[论文解读] Overlapping neural representations for the position of visible and imagined objects

Amanda K. Robinson, Tijl Grootswagers+4 more|arXiv (Cornell University)|Oct 19, 2020

Visual perception and processing mechanisms被引用 4

一句话总结

本研究利用脑电图（EEG）和时间解析多变量模式分析（MVPA），表明想象中的物体位置的神经表征与可见物体的神经表征重叠，其机制依赖于中、高级视觉处理，而非早期的视网膜拓扑通路。与可见物体相比，想象中的位置在更早阶段即可被解码，且神经信号更微弱，表明内部空间表征由自上而下的预测机制驱动。

ABSTRACT

Humans can covertly track the position of an object, even if the object is temporarily occluded. What are the neural mechanisms underlying our capacity to track moving objects when there is no physical stimulus for the brain to track? One possibility is that the brain 'fills-in' information about imagined objects using internally generated representations similar to those generated by feed-forward perceptual mechanisms. Alternatively, the brain might deploy a higher order mechanism, for example using an object tracking model that integrates visual signals and motion dynamics. In the present study, we used EEG and time-resolved multivariate pattern analyses to investigate the spatial processing of visible and imagined objects. Participants tracked an object that moved in discrete steps around fixation, occupying six consecutive locations. They were asked to imagine that the object continued on the same trajectory after it disappeared and move their attention to the corresponding positions. Time-resolved decoding of EEG data revealed that the location of the visible stimuli could be decoded shortly after image onset, consistent with early retinotopic visual processes. For processing of unseen/imagined positions, the patterns of neural activity resembled stimulus-driven mid-level visual processes, but were detected earlier than perceptual mechanisms, implicating an anticipatory and more variable tracking mechanism. Encoding models revealed that spatial representations were much weaker for imagined than visible stimuli. Monitoring the position of imagined objects thus utilises similar perceptual and attentional processes as monitoring objects that are actually present, but with different temporal dynamics. These results indicate that internally generated representations rely on top-down processes, and their timing is influenced by the predictability of the stimulus.

研究动机与目标

• 探究想象中物体位置的神经表征是否与可见物体的神经表征共享机制。
• 利用时间解析解码方法，确定想象与可见刺激的神经处理时间动态。
• 评估空间位置的内部表征是否依赖于感知机制或更高阶的注意力机制。
• 在图像化任务中解码空间表征时，控制眼动等混淆因素的影响。

提出的方法

• 受试者追踪一个在六个位置间移动的物体，随后在物体消失后想象其沿相同轨迹继续移动。
• 在可见追踪和想象延续阶段均采集了EEG数据。
• 采用时间解析多变量模式分析（MVPA）从神经活动模式中解码空间位置。
• 使用随机化可见刺激序列的独立模式估计器训练解码器，以确保其能泛化至追踪任务。
• 从模式估计器向追踪任务执行交叉解码，以隔离与眼动无关的位置特异性神经信号。
• 分别分析后部和前部电极区域，以验证空间解码源自视觉处理区域。

实验结果

研究问题

• RQ1 想象中物体位置的神经表征是否与可见物体共享空间编码机制？
• RQ2 想象位置的神经处理时间动态与可见刺激相比如何？
• RQ3 早期视网膜拓扑视觉过程在想象中物体位置表征中起多大作用？
• RQ4 眼动是否能解释图像化任务中观察到的空间位置解码？
• RQ5 刺激可预测性在多大程度上影响内部空间表征的强度与启动时间？

主要发现

• 想象中物体位置的神经模式比可见刺激更早被解码，表明内部表征中存在预期性处理。
• 编码模型性能显示，想象刺激的神经表征强度显著弱于可见刺激。
• 从想象刺激中解码空间位置依赖于中、高级视觉处理，而非早期视网膜拓扑机制。
• 使用随机化可见刺激序列训练的模式估计器向想象追踪任务执行的交叉解码结果高于随机水平，表明存在与眼动无关的共享神经编码。
• 后部电极分析确认，空间解码模式与视觉系统活动一致，而非前额区域伪影或眼动信号。
• 结果表明，自上而下的预测机制驱动内部空间表征，且其时间特性受刺激可预测性调节。