An ERP Assessment of Hemispheric Projections in Foveal and Extrafoveal Word Recognition

BackgroundThe existence and function of unilateral hemispheric projections within foveal vision may substantially affect foveal word recognition. The purpose of this research was to reveal these projections and determine their functionality. MethodologySingle words (and pseudowords) were presented to the left or right of fixation, entirely within either foveal or extrafoveal vision. To maximize the likelihood of unilateral projections for foveal displays, stimuli in foveal vision were presented away from the midline. The processing of stimuli in each location was assessed by combining behavioural measures (reaction times, accuracy) with on-line monitoring of hemispheric activity using event-related potentials recorded over each hemisphere, and carefully-controlled presentation procedures using an eye-tracker linked to a fixation-contingent display. Principal FindingsEvent-related potentials 100–150 ms and 150–200 ms after stimulus onset indicated that stimuli in extrafoveal and foveal locations were projected unilaterally to the hemisphere contralateral to the presentation hemifield with no concurrent projection to the ipsilateral hemisphere. These effects were similar for words and pseudowords, suggesting this early division occurred before word recognition. Indeed, event-related potentials revealed differences between words and pseudowords 300–350 ms after stimulus onset, for foveal and extrafoveal locations, indicating that word recognition had now occurred. However, these later event-related potentials also revealed that the hemispheric division observed previously was no longer present for foveal locations but remained for extrafoveal locations. These findings closely matched the behavioural finding that foveal locations produced similar performance each side of fixation but extrafoveal locations produced left-right asymmetries. ConclusionsThese findings indicate that an initial division in unilateral hemispheric projections occurs in foveal vision away from the midline but is not apparent, or functional, when foveal word recognition actually occurs. In contrast, the division in unilateral hemispheric projections that occurs in extrafoveal locations is still apparent, and is functional, when extrafoveal word recognition takes place.研究背景：中央凹视觉内的单侧半球投射的存在及其功能，可能对中央凹词汇识别产生显著影响。本研究旨在揭示这类单侧半球投射的特征，并明确其功能机制。 研究方法：本研究将单个词汇（及假词（pseudowords））呈现在注视点左侧或右侧，且完全处于中央凹或副中央凹视觉范围内。为最大化中央凹呈现刺激时出现单侧半球投射的概率，中央凹视觉中的刺激被呈现在偏离中线的位置。通过结合行为学指标（反应时、准确率）与采用事件相关电位（event-related potentials, ERP）记录双侧半球活动以开展在线监测，并借助连接注视依存呈现装置（fixation-contingent display）的眼动仪（eye-tracker）实现严格控制的刺激呈现流程，对不同位置的刺激加工过程进行评估。 主要研究结果：刺激呈现后100~150ms与150~200ms时段的事件相关电位结果显示，副中央凹与中央凹位置的刺激均仅向刺激呈现半视野的对侧半球进行单侧投射，未同时向同侧半球投射。该效应在词汇与假词中表现一致，表明这种早期半球分区现象发生于词汇识别之前。进一步分析发现，在刺激呈现后300~350ms时段，事件相关电位可在中央凹与副中央凹位置均检测到词汇与假词间的差异，表明此时词汇识别过程已完成。但该时段的事件相关电位同时显示，此前观察到的半球分区现象在中央凹位置已消失，而在副中央凹位置仍持续存在。这一结果与行为学发现高度契合：中央凹位置两侧的注视旁刺激任务绩效无显著差异，而副中央凹位置的刺激则表现出左右不对称性。 研究结论：本研究结果表明，偏离中线的中央凹视觉中会出现初始的单侧半球投射分区，但在实际进行中央凹词汇识别时，该分区现象并不显著，亦不具备功能意义。与之相对，副中央凹位置出现的单侧半球投射分区，在副中央凹词汇识别完成时仍清晰可见，且具备相应功能。