Nakahara Hiroyuki, Morita Kenji, Wurtz Robert H, Optican Lance M
Laboratory for Mathematical Neuroscience and for Integrated Theoretical Neuroscience, RIKEN Brain Science Institute, Saitama, Japan.
J Neurophysiol. 2006 Aug;96(2):765-74. doi: 10.1152/jn.01372.2005. Epub 2006 May 3.
The superior colliculus (SC) receives a retinotopic projection of the contralateral visual field in which the representation of the central field is expanded with respect to the peripheral field. The visual projection forms a nonlinear, approximately logarithmic, map on the SC. Models of the SC commonly assume that the function defining the strength of neuronal connections within this map (the kernel) depends only on the distance between two neurons, and is thus isotropic and homogeneous. However, if the connection strength is based on the distance between two stimuli in sensory space, the kernel will be asymmetric because of the nonlinear projection onto the brain map. We show, using a model of the SC, that one consequence of these asymmetric intrinsic connections is that activity initiated at one point spreads across the map. We compare this simulated spread with the spread observed experimentally around the time of saccadic eye movements with respect to direction of spread, differing effects of local and global inhibition, and the consequences of localized inactivation on the SC map. Early studies suggested that the SC spread was caused by feedback of eye displacement during a saccade, but subsequent studies were inconsistent with this feedback hypothesis. In our new model, the spread is autonomous, resulting from intrinsic connections within the SC, and thus does not depend on eye movement feedback. Other sensory maps in the brain (e.g., visual cortex) are also nonlinear and our analysis suggests that the consequences of asymmetric connections in those areas should be considered.
上丘（SC）接收对侧视野的视网膜拓扑投射，其中中央视野的表征相对于外周视野有所扩展。视觉投射在上丘上形成一个非线性的、近似对数的图谱。上丘模型通常假定，定义该图谱内神经元连接强度的函数（核）仅取决于两个神经元之间的距离，因此是各向同性且均匀的。然而，如果连接强度基于感觉空间中两个刺激之间的距离，由于在脑图谱上的非线性投射，核将是不对称的。我们使用上丘模型表明，这些不对称内在连接的一个结果是，在某一点引发的活动会在图谱上扩散。我们将这种模拟的扩散与在扫视眼动时实验观察到的扩散在扩散方向、局部和全局抑制的不同影响以及上丘图谱局部失活的后果方面进行比较。早期研究表明，上丘的扩散是由扫视期间眼位移的反馈引起的，但随后的研究与这种反馈假说不一致。在我们的新模型中，扩散是自主的，由上丘内的内在连接导致，因此不依赖于眼动反馈。大脑中的其他感觉图谱（如视觉皮层）也是非线性的，我们的分析表明，应该考虑这些区域中不对称连接的后果。
