Paré M, Guitton D
Montréal Neurological Institute, Canada.
Exp Brain Res. 1994;101(1):109-22. doi: 10.1007/BF00243221.
The superior colliculus has long been recognized as an important structure in the generation of saccadic displacements of the visual axis. Neurons with presaccadic activity encoding saccade vectors are topographically organized and form a "motor map." Recently, neurons with fixation-related activity have been recorded at the collicular rostral pole, at the area centralis representation or fixation area. Another collicular function which deals with the maintenance of fixation behavior by means of active inhibition of orientation commands was then suggested. We tested that hypothesis as it relates to the suppression of gaze saccades (gaze = eye in space = eye in head + head in space) in the head-free cat by increasing the activity of the fixation cells at the rostral pole with electrical microstimulation. Long stimulation trains applied before gaze saccades delayed their initiation. Short stimuli, delivered during the gaze saccades, transiently interrupted both eye and head components. These results provide further support for a role in fixation behavior for collicular fixation neurons. Brainstem omnipause neurons also exhibit fixation-related activity and have been shown to receive a direct excitatory input from the superior colliculus. To determine whether the collicular projection to omnipause neurons arises from the fixation area, the deep layers of the superior colliculus were electrically stimulated either at the rostral pole including the fixation area or in more caudal regions where stimulation evokes orienting responses. Forty-nine neurons were examined in three cats. 61% of the neurons were found to be orthodromically excited by single-pulse stimulation of the rostral pole, whereas only 29% responded to caudal stimulation. In addition, stimuli delivered to the rostral pole activated, on average, omnipause neurons at shorter latencies and with lower currents than those applied in caudal regions. These results suggest that excitatory inputs to omnipause neurons from the superior colliculus are principally provided by the fixation area, via which the superior colliculus could play a role in suppression of gaze shifts.
上丘长期以来一直被认为是视觉轴扫视位移产生过程中的一个重要结构。具有编码扫视向量的扫视前活动的神经元在地形上有组织地排列，形成一个“运动图谱”。最近，在丘脑中喙极、中央区表征或注视区记录到了具有注视相关活动的神经元。随后有人提出上丘的另一个功能，即通过对定向指令的主动抑制来维持注视行为。我们通过电微刺激增加中喙极注视细胞的活动，来测试这一与无头部猫的注视扫视（注视=空间中的眼睛=头部中的眼睛+空间中的头部）抑制相关的假设。在注视扫视之前施加的长时间刺激序列会延迟其起始。在注视扫视期间施加的短刺激会短暂中断眼睛和头部的运动成分。这些结果为上丘注视神经元在注视行为中的作用提供了进一步的支持。脑干全暂停神经元也表现出注视相关活动，并且已被证明接受来自上丘的直接兴奋性输入。为了确定上丘向全暂停神经元的投射是否起源于注视区，对上丘深层在包括注视区的中喙极或在更靠尾侧的区域进行电刺激，在更靠尾侧的区域刺激会引发定向反应。在三只猫中检查了49个神经元。发现其中61%的神经元通过对中喙极的单脉冲刺激产生顺向兴奋，而只有29%的神经元对尾侧刺激有反应。此外，与施加在尾侧区域的刺激相比，施加到中喙极的刺激平均以更短的潜伏期和更低的电流激活全暂停神经元。这些结果表明，上丘向全暂停神经元的兴奋性输入主要由注视区提供，通过这一途径上丘可能在抑制注视转移中发挥作用。
