Stimulus-choice (mis)alignment in primate area MT

For stimuli near perceptual threshold, the trial-by-trial activity of single neurons in many sensory areas is correlated with the animal's perceptual report. This phenomenon has often been attributed to feedforward readout of the neural activity by the downstream decision-making circuits. The interpretation of choice-correlated activity is quite ambiguous, but its meaning can be better understood in the light of population-wide correlations among sensory neurons. Using a statistical nonlinear dimensionality reduction technique on single-trial ensemble recordings from the middle temporal (MT) area during perceptual-decision-making, we extracted low-dimensional latent factors that captured the population-wide fluctuations. We dissected the particular contributions of sensory-driven versus choice-correlated activity in the low-dimensional population code. We found that the latent factors strongly encoded the direction of the stimulus in single dimension with a temporal signature similar to that of single MT neurons. If the downstream circuit were optimally utilizing this information, choice-correlated signals should be aligned with this stimulus encoding dimension. Surprisingly, we found that a large component of the choice information resides in the subspace orthogonal to the stimulus representation inconsistent with the optimal readout view. This misaligned choice information allows the feedforward sensory information to coexist with the decision-making process. The time course of these signals suggest that this misaligned contribution likely is feedback from the downstream areas. We hypothesize that this non-corrupting choice-correlated feedback might be related to learning or reinforcing sensory-motor relations in the sensory population.

当刺激处于知觉阈值附近时，多个感觉脑区内单个神经元的逐试次活动与动物的知觉报告存在相关性。该现象常被归因于下游决策回路对神经活动的前向读取。与选择相关的神经活动的解释历来颇具歧义，但结合感觉神经元群体层面的相关性，我们可更好地理解其内涵。我们在知觉决策任务中，对来自中颞叶（MT）区的逐试次集群记录数据采用统计非线性降维技术，提取出了能够捕捉群体水平波动的低维潜在因子。我们解析了低维群体编码中，感觉驱动活动与选择相关活动各自的特有贡献。我们发现，潜在因子在单个维度上可强烈编码刺激方向，其时程特征与单个MT神经元的时程特征相似。若下游回路能最优利用该信息，则与选择相关的信号应与该刺激编码维度保持对齐。令人意外的是，我们发现大量选择相关信息存在于与刺激表征正交的子空间中，这与最优读取的观点相悖。这种未对齐的选择相关信息使得前向感觉信息能够与决策过程共存。这些信号的时程特征表明，这种未对齐的贡献大概率来自下游脑区的反馈投射。我们据此提出假说：这种无干扰的选择相关反馈可能与感觉群体内感觉-运动关系的学习或强化有关。