Tehovnik E J, Lee K
Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge 02139.
Exp Brain Res. 1993;96(3):430-42. doi: 10.1007/BF00234111.
The dorsomedial frontal cortex (DMFC) of monkeys has been implicated in mediating visually guided saccadic eye movements. The purpose of this study was to determine whether the DMFC has a topographic map coding final eye position, and to ascertain whether this region subserves the maintenance of eye position. The DMFC was stimulated electrically while monkeys fixated a target presented somewhere in visual space. A series of parametric tests was conducted to ascertain the best stimulation parameters to evoke saccades. Electrical stimulation typically produced contraversive saccades that converged onto a region of space, the termination zone. For some stimulation sites, however, stimulation produced ipsiversive saccades. This occurred when the termination zone was located straight ahead of the monkey. Convergence onto an orbital position was never observed during stimulation of the frontal eye fields (FEF), stimulation of which evoked fixed-vector saccades. The latency to evoke a saccade from the DMFC varied with fixation position, such that it increased monotonically the closer the fix spot was to the termination zone. Moreover, the probability of evoking a saccade from the DMFC decreased the closer the fix spot was to the termination zone. The latency for evoking a saccade and the probability of evoking a saccade from the FEF did not vary with fixation position. Horizontal head movements were not evoked from the DMFC while a monkey fixated targets presented in different positions of visual space. Moveover, changing the position of the head with respect to the body did not change the location of a termination zone with respect to the head. The DMFC was found to contain a topographic coding of termination zones, with rostral sites representing zones in extreme contralateral visual space, and caudal sites representing zones straight ahead or ipsilaterally. Furthermore, lateral sites represented zones in upper visual space, whereas medial sites represented zones in lower visual space. Once the eyes were positioned within a termination zone, further stimulation fixed the gaze and inhibited visually evoked saccades. Following release from inhibition, which occurred shortly after the end of stimulation, the saccades reached the visual target accurately. This shows that the stimulation delayed the execution of the saccades without actually aborting their execution. We conclude that the DMFC contains a map representing eye position in craniotopic coordinates, and we argue that this map is utilized to maintain eye position.
猴子的背内侧额叶皮层（DMFC）被认为参与介导视觉引导的眼球跳动。本研究的目的是确定DMFC是否具有编码最终眼球位置的地形图，并确定该区域是否有助于维持眼球位置。当猴子注视视觉空间中某个位置呈现的目标时，对DMFC进行电刺激。进行了一系列参数测试以确定诱发眼球跳动的最佳刺激参数。电刺激通常会产生向对侧的眼球跳动，这些跳动汇聚到一个空间区域，即终止区。然而，对于某些刺激部位，刺激会产生同侧眼球跳动。当终止区位于猴子正前方时就会出现这种情况。在刺激额叶眼区（FEF）时，从未观察到向眼眶位置的汇聚，刺激FEF会诱发固定矢量的眼球跳动。从DMFC诱发眼球跳动的潜伏期随注视位置而变化，即注视点越靠近终止区，潜伏期单调增加。此外，注视点越靠近终止区，从DMFC诱发眼球跳动的概率越低。从FEF诱发眼球跳动的潜伏期和概率不随注视位置而变化。当猴子注视视觉空间不同位置呈现的目标时，未从DMFC诱发水平头部运动。此外，改变头部相对于身体的位置不会改变终止区相对于头部的位置。发现DMFC包含终止区的地形图编码，前部区域代表对侧极端视觉空间中的区域，后部区域代表正前方或同侧的区域。此外，外侧区域代表上视觉空间中的区域，而内侧区域代表下视觉空间中的区域。一旦眼睛定位在终止区内，进一步刺激会固定注视并抑制视觉诱发的眼球跳动。在刺激结束后不久解除抑制后，眼球跳动准确地到达视觉目标。这表明刺激延迟了眼球跳动的执行，但实际上并未中止其执行。我们得出结论，DMFC包含一个以颅骨坐标表示眼球位置的图谱，并且我们认为该图谱用于维持眼球位置。
