Talwar S K, Gerstein G L
Department of Neuroscience, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
J Neurophysiol. 2001 Oct;86(4):1555-72. doi: 10.1152/jn.2001.86.4.1555.
In common with other sensory cortices, the mammalian primary auditory cortex (AI) demonstrates the capacity for large-scale reorganization following many experimental situations. For example, training animals in frequency-discrimination tasks has been shown to result in an increase in cortical frequency representation. Such central changes-most commonly, an increase in central representation of specific stimulus parameters-have been hypothesized to underlie the improvements in perceptual acuity (perceptual learning) seen in many learning situations. The actual behavioral relevance of central reorganizations, however, remains speculative. Here, we directly examine this issue. We first show that stimulating the AI cortex of the awake rat with a weak electric current (intracortical microstimulation or ICMS) has the effect of inducing central reorganizations similar to those accompanying the traditional plasticity experiments (a result previously noted only in anesthetized preparations). Depending on the site of AI stimulation, ICMS enlarged the cortical representation of certain frequencies. Next we examined the direct perceptual consequences of ICMS-induced AI reorganization for the rat's ability to discriminate frequencies. Over the course of the experiment, we also detailed, and made comparisons between, the frequency-response characteristics of rat AI cortex in the awake and ketamine-anesthetized animal. AI cells that responded to pure tones were divided into two categories--strongly and weakly responsive--based on the strength of their evoked discharge. Individual cells maintained their respective response strengths in both awake and anesthetized conditions. Strongly responsive cells showed at least four different temporal responses and tended to be narrowly tuned. Their responses were stable over the long term. In general frequency-response characteristics were qualitatively similar in the anesthetized and awake animal; bandwidths tended to be broader in awake animals. Although both strong and weak cell populations respond to tones, only the strongly responsive cells fit into a tonotopically organized scheme. By contrast, weakly responsive cells did not exhibit a frequency mapping and may represent a more diffuse input to AI than that underlying strongly responsive cells. In general, the overall frequency organization of AI was found to be equally well expressed in both the awake and anesthetized rat. ICMS reorganization of AI did not alter frequency-discrimination behavior in the rat--either signal detectability or response bias--suggesting that an increase in central representation, by itself, is insufficient to account for perceptual learning. It is likely that cortical reorganizations that accompany perceptual learning are strongly keyed to specific behavioral contexts.
与其他感觉皮层一样，哺乳动物的初级听觉皮层（AI）在许多实验情况下都表现出大规模重组的能力。例如，训练动物完成频率辨别任务已被证明会导致皮层频率表征增加。这种中枢变化——最常见的是特定刺激参数的中枢表征增加——被认为是许多学习情况下感知敏锐度提高（感知学习）的基础。然而，中枢重组的实际行为相关性仍具有推测性。在此，我们直接研究这个问题。我们首先表明，用弱电流（皮层内微刺激或ICMS）刺激清醒大鼠的AI皮层，会产生与传统可塑性实验伴随的中枢重组相似的效果（这一结果此前仅在麻醉制剂中被注意到）。根据AI刺激的部位，ICMS扩大了某些频率的皮层表征。接下来，我们研究了ICMS诱导的AI重组对大鼠频率辨别能力的直接感知后果。在实验过程中，我们还详细描述并比较了清醒和氯胺酮麻醉动物中大鼠AI皮层的频率响应特性。对纯音有反应的AI细胞根据其诱发放电的强度分为两类——强反应型和弱反应型。单个细胞在清醒和麻醉条件下都保持各自的反应强度。强反应型细胞表现出至少四种不同的时间反应，并且倾向于具有窄调谐。它们的反应长期稳定。一般来说，麻醉和清醒动物的频率响应特性在定性上相似；清醒动物的带宽往往更宽。虽然强反应型和弱反应型细胞群体都对音调有反应，但只有强反应型细胞符合音调拓扑组织模式。相比之下，弱反应型细胞没有表现出频率映射，可能代表了比强反应型细胞基础输入更分散的输入到AI。一般来说，发现AI的整体频率组织在清醒和麻醉大鼠中表达同样良好。AI的ICMS重组并没有改变大鼠的频率辨别行为——无论是信号可检测性还是反应偏差——这表明中枢表征的增加本身不足以解释感知学习。伴随感知学习的皮层重组很可能与特定的行为背景密切相关。
