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B. A. Ekesten, P. Gouras; Fast, Positive Evoked Potentials From the Surface of Murine Striate Cortex. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3308.
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To demonstrate a rapid and relatively large positive evoked potential at the surface of murine striate cortex and compare it to a similar negative intracortical response.
Glass micropipettes (3-8 Mohms) were used to record from the surface and within striate cortex of mice anesthetized with sodium pentobarbithal. The stimuli were diffuse light pulses obtained from light emitting diodes (370 and 505 nm); in some cases a hand held light projector was used to produce moveable slits of white light on a tangent screen before the contralateral eye. A strong yellow adapting field suppressed rod activity. Electrocardiogram and respiratory rate were monitored and body temperature maintained at 37 C. The bone and dura over striate cortex was removed and the surface covered with healon.
We can detect a fast, positive potential, about 1-1.5 mV in amplitude and 5-7 ms in duration at both on and off phases of light stimuli. Its amplitude saturates over 2-3 log units, but decreases near threshold. Large, light-driven and spontaneous oscillations follow the fast response. Intra-cortically, a similar negative potential associated with light driven impulses of neurons, often of large amplitude, at its peak and rising phase is always detectable through all cortical layers. There are oscillations of the light driven impulses, which seem to correspond to the waves observed after the fast positive response at the surface. The initial latency of both the superficial positive and intracortical negative potential is 28-30 ms at maximum stimulation.
Micropipettes detect fast positive responses at the surface of murine striate cortex. Some investigators have reported surface positive evoked potentials in murine striate cortex but with a slower waveform, 40-50 msec in duration (Porciatti et al Doc Ophthal 104:69, 2002); others have only reported negative evoked potentials from the surface (Ridder & Nusinowitz Vis Res 46:902, 2006). We suspect that our fast potential is due to stray current from intracortical activity and its extreme rapidity due to the very synchronous discharges of the retinogeniculate input produced in the small eye of the light adapted mouse. Why it contains large oscillatory waves that correspond to oscillations of impulses intracortically is less easy to explain unless these oscillations are highly synchronous over large areas of striate cortex.
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