刺激相對競爭性對干擾物處理的影響

Abstract

本篇研究旨在探討刺激間的相對競爭性如何影響干擾物排除的時機，及其相關的神經活動指標。過去研究以側翼作業（flanker task）探討干擾物對目標物反應判斷的影響，比較干擾物與目標物反應不相容嘗試與相容嘗試間反應時間的差異（反應時間相容效果），以作為干擾物處理至反應決策歷程的指標。因反應時間資料僅為所有認知歷程的加總結果，本研究以事件相關電位研究進一步地回答在側翼作業脈絡中，干擾物競爭力被稀釋的確切時機。實驗一發現，當干擾物能與目標物以突顯顏色群組在一起時（稀釋情境），會引發比其它情境（高負荷量與低負荷量）下干擾物所引發的N2pc振幅較大，表示初始注意力分布會受到知覺組織的效果影響。有趣的是，干擾物在早期注意力分布時與晚期反應決策階段時的競爭力具有解離性：低負荷量情境有P300相容性效果，稀釋情境則無。根據雙機制競爭模形（Yeh & Lin, 2012）注意力再導引會提高了非目標物活化量，進而稀釋了干擾物的表徵，使其競爭性無法延續到晚期階段。由實驗二反應時間相容性的結果可知，在多重目標畫面（redundant target display）的脈絡之下，干擾物位置的不確定性在提升干擾物競爭性上扮演重要角色，但干擾物距離則否。承接實驗二的結果，實驗三A同時使用電生理與行為指標進一步地探討，在固定距離下干擾物的大小如何影響干擾物的競爭性而影響神經活動。結果顯示，在多重目標畫面的刺激脈絡之下，干擾物會於N2時窗（time window）會引發Pd成份波，表示在初始階段即對干擾物產生抑制。另外，如同實驗一，實驗三A也發現干擾物的處理在早期與晚期有解離性存在：即便干擾物大小不會調控Pd成份波的振幅，但只有當干擾物較大時，才會出現P300振幅相容性效果並產生較大的反應時間相容效果。綜合三個實驗的結果，注意力的選擇歷程一個是連續且有多個機制共同交互所完成的結果。

The present study aims to explore how the competitions among task-relevant and task-irrelevant stimuli affect distractor processing using the recording of event-related potentials (ERP). In most of the previous studies, distractor processing was often evaluated indirectly by reaction times to target selection. Yet, reaction time simply reflects the consequence of all intermediated processes. Therefore, we used ERP experiments to underpin the different neural mechanisms underlying distractor processes in two experimental contexts. ERPs were monitored while participants performed a flanker task. In Experiment 1, stimulus competition was manipulated through varying whether the distractor can be perceptually grouped with the task-relevant target assuming that distractor grouped with the target is a strong competitor. Participants performed three experimental conditions: low-load, high-load, and dilution conditions. Target-nontarget feature similarity was low in the low-load condition and was high in the latter two conditions. More importantly, target and distractor shared the same color to reduce target processing load in the dilution condition. The results showed that that a highly salient distractor grouped with the target in color elicited strong activation in attention allocation under the dilution condition. As a result, the amplitude of distractor-elicited N2pc was greater under dilution than under the low-load and high-load conditions. Interestingly, the strong activation did not carry over downstream in processing to produce a strong competition against the target in response decision. Distractor compatibility did not modulate P300 amplitude in the dilution and high-load conditions but manifested in the low-load condition. These results suggest that the representation of distractor was diluted between the initial attention allocation and the response selection stages under the dilution condition. In Experiment 2, redundant target display was adopted to enhance target competition and the reaction time compatibility effect revealed that the location uncertainty played a role in enhancing distractor competition while the distractor distance did not. In Experiment 3A, redundant target display was also adopted to enhance target competition while distractor competition was manipulated by varying distractor size assuming that a large distractor is a strong competitor. The results showed that the high activation of redundant targets was able to suppress distractor activation during the stage of attention allocation as reflected by a distractor-elicited Pd. Distractor size did not modulate the distractor-elicited Pd but only the large distractor produced compatibility effect on P300. This finding suggests that a large distractor produced competition against the target in response decision even though it was suppressed at an early stage. Taken together, the results from two experiments support the contention that biased competition characterizes visual selection and relative stimulus activation strength influences biased competition at different stages of information processing.