Qui sotto trovate una breve review che ho redatto sull'evoluzione del lessico nell'arco della vita.
Si sente molto parlare del lessico nell'arco dello sviluppo del bambino, credo sia interessante approfondire come il lessico si sviluppi e come esso rappresenti una delle funzioni che evolvono nell'intero arco della vita.
(Il testo è presente soltanto in lingua inglese).
The Lexicon Across The Life Span:
Rise And Fall Of A Crucial Component Of Language
Lexicon is a crucial formal component of language that goes along the life of every person. Its development starts from the first months of life and specifically crucial is the emergence of the first words, around 10-12 months.
It is possible to define what is a ‘word’ adopting the definition given by Vihman and McCune (1994): a word needs two criteria a) it must be similar to the adult form; b) it is used as a referent for something. In this definition two aspects are of paramount: the form and the function.
The development of the lexical ability as well as the ability to retrieve words led to different paths of research. Some of these included the description of the interaction between the context and words, distinguishing between a referential meaning (the meaning of the word per se) and a combinatory meaning (the meaning of the word in relation with the context) (Greenfield & Smith, 1976).
It is also possible to distinguish a non-referential use, when the words accompany the action, and a referential use, when the words are used independently from the actions (Bruner, 1975; Bates et al. 1979). This is a fundamental step for the lexical development, since it states the comprehension of the arbitrary nature between sounds and referents.
Another important point is the function of words in social games, where children can build and share the meanings of the words since the actions in the game are unhooked form the consequences (Bruner, 1975), and the attention could be focused on the communication.
The development of lexicon is well studied in infancy, and it is well established that it improves along the lifespan (Bowels & Salthouse, 2008; McArdle et al., 2002).
There are fewer studies that investigate how lexical ability develops through the whole lifespan, especially in healthy aging.
The lexical ability allows to the human being to communicate referents through the use of abstract and arbitrarily established sequences of sounds, and allows the development of social interactions in everyday life; thus, some articles are reviewed here, in order to clarify the trend of lexical ability throughout the lifespan.
A special attention is given to healthy aging, disentangling whether the changes uncovered in some studies (Newman and German, 2005; Zec et al., 2005, 2007; Connor et al., 2004; Kavé, 2009) are due to the involvement of lexical stored knowledge, or to other integrative processing.
In the study presented by Kavé, Knafo, Gilboa (2010) the ability of naming of 1145 Hebrew-speaking participants, aged between 5-86, was tested. A Hebrew naming test (Kavé, 2005), with 48 black and white line drawings, was used to test word retrieval.
The results showed that the correct naming scores increase dramatically during childhood and then decrease modestly during old age. A linear age effect and a quadratic effect of squared age successfully predicted naming scores. In the regression analysis, using a quadratic regression model fitted to the data, the two effects accounted for the 47% of the variance.
Further analysis showed that when naming fails, older adults arrive at the right word more often than younger using the given cues. The hypothesis is that the development of lexicon derives from an enrichment of knowledge, but also from an increasing ability in the control of processes that allow access to stored information.
The authors focus on the importance of the context for older people to create a meaningful representation of the new words acquired, and this seems to help older people in words retrieval as a backup assistant.
In the study immigrants were compared with non-immigrants, this showed a lower number of words correctly retrieved for immigrants, but the access score did not show any significant difference, and the age match between immigrants and non-immigrants showed that the age-associated access difficulties could be dissociated from the effects of vocabulary exposure.
In conclusion the authors state that aging-associated naming difficulties seem to be related to an unsuccessful access to existing knowledge, rather than to a deterioration of stored knowledge.
The article outline how lexicon vary across the lifespan, and introduce the possibility that aging lead to a decreasing in the ability to access to knowledge, even though the authors do not clearly disentangle this hypothesis, probably because the two processes are intertwined.
Moving from this hypothesis other studies have been conducted, using ERP and neuroimaging technique.
Huang, Meyer and Federmeier (2012) studied how age-related changes in basic integrative processes in language could be revealed using event related potentials.
Their aim was to investigate if age-related changes extended even to simpler language unit, such as the relations between noun and adjective.
