Balderston Nicholas L, Schultz Douglas H, Baillet Sylvain, Helmstetter Fred J
Department of Psychology, University of Wisconsin-Milwaukee.
J Vis Exp. 2013 Jun 3(76):50212. doi: 10.3791/50212.
In trace fear conditioning a conditional stimulus (CS) predicts the occurrence of the unconditional stimulus (UCS), which is presented after a brief stimulus free period (trace interval)(1). Because the CS and UCS do not co-occur temporally, the subject must maintain a representation of that CS during the trace interval. In humans, this type of learning requires awareness of the stimulus contingencies in order to bridge the trace interval(2-4). However when a face is used as a CS, subjects can implicitly learn to fear the face even in the absence of explicit awareness*. This suggests that there may be additional neural mechanisms capable of maintaining certain types of "biologically-relevant" stimuli during a brief trace interval. Given that the amygdala is involved in trace conditioning, and is sensitive to faces, it is possible that this structure can maintain a representation of a face CS during a brief trace interval. It is challenging to understand how the brain can associate an unperceived face with an aversive outcome, even though the two stimuli are separated in time. Furthermore investigations of this phenomenon are made difficult by two specific challenges. First, it is difficult to manipulate the subject's awareness of the visual stimuli. One common way to manipulate visual awareness is to use backward masking. In backward masking, a target stimulus is briefly presented (< 30 msec) and immediately followed by a presentation of an overlapping masking stimulus(5). The presentation of the mask renders the target invisible(6-8). Second, masking requires very rapid and precise timing making it difficult to investigate neural responses evoked by masked stimuli using many common approaches. Blood-oxygenation level dependent (BOLD) responses resolve at a timescale too slow for this type of methodology, and real time recording techniques like electroencephalography (EEG) and magnetoencephalography (MEG) have difficulties recovering signal from deep sources. However, there have been recent advances in the methods used to localize the neural sources of the MEG signal(9-11). By collecting high-resolution MRI images of the subject's brain, it is possible to create a source model based on individual neural anatomy. Using this model to "image" the sources of the MEG signal, it is possible to recover signal from deep subcortical structures, like the amygdala and the hippocampus*.
在痕迹恐惧条件反射中，条件刺激（CS）预示着无条件刺激（UCS）的出现，无条件刺激在短暂的无刺激期（痕迹间隔）后呈现（1）。由于条件刺激和无条件刺激在时间上不同时出现，主体必须在痕迹间隔期间保持对该条件刺激的表征。在人类中，这种类型的学习需要对刺激的意外情况有认知，以便跨越痕迹间隔（2 - 4）。然而，当使用面部作为条件刺激时，即使在没有明确认知的情况下，主体也可以隐性地学会对面部产生恐惧*。这表明可能存在额外的神经机制，能够在短暂的痕迹间隔期间维持某些类型的“生物相关”刺激。鉴于杏仁核参与痕迹条件反射，并且对面部敏感，该结构有可能在短暂的痕迹间隔期间维持对面部条件刺激的表征。理解大脑如何将未被感知的面部与厌恶结果联系起来具有挑战性，即使这两种刺激在时间上是分开的。此外，对这一现象的研究因两个特定挑战而变得困难。首先，很难操纵主体对视觉刺激的认知。操纵视觉认知的一种常见方法是使用逆向掩蔽。在逆向掩蔽中，目标刺激被短暂呈现（<30毫秒），并立即随后呈现一个重叠的掩蔽刺激（5）。掩蔽刺激的呈现使目标不可见（6 - 8）。其次，掩蔽需要非常快速和精确的时间安排，这使得使用许多常见方法研究被掩蔽刺激诱发的神经反应变得困难。血氧水平依赖（BOLD）反应的解析时间尺度对于这种方法来说太慢，而脑电图（EEG）和脑磁图（MEG）等实时记录技术在从深部来源恢复信号方面存在困难。然而，用于定位MEG信号神经源的方法最近有了进展（9 - 11）。通过收集受试者大脑的高分辨率MRI图像，可以基于个体神经解剖结构创建一个源模型。使用这个模型对MEG信号的源进行“成像”，就有可能从深部皮层下结构，如杏仁核和海马体*恢复信号。
