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Chapter 5 Future work

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As discussed in Section 3.5.4, some fraction of the measured thresholds may represent stimulation of dendrites rather than axons. Since both axons and dendrites share the same basic cylindrical geometry, the identity of the target little effects the primary result of this thesis. Still, a more definitive statement about the stimulation target could be made if the stimulating and recording clusters were placed further apart than the largest dendritic spreads, perhaps 1mm or more. In the present experiments the cluster separation was kept small to reduce the sensitivity of the measurement to imperfect vertical alignment of axons. At larger separations, the chances of recording from a cell whose axon ran through the stimulating cluster would be reduced. The experimental setup will soon be modified, however, to permit single needle recording (to better isolate cells) while stimulating through an electrode array. The modified setup should facilitate threshold measurements with large stimulating-recording electrode separations, so that the measurements described in Chapter 3 can be repeated under conditions in which dendrite stimulation would be highly unlikely. 5.2 Field direction This thesis suggests that axon thresholds can be raised through a strategic choice of electrode geometry. Thresholds for the transverse bipolar geometry, however, were very sensitive to fiber position, only becoming substantially elevated relative to thresholds for monopolar or longitudinal bipolar stimulation when the fiber under study was well-centered between the electrode poles. Longitudinal fringing fields near the electrode poles provide the most plausible explanation for this threshold profile. Hence future in vitro experiments might endeavor to design novel electrode geometries with better field directionality and minimal fringing. One candidate design inspired by previous work (Grumet, 1994) is shown in Fig

Year: 2008
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