In addition to being the sincerest form of flattery, imitation is an important learning technique. Babies learn to talk and songbirds learn to sing by mimicking adults. Does the same mechanism yield speech and song production in people and songbirds, respectively? Haesler et al. report that a gene associated with speech deficits in people is important in song production in zebra finches in a recent article in PLoS Biology.
People with the speech disorder developmental verbal dyspraxia (DVD) show deficits in language production and comprehension. DVD is associated with haploinsufficiency of the forkhead box transcription factor (FOXP2). FoxP2 expression is similar in humans and songbirds, localizing to basal ganglia, thalamus and cerebellum.
In most songbirds, males sing to attract mates. Area X is the song nucleus in the songbird basal ganglia. FoxP2 expression in Area X increases at the age when zebra finches first learn to sing, approximately 25 days after hatching. What is the role for FoxP2 in song development?
The authors bilaterally injected Area X with short hairpin RNA (shRNA) directed against FoxP2 or control nondirected shRNA in 23-day-old zebra finches. On day 30 after hatching, the authors placed treated zebra finches (pupils) in sound-proof chambers with adult males (tutors). They recorded pupil performance on days 65, 80, and 90. On day 50, when FoxP2 expression normally peaks, and on day 130, zebra finches treated with FoxP2 shRNA showed a 70% reduction in FoxP2 relative to controls, suggesting that one dose of FoxP2 shRNA reduces FoxP2 expression for several months.
FoxP2 knockdown pupils showed deficits in song imitation. On day 90, zebra finches treated with FoxP2 and control shRNA sang with similar pitch, frequency and duration. However, FoxP2 knockdown pupils were less faithful to tutor songs than were control pupils. FoxP2 knockdown pupils imitated fewer tutor 'syllables' (discrete sections of song) than did control pupils. Of the syllables they did imitate, FoxP2 knockdown pupils showed reduced accuracy relative to control pupils. Although none of the tutors or control pupils repeated syllables within a song, approximately 60% of FoxP2 knockdown pupils repeated single or paired syllables.
FoxP2 knockdown pupils also showed deficits in song development. From days 65-90, song accuracy improved in control pupils. FoxP2 knockdown pupils were less accurate than control pupils on day 65. Although song accuracy increased slightly over time in FoxP2 knockdown pupils, it peaked on day 80, suggesting that the song learning process ended earlier in FoxP2 knockdown relative to control pupils, according to the authors. On day 65, song variability was similar in FoxP2 knockdown and control pupils, and song variability did not change in control pupils over time. However, song variability increased over time for FoxP2 knockdown pupils.
According to the authors, these data suggest that knockdown zebra finches are deficient in the motor learning necessary to adjust and improve their songs. Although DVD was thought to impair motor performance during speech production, they suggest that DVD may instead impair motor learning during speech development.