Comprehending auditory speech:previous and potential contributions of functional MRI

Functional neuroimaging revolutionised the study of human language in the late twentieth century, allowing researchers to investigate its underlying cognitive processes in the intact brain. Here, we review how functional MRI (fMRI) in particular has contributed to our understanding of speech comprehension, with a focus on studies of intelligibility. We highlight the use of carefully controlled acoustic stimuli to reveal the underlying hierarchical organisation of speech processing systems and cortical (a)symmetries, and discuss the contributions of novel design and analysis techniques to the contextualisation of perisylvian regions within wider speech processing networks. Within this, we outline the methodological challenges of fMRI as a technique for investigating speech and describe the innovations that have overcome or mitigated these difficulties. Focussing on multivariate approaches to fMRI, we highlight how these techniques have allowed both local neural representations and broader scale brain systems to be described

Using fMRI and Behavioural Measures to Investigate Rehabilitation in Post-Stroke Aphasic Deficits

In this thesis I investigated whether an intensive computerised, home-based therapy programme could improve phonological discrimination ability in 19 patients with chronic post-stroke aphasia. One skill specifically targeted by the treatment demonstrated an improvement due to the therapy. However, this improvement did not generalise to untreated items, and was only effective for participants without a lesion involving the frontal lobe, indicating a potentially important role for this region in determining outcome of aphasia therapy. Complementary functional imaging studies investigated activity in domain-general and domain-specific networks in both patients and healthy volunteers during listening and repeating simple sentences. One important consideration when comparing a patient group with a healthy population is the difference in task difficulty encountered by the two groups. Increased cognitive effort can be expected to increase activity in domain-general networks. I minimised the effect of this confound by manipulating task difficulty for the healthy volunteers to reduce their behavioural performance so that it was comparable to that of the patients. By this means I demonstrated that the activation patterns in domain-general regions were very similar in the two groups. Region-of-interest analysis demonstrated that activity within a domain-general network, the salience network, predicted residual language function in the patients with aphasia, even after accounting for lesion volume and their chronological age. I drew two broad conclusions from these studies. First, that computer-based rehabilitation can improve disordered phonological discrimination in chronic aphasia, but that lesion distribution may influence the response to this training. Second, that the ability to activate domain-general cognitive control regions influences outcome in aphasia. This allows me to propose that in future work, therapeutic strategies, pharmacological or behavioural, targeting domain-general brain systems, may benefit aphasic stroke rehabilitation.Open Acces

