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，诸多感觉皮层区域内的单个神经元，其逐试次活动会与动物的知觉报告呈现相关性。该现象常被归因于下游决策环路对神经活动的前馈读取。尽管针对选择相关活动的解释存在较大歧义，但结合感觉神经元的群体水平相关性，我们可以更好地阐释其意义。本研究针对知觉决策任务中采集的中颞区（MT）单试次集群记录数据，运用统计非线性降维技术，提取出可捕捉群体水平波动的低维隐因子。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的具体贡献。研究结果显示，这些隐因子在单个维度上可强烈编码刺激方向，其时间特征与单个MT神经元的时间特征高度相似。若下游环路能够最优利用该信息，则选择相关信号应与该刺激编码维度保持对齐。但令人意外的是，我们发现大量选择信息存在于与刺激表征正交的子空间中，这与最优读取假说的预测并不一致。这种未对齐的选择信息使得前馈感觉信息能够与决策过程共存。此类信号的时间进程表明，这类未对齐的贡献大概率来源于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神经元群体内的感觉运动关联学习或强化过程相关。