Bartlett John R, DeYoe Edgar A, Doty Robert W, Lee Barry B, Lewine Jeffrey D, Negrão Nubio, Overman William H
Department of Neurobiology and Anatomy, Box 603, University of Rochester Medical Center, Rochester, NY 14642, USA.
J Neurophysiol. 2005 Nov;94(5):3430-42. doi: 10.1152/jn.00406.2005. Epub 2005 Aug 3.
Macaques indicated their detection of onset or alteration of 0.2-ms pulses applied in various configurations through electrodes implanted in striate cortex. When microelectrodes were introduced and left in place, the threshold for detection of 100-Hz pulses nearly doubled within 24 h. However, for chronically implanted platinum-alloy macroelectrodes detection thresholds usually remained stable for many months, independently of location within striate cortex or its immediately subjacent white matter. Thresholds were unaffected by the visual conditions, such as light versus darkness, or movement of the eyes; but in one animal blind after acute glaucoma thresholds for loci in striate cortex were permanently decreased by about 50%. Learning to respond to electrical stimulation of the optic tract produced no tendency to respond to such stimulation of striate cortex. Onset of stimulation at a given locus could be detected even in the face of continuous supraliminal stimulation at four surrounding loci on a 3-mm radius. The surround stimulation did alter the threshold of the central locus, but such stimuli could not summate if they were subliminal by some 10%. Cessation of stimulation that had been continuing for 1 min to 1 h could be detected if it were being applied at a level 20-75% above that needed for detection of stimulus onset. Continuous stimulation had a pronounced "priming" effect, in that modulation of frequency or intensity of such stimulation by as little as 5% could be detected (e.g., 20 microA in a background of 500 microA, or <2-ms interpulse interval with pulses at 50 Hz). Using pulses inserted in various phase relations to ongoing pulses at 2-5 Hz, it could be determined that stimulus pulses were surrounded by a strong facilitatory period for about 30 ms, which was then replaced by refractoriness. Given the congruence of macaque and human visual anatomy and psychophysics, these results further encourage efforts to develop a cortical prosthesis for the blind.
猕猴通过植入纹状皮层的电极，表明它们能够检测到以各种配置施加的0.2毫秒脉冲的开始或变化。当微电极插入并留在原位时，100赫兹脉冲的检测阈值在24小时内几乎翻倍。然而，对于长期植入的铂合金宏观电极，检测阈值通常在许多个月内保持稳定，与纹状皮层内或其紧邻的白质中的位置无关。阈值不受视觉条件的影响，如明与暗或眼睛的运动；但在一只因急性青光眼而失明的动物中，纹状皮层中各部位的阈值永久性降低了约50%。学会对视神经束的电刺激做出反应并没有产生对纹状皮层这种刺激做出反应的倾向。即使在半径为3毫米的四个周围部位持续进行阈上刺激的情况下，给定部位的刺激开始仍能被检测到。周围刺激确实改变了中央部位的阈值，但如果这些刺激低于阈值约10%，它们就不会叠加。如果以比检测刺激开始所需水平高20% - 75%的水平施加持续1分钟至1小时的刺激停止，就可以被检测到。持续刺激有明显的“启动”效应，即这种刺激的频率或强度只要有低至5%的调制就能被检测到（例如，在500微安的背景下为20微安，或在50赫兹时脉冲的脉冲间隔小于2毫秒）。使用与2 - 5赫兹的持续脉冲具有各种相位关系的脉冲，可以确定刺激脉冲被大约30毫秒的强易化期所包围，然后被不应期所取代。鉴于猕猴和人类视觉解剖学与心理物理学的一致性，这些结果进一步鼓励了为盲人开发皮层假体的努力。
