1Department of Biology, Emory University, 2Neuroscience Graduate Program, Emory University, 3Program in Neuroscience and Behavioral Biology, Emory University
Hoffmann, L. A., Kelly, C. W., Nicholson, D. A., Sober, S. J. A Lightweight, Headphones-based System for Manipulating Auditory Feedback in Songbirds. J. Vis. Exp. (69), e50027, doi:10.3791/50027 (2012).
Experimental manipulations of sensory feedback during complex behavior have provided valuable insights into the computations underlying motor control and sensorimotor plasticity1. Consistent sensory perturbations result in compensatory changes in motor output, reflecting changes in feedforward motor control that reduce the experienced feedback error. By quantifying how different sensory feedback errors affect human behavior, prior studies have explored how visual signals are used to recalibrate arm movements2,3 and auditory feedback is used to modify speech production4-7. The strength of this approach rests on the ability to mimic naturalistic errors in behavior, allowing the experimenter to observe how experienced errors in production are used to recalibrate motor output.
Songbirds provide an excellent animal model for investigating the neural basis of sensorimotor control and plasticity8,9. The songbird brain provides a well-defined circuit in which the areas necessary for song learning are spatially separated from those required for song production, and neural recording and lesion studies have made significant advances in understanding how different brain areas contribute to vocal behavior9-12. However, the lack of a naturalistic error-correction paradigm - in which a known acoustic parameter is perturbed by the experimenter and then corrected by the songbird - has made it difficult to understand the computations underlying vocal learning or how different elements of the neural circuit contribute to the correction of vocal errors13.
The technique described here gives the experimenter precise control over auditory feedback errors in singing birds, allowing the introduction of arbitrary sensory errors that can be used to drive vocal learning. Online sound-processing equipment is used to introduce a known perturbation to the acoustics of song, and a miniaturized headphones apparatus is used to replace a songbird's natural auditory feedback with the perturbed signal in real time. We have used this paradigm to perturb the fundamental frequency (pitch) of auditory feedback in adult songbirds, providing the first demonstration that adult birds maintain vocal performance using error correction14. The present protocol can be used to implement a wide range of sensory feedback perturbations (including but not limited to pitch shifts) to investigate the computational and neurophysiological basis of vocal learning.
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
2. Attach Headphones Frame to Bird
3. Assemble Electronics
4. Connect Headphones Electronics to Power and Signal Processing Equipment
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.
Figure 6 shows a representative experiment performed on an adult Bengalese finch. Here, the headphones system was used to increase the pitch of auditory feedback by one semitone (one twelfth of an octave, representing an approximately 6% change in absolute frequency) for 16 days. This manipulation resulted in a gradual reduction of the pitch of all song syllables (colored lines). This change in the vocal motor program resulted in a reduction in the auditory error experienced by the bird (dashed line), 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.
The data shown in Figure 6 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 5,15, and in the present paradigm likely reflects a partial reliance on nonauditory (e.g. proprioceptive) signals as songbirds evaluate their ongoing vocal performance.
Figure 1. Headphones frame assembly. a. Crossbar assembly. Attach screws, crossbar, hex nuts, and syringe needles (red) using epoxy adhesive (blue) as shown. Cover the tip of each screw with mineral oil (green) to prevent epoxy from bonding to screws. b. Earbud assembly. Attach post, cylinder, and foam pad (orange) with epoxy. c. Fitting headphones frame. Prior to surgery, thread stereotax earbars (black) through earbuds. Attach crossbar to skull using epoxy or dental acrylic (blue), then attach earbuds to crossbar using alligator clips to gently press the foam pads against the bird's head. d. Left, side view illustrating positioning of crossbar and earbuds on bird's head. Right, side view of the hex nut assembly attached to the skull after the headphones frame has been removed by detaching the screws. Scale bar in b pertains to all panels.
Figure 2. Electronics assembly. a. Make an adapter by inserting a pipet tip into a scrap piece of carbon fiber cylinder and cutting pipet tip to length. b. Using epoxy, glue one speaker and headphones mic together separated by a piece of tape. c. Glue adapter onto speaker (shown) and speaker/phones mic component. d. Solder wires from speakers and headphones mic to a connector strip socket and glue the socket to the top of the headphones frame. White dot, alignment mark on connector strip socket. e. Wiring diagram showing connections between speakers, headphones mic, and socket.
Figure 3. Lead assembly. a. Fabricate a flexible lead by soldering four 15 cm lengths of wire to the pins on one side of a connector strip header and braid wires together. Solder the other end of the lead wires to an adapter connecting to commutator. White dot, alignment mark on connector strip header. b. Plug lead into connector strip socket on headphones to carry altered auditory feedback to headphones speaker, power the headphones mic, and record the signals from the headphones mic. Align dots on connector strip header and socket to ensure correct connectivity.
Figure 4. Circuit summary. Flowchart summarizing system connectivity. The three data channels record (1) the unshifted signal from the cage microphone, (2) a copy of the pitch-shifted signal sent to the headphones speakers, and (3) the sound waveform recorded by the headphones mic. Preamp, microphone preamplifier; LPF, low-pass filter.
Figure 5. Testing the system. a. Spectrograms of sound recorded on the three data channels. Each data channel shows three song syllables. Color represents power at each time and acoustic frequency. b. Power spectrum at the time indicated by the vertical red lines in a. Note that the peaks in the headphones mic spectrum (green) match the peaks in the pitch-shifted (red) rather than the unshifted (black) signal, indicating that the sound reaching the bird's ear is dominated by the shifted feedback.
