  The effect of age at injury on outcome after acquired brain injury (ABI) has
been the subject of much debate. Many argue that young brains are relatively
tolerant of injury. A contrasting viewpoint due to Hebb argues that greater
system integrity may be required for the initial establishment of a function
than for preservation of an already-established function. A neuro-computational
model of cortical map formation was adapted to examine effects of focal and
distributed injury at various stages of development. This neural network model
requires a period of training during which it self-organizes to establish
cortical maps. Injuries were simulated by lesioning the model at various stages
of this process and network function was monitored as "development" progressed
to completion. Lesion effects are greater for larger, earlier, and distributed
(multifocal) lesions. The mature system is relatively robust, particularly to
focal injury. Activities in recovering systems injured at an early stage show
changes that emerge after an asymptomatic interval. Early injuries cause
qualitative changes in system behavior that emerge after a delay during which
the effects of the injury are latent. Functions that are incompletely
established at the time of injury may be vulnerable particularly to multifocal
injury.
