Weinberger N M, Bakin J S
Department of Psychobiology, University of California, Irvine 92697-3800, USA.
Audiol Neurootol. 1998 Mar-Jun;3(2-3):145-67. doi: 10.1159/000013787.
It is well established that the functional organization of adult sensory cortices, including the auditory cortex, can be modified by deafferentation, sensory deprivation, or selective sensory stimulation. This paper reviews evidence establishing that the adult primary auditory cortex develops physiological plasticity during learning. Determination of frequency receptive fields before and at various times following aversive classical conditioning and instrumental avoidance learning in the guinea pig reveals increased neuronal responses to the pure tone frequency used as a conditioned stimulus (CS). In contrast, responses to the pretraining best frequency and other non-CS frequencies are decreased. These opposite changes are often sufficient to shift cellular tuning toward or even to the frequency of the CS. Learning-induced receptive field (RF) plasticity (i) is associative (requires pairing tone and shock), (ii) highly specific to the CS frequency (e.g., limited to this frequency +/- a small fraction of an octave), (iii) discriminative (specific increased response to a reinforced CS+ frequency but decreased response to a nonreinforced CS- frequency), (iv) develops extremely rapidly (within 5 trials, the fewest trials tested), and (v) is retained indefinitely (tested to 8 weeks). Moreover, RF plasticity is robust and not due to arousal, but can be expressed in the deeply anesthetized subject. Because learning- induced RF plasticity has the major characteristics of associative memory, it is therefore referred to as "physiological memory". We developed a model of RF plasticity based on convergence in the auditory cortex of nucleus basalis cholinergic effects acting at muscarinic receptors, with lemniscal and nonlemniscal frequency information from the ventral and magnocellular divisions of the medial geniculate nucleus, respectively. In the model, the specificity of RF plasticity is dependent on Hebbian rules of covariance. This aspect was confirmed in vivo using microstimulation techniques. Further, the model predicts that pairing a tone with activation of the nucleus basalis is sufficient to induce RF plasticity similar to that obtained in behavioral learning. This prediction has been confirmed. Additional tests of the model are described. RF plasticity is thought to translate the acquired significance of sound into an increased frequency representation of behaviorally important stimuli.
众所周知，包括听觉皮层在内的成年感觉皮层的功能组织可通过传入神经阻滞、感觉剥夺或选择性感觉刺激而发生改变。本文综述了相关证据，这些证据表明成年初级听觉皮层在学习过程中会产生生理可塑性。在豚鼠进行厌恶性经典条件反射和操作性回避学习之前及之后的不同时间，对频率感受野进行测定，结果显示神经元对用作条件刺激（CS）的纯音频率的反应增强。相比之下，对训练前最佳频率和其他非CS频率的反应则减弱。这些相反的变化通常足以使细胞调谐朝着甚至转向CS的频率。学习诱导的感受野（RF）可塑性具有以下特点：（i）是关联性的（需要将音调与电击配对），（ii）对CS频率具有高度特异性（例如，限于该频率±一个八度的一小部分），（iii）具有辨别性（对强化的CS +频率有特异性增强反应，但对未强化的CS -频率反应减弱），（iv）发展极其迅速（在5次试验内，这是测试的最少试验次数），并且（v）可无限期保留（测试至8周）。此外，RF可塑性很强，并非由唤醒引起，而是可以在深度麻醉的受试者中表现出来。由于学习诱导的RF可塑性具有联想记忆的主要特征，因此被称为“生理记忆”。我们基于基底核胆碱能效应在毒蕈碱受体上的汇聚，以及分别来自内侧膝状体腹侧和大细胞部分的lemniscal和非lemniscal频率信息，在听觉皮层中建立了一个RF可塑性模型。在该模型中，RF可塑性的特异性取决于协方差的赫布规则。这一点在体内使用微刺激技术得到了证实。此外，该模型预测，将音调与基底核的激活配对足以诱导类似于行为学习中获得的RF可塑性。这一预测已得到证实。还描述了对该模型的其他测试。RF可塑性被认为是将声音获得的重要性转化为行为上重要刺激的频率表征增加。
