Sounds and silence: an optical topography study of language recognition at birth

Abstract

Does the neonate's brain have left hemisphere (LH) dominance for speech? Twelve full-term neonates participated in an optical topography study designed to assess whether the neonate brain responds specifically to linguistic stimuli. Participants were tested with normal infant-directed speech, with the same utterances played in reverse and without auditory stimulation. We used a 24-channel optical topography device to assess changes in the concentration of total hemoglobin in response to auditory stimulation in 12 areas of the right hemisphere and 12 areas of the LH. We found that LH temporal areas showed significantly more activation when infants were exposed to normal speech than to backward speech or silence. We conclude that neonates are born with an LH superiority to process specific properties of speech.

Fig. 1.

Testing protocol. Neonates were tested by using 10 blocks per experimental condition: FW (a), BW (b), and SIL (c). Only three of 10 blocks are illustrated for each condition. A block begins with the presentation of 15 s of the stimulus. A period of silence of varying duration (from 25 to 35 s) follows the stimulation period. In c, the epochs that correspond to stimulation epochs in conditions FW and BW are silent. The arrows indicate the onset of the periods of 15 s used to align SIL with FW and with BW during the analyses. The periods used for statistical analyses start with the onset of the block and finish 30 s later (indicated by the dotted line).

Fig. 2.

Positioning of the OT probes and observed results. (a) OT channels projected on an MR image of a 2-month-old infant. Red dots correspond to emitter and blue dots to detector optical fibers. The numbers on the black dotted lines, between adjacent emitter–detector pairs of fibers, correspond to the channels from which changes in Hb concentration were estimated. Indicated skull landmarks (inion, nasion, tragus, and vertex) were used to place the probes. (b) The numbers above the plots correspond to channel numbers in a. The plots show the grand average of the mean of total Hb (mmol·mm) for successive 5-s windows. The first window begins 5 s before the onset of a block. The vertical black line in channel 1 of the LH indicates the range of total Hb concentration in mmol·mm valid for all of the channels. Total Hb is plotted in red for FW, in green for BW, and in blue for SIL. Ascending bars indicate SDs. The six channels enclosed within dotted lines (–12) cover the temporal regions below the Sylvian fissure (lower channels). Channels 1–6 were placed over the frontoparietal regions above the Sylvian fissure (upper channels).