The Discovery of Babbling on the Human Hand
In the course of conducting research on deaf infants’ transition from pre-linguistic gesturing to first signs (9-12 months), Petitto first discovered a class of hand activity that contained linguistically-relevant units that was different from all other hand activity at this time. Her data showed that deaf infants appeared to be babbling with their hands. Additional studies were undertaken to understand the basis of this extraordinary behavior. The findings that were reported in Science revealed unambiguously a discrete class of hand activity in deaf infants that was structurally identical to vocal babbling observed in hearing infants.Like vocal babbling, manual babbling was found to possess:(i) Have a restricted set of phonetic units (unique to signed languages)(ii) Possess syllabic organization(iii) Be used without meaning or reference.This hand activity was also wholly distinct from all infants’ rhythmic hand activity, be they deaf or hearing.
Even its structure was wholly distinct from all infants’ communicative gestures.The discovery of babbling in another modality challenged our conception of the nature of language as being tied to speech. In this radical discovery, it pulled apart speech and language, suggesting that they were not one and the same thing.The discovery of babbling in another modality further confirmed Petitto’s hypothesis that babbling represents a distinct and critical stage in the ontogeny of human language. However, it disconfirmed existing hypotheses about why babbling occurs, i.e., that babbling is neurologically determined by the maturation of the speech-production mechanisms, per se.Specifically, it was thought that the “bababa,” that infants produce is determined by the rhythmic opening and closing of the mandible (jaw). But manual babbling is also produced with rhythmic, syllabic (open-close, hold-movement hand) alternations. Subsequent studies were conducted to examine the physical basis of this extraordinary phenomenon. (See Petitto & Marentette, 1991, Babbling in the manual mode: Evidence for the ontogeny of language. Science).
The Physics of Manual Babbling: The OPTOTRAK Study
Where do the common structures in vocal and manual babbling come from? Is manual babbling really different from all babies’ other rhythmic hand movements and early hand gestures? Petitto hypothesized that the common structure observed across manual and vocal babbling is due to the existence of “supra-modal constraints,” with the rhythmic-temporal oscillations of babbling being key. Both types of babbling are produced in rhythmic, temporally-oscillating bundles (though their absolute HZ are not identical and, crucially, need not be so due to predicted structure and modality-specific differences), which may, in turn, be yoked to constraints on the infant’s perceptual systems.
Petitto’s study of manual babbling was conducted with McGill colleague David Ostry, and students Siobhan Holowka, Lauren Sergio, and Bronna Levy,using the “OPTOTRAK Computer Visual-Graphic Analysis System. The precise physical properties of all infants’ manual activity were measured by placing tiny Light-Emitting Diodes (LEDs) on infants’ hands and feet (control site).
The LEDs transmitted light impulses to cameras that, in turn, sent signals into the OPTOTRAK system. This information was then fed into the computer software that Petitto and her colleagues designed to provide information analogous to the spectrographic representation of speech (“Speech Spectrogram’), but adapted here for the spectrographic representation of sign. For the first time, researchers were able to obtain recordings of the timing, rate, path movement, velocity, and “fo” for all infant hand activity, and to obtain sophisticated, 3-D graphic displays of each.
In the video clip below, you will first see a 10 month old SPEECH-exposed hearing boy producing one example of the high frequency hand movements mentioned in the above article. Second, and following the first, you will see a 10 month old SIGN-exposed hearing girl producing one example of the low frequency hand movements. While the naked eye using videotape analysis alone may impart the erroneous perception that the two babies’ hand movements are the same, the Optotrak technology reveals the stunning ways in which these two babies’ hand movements are systematically different, and it further reveals the linguistic principles that bind one class of hand movements (i.e., the baby girl’s) but not the other (i.e., the baby boy’s).The work was published in Nature.