Ichida Jennifer M, Schwabe Lars, Bressloff Paul C, Angelucci Alessandra
Department of Ophthalmology, University of Utah, Salt Lake City, UT 84132, USA.
J Neurophysiol. 2007 Oct;98(4):2168-81. doi: 10.1152/jn.00298.2007. Epub 2007 Aug 8.
In primary visual cortex (V1), neuronal responses to optimally oriented stimuli in the receptive field (RF) center are usually suppressed by iso-oriented stimuli in the RF surround. The mechanisms and pathways giving rise to surround modulation, a possible neural correlate of perceptual figure-ground segregation, are not yet identified. We previously proposed that highly divergent and fast-conducting top-down feedback connections are the substrate for fast modulation arising from the more distant regions of the surround. We have recently implemented this idea into a recurrent network model (Schwabe et al. 2006). The purpose of this study was to test a crucial prediction of this feedback model, namely that the suppressive "far" surround of V1 neurons can be facilitatory under conditions that weakly activate neurons in the RF center. Using single-unit recordings in macaque V1, we found iso-orientation far-surround facilitation when the RF center was driven by a low-contrast stimulus and the far surround by a small annular stimulus. Suppression occurred when the center stimulus contrast or the size of the surround stimulus was increased. This suggests that center-surround interactions result from excitatory and inhibitory mechanisms of similar spatial extent, and that changes in the balance of local excitation and inhibition, induced by surround stimulation, determine whether facilitation or suppression occurs. In layer 4C, the main target of geniculocortical afferents, lacking long-range intra-cortical connections, far-surround facilitation was rare and large surround fields were absent. This strongly suggests that feedforward connections do not contribute to far-surround modulation and that the latter is generated by intra-cortical mechanisms, likely involving top-down feedback.
在初级视皮层（V1）中，感受野（RF）中心对最佳朝向刺激的神经元反应通常会受到RF外周同朝向刺激的抑制。导致外周调制（一种可能与知觉图形 - 背景分离相关的神经关联）的机制和途径尚未明确。我们之前提出，高度发散且传导速度快的自上而下反馈连接是外周较远区域产生快速调制的基础。我们最近将这一想法应用于一个循环网络模型（施瓦贝等人，2006年）。本研究的目的是检验该反馈模型的一个关键预测，即V1神经元的抑制性“远”外周在弱激活RF中心神经元的条件下可能具有易化作用。通过对猕猴V1进行单单位记录，我们发现当RF中心由低对比度刺激驱动且远外周由小环形刺激驱动时，存在同朝向远外周易化现象。当中心刺激对比度或外周刺激大小增加时则出现抑制。这表明中心 - 外周相互作用是由空间范围相似的兴奋性和抑制性机制导致的，并且外周刺激引起的局部兴奋和抑制平衡的变化决定了易化还是抑制的发生。在4C层，即丘脑皮质传入纤维的主要靶点，由于缺乏长程皮质内连接，远外周易化很少见且不存在大的外周区域。这有力地表明前馈连接对外周远调制没有贡献，后者是由皮质内机制产生的，可能涉及自上而下的反馈。
