Shen L, Alexander G E
Department of Neurology, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
J Neurophysiol. 1997 Mar;77(3):1195-212. doi: 10.1152/jn.1997.77.3.1195.
The dorsal premotor area (PMd) of monkeys has been implicated in processes relating to movement preparation and movement selection. In the present study, we sought to determine whether PMd neurons that are activated during a delayed reaching task have directional responses that reflect either the target (i.e., the goal) of an intended movement or the physical properties of the movement itself. Two macaque monkeys were trained to perform a visually instructed, delayed reaching task with indirect visual feedback. The subjects and methods were identical to those described in the preceding paper. In the behavioral task, each subject moved a two-dimensional joystick with the right forelimb to align a cursor with targets presented on a video display. The paradigm dissociated the direction of forelimb movement from the spatial location of the target. This was accomplished by varying the spatial mappings between joystick and cursor. A variable delay separated the visual stimulus that instructed the target location (IS) from the visual stimulus that triggered the instructed movement (TS). Task-related activity was recorded from a total of 181 PMd neurons. The focus of this study was on directionally tuned neuronal responses that included 1) stimulus-related activity (phasic, following IS); 2) set-related activity (phasic, following IS and TS); and 3) movement-related activity (phasic, following TS). Of the entire sample of PMd neurons with directionally tuned activity, 114 were tested with two joystick/cursor mappings, permitting dissociation of directional responses that depended on limb trajectory from those that depended on target location. Task-related neuronal activity was classified as target-dependent if it covaried exclusively with target location across both conditions, and as limb-dependent if it covaried exclusively with limb trajectory. Directional activity that changed significantly across rotation conditions was classified as complex. Approximately one half of the sample of PMd neurons showed stimulus-related activity that was directionally tuned (56%, 64 of 114). Nearly all of the directionally classifiable stimulus-related activity was target dependent (94%, 44 of 47 responses), and none was limb dependent. A small proportion was classified as complex (6%, 3 of 47 responses). More than two thirds of the PMd neurons showed set-related activity that was directionally tuned (69%, 79 of 114). Among cells with set-related activity that was directionally classifiable, there were approximately 9 times as many target-dependent responses (76%, 48 of 63) as there were limb-dependent responses (8%, 5 of 63), with the remainder being complex (16%, 10 of 63). Approximately three quarters of the sample of PMd neurons showed early movement-related activity (before movement onset) that was directionally tuned (78%, 89 of 114). Among those cells whose early movement-related activity was directionally classifiable, there were > 3 times as many target-dependent responses ((51%, 34 of 66) as limb-dependent responses (14%, 9 of 66), with the remainder being complex (35%, 23 of 66). Approximately two thirds of the sample showed late movement-related responses (after movement onset) that were directionally tuned (68%, 78 of 114). Among those cells whose late movement-related activity was directionally classifiable, there were comparable numbers of target-dependent (25%, 15 of 61) and limb-dependent responses (28%, 17 of 61), with the remainder being complex (47%, 29 of 61). These results indicate a preferential representation of target location rather than limb trajectory among PMd neurons. Over the extended interval from IS to motor response, there was a gradual decline in the frequency of target-dependent activity and corresponding increases in the frequencies of both limb-dependent and complex activity. These findings suggest that PMd neurons may participate in mediating the sensory-to-motor transformation required by the delayed reaching
猴子的背侧运动前区（PMd）与运动准备和运动选择相关的过程有关。在本研究中，我们试图确定在延迟伸手任务中被激活的PMd神经元是否具有方向反应，这些反应反映了预期运动的目标（即终点）或运动本身的物理特性。两只猕猴经过训练，执行一项带有间接视觉反馈的视觉指示延迟伸手任务。实验对象和方法与前一篇论文中描述的相同。在行为任务中，每个实验对象用右前肢移动一个二维操纵杆，使光标与视频显示器上呈现的目标对齐。该范式将前肢运动的方向与目标的空间位置分离开来。这是通过改变操纵杆和光标的空间映射来实现的。一个可变延迟将指示目标位置的视觉刺激（IS）与触发指示运动的视觉刺激（TS）分隔开。共记录了181个PMd神经元的任务相关活动。本研究的重点是方向调谐神经元反应，包括1）刺激相关活动（相位性，跟随IS）；2）集合相关活动（相位性，跟随IS和TS）；以及3）运动相关活动（相位性，跟随TS）。在具有方向调谐活动的PMd神经元的整个样本中，114个神经元用两种操纵杆/光标映射进行了测试，从而能够区分依赖于肢体轨迹的方向反应和依赖于目标位置的方向反应。如果任务相关神经元活动在两种条件下都仅与目标位置协变，则被分类为目标依赖型；如果仅与肢体轨迹协变，则被分类为肢体依赖型。在旋转条件下显著变化的方向活动被分类为复杂型。PMd神经元样本中约有一半显示出方向调谐的刺激相关活动（56%，114个中的64个）。几乎所有可分类方向的刺激相关活动都是目标依赖型的（94%，47个反应中的44个），没有一个是肢体依赖型的。一小部分被分类为复杂型（6%，47个反应中的3个）。超过三分之二的PMd神经元显示出方向调谐的集合相关活动（69%，114个中的79个）。在具有可分类方向的集合相关活动的细胞中，目标依赖型反应（76%，63个中的48个）大约是肢体依赖型反应（8%，63个中的5个）的9倍，其余为复杂型（16%，63个中的10个）。PMd神经元样本中约四分之三显示出方向调谐的早期运动相关活动（运动开始前）（78%，114个中的8个）。在那些早期运动相关活动可分类方向的细胞中，目标依赖型反应（51%，66个中的34个）是肢体依赖型反应（14%，66个中的9个）的3倍多，其余为复杂型（35%，66个中的23个）。约三分之二的样本显示出方向调谐的晚期运动相关反应（运动开始后）（68%，114个中的78个）。在那些晚期运动相关活动可分类方向的细胞中，目标依赖型反应（25%，61个中的15个）和肢体依赖型反应（28%，61个中的17个）数量相当，其余为复杂型（47%，61个中的29个）。这些结果表明，PMd神经元中目标位置的表征比肢体轨迹更具优势。在从IS到运动反应的延长间隔内，目标依赖型活动的频率逐渐下降，而肢体依赖型和复杂型活动的频率相应增加。这些发现表明，PMd神经元可能参与介导延迟伸手所需的感觉-运动转换。
