Memory in the neonate brain

Abstract

Background: The capacity to memorize speech sounds is crucial for language acquisition. Newborn human infants can discriminate phonetic contrasts and extract rhythm, prosodic information, and simple regularities from speech. Yet, there is scarce evidence that infants can recognize common words from the surrounding language before four months of age.

Methodology/principal findings: We studied one hundred and twelve 1-5 day-old infants, using functional near-infrared spectroscopy (fNIRS). We found that newborns tested with a novel bisyllabic word show greater hemodynamic brain response than newborns tested with a familiar bisyllabic word. We showed that newborns recognize the familiar word after two minutes of silence or after hearing music, but not after hearing a different word.

Conclusions/significance: The data show that retroactive interference is an important cause of forgetting in the early stages of language acquisition. Moreover, because neonates forget words in the presence of some--but not all--sounds, the results indicate that the interference phenomenon that causes forgetting is selective.

Figure 1. Experimental paradigm and results.

A. Schematic diagrams of the procedure used in the experiments. During the familiarization phase, all the neonates were presented with 10 blocks composed of 6 identical words. A period of silence of varying duration (from 25s to 35s) followed each block. In the test, 5 blocks of the same word heard during familiarization were presented to half of the neonates while the other half heard a novel word. In Study 1, a silent 2-minute interval intervened between familiarization and test. In studies 2 and 3, the silent interval was filled by music (Study 2) or speech stimuli (Study 3). B-C-D) Time courses of the relative hemodynamic changes averaged across all the channels and subjects per group. The dashed line indicates the time series for the group that heard the same word before and after the pause; the continued line represents the group that heard a novel word in the test. Error bars indicate standard errors. The x-axis shows number of blocks; in the y-axis the changes in concentration of Oxy-hemoglobin in mmol*mm is displayed. The neonates who heard a novel word after a silent period showed greater cortical Oxy-Hb concentration changes in the test than neonates who heard the same word before and after the silent pause. The presence of speech stimuli during the interval affects recognition memory in neonates. No interference was found when music was presented during the interval (*, p<0.01; **, p<0.0001).

Figure 2. Statistical maps on the schematic neonate brain.

The graph depicts the comparison between the Novel-word and Same-word conditions in the first block of the test -block 11, Study 1-. Significance levels for each channel (p-values corrected by false discovery rate [38]) are color-coded as indicated on the color bar. Grey circles indicate no significant differences between conditions.

Figure 3. OxyHb changes from the last familiarization block to the first test block (Study 1).

Channels bilaterally located in frontal, temporal and parietal areas show a decrease in the concentration of oxyHb when neonates hear the same word before and after the pause (white bar). In contrast, when neonates are confronted with a novel word in the test (black bar) the concentration from the familiarization phase to the test increases. Colored ellipses on the schematic neonate brain indicate the localization of the channels included in the areas of interest.

Figure 4. OxyHb changes from the last familiarization block to the first test block (Studies 2 and 3).

A. When the 2-minute pause was filled with music, channels in both hemispheres showed a decrement in the concentration of oxyHb from the familiarization to the test in the Same-word condition, and an increment in the Novel-word condition. B. There were no significant changes in oxyHb concentration from the familiarization to the test phases when the interval was filled with speech stimuli.