A Lightweight, Headphones-based System for Manipulating Auditory Feedback in Songbirds

Implementing the headphones system consists of four major steps. Section 1 below details the assembly of the headphones frame, which houses the electronics (speakers and a miniaturized microphone). Section 2 describes how the frame is attached to the bird. Section 3 describes the assembly of the electronics. Section 4 explains how the electronics are connected to sound-processing and data-collection hardware and details a procedure for testing that the system is functioning correctly.

1. Fabricate Headphone Frame

1. Cut carbon fiber components for crossbar and earbuds. Use the cutting wheel on a Dremel tool to prepare the following pieces:
   1 crossbar (3 cm of 1 x 3 mm carbon fiber strip)
   2 vertical struts (1.5 cm of 1 x 3 mm carbon fiber strip)
   2 earbuds (4 mm of 4 mm ID/ 6 mm OD carbon fiber cylinder)
2. Assemble crossbar. Drill two small holes 4.5 mm apart in the center of the crossbar (Figure 1a, top) using a 1.4 mm diameter bit. Holes should be just large enough to accommodate the 0-80 screw. Insert two 0-80 screws though the holes and attach hex nuts. Cut two 6 mm strips of syringe needle (any small-diameter needle will work) and flatten with pliers (needle strips shown in red in Figure 1a). 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 (shown in green in Figure 1a) on the tip of each screw. Place the two needle segments on top of the hex nuts. Epoxy hex nuts to each other and the needle segments, covering both (Figure 1a, bottom). After epoxy has fully cured (24 hr), remove both screws. The crossbar unit should separate into four components: two screws, the 3 cm length of carbon fiber, and a single piece consisting of 2 hex nuts and 2 needle segments glued together with epoxy. Reassemble crossbar unit.
3. Fabricate foam rings. Use a 1/4 in (6.35 mm) hole punch to cut a foam cylinder from a foam earplug. Use scissors to cut perpendicular to the long axis of the foam cylinder yielding a 2 mm (length) by 6.35 mm (diameter) cylinder. Then use a 1/8 in (3.175 mm) hole punch to cut out the center of the 2 mm foam cylinder, forming a foam ring (shown in orange in Figure 1b).
4. Assemble earbuds. Use epoxy to glue vertical struts (1.5 cm length of carbon fiber strip) to the 4mm length of carbon fiber cylinder as shown in Figure 1b, bottom. When epoxy has hardened, use a small amount of epoxy to glue the foam ring to the end of the cylinder (Figure 1b, bottom). The foam ring will rest against the bird’s head.

2. Attach Headphones Frame to Bird

1. Prepare stereotax. Prior to surgery, insert stereotax earbars through earbuds with the foam pads facing inwards (Figure 1c, top).
2. Attach crossbar. Anesthetize the bird using isoflurane and/or ketamine plus midazolam. The anesthetized bird should have closed eyes and be unresponsive to toe pinch. Place bird in stereotax and make a 5-10 mm incision in scalp. The incision should be along the midline and extend anterior from a point 2 mm anterior of the attachment of the neck muscles at the back of the skull. Rub the exposed skull with 70% ethanol to dry the bone surface. Use an alligator clip holder to place crossbar such that the epoxy-covered hex nuts rest on the top of the skull (Figure 1c, d). Lift the crossbar slightly and glue the hex nuts (with epoxy or dental acrylic) to the skull, being careful that the adhesive touches ONLY the hex nuts and not the crossbar itself. This will allow you to remove the headphones (by removing the screws), leaving only the hex nuts permanently attached to the head (Figure 1d).
3. Attach earbuds. Slide earbuds along ear bars until foam pads rest against the bird’s head (Figure 1c, top). Use alligator clips on the ear bars to press foam pads firmly against the bird’s head (Figure 1c, bottom), this is important to getting a good acoustic seal. Rotate earbuds so that the vertical posts touch the back of the crossbar, leaving space for the connector strip socket (Figure 2) in front of the post. Glue posts to crossbar using epoxy or acrylic.
4. Recovery. Allow 24 hr for epoxy/acrylic to fully cure and bird to recover. After recovery, examine frames to make sure foam pads form a tight seal around ear canals. If seal is loose, foam pads can be removed and replaced with larger ones to improve acoustic seal. Gently remove screws using a 0.05 in (1.27 mm) hexagonal wrench and remove assembled headphones frame from the bird.

