The role of over and under activation in the emergence of spoken language deficits

Abstract

In the current thesis, I advance the idea that semantic interference and semantic errors are bound to occur in healthy individual as an effect of experimental conditions and especially when language areas of the brain are compromised following a brain injury. My thesis program can be described as a series of steps, in which I tested different models of lexical retrieval by means of specific methods on both healthy and aphasic population.The first step investigated the extent to which the amount of semantic interference may be modulated by individual predisposition towards the perception of part of a context a sdiscrete from the surrounding field: field dependent and field independent cognitive styles.I found a relationship between semantic interference in naming and cognitive styles.The second step aimed to gather evidence about the long-lasting effect of cumulative semantic interference. By providing two alternative versions of the continuous picture naming task, I explored, respectively: a) the extent to which the increasing of semantic interference accrues over the ordinal positions described in the literature; b) whether semantic interference decayed after an amount of time. I found that the activation of a target representation dissipates after an unfilled delay and that the strength of interference tapers off after presentation of distractors.The third step aimed to disentangle the contribution of bottom-up and top-down mechanisms in the emergence of lexical deficits in a population of aphasic patients. I compared the performance of aphasic patients and healthy individuals in naming tasks inducing semantic interference, in a short-term memory task and, finally, in a Stroop task.Our patients showed two distinct patterns consistent with a damage to activation vs inhibition mechanisms. In conclusion, semantic interference and semantic errors offer an important view to a better understanding of the cognitive mechanisms underpin normal and pathological word retrieval.