The Biological Signal as recorded was 70μV. Neither this native amplitude nor slight amplification to 100μV induced detectable network activity. However, amplification to 1mV induced reproducible activity. We did not attempt to determine the absolute minimum amplitude necessary for the Biological Signal to evoke network activity. We also did not attempt to alter the duration of the Biological Signal, which would likely impact its efficacy. The absolute value of these parameters would likely depend upon the particular Biological Signal utilized for stimulation. Notably, the 1mV Biological Signal as utilized herein was at least as effective as a substantially larger (700mV) square pulse. We have not determined whether or not the complexity of the Biological Signal is essential to evoke responses, or whether a biphasic square signal of similar low amplitue, or tetanic stimulation with such a signal (1), would achieve similar results.
The asymmetric profile of the Biological Signal allowed application in native or inverted orientation. We observed stimulation or suppression of spontaneous signaling using native or inverted Biological Signal, indicating that neuronal subpopulations were opposingly affected depending upon signal orientation. Inhibition of spontaneous signaling suggests a predominant impact on inhibitory neurons, consistent with the prior demonstration that GABAergic neurons are responsible for synchronized bursts in mature signal patterns (19, 22).
These results elucidate inherent limitations of current MEA technology for use as extracellular neuronal stimulation platforms. At present, there is no ability to switch an electrode directly to ground (without resistance). This could be overcome by incorporating digitally controlled grounding switches into the stimulus equipment. In addition, the bath, reference and stimulus electrodes must all fall in a straight line in order to minimize current sinking at the bath ground electrode and generation of two electrical fields, which would preclude establishment of a local circuit. This inherent limitation cannot be overcome at present, since the bath must be grounded to achieve an acceptable signal-to-noise ratio. However, the use of a narrower bath electrode, possibly with a pointed end, would reduce field fringing around the signal reference electrode. Despite these limitations, the temporal accuracy of the responses to the Biological Signal and its ability to provoke or inhibit spontaneous signaling suggests that our localized electrode configuration provides the ability to monitor the activity of localized populations within networks.
Acknowledgments
This work was supported by the U.S. Army Research Office, contract number W911NF-06-C-0094 and W911NF-08-1-0222. We are grateful for the continued advice of Dr. Elmar Schmeisser.
Competing interests
The authors declare no competing interests.
Correspondence Address correspondence to Thomas B. Shea, Center for Cellular Neurobiology and Neurodegeneration Research, University of Massachusetts⋅Lowell, Lowell, MA, USA. Email: [email protected]">[email protected]References
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