DeYoe Edgar A, Lewine Jeffrey D, Doty Robert W
Department of Neurobiology and Anatomy, Box 603, University of Rochester Medical Center, Rochester, NY 14642, USA.
J Neurophysiol. 2005 Nov;94(5):3443-50. doi: 10.1152/jn.00407.2005. Epub 2005 Aug 3.
Macaques were trained to signal their detection of electrical stimulation applied by a movable microelectrode to perifoveal striate cortex. Trains of < or =100 cathodal, 0.2-ms, constant current pulses were delivered at 50 or 100 Hz. The minimum current that could be reliably detected was measured at successive depths along radial electrode penetrations through the cortex. The lowest detection thresholds were routinely encountered when the stimulation was applied to layer 3, particularly just at the juncture between layers 3 and 4A. On the average, there was a twofold variation in threshold along the penetrations, with the highest intracortical thresholds being in layers 4C and 6. Variations as high as 20-fold were obtained in some individual penetrations, whereas relatively little change was observed in others. The minimum detectable current was 1 muA at a site in layer 3, i.e., 10-100 times lower than that for surface stimulation. Because macaques, as do human subjects, find electrical stimulation of striate cortex to be highly similar at all loci (a phosphene in the human case), it is puzzling as to how such uniformity of effect evolves from the exceedingly intricate circuitry available to the effective stimuli. It is hypothesized that the stimulus captures the most excitable elements, which then suppress other functional moieties, producing only the luminance of the phosphene. Lowest thresholds presumably are encountered when the electrode lies among these excitable elements that can, with higher currents, be stimulated directly from some distance or indirectly by the horizontal bands of myelinated axons, the stria of Baillarger.
猕猴经过训练，以发出信号表明它们检测到可移动微电极对中央凹周围纹状皮层施加的电刺激。以50或100赫兹的频率施加一系列≤100个阴极、0.2毫秒的恒流脉冲。沿着径向电极穿透皮层的连续深度测量能够可靠检测到的最小电流。当刺激施加到第3层时，通常会遇到最低的检测阈值，特别是在第3层和4A层之间的交界处。平均而言，沿穿透方向阈值有两倍的变化，皮层内最高阈值出现在第4C层和第6层。在一些个体穿透中获得了高达20倍的变化，而在其他穿透中观察到的变化相对较小。在第3层的一个部位，最小可检测电流为1微安，即比表面刺激低10 - 100倍。因为猕猴和人类受试者一样，发现在所有部位对纹状皮层进行电刺激非常相似（在人类中会产生光幻视），所以令人困惑的是，如此一致的效果是如何从有效刺激所可用的极其复杂的神经回路中产生的。据推测，当电极位于这些最易兴奋的元素之间时会遇到最低阈值，在较高电流下，这些元素可以从一定距离直接受到刺激，或者通过有髓轴突的水平带（拜尔林格纹）间接受到刺激。
