects on cognitive, sensorimotor, and affective outcome. On the other hand, the study of developmental disorders, such as attention-deficit disorder, dyslexia, or autism, proceeds from a diagnostic profile of cognitive–behavioral symptoms to the exploration of underlying neurodevelopmental disturbances. The two approaches shed light on the malleability of the developing brain in opposite, yet complementary ways.

Research on the effects of early structural lesions has produced evidence for the brain's astounding capacity to compensate for loss of neural tissue. Extreme examples are studies of patients with resection or disconnection of a complete forebrain hemisphere. While extensive brain damage in adults results in severe and persistent region-specific deficits (as evidenced by aphasia following a left perisylvian lesion: Pedersen et al., 1995; Benson and Ardila, 1996), left hemispherectomy after an early lesion is often associated with good long-term language outcome if the right hemisphere is intact (Basser, 1962; Ogden, 1988; Vargha-Khadem and Polkey, 1992; Vargha-Khadem et al., 1997). The different meanings and implications of the broad term early lesion will be discussed later in this chapter. Roughly, this term will be used in the sense of acquired structural damage affecting one or several brain regions before these have fully matured.