Speech Perception in Infants Peter D. Eimas, Einar R. Siqueland, Peter Jusczyk, and James Vigorito 1971

Purpose of the Study To compare discriminability of two synthetic speech sounds separated by a fixed difference in VOT in infants under two conditions: –condition 1: the two stimuli lay on opposite sides of the adult phonemic boundary –condition 2: the two stimuli are from the same phonemic category

Rationale Perception cue for voicing in English stop consonants in initial position is the onset of the first formant relative to the second and third formants in adults. Perception of the cue is categorical—speech is special. “Given the strong evidence for universal—and presumably biologically determined—modes of production for the voicing distinction, we should suppose that there might exist a complementary process of perception.” (pg. 304)

Participants 1 and 4 month old infants 8 from each age level randomly assigned to the two conditions 10 from each age level assigned to control condition

High Amplitude Sucking Paradigm Typically, the experiment takes place in a sound-attenuated chamber. The infant is installed in a reclining seat, and a flexible arm holds a pacifier in his/her mouth. A pressure transducer measures the air pressure inside the pacifier, and sends the signal into a computer.

HASP continued… The experimentation program detects sucks, computes their amplitude and plays a stimulus back whenever a suck is considered as having a high amplitude. Thus, the reinforcing auditory stimulus is presented contingent on the infants’ sucking responses. The more the baby sucks, the more he/she can listen to the sounds

HASP continued… Baseline measure of sucking is gathered Followed by operant conditioning and habituation phases When a criterion is met (that the number of sucks per minute significantly decreases by at least 20% for 2 consecutive minutes), the experimental groups switch to the test phase, when they listen to the new stimuli (this is the “shift”).

Results When the babies perceived a difference between the two classes of stimuli, they usually responded to the novelty by increasing their sucking in the minutes following the change of stimuli. They noticed more of a difference when the stimuli were from different adult phonemic categories Figure 2

Results continued… The shift data for younger infants differed slightly from the older infants when the stimuli were from the same phonemic category Figure 3

Implications The authors suggest: Infants discriminate voiced and voiceless stop consonants in a manner approximating categorical perception Because of infants’ limited exposure to speech and lack of production experience, this categorical perception in a linguistic mode may be innate