Rushworth M F S, Hadland K A, Paus T, Sipila P K
Department of Experimental Psychology, University of Oxford, Oxford OX1 3UD, United Kingdom.
J Neurophysiol. 2002 May;87(5):2577-92. doi: 10.1152/jn.2002.87.5.2577.
We used event-related functional magnetic resonance imaging (fMRI) to measure brain activity when subjects were performing identical tasks in the context of either a task-set switch or a continuation of earlier performance. The context, i.e., switching or staying with the current task, influenced medial frontal cortical activation; the medial frontal cortex is transiently activated at the time that subjects switch from one way of performing a task to another. Two types of task-set-switching paradigms were investigated. In the response-switching (RS) paradigm, subjects switched between different rules for response selection and had to choose between competing responses. In the visual-switching (VS) paradigm, subjects switched between different rules for stimulus selection and had to choose between competing visual stimuli. The type of conflict, sensory (VS) or motor (RS), involved in switching was critical in determining medial frontal activation. Switching in the RS paradigm was associated with clear blood-oxygenation-level-dependent signal increases ("activations") in three medial frontal areas: the rostral cingulate zone, the caudal cingulate zone, and the presupplementary motor area (pre-SMA). Switching in the VS task was associated with definite activation in just one medial frontal area, a region on the border between the pre-SMA and the SMA. Subsequent to the fMRI session, we used MRI-guided frameless stereotaxic procedures and repetitive transcranial magnetic stimulation (rTMS) to test the importance of the medial frontal activations for task switching. Applying rTMS over the pre-SMA disrupted subsequent RS performance but only when it was applied in the context of a switch. This result shows, first, that the pre-SMA is essential for task switching and second that its essential role is transient and limited to just the time of behavioral switching. The results are consistent with a role for the pre-SMA in selecting between response sets at a superordinate level rather than in selecting individual responses. The effect of the rTMS was not simply due to the tactile and auditory artifacts associated with each pulse; rTMS over several control regions did not selectively disrupt switching. Applying rTMS over the SMA/pre-SMA area activated in the VS paradigm did not disrupt switching. This result, first, confirms the limited importance of the medial frontal cortex for sensory attentional switching. Second, the VS rTMS results suggest that just because an area is activated in two paradigms does not mean that it plays the same essential role in both cases.
我们使用事件相关功能磁共振成像（fMRI）来测量受试者在任务集切换或延续先前表现的情境下执行相同任务时的大脑活动。情境，即切换或继续当前任务，会影响内侧前额叶皮质的激活；当受试者从一种执行任务的方式切换到另一种方式时，内侧前额叶皮质会短暂激活。我们研究了两种类型的任务集切换范式。在反应切换（RS）范式中，受试者在不同的反应选择规则之间切换，必须在相互竞争的反应之间做出选择。在视觉切换（VS）范式中，受试者在不同的刺激选择规则之间切换，必须在相互竞争的视觉刺激之间做出选择。切换中涉及的冲突类型，即感觉（VS）或运动（RS）冲突，对于确定内侧前额叶激活至关重要。在RS范式中的切换与三个内侧前额叶区域明显的血氧水平依赖信号增加（“激活”）相关：喙扣带区、尾扣带区和补充运动前区（pre - SMA）。在VS任务中的切换仅与一个内侧前额叶区域的明确激活相关，该区域位于pre - SMA和SMA之间的边界上。在fMRI实验之后，我们使用MRI引导的无框架立体定向程序和重复经颅磁刺激（rTMS）来测试内侧前额叶激活对任务切换的重要性。在pre - SMA上方施加rTMS会干扰后续的RS表现，但仅在切换的情境下施加时才会如此。这一结果首先表明，pre - SMA对任务切换至关重要，其次表明其关键作用是短暂的，且仅限于行为切换的时刻。这些结果与pre - SMA在上级水平上在反应集之间进行选择而非在选择单个反应方面的作用一致。rTMS的效果并非仅仅归因于与每个脉冲相关的触觉和听觉伪影；在几个对照区域施加rTMS并没有选择性地干扰切换。在VS范式中激活的SMA/pre - SMA区域上方施加rTMS并没有干扰切换。这一结果首先证实了内侧前额叶皮质对感觉注意切换的重要性有限。其次，VS的rTMS结果表明，仅仅因为一个区域在两种范式中都被激活，并不意味着它在两种情况下都发挥相同的关键作用。
