Forskningsassistent i Fonetiklaboratoriet
Igenkänningsprocessen i mänsklig talperception
Lexikal access är en central komponent i talförståelsen. För att förstå hur lexikal access fungerar är det viktigt att identifiera vilka typer av akustisk information som finns representerade i långtidsminnet och att undersöka hur hjärnan använder denna typ av information.
I min avhandling undersöker jag om det finns så kallade minnesspår för prosodisk information i lexikal access och om prosodisk information uppfattas innan uppmärksamheten riktas mot signalen. Avhandlingen jämför tre typologiskt distinkta språk (engelska, turkiska, och svenska) och testar i vilken utsträckning betydelsen av prosodiska drag för lexikal access är språkspecifik och skiljer sig åt mellan olika språktyper.
För att förstå lexikal access i grunden är det nödvändigt att kunna obeservera igenkänningsprocessens utveckling och att kunna uppskatta tidsutvecklingen i mänsklig talperception. För att kunna göra detta behöver man kunna följa och registrera hjärnaktivitet i realtid. I min avhandling använder jag Elektroencefalografi (EEG) som är en elektrofysiologisk teknik för att studera elektrisk aktivitet i hjärnan med god tidsupplösning, vilket möjliggör observationer av tidsutvecklingen i lexikal processning. EEG-tekniken fångar neurala responser som är direkt relaterade till specifika sensoriska och kognitive processer och är mycket väl lämpad för undersökningar av språkbearbetning i hjärnan.
PublikationerI urval från Stockholms universitets publikationsdatabas
2016. Hatice Zora (et al.). Frontiers in Neuroscience
Avhandling (Dok) Mapping prosody onto the lexicon2016. Hatice Zora (et al.).
Lexical access, the matching of auditory information onto lexical representations in the brain, is a crucial component of online language processing. To understand the nature of lexical access, it is important to identify the kind of acoustic information that is stored in the long-term memory and to study how the brain uses such information. This dissertation investigates the contribution of prosodic information to lexical access and examines language-specific processing mechanisms by studying three typologically distinct languages: English, Turkish, and Swedish. The main research objective is to demonstrate the activation of long-term memory traces for words on the sole basis of prosodic information and to test the accuracy of typological phonological descriptions suggested in the literature by studying electrophysiological measurements of brain activation. A secondary research objective is to evaluate three distinct electrophysiological recording systems. The dissertation is based on three papers, each examining neural responses to prosodic changes in one of the three languages with a different recording system. The first two papers deal directly with the interplay between prosody and the lexicon, and investigate whether prosodic changes activate memory traces associated with segmentally identical but prosodically different words; the third paper introduces morphology to this process and investigates whether prosodic changes activate memory traces associated with potential lexical derivations. Neural responses demonstrate that prosodic information indeed activates memory traces associated with words and their potential derivations without any given context. Strongly connected neural networks are argued to guarantee neural activation and implementation of long-term memory traces. Regardless of differences in prosodic typology, all languages exploit prosodic information for lexical processing, although to different extents. The amount of neural activation elicited by a particular piece of prosodic information is positively correlated with the strength of its lexical representation in the brain, which is called lexical specification. This dissertation could serve as a first step towards building an electrophysiological-perceptual taxonomy of prosodic processing based on lexical specification.
Artikel Perceptual correlates of Turkish word stress and their contribution to automatic lexical access2016. Hatice Zora, Mattias Heldner, Iris-Corinna Schwarz. Frontiers in Neuroscience 10
Perceptual correlates of Turkish word stress and their contribution to lexical access were studied using the mismatch negativity (MMN) component in event-related potentials (ERPs). The MMN was expected to indicate if segmentally identical Turkish words were distinguished on the sole basis of prosodic features such as fundamental frequency (f0), spectral emphasis (SE) and duration. The salience of these features in lexical access was expected to be reflected in the amplitude of MMN responses. In a multi-deviant oddball paradigm, neural responses to changes in f0, SE, and duration individually, as well as to all three features combined, were recorded for words and pseudowords presented to 14 native speakers of Turkish. The word and pseudoword contrast was used to differentiate language-related effects from acoustic-change effects on the neural responses. First and in line with previous findings, the overall MMN was maximal over frontal and central scalp locations. Second, changes in prosodic features elicited neural responses both in words and pseudowords, confirming the brain’s automatic response to any change in auditory input. However, there were processing differences between the prosodic features, most significantly in f0: While f0 manipulation elicited a slightly right-lateralized frontally-maximal MMN in words, it elicited a frontal P3a in pseudowords. Considering that P3a is associated with involuntary allocation of attention to salient changes, the manipulations of f0 in the absence of lexical processing lead to an intentional evaluation of pitch change. f0 is therefore claimed to be lexically specified in Turkish. Rather than combined features, individual prosodic features differentiate language-related effects from acoustic-change effects. The present study confirms that segmentally identical words can be distinguished on the basis of prosodic information alone, and establishes the salience of f0 in lexical access.
2015. Hatice Zora, Iris-Corinna Schwarz, Mattias Heldner. NeuroReport 26 (13), 791-796
Neural correlates of lexical stress were studied using the mismatch negativity (MMN) component in event-related potentials. The MMN responses were expected to reveal the encoding of stress information into long-term memory and the contributions of prosodic features such as fundamental frequency (F0) and intensity toward lexical access. In a passive oddball paradigm, neural responses to changes in F0, intensity, and in both features together were recorded for words and pseudowords. The findings showed significant differences not only between words and pseudowords but also between prosodic features. Early processing of prosodic information in words was indexed by an intensity-related MMN and an F0-related P200. These effects were stable at right-anterior and mid-anterior regions. At a later latency, MMN responses were recorded for both words and pseudowords at the mid-anterior and posterior regions. The P200 effect observed for F0 at the early latency for words developed into an MMN response. Intensity elicited smaller MMN for pseudowords than for words. Moreover, a larger brain area was recruited for the processing of words than for the processing of pseudowords. These findings suggest earlier and higher sensitivity to prosodic changes in words than in pseudowords, reflecting a language-related process. The present study, therefore, not only establishes neural correlates of lexical stress but also confirms the presence of long-term memory traces for prosodic information in the brain.