The Lateralization of Expectations: Hemispheric Differences in Top-down and Bottom-up Word Processing in Context

Abstract

The current work investigates how preexisting mental representations of the meaning of an utterance (top-down processing) affect the comprehension of external perceptual properties of the linguistic input (bottom-up processing). When it comes to top-down bottom-up processing in the brain previous findings report a division of focus between left and right hemispheric mechanisms. The PARLO sentence comprehension model posits that the LH employs top-down mechanisms which allow for efficient anticipatory processing, while the RH relies more on bottom-up mechanisms. A shortcoming of the PARLO model is that it’s based on experiments manipulating solely top-down contextual constraint, leading to conclusions that hemispheric asymmetries are a function of differences in the efficiency of top-down rather than bottom-up mechanisms. Up until now, there has been no investigation of asymmetries in bottom-up processing, nor an investigation of the potential interactions between that and top-down processing for each hemisphere. This thesis consists of four event-related potential (ERP) experiments divided into two parts. Experiments 1 (central presentation) and 2 (hemispheric presentation) manipulate the bottom-up lexical frequency of critical words in high and low predictability contexts. Experiments 3 (central presentation) and 4 (hemispheric presentation) manipulate bottom-up word status, presenting critical words and pseudowords in the same high and low predictability contexts. The results allow us to extend previous findings and present the Spotlight Theory of Hemispheric Comprehension. We argue that the LH employs a kind of spotlight focus, which affords very efficient top-down processing of the expected input, since only highly predictable inputs receive additional facilitation based their bottom-up features. Alternatively, the RH lack of spotlight mechanism and focus on bottom-up lexical properties allows for the reliable processing of less predictable and irregular inputs. In combination, these complementary processing strategies provide the comprehension system with the efficiency and robustness required in a wide range of communicative situations.