The two studies by Dehaene-Lambertz, Dehaene, and HertzPannier (2002), as well as, Homae, Watanabe, and Taga (2014) will be compared. The studies measure the perception of speech sounds in the infant brain using functional magnetic resonance imaging (fMRI). Each set of researchers collected data from young infants around 3 months. Different qualities of sound were presented to the infants in order to measure the activation of certain structures in the brain.
The hemodynamic responses to different qualities of sound allowed the two sets of researchers to determine the general functions of the different areas of the infant brain while perceiving speech. The presentation of speech sounds activated the left temporal lobe and the frontal regions. The two sets of researchers were able to conclude that these findings of significant activation show that the infant brain is already forming a web for speech processing.
Introduction Functional neuroimaging of speech perception in infants The purpose of the study conducted by Dehaene-Lambertz, Dehaene, and Hertz-Pannier (2002) is to understand the brain structures and mechanisms that allow for the learning of language to take place during the first year of an infant’s life. Research using fMRI was previously done on infants, though the results were very contradictory, as the infants were unconscious. Dehaene-Lambertz et al. (2002) collected images from infants that were conscious, in order to find the specific areas in the infant brain that are active while processing speech.
The neural substrates of infant speech perception Homae, Watanabe, and Gentaro (2014) claim that the primary objective of their study is “to examine whether 3-month-old infants showed selective activation in response to speech sounds, by directly comparing cortical activation in speech sound and SWS conditions. ” The general purpose for their study was to figure out how language is acquired by infants as well as which areas of the brain are activated using different auditory stimuli. Homae et al. (2014) looked at the second step in linguistic processing in the developing infant brain.
Their study is important because they prove that infants take in more information than is generally understood. Comparison Dehaene-Lambertz et al. (2002) and Homae et al. (2014) are expanding on the research done on sound activation in the infant brain while unconscious. Both sets of researchers studied the activity of the infant brain when exposed to different types of sounds using fMRI. The findings allow the labeling of structures in the infant brain that are responsible for speech perception.
Methods Functional neuroimaging of speech perception in infants Dehaene-Lambertz et al. 2002) presented 20 seconds of speech with an alternate of 20 seconds of silence to 20 infants. The native language of the infants was French; they were tested at 2-3 months of age. Dehaene-Lambertz et al. (2002) presented a recording of a female voice reading a childrens book in French contrasted with the recording being played backwards to the conscious infants. It was predicted that forward speech would call for a stonger activation in the brain areas that allow for segmental and suprasegmental properties since backward speech violates these properties.
These activated areas were measured using the hemodynamic response function. The neural substrates of infant speech perception Thirty 3-month-old Japanese infants were presented with three types of auditory stimuli, while they watched a silent movie (Homae et al. , 2014). Speech sounds, Sine Wave Speech (SWS), and three synthesized pure tones were presented to the infants 3-6 times per stimuli. The stimuli were presented using a speaker that was placed directly in front of the infant, which did not exceed a maximum amplitude of 65 decibels (Homae et al. 2014).
NIRs instruments with 48 different measurement channels were placed on each infant’s head using a flexible cap. Homae et al. (2014) predicted that they would observe three types of activation patterns in the brain. The most important of these patterns being that a different part of the brain would be activated when the infants were presented with the speech sounds compared to SWS. These parts of the brain would then be labeled as the speech-selective regions of the infant brain. Comparison Homae et al. had a larger pool of participants at 30 than Dehaene-Lambertz had with 20.
In the explanation of their methods, Homae et al. (2014) seemed to be more concise in their methods. The two methods also differed in the fact that Dehaene-Lambertz et al. (2002) used one recording of a French woman for each speech stimuli, while Homae et al. used 6 different speech recordings of a Japanese woman. These two languages work in different ways; therefore, there could be differentiation between the languages. Using one recording compared to six different recordings could also have affected the results. Homae et al. xposed their infant participants to 4 second recordings, while Dehaene-Lambertz (2002) exposed their participants to a twenty second recording. Both sets of researchers examined the differentiation in the oxygenated hemoglobin. These signals “estimate the changes in the regional cerebral blood oxygenation during brain activation” (Homae et al. , 2014).
Results Functional neuroimaging of speech perception in infants Dehaene-Lambertz et al. (2002) found that “the left angular gyrus and left mesial parietal lobe (precuneus) were significantly more activated by forward speech than by backward speech. They also found that the right dorsolateral prefrontal complex, as well as, the posterior part of the superior temporal sulci was more active when the infant was exposed to forward than when exposed to backward speech. The neural substrates of infant speech perception The SWS and speech stimuli both had larger hemodynamic responses than the synthesized pure tones (Homae et al. 2014). The left posterior temporal and frontal channels had a larger reaction to speech sounds than both SWS and pure tones Homae et al. (2014).
This proves that infants perceive speech differently than they perceive bot SWS and pure tones. Comparison Both sets of researchers found that compared to the other nonspeech stimuli, the speech stimuli produced a significant activation of the left posterior temporal cortex as well as the right prefrontal complex. Discussion Functional neuroimaging of speech perception in infants The activation of the infant brain mimicked the patterns of normal adults, but the researchers concluded that the infant brain is still rapidly changing at three months.
The results showed significant left lateralization, but the researchers cannot tell whether this is due to specialization for speech or just greater responsitivity of the left temporal cortex. The neural substrates of infant speech perception “The statistical analysis revealed that the left posterior temporal region was selectively involved in the processing of speech sounds” Homae et al. (2014). In previous studies this region was found to be involved in phonological processing in the adult brain.
Using this information, Homae et al. (2014) concluded hat the infant brain is sensitive to and is able to process speech sounds. Homae et al. (2014) also concluded that the left frontotemporal network of the infant brain processes the linguistic information from speech sounds. Comparison Both sets of researchers concluded that an infant’s brain starts to specialize structures for speech perception and processing. Different structures in the infant brain were activated when speech sounds were presented than when compared to other sounds: therefore, the researchers concluded that infants perceive speech sounds differently.
The infant brain processes the native language “in the left angular gyrus, the left precuneus, and in awake babies only, the right prefrontal cortex (DehaeneLambertz et al. 2002). These findings, supported by both sets of researchers, allow the researchers to suggest that the temporal and frontal regions form a network that allows the infants to process speech sounds (Homae et al. 2014). My Impression Functional neuroimaging of speech perception in infants The article by Dehaene-Lambertz et al. (2002) seemed very simple, but it had a lot of information packed into three pages.
The article was not organized well. It would be hard to find specific information without reading through it. Using fMRI would be something that I can imagine using in the clinics. The article was pretty vague, it did not say how they presented the sounds to the infants. I am not sure if they used headphones or a free field, but headphones would be something we would use in the clinic. The neural substrates of infant speech perception The article by Homae et al. (2014) was organized very nicely and easy to follow. The headings allow the reader to quickly find information.
Homae et al. (2014) mention that they used a “sound-attenuated room. ” In class we talked about attenuating sound in the Frequency Selectivity lecture by using filters. The sounds were presented binaurally using a speaker in front of the infant. In a clinical setting, sounds are presented monaurally using headphones. Homae et al. (2014) used the binaural presentation to mimic real sounds in a sound field. Comparison These studies were very important in giving more information about how the infant brain works and which structures are important to the perception of speech sounds.
Most of the structures in the infant brain correspond in the adult brain. This alludes that, the infant brain, while it is still growing and changing, is already starting to specialize at just 3 months. A network is starting to form in order to perceive the speech information. In the future, there needs to be more research done on what each of these parts of the brain specifically do to function in perceiving speech. This will help to understand how the infant brain starts to understand speech and form.