Categorical representations of phonemic vowels investigated with fMRI

The present thesis investigates the sensitivity of the human auditory cortex (AC) to the contrast between prototype and nonprototype vowels as well as between phonemic and nonphonemic vowels. Activations to vowels were measured with functional magnetic resonance imaging (fMRI), which was also used to analyze the effect of categorical processing on modulations in AC and adjacent inferior parietal lobule (IPL) observed during active listening tasks. A prominent theoretical view suggests that native phonemic vowels (i.e., phonemes) are represented in the human brain as categories organized around a best representative of the category (i.e., phoneme prototype). This view predicts systematic differences in the neural representations and processing of phoneme prototypes, nonprototypes and nonphonemic vowels. In three separate studies, subjects were presented with vowel pairs and visual stimuli during demanding auditory and visual tasks. Study I compared activations to prototypical and nonprototypical vowels, whereas Study II focused on the contrast between phonemic and nonphonemic vowels. Study II also tested whether activations in IPL during a categorical vowel memory task depend on whether the task is performed on phonemic (easy to categorize) or nonphonemic (harder to categorize) vowels. Study III was designed to replicate the key findings of Studies I and II. Further, Study III compared activations to identical vowels presented during a number of different task conditions requiring analysis of the acoustical or categorical differences between the vowels. The results of this thesis are in line with the general theoretical view that phonemic vowels are represented in a categorical manner in the human brain. Studies I–III showed that information about categorical vowel representations is present in human AC during active listening tasks. Areas of IPL, in turn, were implicated in general operations on categorical representations rather than in categorization of speech sounds as such. Further, the present results demonstrate that task-dependent activations in AC and adjacent IPL strongly depend on whether the task requires analysis of the acoustical or categorical features of the vowels. It is important to note that, in the present studies, surprisingly small differences in the characteristics of the vowel stimuli or the tasks performed on these vowels resulted in significant and widespread activation differences in AC and adjacent regions. As the key findings of Studies I and II were also quite successfully replicated in Study III, these results highlight the importance of carefully controlled experiments and replications in fMRI research.Vallitsevan teorian mukaan äidinkielen foneemisten vokaalien (eli foneemien) representaatiot ovat luonteeltaan kategorisia eli äänteet ovat järjestyneet kategorian parhaan esiintymän (eli foneemin prototyypin) ympärille. Teoria ennustaa, että foneemiprototyyppien, nonprototyyppien ja nonfoneemisten vokaalien representaatiot ja niiden käsittely aivoissa eroavat toisistaan. Tässä väitöskirjassa selvitetään toiminnallisen magneettikuvauksen (fMRI) avulla, eroaako ihmisen kuuloaivokuoren ja sen lähialueiden aktivaatio prototyyppisten ja nonprototyyppisten sekä foneemisten ja nonfoneemisten vokaalien käsittelyn aikana ja miten aktiiviset kuuntelutehtävät vaikuttavat näihin eroihin. Väitöskirjan kolmessa osatutkimuksessa koehenkilöille esitettiin vokaalipareja ja visuaalisia ärsykkeitä kuuntelu- ja katselutehtävien aikana. Tutkimuksessa I vokaaliparit koostuivat prototyyppisistä ja nonprototyyppisistä foneemisista vokaaleista. Tutkimuksessa II puolestaan tarkasteltiin foneemisten ja nonfoneemisten vokaalien käsittelyn eroa. Lisäksi tutkimuksessa II selvitettiin, riippuuko kuuloaivokuoren ja sen lähialueiden tehtäväsidonnainen aktivaatio siitä, suoritetaanko kuuntelutehtävää helposti (foneemiset vokaalit) vai vaikeasti (nonfoneemiset vokaalit) kategorisoitavilla vokaaleilla. Viimeisessä tutkimuksessa (III) toistettiin tutkimusten I ja II päätulokset. Tutkimuksessa III selvitettiin myös sitä, miten aktivaatio äänten erottelutehtävän aikana eroaa silloin, kun vokaaleja erotellaan niiden fysikaalisten tai kategoristen ominaisuuksien perusteella. Väitöskirjan tutkimuksissa saadut tulokset tukevat oletetusta siitä, että foneemisten vokaalien representaatiot ovat luonteeltaan kategorisia. Tulokset osoittavat, että tieto vokaalikategorioiden representaatioista on käytettävissä kuuloaivokuorella aktiivisten kuuntelutehtävien aikana. Lisäksi tutkimuksissa havaittiin, että päälakilohkon alaosien aktivaatio voimistui niiden tehtävien aikana, jotka vaativat kategorisen tiedon käsittelyä. Näiden aivoalueiden aktivaatio ei kuitenkaan näytä liittyvän kategorioiden muodostamiseen (puheäänten kategorisointiin) sinänsä. On merkillepantavaa, että tutkimuksissa I–III näennäisen pienet erot vokaaliärsykkeissä ja kuuntelutehtävissä johtivat huomattaviin aktivaatioeroihin kuuloaivokuorella ja sen läheisillä aivoalueilla. Tämä korostaa huolellisesti kontrolloitujen koeasetelmien ja etenkin replikaatiotutkimusten tärkeyttä fMRI -tutkimuksissa

Neural Correlates of Multisensory Enhancement in Audiovisual Narrative Speech Perception: A fMRI investigation

This fMRI study investigated the effect of seeing articulatory movements of a speaker while listening to a nat- uralistic narrative stimulus. It had the goal to identify regions of the language network showing multisensory enhancement under synchronous audiovisual conditions. We expected this enhancement to emerge in regions known to underlie the integration of auditory and visual information such as the posterior superior temporal gyrus as well as parts of the broader language network, including the semantic system. To this end we presented 53 participants with a continuous narration of a story in auditory alone, visual alone, and both synchronous and asynchronous audiovisual speech conditions while recording brain activity using BOLD fMRI. We found multi- sensory enhancement in an extensive network of regions underlying multisensory integration and parts of the semantic network as well as extralinguistic regions not usually associated with multisensory integration, namely the primary visual cortex and the bilateral amygdala. Analysis also revealed involvement of thalamic brain regions along the visual and auditory pathways more commonly associated with early sensory processing. We conclude that under natural listening conditions, multisensory enhancement not only involves sites of multisensory in- tegration but many regions of the wider semantic network and includes regions associated with extralinguistic sensory, perceptual and cognitive processing