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Corollary discharge and spatial updating: when the brain is split, is space still unified?

机译：推论和空间更新：当大脑分裂时，空间是否仍然统一？

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How does the brain keep track of salient locations in the visual world when the eyes move? In parietal, frontal and extrastriate cortex, and in the superior colliculus, neurons update or 'remap' stimulus representations in conjunction with eye movements. This updating reflects a transfer of visual information, from neurons that encode a salient location before the saccade, to neurons that encode the location after the saccade. Copies of the oculomotor command - corollary discharge signals - must initiate this transfer. We investigated the circuitry that supports spacial updating in the primate brain. Our central hypothesis was that the forebrain commissures provide the primary route for remapping spatial locations across visual hemifields, from one cortical hemisphere to the other. Further, we hypothesized that these commissures provide the primary route for communicating corollary discharge signals from one hemisphere to the other. We tested these hypotheses using the double-step task and subsequent physiological recording in two split-brain monkeys. In the double-step task, monkeys made sequential saccades to two briefly presented targets, T1 and T2. In the visual version of the task, the representation of T2 was updated either within the same hemifield ("visual-within"), or across hemifields ("visual-across"). In the motor version, updating of the visual stimulus was always within-hemifield. The corollary discharge signal that initiated the updating, however, was generated either within the same hemisphere ("motor-within") or in the opposite hemisphere ("motor-across"). We expected that, in the absence of the forebrain commissures, both visual-across and motor-across conditions would be impaired relative to their "within" controls. In behavioral experiments, we observed striking initial impairments in the monkeys' ability to update stimuli across visual hemifields. Surprisingly, however, both animals were ultimately capable of performing the visual-across sequences of the double-step task. In subsequent physiological experiments, we found that neurons in lateral intraparietal cortex (LIP) can remap stimuli across visual hemifields, albeit with a reduction in the strength of remapping activity. These behavioral and neural findings indicate that the transfer of visual information is compromised, but by no means abolished, in the absence of the forebrain commissures. We found minimal evidence of impairment of the motor-across condition. Both monkeys readily performed the motor-across sequences of the double-step task, and LIP neurons were robustly active when within-hemifield updating was initiated by a saccade into the opposite hemifield. These results indicate that corollary discharge signals are available bilaterally. Altogether, our findings show that both visual and corollary discharge signals from opposite hemispheres can converge to update spatial representations in the absence of the forebrain commissures. These investigations provide new evidence that a unified and stable representation of visual space is supported by a redundant circuit, comprised of cortical as well as subcortical pathways, with a remarkable capacity for reorganization.
机译：当眼睛移动时，大脑如何跟踪视觉世界中的重要位置？在顶叶，额叶和外皮层以及上丘中，神经元会随着眼球运动而更新或“重映射”刺激表现。此更新反映了视觉信息从编码扫视之前的显着位置的神经元到编码扫视之后的位置的神经元的传递。动眼运动命令的副本-必然放电信号-必须启动此传输。我们研究了支持灵长类动物大脑中空间更新的电路。我们的中心假设是，前脑连合提供了重新映射从一个皮层半球到另一个皮层半球的视觉半场的空间位置的主要途径。此外，我们假设这些连合提供了从一个半球向另一个半球传递推论放电信号的主要途径。我们使用了双重步骤并随后在两只裂脑猴中进行了生理记录测试了这些假设。在双步任务中，猴子对两个简要介绍的目标T1和T2进行了连续扫视。在任务的可视版本中，T2的表示在同一半场内（“内部可视”）或跨半场（“跨视觉”）更新。在运动版中，视觉刺激的更新始终在半视野内。然而，引发更新的必然放电信号是在相同的半球（“内部电动机”）或相反的半球（“横跨电动机”）内产生的。我们预期，在没有前脑连合的情况下，相对于其“内部”对照，视觉交叉和运动交叉条件都会受到损害。在行为实验中，我们观察到猴子在视觉半视野内更新刺激的能力受到了明显的损害。然而，令人惊讶的是，两只动物最终都能够执行跨步任务的跨视觉序列。在随后的生理实验中，我们发现侧顶壁皮层（LIP）中的神经元可以在视觉半场中重新映射刺激，尽管降低了重新映射活动的强度。这些行为和神经方面的发现表明，在没有前脑连合的情况下，视觉信息的传递受到了损害，但绝没有消除。我们发现运动障碍情况受损的证据极少。两只猴子都轻松执行了跨步任务的跨运动序列，并且当通过扫视向相对的半场发起半场内更新时，LIP神经元活跃。这些结果表明推论放电信号是双向可用的。总而言之，我们的发现表明，在没有前脑连合的情况下，来自相对半球的视觉和推论放电信号都可以收敛以更新空间表示。这些研究提供了新的证据，表明视觉空间的统一和稳定表示由包含皮层和皮层下通道的冗余回路支持​​，并具有显着的重组能力。

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