The overall goal of this procedure is to build a set of lightweight headphones for manipulating auditory feedback in a vocalizing songbird. This is accomplished by first fabricating the components that make up the frame for the headphones. The second step is to assemble the headphones frame and attach it to the bird's head.
Next, the frame is removed and fitted with miniature speakers and a miniature microphone used to monitor system performance. The final step is to reattach the headphones and use online sound processing hardware to perturb auditory feedback during singing. Ultimately, this virtual auditory feedback system facilitates investigation on how the brain uses sensory feedback to optimize vocal behavior.
The main advantage of this technique over other methods for driving vocal plasticity like deafening or reinforcement based techniques, is that this paradigm lists error correction by mimicking a naturalistic vocal error and allowing the somber to generate a corrective response. Although this method was developed for use in the Bengalese Finch, it may be applied to any animal which uses auditory feedback to regulate vocal behavior. To begin fabrication of the headphone frame, cut carbon fiber components for the crossbar and earbuds.
First, use the cutting wheel on a Dremel tool to prepare one crossbar from three centimeters of a one by three millimeter carbon fiber strip. Then prepare vertical struts from 1.5 centimeters of a one by three millimeter carbon fiber strip. Finally, prepare two earbuds from four millimeters of a four millimeter inner diameter, six millimeter outer diameter carbon fiber cylinder.
Because the headphone components are very small, they're typically assembled and soldered under a microscope. Assemble the crossbar by drilling two small holes, 4.5 millimeters apart in the center of the crossbar using a 1.4 millimeter diameter bit. Holes should be just large enough to accommodate a zero 80 screw.
Insert 2 0 80 screws through the holes and attach hex nuts. Then cut two six millimeter strips of a small diameter syringe needle and score with pliers for better epoxy adhesion. Turn the crossbar over so the hex nuts lie on top to prevent epoxy from gluing the tip of the screws to the hex nuts, put a drop of mineral oil on the tip of each screw.
A critical aspect of this procedure is to minimize total weight of the headphones apparatus. This can be achieved by using as little epoxy and acrylic as possible. Place the two needle segments on top of the hex nuts, epoxy the hex nuts to each other and to the needle segments covering both.
After the epoxy is fully cured, remove both screws following fabrication of foam rings as described in the written protocol. Accompanying this video assemble earbuds by gluing the vertical struts to the four millimeter length of carbon fiber cylinder. When the epoxy has hardened, use a small amount of epoxy to glue the foam ring to the end of the cylinder.
The foam ring will rest against the bird's head. In this demonstration, attachment of the headphones is shown on a dummy without an actual bird undergoing surgery. When performing surgery, the first step is anesthetization of the bird as described in the written procedure, place the bird in the prepared stereo attacks and make a five to 10 millimeter incision in the scalp.
The incision should be along the midline and extend anterior from a 0.2 millimeters anterior of the attachment of the neck muscles at the back of the skull. After rubbing the exposed skull with 70%ethanol to dry the bone surface, use an alligator clip holder to place the crossbar such that the epoxy covered hex nuts. Rest on the top of the skull, lift the crossbar slightly and glue the hex nuts to the skull.
Being careful that the adhesive touches only the hex nuts and not the crossbar itself. This will allow removal of the headphones by removing the screws, leaving only the hex nuts permanently attached to the head as shown here to attach the earbuds, slide them along the ear bars until the foam pads rest against the bird's head. Use alligator clips on the ear bars to press the foam pads firmly against the bird's head.
This is important to getting a good acoustic seal. Rotate the earbuds so that the vertical posts touch the back of the crossbar, leaving space for the connector strip socket in front of the post glue posts to the crossbar using epoxy or acrylic, and then allow 24 hours for the epoxy or acrylic to fully cure, and for the bird to recover. After 24 hours, examine the frames to make sure the foam pads form a tight seal around the ear canals.
If the seal is loose, the foam pads can be removed and replaced with larger ones. To improve acoustic seal, gently remove the screws using a 0.05 inch hexagonal wrench and remove the assembled headphones. Frame from the bird to fabricate the speaker adapters.
