Neurotrophic electrode: method of assembly and implantation into human motor speech cortex

Abstract

The neurotrophic electrode (NE) is designed for longevity and stability of recorded signals. To achieve this aim it induces neurites to grow through its glass tip, thus anchoring it in neuropil. The glass tip contains insulated gold wires for recording the activity of the myelinated neurites that grow into the tip. Neural signals inside the tip are electrically insulated from surrounding neural activity by the glass. The most recent version of the electrode has four wires inside its tip to maximize the number of discriminable signals recorded from ingrown neurites, and has a miniature connector. Flexible coiled, insulated gold wires connect to electronics on the skull that remain subcutaneous. The implanted electronics consist of differential amplifiers, FM transmitters, and a sine wave at power up for tuning and calibration. Inclusion criteria for selecting locked-in subjects include medical stability, normal cognition, and strong caregiver support. The implant target is localized via an fMRI-naming task. Final localization at surgery is achieved by 3D stereotaxic localization. During recording, implanted electronics are powered by magnetic induction across an air gap. Coiled antennas placed on the scalp over the implanted transmitters receive the amplified FM transmitter outputs. Data is processed as described elsewhere where stability and longevity issues are addressed. Five subjects have been successfully implanted with the NE. Recorded signals persisted for over 4 years in two subjects who died from underlying illnesses, and continue for over 3 years in our present subject.

Figure 1

Schematic representation of the NE's electronic configuration. See text for description.

Figure 2

The NE in its intended placement in layer V of the neocortex. The inset photograph shows the NE tip with four wires inside. The scale bar is 500 microns.

Figure 3

Photograph of four gold wires hand coiled for strain relief.

Figure 4

Forth-five degree bends in coiled wires that limit depth of cone tip insertion.

Figure 5

Omnetics connector with eight wires covered with acrylic cement for insulation.

Figure 6

Omnetics connector with two sets of four coiled wires with connections covered in acrylic.

Figure 7

Glass cone tip is maneuvered onto three gold wires using a pience of hair as a “handle”.

Fignre 8

View of the glass cone tip containing four wires.

Figure 9

The cone tip has a shelf for gluing wires securely into position.

Figure 10

Image of old and new 8 pin connectors showing the dramatic reduction in size.

Figure 11

A handle is used to maneuver the finished electrode at surgery.

Figure 12

Example, of high resolution presurgical fMRI mapping of the motor area in a patient with brain tumor.

Figure 13

Implanted Neurotrophic Electrode and telemetry system.

Figure 14

Surgical implant site shows electronics (A), cement (B) and gelfoam (C). The nearly invisible electrode coils are dead center exiting from the cement and running upwards (D).

Figure 15

Evolution of completed electrodes from 1986 to 2007.

Figure 16

BOLD MRI signal time courses in pixels within a Region of Interest (yellow box) of left pre-motor area, depicted the activation signal pattern in activated pixels while non-activated pixels remained at the baseline. The fMRI exam task was to imagine right-handed finger movements in a block design with 3 ON and four OFF periods (30 seconds/block).

Figure 17

Area in the left superior frontal gyrus was activated in the confrontation naming task in our present subject ER.

Figure 18

X ray Image of Implanted Recording Electronics

Figure 19

3D CT scan of ER with electronics (on vertex) and electrode wire (center of craniotomy opening) two years post implantation.