Depth-dependent detection of microampere currents delivered to monkey V1.

E.J. Tehovnik, W.M. Slocum and P.H. Schiller, 2007
Soc Neurosci. Abstr., 279.14.

Abstract
Electrical stimulation of area V1 can be detected by monkeys (Doty 1965; Murphey and Maunsell 2007). Our study examined to what extent such stimulation is layer dependent. The following parameters of electrical microstimulation were studied as a function of cortical depth: (1) the current threshold for detection; (2) the chronaxies for detection; (3) the anode-cathode ratios for detection; and finally (4) the latency for detection. While remaining fixated on a spot of light, a rhesus monkey was required to detect a 100 ms train of electrical stimulation delivered to sites within area V1. The monkey signaled the delivery of stimulation by depressing a lever after which he was rewarded with a drop of apple juice. Control trials were interleaved during which time no stimulation was delivered and the monkey was rewarded for not depressing the lever. The frequency of stimulation was maintained at 200 Hz and the current ranged from 1 to 30 uA. Typically, 0.2 ms cathode-first biphasic pulses were used, but for some experiments the pulse duration ranged from 0.05 to 0.7 ms and anode-first biphasic pulses were used. We found that the current threshold for detecting cathode-first pulses 50% of the time was the lowest (< 10 uA) when stimulation was delivered to the deepest layers of V1 (between 1.5 and 2.5 mm below the cortical surface). Also the shortest chronaxies (less than 0.2 ms) and the shortest latencies for detecting the stimulation (less than 325 ms) were observed at these depths. We also found that anode-first pulses were most effective at evoking a detection response in superficial V1 and cathode-first pulses were most effective at evoking a detection response in deep V1 (> 1.75 mm below the cortical surface). This suggests that stimulation of superficial V1 activates mainly cell bodies and terminals, whereas stimulation of deep V1 activates mainly axons. We conclude that the deepest layers of V1 are the most sensitive for the induction of a detection response in monkeys. Our results are consistent with those observed for the electrical evocation and interference of saccadic eye movements elicited from monkey V1 (Tehovnik et al. 2005) and with those reported for the electrical elicitation of phosphenes from human V1 (Bak et al. 1990; Rushton and Brindley 1977). Our work has relevance to investigators interested in targeting the most excitable layers of primate V1 for the development of a visual prosthesis for the blind.

Support Contributed by: NIH Grant EY014884

Citation: E. J. Tehovnik, W. M. Slocum and P. H. Schiller. Depth-dependent detection of microampere currents delivered to monkey V1. Program No. 279.14. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2007. Online.



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