Insert a pipette tip that is 5.2 centimeters long with a diameter that tapers from five millimeter to one millimeter into a scrap piece of carbon fiber cylinder. Then cut the pipette tip one millimeter past the end of the cylinder. Remove the tip from the cylinder and cut it so that the total length of the adapter is three millimeters.
Next, connect the speakers by soldering five centimeter lengths of insulated 36 gauge wire to the positive and negative terminals of both speakers. For assembly of the speaker microphone component ep a strip of electrically insulating tape to one side of one speaker and epoxy the headphones microphone on top of the tape solder the ground wire from the microphone to the negative terminal on the speaker. Then epoxy both the speaker and the speaker microphone component into the wider end of each adapter.
Both the speaker and microphone port will just fit inside the pipette. Be sure that epoxy does not cover either port. Gently insert adapters into the earbuds.
Glue each adapter to its earbud with a small drop of epoxy. To fabricate the connector strip, cut a segment of connector strip socket consisting of four terminals and place a reference mark on one corner of the socket. Position the connector strip near one of the vertical posts, solder the speaker and microphone wires to the pins.
Use an ome to check for short circuits on the connector, strip and repair if necessary, epoxy the connector. Strip to the headphones frame to collect, filter pit shift, and amplify acoustic input. First, hang an omnidirectional condenser microphone directly above the bird's cage.
Pre amplify and lowpass. Filter this signal and input it into the harmonizer. Use the pit shifter module on the harmonizer to generate a shifted acoustic signal.
Amplify this signal and route it via the commutator and flexible lead to the positive channel on the speaker headphones. Next, provide power to the phone's microphone by attaching a nine volt battery to the wall of the bird sound box. Connect the positive terminal of the battery to the V channel via the commutator and flexible lead, connecting the battery's negative terminal to a common ground on the signal processing equipment.
Connect this common ground to the speaker and phone's microphone ground. To calibrate feedback gain. Attach the finished headphones to the hex nuts on the bird's head using zero 80 screws.
When the bird begins singing, record three channels of data, including cage, microphone, unshifted signal shifted signal, and phone's microphone as shown in the schematic, which can be found in the written protocol. Accompanying this video, examine the sound recorded from the phone's microphone. This channel will record the superposition of the virtual feedback played through the headphone speaker and the bird's actual song leaking directly into the headphones frame.
Adjust the amplitude setting of the speaker amplifier so that the virtual auditory feedback is 15 to 20 decibels louder than the direct leaking signal. Monitor system performance by checking headphones twice daily to ensure that both speakers and the microphone are functioning properly.Shown. Here is an example of a well calibrated system in which the signal recorded by the headphones microphone is dominated by the pitch shifted signal rather than the direct leaking signal.
Here, the signal recorded through the headphones microphone at the time indicated by the vertical red line in the spectrogram is dominated by the pitch shifted signal, which is represented by the red plot in the power spectrum. This is in contrast to the bird's natural vocalization, which is represented by the black plot in the power spectrum. The results indicate that the sound reaching the bird's ear is dominated by the shifted feedback a representative experiment performed on an adult bengalese Finch is shown here.
The headphone system was used to increase the of auditory feedback by one semitone for 16 days. This manipulation resulted in a gradual reduction of the pitch of all seven song syllables. This change in the vocal motor program resulted in a reduction in the auditory error experienced by the bird, demonstrating the bird's reliance on auditory feedback to correct apparent vocal errors.
When the pitch shift was removed after day 16, the pitch of song eventually returned to baseline. These data are typical in that they reflect incomplete adaptation. Here, although the pitch of auditory feedback was shifted by 1.0 semitones, the bird changed the pitch of his song by only about 0.4 semitones across species and systems incomplete.
Adaptation is the norm when virtual feedback is used to perturb a single sensory modality and in the present paradigm, likely reflects a partial reliance on non-audit auditory signals. As songbirds evaluate their ongoing vocal performance. Once mastered, the headphones can be assembled and attached in two working days.
Once again, it's important to remember to use as little epoxy or acrylic as possible in order to minimize the total weight of the headphones.
