Scanning electrochemical microscopy as a novel proximity sensor for atraumatic cochlear implant insertion

A growing number of minimally invasive surgical and diagnostic procedures require the insertion of an optical, mechanical, or electronic device in narrow spaces inside a human body. In such procedures, precise motion control is essential to avoid damage to the patient's tissues and/or the device itself. A typical example is the insertion of a cochlear implant which should ideally be done with minimum physical contact between the moving device and the cochlear canal walls or the basilar membrane. Because optical monitoring is not possible, alternative techniques for sub millimeter-scale distance control can be very useful for such procedures. The first requirement for distance control is distance sensing. We developed a novel approach to distance sensing based on the principles of scanning electrochemical microscopy (SECM). The SECM signal, i.e., the diffusion current to a microelectrode, is very sensitive to the distance between the probe surface and any electrically insulating object present in its proximity. With several amperometric microprobes fabricated on the surface of an insertable device, one can monitor the distances between different parts of the moving implant and the surrounding tissues. Unlike typical SECM experiments, in which a disk-shaped tip approaches a relatively smooth sample, complex geometries of the mobile device and its surroundings make distance sensing challenging. Additional issues include the possibility of electrode surface contamination in biological fluids and the requirement for a biologically compatible redox mediator.

Magnetic sources of the M50 response are localized to frontal cortex

OBJECTIVE

To determine the source localization(s) of the midlatency auditory magnetic response M50, the equivalent of the P50 potential, a sleep state-dependent waveform known to habituate to repetitive stimulation.

METHODS

We used a paired stimulus paradigm at interstimulus intervals of 250, 500 and 1000 ms, and magnetoencephalographic (MEG) recordings were subjected to computational methods for current density reconstruction, blind source separation, time-frequency analysis, and data visualization to characterize evoked dynamics.

RESULTS

Each subject showed localization of a source for primary auditory evoked responses in the region of the auditory cortex, usually at a 20-30 ms latency. However, responses at 40-70 ms latency that also decreased following the second stimulus of a pair were not localizable to the auditory cortex, rather showing multiple sources usually including the frontal lobes.

CONCLUSIONS

The M50 response, which shows habituation to repetitive stimulation, was not localized to the auditory cortex, but showed multiple sources including frontal lobes.

SIGNIFICANCE

These MEG results suggest that sources for the M50 response may represent non-auditory, perhaps arousal-related, diffuse projections to the cortex.

Entorhinal cortex long-term potentiation evoked by theta-patterned stimulation of associative fibers in the isolated in vitro guinea pig brain

Long-term potentiation (LTP) induced in the lateral entorhinal cortex by theta-patterned tetanic stimulation of the piriform cortex was analyzed in the isolated guinea pig brain maintained in vitro. Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by stimulation of the piriform cortex are composed of an early and late component selectively blocked by non-N-methyl-D-aspartate (non-NMDA) and NMDA receptor antagonists, respectively. LTP induction was dependent on NMDA receptor activation, being blocked by perfusing the preparation with 2-amino-5-phosphonovalerate (AP-5). LTP was expressed through synaptic enhancement of both early non-NMDA and late, possibly NMDA receptor-mediated responses.