Publications

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.