3. Assemble Electronics

1. Fabricate speaker adapters. Make speaker adapters by inserting a pipet tip (a 5.2 cm long tip with a diameter that tapers from 5 mm to 1 mm) into a scrap piece of carbon fiber cylinder (use the same 4 mm ID/ 6 mm OD cyinder stock as for the earbuds; length of the cylinder scrap is not important), then cut pipet tip 1 mm past the end of the cylinder (Figure 2a). Remove from cylinder and cut so that the total length of the adapter is 3 mm.
2. Connect speakers. Solder 5 cm lengths of insulated 36 ga. wire to the positive and negative terminals of both speakers (green and black wires, respectively, in Figure 2d, e). Use an ohmmeter to verify that neither terminal is in electrical contact with the speaker’s metal casing.
3. Assemble speaker/microphone component. Epoxy a strip of tape (any electrically insulating tape will work) to one side of one speaker and epoxy the headphones mic on top of the tape (Figure 2b). Solder the ground wire from the microphone to the negative terminal on the speaker (connection shown in black in Figure 2e).
4. Epoxy both the speaker and the speaker/microphone component into the wider end of each adapter (Figure 2c, right). Both the speaker and microphone port will (just) fit inside the pipet. Be sure that epoxy does not cover either port.
5. Gently insert adapters into the earbuds (Figure 2d, right). Glue each adapter to its earbud with a small drop of epoxy.
6. Fabricate connector strip. Cut a segment of connector strip socket consisting of four terminals and place a reference mark (white dot in Figure 2d) 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 in the configuration shown in Figure 2e. Use an ohmmeter to check for short circuits on connector strip and repair if necessary. Epoxy the connector strip to the headphones frame. Wrap strips of electrical or scotch tape around exposed wires to protect wires and add thin layer of epoxy over tape ends to prevent unraveling. The finished weight of the headphones (including frame and electronics) should be 1.3-1.5 g.
7. Build a flexible lead by soldering four 15 cm lengths of insulated 36 ga. wire to a segment of connector strip header consisting of four terminals (Figure 3a). Place a reference mark (white dot in Figure 3) on one corner of the header to ensure that the lead is plugged into the socket in the correct orientation. Braid wires and finish lead by soldering wires to an adapter that plugs into commutator (Figure 3b).

4. Connect Headphones Electronics to Power and Signal Processing Equipment

1. Collect, filter, pitch shift, and amplify acoustic input. Hang an omnidirectional condenser microphone (“Cage mic,” Figure 4) directly above the bird’s cage. Preamplify and low-pass filter this signal (cutoff at 10 kHz for finch song) and input it into the Harmonizer (or other realtime sound-processing hardware). Use the pitch 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 (green in Figure 2e).
2. Provide power to phones mic. Attach a 9V battery to the wall of the bird’s soundbox. Connect the positive terminal of the battery to the V+ channel (red in Figure 2e) 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 phones mic ground (black in Figure 2e).
3. Calibrate feedback gain. Attach finished headphones to the hex nuts on bird’s head using 0-80 screws. When bird begins singing, record three channels of data (cage mic/unshifted signal, shifted signal, and phones mic) as shown in Figure 4. Examine the sound recorded from the phones mic. This channel will record the superposition of the virtual feedback (played through the headphones speaker) and the bird’s actual song “leaking” directly into the headphones frame. Adjust amplitude setting of speaker amplifier (Figure 4) so that the virtual auditory feedback is 15-20 dB louder than the direct/leaking signal. Note that increasing the amplitude beyond this threshold can result in the virtual signal bleeding through the headphones and being picked up by the cage mic, resulting in feedback distortion. An example of a well-calibrated system (in which the signal recorded by the headphones mic is dominated by the pitch-shifted signal rather than the direct/leaking signal) is shown in Figure 5. Here, the signal recorded through the headphones microphone (Figure 5a, right) is dominated by the pitch-shifted signal (rather than the bird’s natural vocalization), as shown in the example power spectrum in Figure 5b.
4. Monitor system performance. Check headphones twice daily to insure that both speakers and the microphone are functioning properly. Note that the headphones mic only monitors one of the speakers. To test the other speaker, tap the cage mic while holding the non-monitored speaker next to your ear – you will hear a clicking sound if the non-monitored speaker is working. Another recommended check is to disconnect the power source and signal output from the phone mic and use an ohmmeter to measure the resistance across the speakers (black and green in Figure 2e). When both speakers are connected, this will be half of the resistance across a single speaker since speakers are connected in parallel.

5. A Note on the Cost of Materials

With two notable exceptions, all items listed in the Table of Materials are relatively inexpensive (less than a few hundred US dollars). The most costly components are the commutator and the Harmonizer listed in the Table, which each cost 2,000 US dollars or more. We note that less expensive versions of both items may be available from different manufacturers than the ones listed (although we have not tested them) and might allow researchers to implement this protocol at a lower cost.