Talk Andrew C, Gandhi Chetan C, Matzel Louis D
Beckman Institute, University of Illinois, Urbana, USA.
Hippocampus. 2002;12(5):648-56. doi: 10.1002/hipo.10098.
In laboratory studies, the assessment of memory is typically associated with overt behavioral responses. Thus, it has been difficult to determine whether the enhancement of hippocampal sensory-evoked potentials that often accompany memory formation are the neurophysiological manifestation of a memory "trace" or are a secondary product of the behavioral expression of the memory. We addressed this issue by examining changes in evoked hippocampal field potentials during sensory preconditioning, a form of behaviorally silent relational learning that requires an intact hippocampus for execution. Rats were exposed to presentations of a white noise (S1) that terminated with a tone (S2). These pairings of ostensibly "neutral" stimuli supported no change in the behavior elicited by the noise. However, if the tone was subsequently paired with mild footshock (US), suppression of ongoing licking behavior (indicative of fear) was elicited by the noise, indicating that the animal had associated the noise with tone (S1-S2), and had represented the noise-tone-shock (S1-S2-US) relationship. Pre-training neurotoxic lesions of the hippocampus had no effect on conditioned suppression to tone after tone-shock (S2-US) pairings, but disrupted the expression of continued suppression to noise (S1) after tone-shock pairings. In a second experiment, sensory-evoked field potentials in the dorsal hippocampus were recorded with extracellular electrodes. No changes in the hippocampal response evoked by white noise were observed after pairings of noise and tone, i.e., no evidence for a memory trace could be detected. In contrast, after tone was paired with footshock, two short-latency negative potentials within the noise-evoked field response increased in amplitude, a response often presumed to reflect a neurophysiological correlate of memory storage. In total, these results suggest that although the hippocampus critically contributes to the processing of a behaviorally silent associative memory, there may be no role for changes in the amplitude of hippocampal sensory-evoked field potentials in storing representations of the relationships between sensory experiences.
在实验室研究中，记忆评估通常与明显的行为反应相关联。因此，很难确定常伴随记忆形成的海马体感觉诱发电位增强是记忆“痕迹”的神经生理表现，还是记忆行为表达的次级产物。我们通过检查感觉预适应过程中海马体诱发场电位的变化来解决这个问题，感觉预适应是一种行为上无明显表现的关系学习形式，执行该过程需要完整的海马体。将大鼠暴露于以一个纯音（S2）结束的白噪声（S1）刺激中。这些表面上“中性”刺激的配对并未引起噪声所引发行为的变化。然而，如果随后将纯音与轻度足部电击（US）配对，噪声会引发正在进行的舔舐行为受到抑制（表明恐惧），这表明动物已将噪声与纯音（S1 - S2）联系起来，并表征了噪声 - 纯音 - 电击（S1 - S2 - US）的关系。海马体的训练前神经毒性损伤对纯音 - 电击（S2 - US）配对后对纯音的条件性抑制没有影响，但破坏了纯音 - 电击配对后对噪声（S1）持续抑制的表达。在第二个实验中，用细胞外电极记录背侧海马体的感觉诱发电场电位。在噪声和纯音配对后，未观察到白噪声诱发的海马体反应有变化，即未检测到记忆痕迹的证据。相比之下，在纯音与足部电击配对后，噪声诱发场反应中的两个短潜伏期负电位幅度增加，这种反应通常被认为反映了记忆存储的神经生理相关物。总的来说，这些结果表明，尽管海马体对行为上无明显表现的联想记忆处理起着关键作用，但海马体感觉诱发电场电位幅度的变化在存储感觉体验之间关系的表征方面可能没有作用。