Figure 6. Using the headphones system to drive vocal learning. Vocal error correction in an adult Bengalese finch. Colored lines show changes in the pitch of seven different song syllables during a 16-day period (gray box) in which the headphones system was used to shift the pitch of auditory feedback upwards by one semitone. Solid black line, mean pitch change across all song syllables. Dashed black line shows the mean pitch error experienced by the subject during the shift epoch. Note that the change in song pitch serves to reduce the experienced pitch error. After the pitch shift was set back to zero on day 17 pitch approaches baseline. On day 24 the headphones were removed, and then replaced on day 46, at which time pitch had recovered back to its baseline value.
The protocol presented here allows the experimenter to manipulate auditory feedback in singing birds. The lightweight construction allows such manipulations to be sustained over long periods, and birds will sing prolifically while wearing headphones for a month or more. Although some songbirds will sing for as long as 10 weeks wearing headphones, in some cases the amount of singing begins to decline after ~5 weeks of use. For this reason, we typically limit experiments to 4 weeks. In our experience, every songbird fitted with headphones can be expected to sing 100+ song bouts per day (and sometimes much more). Therefore, if properly employed, the headphones system offers a nearly 100% success rate (if success is defined by the acquisition of data from singing birds). Furthermore, after completing one learning experiment the headphones can be removed and subsequently reattached for further data collection. Provided that the animal is in good general health reattachment can take place at any time.
One important determinant of success is minimizing the weight and optimizing the comfort of the headphones. During construction, care should be taken to minimize the amount of epoxy or dental acrylic used as excess adhesive will increase the overall weight of the apparatus and potentially reduce the bird's willingness to sing. Additionally, several days after attaching the headphones, the apparatus should be briefly removed to verify that the skin around the ear canals has not become irritated by the earbuds, which can occur if the earbuds are too tight. The ear canals should appear just as they did at the time of headphones attachment (open and with no signs of redness or swelling). If irritation occurs, pressure can be alleviated by reducing the thickness of the foam pads. Take care to ensure that foam hardened by dried epoxy does not contact the bird's skin, as this will also cause irritation.
It is important to note that in addition to the pitch shift selected by the experimenter, virtual auditory feedback is also delayed (by ~10 msec, reflecting the processing latency of the Harmonizer) and is introduced at a greater amplitude than the bird's natural auditory feedback (in order to drown out the sound of the bird's natural song "leaking" into the headphones). For this reason, experiments should begin with a baseline period of several days in which the bird sings with the headphones on but with zero pitch shift14, allowing the effect of the pitch shift to be isolated from vocal changes resulting from other factors related to the headphones paradigm. In practice, changes in song pitch or amplitude are seldom observed when birds first begin to sing with headphones in the absence of a pitch shift. Furthermore, we have shown that extended exposure to unshifted feedback delivered via headphones does not cause a change in song pitch14.
We have previously used this design to demonstrate that in adult songbirds, both upward and downward shifts in the pitch of auditory feedback generate adaptive changes in vocal pitch (i.e. changes opposite in sign to the feedback shift) 14. Including both upward and downward shifts in any experiment employing this paradigm is important because such a design can demonstrate that song pitch changes in response to changes in the pitch of auditory feedback (and not in response to the delay or amplitude artifacts introduced by the headphones). Additionally, a key strength of this paradigm is that it can be used to introduce arbitrary auditory manipulations. The Harmonizer system can generate a wide variety of online perturbations, for example by altering the amplitude or spectral envelope of the acoustic signal. Expanding the range of manipulations beyond pitch shifts could therefore be used to examine a variety of vocal learning phenomena. Additionally, the headphones could be used to deliver white noise or other conditional reinforcement signals to drive learning in individual syllables16. Finally, this paradigm could in principle be employed in any small animal system that relies on auditory feedback during vocal behavior.
We note that our technique, which mimics auditory feedback manipulations used to study human speech 4-7, allows vocal plasticity to be investigated in a physiologically accessible animal model. Combining behavioral studies of vocal error correction with brain lesions, pharmacological manipulations, or neural recordings could be used to reveal how particular neural circuits contribute to the correction of errors in vocal performance.
No conflicts of interest declared.
This work was supported by NINDS 5P30NS069250. We thank Diala Chehayeb, Jeffrey Simpson, Taylor Rosenbaum, and Christopher Hoover for technical assistance.
|Hex nuts||Amazon supply||B000FMW43Y|
|0-80 Screws, 1/8"||Amazon supply||B000FN0JXK|
|0.05" Hex wrench||Amazon supply||B003GDISE8|
|Carbon fiber strip, 1 x 3 mm||Hobby Lobby International||GXS1030|
|Carbon fiber cylinder, 6 mm (OD) x 4 mm (ID)||Hobby Lobby International||GXT6040|
|Wire||Cooner Wire & Cable||NUF36-2550|
|Connector strip header||Digikey||ED83100-ND|
|Connector strip socket||Digikey||ED85100-ND|
|Foam earplugs||AO SAFETY||92050|
|1/8" hole punch||Paperwishes||7260197000|
|1/4" hole punch||Paperwishes||7260198000|
|Speaker amplifier (Crown D-45)||Sweetwater sound||D-45|
|Low-pass filter||Krohn-hite||FMB3002AC, 3FS8SL-10kg-N1U1|
|Alligator clip holder||GC Electronics||12-051|