Shenoy K V, Bradley D C, Andersen R A
Division of Biology, California Institute of Technology, Pasadena, California 91125, USA.
J Neurophysiol. 1999 Jun;81(6):2764-86. doi: 10.1152/jn.1999.81.6.2764.
When we move forward, the visual image on our retina expands. Humans rely on the focus, or center, of this expansion to estimate their direction of heading and, as long as the eyes are still, the retinal focus corresponds to the heading. However, smooth rotation of the eyes adds nearly uniform visual motion to the expanding retinal image and causes a displacement of the retinal focus. In spite of this, humans accurately judge their heading during pursuit eye movements and during active, smooth head rotations even though the retinal focus no longer corresponds to the heading. Recent studies in macaque suggest that correction for pursuit may occur in the dorsal aspect of the medial superior temporal area (MSTd) because these neurons are tuned to the retinal position of the focus and they modify their tuning during pursuit to compensate partially for the focus shift. However, the question remains whether these neurons also shift focus tuning to compensate for smooth head rotations that commonly occur during gaze tracking. To investigate this question, we recorded from 80 MSTd neurons while monkeys tracked a visual target either by pursuing with their eyes or by vestibulo-ocular reflex cancellation (VORC; whole-body rotation with eyes fixed in head and head fixed on body). VORC is a passive, smooth head rotation condition that selectively activates the vestibular canals. We found that neurons shift their focus tuning in a similar way whether focus displacement is caused by pursuit or by VORC. Across the population, compensation averaged 88 and 77% during pursuit and VORC, respectively (tuning shift divided by the retinal focus to true heading difference). Moreover the degree of compensation during pursuit and VORC was correlated in individual cells (P < 0.001). Finally neurons that did not compensate appreciably tended to be gain-modulated during pursuit and VORC and may constitute an intermediate stage in the compensation process. These results indicate that many MSTd cells compensate for general gaze rotation, whether produced by eye-in-head or head-in-world rotation, and further implicate MSTd as a critical stage in the computation of heading. Interestingly vestibular cues present during VORC allow many cells to compensate even though humans do not accurately judge their heading in this condition. This suggests that MSTd may use vestibular information to create a compensated heading representation within at least a subpopulation of cells, which is accessed perceptually only when additional cues related to active head rotations are also present.
当我们向前移动时，视网膜上的视觉图像会扩大。人类依靠这种扩大的焦点或中心来估计自己的前进方向，并且只要眼睛保持静止，视网膜焦点就与前进方向相对应。然而，眼睛的平稳转动会给不断扩大的视网膜图像增加几乎均匀的视觉运动，并导致视网膜焦点发生位移。尽管如此，人类在追踪眼球运动期间以及在主动、平稳的头部转动过程中仍能准确判断自己的前进方向，即使视网膜焦点不再与前进方向相对应。最近对猕猴的研究表明，对追踪的校正可能发生在内侧颞上区（MSTd）的背侧，因为这些神经元会根据焦点的视网膜位置进行调谐，并且在追踪过程中会调整其调谐以部分补偿焦点的移动。然而，问题仍然存在，即这些神经元是否也会改变焦点调谐以补偿在注视跟踪过程中常见的平稳头部转动。为了研究这个问题，我们在猴子通过眼睛追踪视觉目标或通过前庭眼反射抵消（VORC；眼睛固定在头部，头部固定在身体上的全身转动）来追踪视觉目标时，记录了80个MSTd神经元的活动。VORC是一种被动的、平稳的头部转动状态，它会选择性地激活前庭管。我们发现，无论焦点位移是由追踪还是由VORC引起的，神经元都会以类似的方式改变其焦点调谐。在整个群体中，追踪和VORC期间的补偿平均分别为88%和77%（调谐变化除以视网膜焦点与真实前进方向的差异）。此外，追踪和VORC期间的补偿程度在单个细胞中是相关联的（P < 0.００１）。最后，那些没有明显补偿的神经元在追踪和VORC期间往往会进行增益调制，并且可能构成补偿过程中的一个中间阶段。这些结果表明，许多MSTd细胞会补偿一般的注视转动，无论这种转动是由头部内的眼睛转动还是由身体周围的头部转动引起的，并且进一步表明MSTd是前进方向计算中的一个关键阶段。有趣的是，VORC期间存在的前庭线索使许多细胞能够进行补偿，尽管人类在这种情况下并不能准确判断自己的前进方向。这表明MSTd可能利用前庭信息在至少一部分细胞内创建一个经过补偿的前进方向表征，只有当与主动头部转动相关的其他线索也存在时，才会在感知上被利用。
