The single current-pulse stimulation of the dorsal root elicits the following optical responses in the dorsal horn [14]: (1) a brief (< 3 ms) and small, almost undetectable, response is evoked at the entry zone of the dorsal root and, occasionally, in the deep dorsal horn by a 0.05-mA current pulse of 0.05 ms duration; (2) by increasing the stimulus intensity to 0.1 mA, an optical response of longer duration (< 100 ms) appears in lamina I extending to the outer part of lamina II, and in lamina III and deeper laminae; (3) additional increases in intensity (> 0.3 mA) and/or duration (> 0.5 ms) lead to the generation of an intense, prolonged (> 200 ms) response in the superficial laminae I-III, most prominently in lamina II.
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