Xing J, Andersen R A
California Institute of Technology, Pasadena, CA 91125, USA.
J Cogn Neurosci. 2000 Jul;12(4):601-14. doi: 10.1162/089892900562363.
Many neurons in the posterior-parietal cortex (PPC) have saccadic responses to visual and auditory targets. The responses are modulated by eye position and head position. These findings suggest that PPC integrates multisensory inputs and may provide information about saccadic targets represented in different coordinate frames. In addition to an eyecentered output representation, PPC may also project to brain areas which contain head-centered and body-centered representations of the space. In this report, possible coordinate transformations in PPC were examined by comparing several sets of models of PPC, each having different representations in the output layer: (i) an eye-centered map only; (ii) a head-centered map only; (iii) an eye-centered map and a head-centered map; and (iv) an eye-centered map, a head-centered map, and a body-centered map. These output maps correctly encoded saccades to visual and auditory targets through training. The units in the hidden layers of the models exhibited the following properties: (1) The units had gain fields (GFs) for eye position, and also for head position if the model had a body-centered output representation; (2) As the result of the GF and the nonlinear activation function of the units, the hidden layers often employed "intermediate" coding, e.g., the hidden units coded targets partially in eye-centered coordinates and, partially, in head-centered coordinates; (3) Different types of coordinate transformations in these models were carried out by different relationships between the receptive fields (RFs) and the GFs of the hidden units; and (4) The properties of PPC neurons are in better accordance with the hidden units of the models that had multiple-output representations than the models that had only one single-output representation. In conclusion, the results show that the GF is an effective mechanism for performing coordinate transformations. The models also suggest that neurons with intermediate coding are to be expected in the process of coordinate transformations.
后顶叶皮质（PPC）中的许多神经元对视觉和听觉目标具有扫视反应。这些反应受眼睛位置和头部位置的调节。这些发现表明，PPC整合多感官输入，并可能提供以不同坐标系表示的扫视目标的信息。除了以眼睛为中心的输出表示外，PPC还可能投射到包含以头部为中心和以身体为中心的空间表示的脑区。在本报告中，通过比较几组PPC模型来研究PPC中可能的坐标转换，每组模型在输出层具有不同的表示：（i）仅以眼睛为中心的地图；（ii）仅以头部为中心的地图；（iii）以眼睛为中心的地图和以头部为中心的地图；以及（iv）以眼睛为中心的地图、以头部为中心的地图和以身体为中心的地图。通过训练，这些输出地图正确地编码了对视觉和听觉目标的扫视。模型隐藏层中的单元表现出以下特性：（1）这些单元具有眼睛位置的增益场（GFs），如果模型具有以身体为中心的输出表示，还具有头部位置的增益场；（2）作为单元的GF和非线性激活函数的结果，隐藏层经常采用“中间”编码，例如，隐藏单元部分以眼睛为中心的坐标编码目标，部分以头部为中心的坐标编码目标；（3）这些模型中不同类型的坐标转换是由隐藏单元的感受野（RFs）和GFs之间的不同关系执行的；（4）PPC神经元的特性与具有多个输出表示的模型的隐藏单元比与仅具有单个输出表示的模型的隐藏单元更符合。总之，结果表明GF是执行坐标转换的有效机制。这些模型还表明，在坐标转换过程中可以预期具有中间编码的神经元。
