Publications

Abstract

Speaking and listening are complex tasks that we perform on a daily basis, almost without conscious effort. Interestingly, speaking almost never occurs without listening: whenever we speak, we at least hear our own speech. The research in this thesis is concerned with how the perception of our own speech influences our speaking behavior. We show that unconsciously, we actively monitor this auditory feedback of our own speech. This way, we can efficiently take action and adapt articulation when an error occurs and auditory feedback does not correspond to our expectation. Processing the auditory feedback of our speech does not, however, automatically affect speech production. It is subject to a number of constraints. For example, we do not just track auditory feedback, but also its consistency. If auditory feedback is more consistent over time, it has a stronger influence on speech production. In addition, we investigated how auditory feedback during speech is processed in the brain, using magnetoencephalography (MEG). The results suggest the involvement of a broad cortical network including both auditory and motor-related regions. This is consistent with the view that the auditory center of the brain is involved in comparing auditory feedback to our expectation of auditory feedback. If this comparison yields a mismatch, motor-related regions of the brain can be recruited to alter the ongoing articulations.

Abstract

Sprekers hebben een sterke neiging om syntactische structuren te hergebruiken in nieuwe zinnen. Wanneer we een situatie beschrijven met een passieve zin bijvoorbeeld: 'De vrouw wordt begroet door de man', zullen we voor de beschrijving van een nieuwe situatie gemakkelijker opnieuw een passieve zin gebruiken. Vooral bij moeilijke syntactische structuren is de neiging om ze te hergebruiken erg sterk. Voor gemakkelijke zinsconstructies geldt dat minder. Maar als deze toch hergebruikt worden dan gaat dit samen met een sneller initiëren van de beschrijving. Ook in het brein zien we dat het herhalen van syntactische structuren de verwerking ervan vergemakkelijkt. Bepaalde hersengebieden die zorgen voor de verwerking van syntactische structuren zijn zeer actief de eerste keer dat een syntactische structuur wordt verwerkt, en minder actief de tweede keer. Het gaat hier om een gebiedje in de frontaalkwab en een gebiedje in de temporaalkwab. Opvallend is ook dat deze gebieden de verwerking van syntactische structuren ondersteunen zowel tijdens het spreken als tijdens het luisteren.

Abstract

In this thesis I approach language as a neurobiological system. I defend a sequence processing perspective on language and on the function of Broca's region in the left inferior frontal gyrus (LIFG). This perspective provides a way to express common structural aspects of language, music and action, which all engage the LIFG. It also facilitates the comparison of human language and structured sequence processing in animals. Research on infants, song-birds and non-human primates suggests an interesting role for non-adjacent dependencies in language acquisition and the evolution of language. In a series of experimental studies using a sequence processing paradigm called artificial grammar learning (AGL), we have investigated sequences with adjacent and non-adjacent dependencies. Our behavioral and transcranial magnetic stimulation (TMS) studies show that healthy subjects successfully discriminate between grammatical and non-grammatical sequences after having acquired aspects of a grammar with nested or crossed non-adjacent dependencies implicitly. There were no indications of separate acquisition/processing mechanisms for sequence processing of adjacent and non-adjacent dependencies, although acquisition of non-adjacent dependencies takes more time. In addition, we studied the causal role of Broca‟s region in processing artificial syntax. Although syntactic processing has already been robustly correlated with activity in Broca's region, the causal role of Broca's region in syntactic processing, in particular syntactic comprehension has been unclear. Previous lesion studies have shown that a lesion in Broca's region is neither a necessary nor sufficient condition to induce e.g. syntactic deficits. Subsequent to transcranial magnetic stimulation of Broca‟s region, discrimination of grammatical sequences with non-adjacent dependencies from non-grammatical sequences was impaired, compared to when a language irrelevant control region (vertex) was stimulated. Two additional experiments show perturbation of discrimination performance for grammars with adjacent dependencies after stimulation of Broca's region. Together, these results support the view that Broca‟s region plays a causal role in implicit structured sequence processing.

Abstract

Many people speak a second language next to their mother tongue. How do they learn this language and how does the brain process it compared to the native language? A second language can be learned without explicit instruction. Our brains automatically pick up grammatical structures, such as word order, when these structures are repeated frequently during learning. The learning takes place within hours or days and the same brain areas, such as frontal and temporal brain regions, that process our native language are very quickly activated. When people master a second language very well, even the same neuronal populations in these language brain areas are involved. This is especially the case when the grammatical structures are similar. In conclusion, it appears that a second language builds on the existing cognitive and neural mechanisms of the native language as much as possible.