Huang and colleagues studied the concreteness (i.e. the characteristic of concepts that can be easily experienced by senses) effects at a single word level, modifying the contextual information which should calls up more abstract or concrete features of that word.
They used ERP to investigate the trend of frontal negativity and N400, already found to be significant in younger participants (Huang et al. 2010), in older adults.
Twenty older adults aged 60-81 were tested, each participant saw 392 triplets composed by adjective, noun (lateralized of 2°, balanced between right and left presentation), adjective in sequence. The two adjectives were one concrete and the other abstract, balanced between first- and third-position presentations.
Two sets were presented, each one with 56 abstract adjective-noun pair, 56 concrete adjective-noun pair, 84 filler trials (with anomalous adjectives as modifier of the noun).
The previous study conducted with younger adults showed concreteness-based predictability effects linked to left hemisphere processing and imagery effects linked to right hemisphere processing at lateralized nouns.
The results of this study, with older participants, showed no concreteness effects on any components in either hemisphere.
Yet, older adults showed clear N400 congruity effect on lateralized nouns in the filler trials.
In conclusion it seems that even though the stored lexical knowledge was preserved, as shown by the N400 congruity effect, the results suggested that older adults are less able to link adjective-noun meaning to form an integrated conceptual representation.
In order to better understand if the changes in the lexicon presented in these two studies are consistent with a change in brain structure, the next study present the effects of vocabulary knowledge on temporal and parietal brain structures across the lifespan.
Richardson et al. demonstrated contrasting effects of vocabulary knowledge in grey matter in the left posterior SMG (supramarginal gyrus), left pSTS (posterior superior temporal sulcus) and left pT-P (posterior temporo-parietal cortex).
The participants were 48, aged 7-73, they took behavioral tests for vocabulary, memory, reading test; they were then scanned in MRI for structural images and fMRI for functional images.
The experimental paradigm was a sentence-processing task, carried out in auditory and visual modalities. The participants listened passively to auditory sentence stimuli, and silently read visual sentences; they also listened to or read scrambled sentences, tapping comprehension at the individual word level. The baseline task consisted in listening rendered meaningless word (digitally reversed word), and reading words presented in an unrecognizable font.
The participants attended four different scanning sessions, two blocks of auditory sentences, two blocks of visual sentences, one block of scrambled auditory sentences, on block of scramble visual sentences, three blocks of auditory baseline and three blocks of visual baseline.
The ROI in fMRI analysis was the SMG, accordingly to a previous study (Lee et al. 2007). Another ROI consisted in a mask of active areas across the whole brain, which survived correction for multiple comparisons.
The results of functional analysis showed a positive correlation between vocabulary score and sentence activation in left pSTS and left pT-P. No significant effects were found in the left pSMG.
These areas seem to be involved in sentence comprehension.
In the structural analysis it was found a positive correlation between vocabulary score and grey matter density in both pSTS and pT-P. No significant effect of vocabulary was found in the pSMG ROIs.
When structural analysis has been performed in teenagers only the grey matter density was significantly correlated with the vocabulary score in both left and right pSMG.
The pSMG may reflect learning through explicit instructions, and this usually occurs during formal education.
This result indicates the effect of vocabulary in grey matter differs across age, and this could lead to different strategies of learning during the lifespan.
In the studies presented in this short review the lexicon was investigated across the lifespan, taking in account the behavioral, structural, functional and physiological points of view.
It is clear that we are far from a full comprehension of the mechanisms underlying the lexical component of language; anyway it is possible to outline some characteristics of its trend across life span.
Different neural structures are involved in different time of life, probably depending on the modality of learning new words, with a prominent role of pSMG during the first two decades of life, involving formal instructions; a continuous positive correlation between vocabulary and pSTS and pT-P may reflect new words learning within the context of everyday language.
These studies revealed as crucial the role of context in which new words are learned, since it seems to have a different impact across the lifespan; as reported above the ability to engage predictive processing meaning and the message-level meaning information seems to decrease in older adults, as shown in the study conducted with ERPs (Huang et al.; 2010).
In conclusion it is necessary to underline that studying the lexicon in an experimental environment is restrictive, so further analyses are very important in order to compare the gained in-laboratory-knowledge to the spontaneous use of lexicon of everyday life.
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