Department of Neurobiology & Anatomy, University of Rochester, Box 603, Rochester, NY 14642, USA.
J Neurophysiol. 2011 Sep;106(3):1260-73. doi: 10.1152/jn.00016.2011. Epub 2011 Jun 15.
Visually guided behavior often involves decisions that are based on evaluating stimuli in the context of those observed previously. Such decisions are made by monkeys comparing two consecutive stimuli, sample and test, moving in the same or opposite directions. We examined whether responses in the motion processing area MT during the comparison phase of this task (test) are modulated by the direction of the preceding stimulus (sample). This modulation, termed comparison signal, was measured by comparing responses to identical test stimuli on trials when it was preceded by sample moving in the same direction (S-trials) with trials when it was preceded by sample moving in a different direction (D-trials). The test always appeared in the neuron's receptive field (RF), whereas sample could appear in the RF or in the contralateral visual field (remote sample). With sample in-RF, we found three types of modulation carried by different sets of neurons: early suppression on S-trials and late enhancement, one on S-trials, and the other on D-trials. Under these conditions, many neurons with and without comparison effects exhibited significant, choice-related activity. Response modulation was also present following the remote sample, even though the information about its direction could only reach MT indirectly via top-down influences. However, unlike on trials with in-RF sample, these signals were dominated by response suppression, shedding light on the contribution of top-down influences to the comparison effects. These results demonstrate that during the task requiring monkeys to compare two directions of motion, MT responses during the comparison phase of this task reflect similarities and differences between the two stimuli, suggesting participation in sensory comparisons. The nature of these signals provides insights into the operation of bottom-up and top-down influences involved in this process.
视觉引导的行为通常涉及基于先前观察到的刺激来评估刺激的决策。猴子通过比较两个连续的刺激，样本和测试，在相同或相反的方向上移动来做出这些决策。我们研究了在这个任务（测试）的比较阶段期间，运动处理区域 MT 中的反应是否受到前一个刺激（样本）方向的调制。这种调制，称为比较信号，通过比较在样本以相同方向（S 试验）移动时的试验和在样本以不同方向（D 试验）移动时的试验中对相同测试刺激的反应来测量。测试总是出现在神经元的感受野（RF）中，而样本可以出现在 RF 或对侧视野（远程样本）中。当样本在 RF 中时，我们发现了由不同神经元集合携带的三种类型的调制：S 试验上的早期抑制和晚期增强，一种在 S 试验上，另一种在 D 试验上。在这些条件下，许多具有和不具有比较效应的神经元表现出显著的、与选择相关的活动。即使关于其方向的信息只能通过自上而下的影响间接到达 MT，远程样本后也存在响应调制。然而，与在 RF 中具有样本的试验不同，这些信号主要由响应抑制主导，这揭示了自上而下的影响对比较效应的贡献。这些结果表明，在要求猴子比较两个运动方向的任务期间，MT 响应在这个任务的比较阶段反映了两个刺激之间的相似性和差异性，表明参与了感觉比较。这些信号的性质为这个过程中涉及的自下而上和自上而下的影响的运作提供了深入了解。