Abstract

In recent decades, neuroimaging studies on the neural infrastructure of language are usually (or mostly) conducted with certain on-line language processing tasks. These functional neuroimaging studies helped to localize the language areas in the brain and to investigate the brain activity during explicit language processing. However, little is known about what is going on with the language areas when the brain is ‘at rest’, i.e., when there is no explicit language processing running. Taking advantage of the fcMRI and DTI techniques, this thesis is able to investigate the language function ‘off-line’ at the neuronal network level and the connectivity among language areas in the brain. Based on patient studies, the traditional, classical model on the perisylvian language network specifies a “Broca’ area – Arcuate Fasciculus – Werinicke’s area” loop (Ojemann 1991). With the help of modern neuroimaging techniques, researchers have been able to track language pathways that involve more brain structures than are in the classical model, and relate them to certain language functions. In such a background, a large part of this thesis made a contribution to the study of the topology of the language networks. It revealed that the language networks form a topographical functional connectivity pattern in the left hemisphere for the right-handers. This thesis also revealed the importance of structural hubs, such as Broca’s and Wernicke’s areas, which have more connectivity to other brain areas and play a central role in the language networks. Furthermore, this thesis revealed both functionally and structurally lateralized language networks in the brain. The consistency between what is found in this thesis and what has been known from previous functional studies seems to suggest, that the human brain is optimized and ‘ready’ for the language function even when there is currently no explicit language-processing running.

Abstract

Baby's begrijpen woorden eerder dan dat ze deze zeggen. Dit stadium is onderbelicht want moeilijk waarneembaar. Caroline Junge onderzocht de vaardigheden die nodig zijn voor het leren van de eerste woordjes: conceptherkenning, woordherkenning en het verbinden van woord aan betekenis. Daarvoor bestudeerde ze de hersenpotentialen van het babybrein tijdens het horen van woordjes. Junge stelt vast dat baby's van negen maanden al woordbegrip hebben. En dat is veel vroeger dan tot nu toe bekend was. Als baby's een woord hoorde dat niet klopte met het plaatje dat ze zagen, lieten ze een N400-effect zien, een klassiek hersenpotentiaal. Uit eerder Duits onderzoek is gebleken dat baby's van twaalf maanden dit effect nog niet laten zien, omdat de hersenen nog niet rijp zouden zijn. Het onderzoek van Junge weerlegt dit. Ook laat ze zien dat als baby's goed woorden kunnen herkennen binnen zinnetjes, dit belangrijk is voor hun latere taalontwikkeling, wat mogelijk tot nieuwe therapieën voor taalstoornissen zal leiden.

Abstract

Functional Magnetic Resonance Imaging (fMRI) and Electropencephalography (EEG) are the two techniques that are most often used to study the working brain. With the first technique we use the MRI machine to measure where in the brain the supply of oxygenated blood increases as result of an increased neural activity with a high precision. The temporal resolution of this measure however is limited to a few seconds. With EEG we measure the electrical activity of the brain with millisecond precision by placing electrodes on the skin of the head. We can think of the EEG signal as a signal that consists of multiple superimposed frequencies that vary their strength over time and when performing a cognitive task. Since we measure EEG at the level of the scalp, it is difficult to know where in the brain the signals exactly originate from. For about a decade we are able to measure fMRI and EEG at the same time, which possibly enables us to combine the superior spatial resolution of fMRI with the superior temporal resolution of EEG. To make this possible, we need to understand how the EEG signal is related to the fMRI signal, which is the central theme of this thesis. The main finding in this thesis is that increases in the strength of EEG frequencies below 30 Hz are related to a decrease in the fMRI signal strength, while increases in the strength of frequencies above 40 Hz is related to an increase in the strength of the fMRI signal. Changes in the strength of the low EEG frequencies are however are not coupled to changes in high frequencies. Changes in the strength of low and high EEG frequencies therefore contribute independently to changes in the fMRI signal.

Abstract

People with grapheme-colour synaesthesia experience colour for letters of the alphabet or digits; A can be red and B can be green. How can it be, that people automatically see a colour where only black letters are printed on the paper? With brain scans (fMRI) I showed that (black) letters activate the colour area of the brain (V4) and also a brain area that is important for combining different types of information (SPL). We found that the location where synaesthetes subjectively experience their colours is related to the order in which these brain areas become active. Some synaesthetes see their colour ‘projected onto the letter’, similar to real colour experiences, and in this case colour area V4 becomes active first. If the colours appear like a strong association without a fixed location in space, SPL becomes active first, similar to what happens for normal memories. In a last experiment we showed that in synaesthetes, attention is captured by real colour very strongly, stronger than for control participants. Perhaps this attention effect of colour can explain how letters and colours become coupled in synaesthetes.

Abstract

Marieke van der Linden investigated the neural mechanisms underlying category formation in the human brain. The research in her thesis provides novel insights in how the brain learns, stores, and uses category knowledge, enabling humans to become skilled in categorization. The studies reveal the neural mechanisms through which perceptual as well as conceptual category knowledge is created and shaped by experience. The results clearly show that neuronal sensitivity to object features is affected by categorization training. These findings fill in a missing link between electrophysiological recordings from monkey cortex demonstrating learning-induced sharpening of neuronal selectivity and brain imaging data showing category-specific representations in the human brain. Moreover, she showed that it is specifically the features of an object that are relevant for its categorization that induce selectivity in neuronal populations. Category-learning requires collaboration between many different brain areas. Together these can be seen as the neural correlates of the key points of categorization: discrimination and generalization. The occipitotemporal cortex represents those characteristic features of objects that define its category. The narrowly shape-tuned properties of this area enable fine-grained discrimination of perceptually similar objects. In addition, the superior temporal sulcus forms associations between members or properties (i.e. sound and shape) of a category. This allows the generalization of perceptually different but conceptually similar objects. Last but not least is the prefrontal cortex which is involved in coding behaviourally-relevant category information and thus enables the explicit retrieval of category membership.