Department of Physiology and the Center for Integrative Neuroscience, University of California San Francisco, San Francisco, California, USA.
PLoS Comput Biol. 2013 Apr;9(4):e1003035. doi: 10.1371/journal.pcbi.1003035. Epub 2013 Apr 18.
Sensory processing in the brain includes three key operations: multisensory integration-the task of combining cues into a single estimate of a common underlying stimulus; coordinate transformations-the change of reference frame for a stimulus (e.g., retinotopic to body-centered) effected through knowledge about an intervening variable (e.g., gaze position); and the incorporation of prior information. Statistically optimal sensory processing requires that each of these operations maintains the correct posterior distribution over the stimulus. Elements of this optimality have been demonstrated in many behavioral contexts in humans and other animals, suggesting that the neural computations are indeed optimal. That the relationships between sensory modalities are complex and plastic further suggests that these computations are learned-but how? We provide a principled answer, by treating the acquisition of these mappings as a case of density estimation, a well-studied problem in machine learning and statistics, in which the distribution of observed data is modeled in terms of a set of fixed parameters and a set of latent variables. In our case, the observed data are unisensory-population activities, the fixed parameters are synaptic connections, and the latent variables are multisensory-population activities. In particular, we train a restricted Boltzmann machine with the biologically plausible contrastive-divergence rule to learn a range of neural computations not previously demonstrated under a single approach: optimal integration; encoding of priors; hierarchical integration of cues; learning when not to integrate; and coordinate transformation. The model makes testable predictions about the nature of multisensory representations.
多感觉整合——将线索组合成对共同潜在刺激的单一估计的任务;坐标变换——通过对中间变量(例如,注视位置)的知识来改变刺激的参考框架(例如,视网膜到身体中心);以及先验信息的纳入。统计上最优的感觉处理要求这些操作中的每一个都保持对刺激的正确后验分布。这些最优性的要素在人类和其他动物的许多行为背景中得到了证明,这表明神经计算确实是最优的。感觉模态之间的关系是复杂和可塑的,这进一步表明这些计算是学习的——但如何学习?我们通过将这些映射的获取视为密度估计的情况来提供一个有原则的答案,这是机器学习和统计学中一个研究充分的问题,其中观察到的数据的分布是根据一组固定参数和一组潜在变量来建模的。在我们的案例中,观察到的数据是单感觉——群体活动,固定参数是突触连接,而潜在变量是多感觉——群体活动。具体来说,我们使用具有生物合理性的对比散度规则训练受限玻尔兹曼机,以学习以前在单一方法下未证明的一系列神经计算:最佳整合;先验编码;线索的分层整合;何时不整合的学习;以及坐标变换。该模型对多感觉表示的性质做出了可测试的预测。