Kadunce D C, Vaughan J W, Wallace M T, Benedek G, Stein B E
Department of Neurobiology and Anatomy, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, North Carolina 27157, USA.
J Neurophysiol. 1997 Dec;78(6):2834-47. doi: 10.1152/jn.1997.78.6.2834.
The present studies were initiated to explore the basis for the response suppression that occurs in cat superior colliculus (SC) neurons when two spatially disparate stimuli are presented simultaneously or in close temporal proximity to one another. Of specific interest was examining the possibility that suppressive regions border the receptive fields (RFs) of unimodal and multisensory SC neurons and, when activated, degrade the neuron's responses to excitatory stimuli. Both within- and cross-modality effects were examined. An example of the former is when a response to a visual stimulus within its RF is suppressed by a second visual stimulus outside the RF. An example of the latter is when the response to a visual stimulus within the visual RF is suppressed when a stimulus from a different modality (e. g., auditory) is presented outside its (i.e., auditory) RF. Suppressive regions were found bordering visual, auditory, and somatosensory RFs. Despite significant modality-specific differences in the incidence and effectiveness of these regions, they were generally quite potent regardless of the modality. In the vast majority (85%) of cases, responses to the excitatory stimulus were degraded by >/=50% by simultaneously stimulating the suppressive region. Contrary to expectations and previous speculations, the effects of activating these suppressive regions often were quite specific. Thus powerful within-modality suppression could be demonstrated in many multisensory neurons in which cross-modality suppression could not be generated. However, the converse was not true. If an extra-RF stimulus inhibited center responses to stimuli of a different modality, it also would suppress center responses to stimuli of its own modality. Thus when cross-modality suppression was demonstrated, it was always accompanied by within-modality suppression. These observations suggest that separate mechanisms underlie within- and cross-modality suppression in the SC. Because some modality-specific tectopetal structures contain neurons with suppressive regions bordering their RFs, the within-modality suppression observed in the SC simply may reflect interactions taking place at the level of one input channel. However, the presence of modality-specific suppression at the level of one input channel would have no effect on the excitation initiated via another input channel. Given the modality-specificity of tectopetal inputs, it appears that cross-modality interactions require the convergence of two or more modality-specific inputs onto the same SC neuron and that the expression of these interactions depends on the internal circuitry of the SC. This allows a cross-modality suppressive signal to be nonspecific and to degrade any and all of the neuron's excitatory inputs.
开展本研究是为了探究当同时呈现或在时间上彼此紧邻呈现两个空间上不同的刺激时，猫上丘（SC）神经元中出现的反应抑制的基础。特别感兴趣的是研究抑制区域是否毗邻单峰和多感觉SC神经元的感受野（RF），以及当被激活时，这些抑制区域是否会降低神经元对兴奋性刺激的反应。研究同时考察了模态内和跨模态效应。前者的一个例子是，RF内对视觉刺激的反应被RF外的第二个视觉刺激所抑制。后者的一个例子是，当来自不同模态（如听觉）的刺激呈现于其（即听觉）RF之外时，视觉RF内对视觉刺激的反应被抑制。发现抑制区域毗邻视觉、听觉和体感RF。尽管这些区域在发生率和有效性方面存在显著的模态特异性差异，但无论模态如何，它们通常都相当有效。在绝大多数（85%）情况下，通过同时刺激抑制区域，对兴奋性刺激的反应降低了≥50%。与预期和先前的推测相反，激活这些抑制区域的效应往往相当具有特异性。因此，在许多无法产生跨模态抑制的多感觉神经元中，可以证明强大的模态内抑制。然而，反之则不然。如果RF外刺激抑制了对不同模态刺激的中心反应，那么它也会抑制对其自身模态刺激的中心反应。因此，当证明存在跨模态抑制时，它总是伴随着模态内抑制。这些观察结果表明，SC中模态内和跨模态抑制有不同的机制。由于一些模态特异性的顶盖传入结构包含其RF毗邻抑制区域的神经元，因此在SC中观察到的模态内抑制可能仅仅反映了在一个输入通道水平上发生的相互作用。然而，一个输入通道水平上的模态特异性抑制对通过另一个输入通道引发的兴奋没有影响。鉴于顶盖传入输入的模态特异性，似乎跨模态相互作用需要两个或更多模态特异性输入汇聚到同一个SC神经元上，并且这些相互作用的表现取决于SC的内部电路。这使得跨模态抑制信号具有非特异性，并能降低神经元的任何和所有兴奋性输入。
