Ramachandran Suchitra, Meyer Travis, Olson Carl R
Center for the Neural Basis of Cognition, Carnegie Mellon University, Pittsburgh, Pennsylvania;
Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, Pennsylvania.
J Neurophysiol. 2017 Jul 1;118(1):374-382. doi: 10.1152/jn.00136.2017. Epub 2017 Apr 19.
Exposing monkeys, over the course of days and weeks, to pairs of images presented in fixed sequence, so that each leading image becomes a predictor for the corresponding trailing image, affects neuronal visual responsiveness in area TE. At the end of the training period, neurons respond relatively weakly to a trailing image when it appears in a trained sequence and, thus, confirms prediction, whereas they respond relatively strongly to the same image when it appears in an untrained sequence and, thus, violates prediction. This effect could arise from prediction suppression (reduced firing in response to the occurrence of a probable event) or surprise enhancement (elevated firing in response to the omission of a probable event). To identify its cause, we compared firing under the prediction-confirming and prediction-violating conditions to firing under a prediction-neutral condition. The results provide strong evidence for prediction suppression and limited evidence for surprise enhancement. In predictive coding models of the visual system, neurons carry signed prediction error signals. We show here that monkey inferotemporal neurons exhibit prediction-modulated firing, as posited by these models, but that the signal is unsigned. The response to a prediction-confirming image is suppressed, and the response to a prediction-violating image may be enhanced. These results are better explained by a model in which the visual system emphasizes unpredicted events than by a predictive coding model.
在数天乃至数周的时间里，让猴子观看按固定顺序呈现的成对图像，使每张引导图像成为相应后续图像的预测信号，这会影响颞下回（TE）区域神经元的视觉反应性。在训练期结束时，当后续图像出现在训练序列中从而证实预测时，神经元对其反应相对较弱；而当同一图像出现在未训练序列中从而违背预测时，神经元对其反应相对较强。这种效应可能源于预测抑制（对可能事件的发生反应时放电减少）或意外增强（对可能事件的缺失反应时放电增加）。为确定其原因，我们将预测证实和预测违背条件下的放电与预测中性条件下的放电进行了比较。结果为预测抑制提供了有力证据，而意外增强的证据有限。在视觉系统的预测编码模型中，神经元携带带符号的预测误差信号。我们在此表明，猴子颞下神经元表现出如这些模型所假定的预测调制放电，但该信号是不带符号的。对预测证实图像的反应受到抑制，而对预测违背图像的反应可能增强。相较于预测编码模型，一个视觉系统强调未预测事件的模型能更好地解释这些结果。
