Effect of task difficulty on blood-oxygen-level-dependent signal: A functional magnetic resonance imaging study in a motion discrimination task

There is much evidence that neural activity in the human brain is modulated by task difficulty, particularly in visual, frontal, and parietal cortices. However, some basic psychophysical tasks in visual perception do not give rise to this expected effect, at least not in the visual cortex. In the current study, we used functional magnetic resonance imaging (fMRI) to record brain activity while systematically manipulating task difficulty in a motion discrimination task, by varying the angular difference between the motion direction of random dots and a reference direction. We used both a blocked and an event-related design, and presented stimuli in both central and peripheral vision. The behavioral psychometric function, across angular differences of 3°, 9°, 15°, or 80°, spanned the full response range, as expected. The mean blood oxygen level dependent (BOLD) signals were also correlated within-participants between the blocked and event-related designs, across all brain areas tested. Within the visual cortex, the voxel response patterns correlated more within-conditions (e.g., 3° and 3°) than between-conditions (e.g., 3° and 9°), in both designs, further attesting to the reasonable quality of the BOLD data. Nevertheless, the BOLD-o-metric functions (i.e., BOLD activity as a function of task difficulty) were flat in the whole-brain and region-of-interest (ROI) analyses, including in the visual cortex, the parietal cortex, in both designs, and in foveal and peripheral visual fields alike. Indeed, there was little difference between BOLD activity during the 3° and 80° conditions. Some suggestive evidence of difficulty modulation was revealed only in the superior and inferior frontal gyri for the blocked design. We conclude that, in motion discrimination, there is no systematic BOLD modulation that accompanies the standard psychometric function across different hierarchies of cortical areas, except for the frontal lobe of the brain.大量研究证据表明，人类大脑的神经活动会随任务难度调控，尤其在视觉皮层（visual cortex）、额叶与顶叶皮层中表现显著。然而，部分视觉感知领域的基础心理物理任务并未呈现出这一预期效应，至少在视觉皮层中未观测到该现象。本研究采用功能磁共振成像（functional magnetic resonance imaging，简称fMRI）技术记录大脑活动，通过改变随机点运动方向与参考方向的角度差，系统调控运动辨别任务中的任务难度；同时采用区组设计（blocked design）与事件相关设计（event-related design），并在中央视觉与外周视觉视野中呈现刺激。在3°、9°、15°与80°四个角度差条件下，行为层面的心理测量函数覆盖了完整的反应范围，与预期一致。在所有测试脑区中，被试内的区组设计与事件相关设计所得到的平均血氧水平依赖（blood oxygen level dependent，简称BOLD）信号之间也存在显著相关。无论是区组设计还是事件相关设计下，视觉皮层内的体素反应模式在同条件（如3°与3°）下的相关性均高于跨条件（如3°与9°），进一步证明了BOLD数据的质量较为可靠。尽管如此，全脑与感兴趣区（region-of-interest，简称ROI）分析结果显示，BOLD-心理测量函数（即BOLD活动随任务难度变化的函数）整体平坦：无论采用两种实验设计、涉及视觉皮层、顶叶皮层，或是中央凹与外周视觉视野，该函数均未随任务难度发生显著变化。事实上，3°与80°条件下的BOLD活动几乎无差异。仅在区组设计中，额上回与额下回内观测到了任务难度调控的提示性证据。综上，在运动辨别任务中，除大脑额叶外，不同层级皮层区域内的标准心理测量函数均未伴随系统性的BOLD活动调控。