The role of reward in word learning and its implications for second language acquisition
Language plays a central role in the lives of all human beings, since the very moment we are born. It is such a powerful communication device that allows us to share our feelings and thoughts in a flexible and unrestrained manner. Being language so important, so fundamental for human behavior, the different buildings blocks of this impressive and complex communication device have been extensively assessed. Since childhood, human beings display an innate motivation to acquire language and to communicate and, thus, language learning has been a subject of especial importance. In particular, many research has been devoted to assess the process of new word learning (i.e., the mapping of an unknown word to an appropriate meaning). Human beings show an astounding capacity to learn new words, especially during childhood, and they do so even when a new word co-occurs with multiple referents in the same context (i.e., in a particular situation a new word can be associated with different meanings). Therefore, an important question arises: when presented with multiple words and multiple referents, how do humans assign correct meaning to an unknown word?
While there have been great advances in the field of word learning, especially after the advent of functional and diffusion weighted Magnetic Resonance Imaging (fMRI and DW-MRI, respectively), there is one human cognitive trait—well shared among species—whose relationship with language learning processes has received little focus: reward. In this dissertation we tried to bridge the gap between these two areas of study, which have historically progressed separately, by assessing new word learning from written context in human adults—a process which usually occurs without the need for explicit reward, feedback or external guidance and that is thought to aid vocabulary growth during human lifespan. This thesis tried—by using neuroimaging techniques such as fMRI and DW-MRI, and neurophysiological measures such electrodermal activity (EDA)—to answer two main questions: i) can language learning, in absence of explicit feedback, be intrinsically rewarding and ii) can this intrinsic reward signal enhance new word learning?
We developed a paradigm, which mimics natural new word learning from context, in which participants could learn the meaning of new words on their own, without external feedback, using the contextual information provided by a duplet of sentences. We hypothesized—based on findings showing that self-generating the solution to a problem can be inherently rewarding—that new word learning from context should also be accompanied by intrinsic feelings of reward. In this vein, we show that a strong functional and anatomical coupling between cortical areas related to language processing and subcortical reward related regions exists during successful new word learning. In two additional studies, we further show that new word learning also enhances electrodermal activity and heightens self-reported ratings of pleasantness.
It is well-known that explicit reward can modulate the entrance of new-information into long-term memory by tapping into a neural system formed by the substantia nigra/ventral tegmental area complex (SN/VTA), the ventral striatum (VS) and the hippocampus (the SN/VTA-Hippocampal loop). The activation of this loop is thought to start when new information that needs to be stored into long-term memory arrives at the hippocampus, which then sends a signal to the midbrain via the VS. Dopaminergic neurons at the SN/VTA start to fire and the loop is closed as dopamine is released back at the hippocampus, which ultimately enhances memory through long-term potentiation processes. In this regard, the activity induced by successful word learning was not restricted to the VS, but was also extended to the SN/VTA and hippocampus. In addition, the level of activity and functional connectivity among the areas of the SN/VTA-Hippocampal loop predicted memory benefits. In two additional studies, new words remembered after long retention delays were associated with enhanced EDA responses and higher pleasantness ratings during encoding. We speculate that the same intrinsic reward that is triggered by learning itself can mediate the encoding of new words into long-term memory by directly modulating the SN/VTA-Hippocampal loop (i.e., by increasing the level of dopamine release at the hippocampus). Our findings suggest that human beings possess a very powerful learning system, also engaged during language learning processes, which is able to fuel itself through intrinsic reward related mechanisms.
The combination of the present results with previous literature allows us to provide a joint neuroanatomical model for word learning, memory and reward, thus bridging the gap between well-known language models of semantic processing and the SN/VTA-Hippocampal loop. I believe that the findings and suggestions presented in this dissertation can contribute to break new ground in the field of language and also may help to spark the creation of new lines of research that further explore the role of reward—and other general domain cognitive capabilities—in word learning.