The overall goal of this procedure is to identify regions for stimulation of the cochlear nucleus and patterns of neural stimulation, which produce frequency sensations distinguishable by the animal. First, a heart rate monitoring devices implanted to allow for measurement of the animal's physiological responses. Next, a neural electrode through which stimulation can be delivered to the auditory system is implanted into the cochlear nucleus.
Then the animal is conditioned to detect frequency differences in tones presented and produce a physiological response. Finally, the animal is tested for the detection of different locations and patterns of neurostimulation of the auditory system. Ultimately, results can be obtained that show which areas of the cochlear nucleus are best suited to stimulation for auditory prosthesis and stimulation patterns.
Best suited for use in such devices through identifying locations and patterns of stimulation which consistently produce distinct frequency sensations. The main advantage of this technique over existing methods, such as neural stimulation and recording in an acute preparation, is that it's possible to identify stimulation targets and techniques which are actually distinguished and detected by the animal. The implications of this technique extend towards therapy for the treatment of deafness in which cochlear implants cannot be used either as a result of a compromised cochlear or damage to auditory nerve.
This technique will help determine stimulation strategies that best mimic normal acoustic processing. Three to five days after electrocardiogram telemetry device implantation, begin brain electrode implantation by first administering analgesia and anesthetic. Once anesthesia has been confirmed, shave the animal's head and wipe with Betadine scrub alcohol, and then Betadine.
Place the animal on a homeo themic plate. Lift and position the animal between the ear bars and slide the first ear bar into the external acoustic atu. Lock the first ear bar in place.
Then slide the second ear bar into position using rat tooth forceps. Open the animal's jaw and hook the upper incisors over the tooth holder. Once in position, slide the nose cone over the nose to start the delivery of isof fluorine, which will continue for the duration of the surgery.
After exposing the parietal bone, scrub the surface using 20%peroxide solution. On a gauze pad, drill a hole approximately two millimeters square into the lateral most extent of the inter parietal bone. Flush the hole using sterile saline to remove any bone, dust, or bone fragments that may damage the electrode working under a microscope, drill a small hole in the left parietal bone and right inter parietal bone.
Next, screw a surgical steel screw into each hole, leaving about 0.5 millimeters between the head of each screw and the skull. Attach the coupling speaker to the left hollow ear bar. Bring the electrode manipulator into place above the opening with a Cato rostral angle of 10 degrees.
Then using the tip of a needle, make an incision in the dura on the sagittal plane. Insert the electrode manually approximately two millimeters into the surface of the brain. Connect the screws to the ground and reference electrode points of the high impedance head stage.
Ensure that the amplifier is turned on. Then seal the recording chamber, commence cyclical delivery of low mid-range and high frequency band pass filtered noise with the maximum delivery rate of one burst every 200 milliseconds. Monitor neural activity at each channel to detect responses to noise.Presentation.
Continue insertion of the electrode until either total inserted distance is approaching eight millimeters or until responses are seen to noise presentations. If the cochlear nucleus has been reached, sites at the tip of the electrode should be showing responses primarily to high frequency stimuli. Continue inserting the electrode until the tip detects.
Responses to low frequency stimuli or auditory driven activity ceases to occur. If activity ceases, the electrode may have passed entirely through the cochlear nucleus and it may be necessary to revise the electrode placement. Construct a frequency amplitude response map of the neurons at electrode sites by presenting pure tones in frequency steps across the desired frequency range at amplitudes of one to 70 decibels with 10 repetitions of each stimulus.
To protect the brain tissue and electrode, apply a thin layer of silicon elastomer slightly above the exposed electrode shanks, such that the elastomer flows down the shanks and coats both the shanks and the exposed surface of the brain. To fix the electrode in place, apply dental cement, connect the electrode ground wire to the screws and apply further dental cement for greater strength. Once the cement has hardened, suture the incision around the electrode, then place the home cage on a heating pad for 24 hours to allow the animal to recover.
After activating the telemetry device, place the animal in the testing chamber and allow it to acclimatize for five minutes prior to commencing conditioning. To perform the conditioning procedure, deliver one randomly selected member of the acoustic stimulus pair repeatedly in 250 millisecond bursts separated by 250 milliseconds of silence for 80 to 170 seconds. Each stimulation presentation must have a rise and fall time of 10 milliseconds.
To avoid a click being perceived, begin alternating. The second member of the acoustic stimulus pair with the first presenting each tone for 250 milliseconds, followed by a 250 millisecond silence. After 9.5 seconds of the ten second period of alternating tone presentation, administer a 0.5 millisecond foot shock, then cease tone presentations for 30 seconds to allow the heart rate to stabilize.
Continue this process until 48 cycles of the procedure have been completed. To ensure sufficient repetitions of each tone pair for analysis, ensure each tone pair is presented at least four times. After attaching the neural stimulation cable and activating the telemetry device under isoflurane anesthesia, allow the animal to recover and acclimatize to the test chamber for 10 minutes prior to testing.
In order to maintain the previous day's conditioning, deliver an alternating acoustic stimulus pair as previously described, followed by a 0.5 millisecond foot shock. After allowing the heart rate to stabilize deliver one randomly selected member of an electrical stimulus pair repeatedly in 250 millisecond bursts, separated by 250 milliseconds of silence for 80 to 170 seconds. Then begin alternating the second member of the electrical stimulus pair with the first presenting each tone for 250 milliseconds, followed by a 250 millisecond silence for 10 seconds total.
Repeat the cycle with the next randomly selected electrical stimulation pair until at least 20 trials of each pair has been delivered at random intervals. Insert an acoustic stimulus pair to maintain conditioning. When testing is complete, deactivate the telemetry device, detach the stimulation cable, and return the animal to its home.
Shown here is the electrophysiological response of a correctly placed electrode. Each histogram presents data for one frequency at one electrode site, and each column represents a 25 millisecond time period on both shanks of the electrode array. Responses are detected at each individual electrode site only in response to a quite narrow band of frequencies, but these narrow bands are distributed across a broad range of frequencies.
Such a distribution is optimal as many distinct frequency associated regions of the brain can be stimulated independently. Ideally, electrode placements should lead to neural responses to acoustic stimulus being detected on several channels with sound amplitude as low as 10 decibels as appears in this electrophysiological response at Channel 10. Shown here is the electrophysiological response across a poorly placed electrode activity occurs at the tips of each shank of the electrode array, but there is little variation in the acoustic stimulus frequency that elicits activity at each electrode site.
Such an implantation does not enable stimulation of distinct frequency layer for discrimination testing. Here, individual traces and mean heart rate proportional data are shown from eight seconds prior to eight seconds after the commencement of alternating electrical stimulus presentation. These data were collected during the seventh testing session and included 12 electrical stimulus presentations.
A significant decrease in heart rate occurs rapidly after introduction of the second neural stimulus, followed by a significant rise in heart rate, suggesting that the animal has detected the difference between the first and second stimulus. The degree of error and variance of the response is shown in the proportional mean plus standard error plot, and the significance of the dip and rise following second stimulus presentation can be verified using 95%confidence intervals While attempting this procedure, it's important to remember that any outside stimuli the animal can detect may lead to changes in heart rate increasing the variability in the results testing and conditioning should be carried out in a consistent and well isolated environment to minimize these effects After its development. This technique paves the way for researchers in the field of bionics to explore better ways to electrically stimulate the brain in a variety of sensory systems to provide behaviorally relevant responses.
