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- Neural dynamics of neglected targets in patients with right hemisphere damage. [JOURNAL ARTICLE]
- Cortex 2013 Apr 9.
We studied the neural correlates of target omissions in five patients with right hemisphere damage and varying signs of left spatial neglect. Benefiting from the high temporal resolution of magneto-encephalography, we directly compared brain regional synchrony events of detected and omitted left-sided targets. Results showed that before stimulus presentation, a low beta synchronization activity was specifically increased within left frontal areas before pathological response omissions of left-sided targets. In the same pre-stimulus period, there were no such beta oscillations when patients correctly detected the target, or when no target was presented. Our findings emphasize the importance of neural activity during the pre-stimulus period on subsequent stimulus processing, and highlight the consequences of episodic interruptions of large-scale interhemispheric networks on target detection. Furthermore, our data suggest that prefrontal activity is not necessarily beneficial to target detection, but can be detrimental to it.
- Sight and sound out of synch: Fragmentation and renormalisation of audiovisual integration and subjective timing. [JOURNAL ARTICLE]
- Cortex 2013 Apr 1.
The sight and sound of a person speaking or a ball bouncing may seem simultaneous, but their corresponding neural signals are spread out over time as they arrive at different multisensory brain sites. How subjective timing relates to such neural timing remains a fundamental neuroscientific and philosophical puzzle. A dominant assumption is that temporal coherence is achieved by sensory resynchronisation or recalibration across asynchronous brain events. This assumption is easily confirmed by estimating subjective audiovisual timing for groups of subjects, which is on average similar across different measures and stimuli, and approximately veridical. But few studies have examined normal and pathological individual differences in such measures. Case PH, with lesions in pons and basal ganglia, hears people speak before seeing their lips move. Temporal order judgements (TOJs) confirmed this: voices had to lag lip-movements (by ∼200 msec) to seem synchronous to PH. Curiously, voices had to lead lips (also by ∼200 msec) to maximise the McGurk illusion (a measure of audiovisual speech integration). On average across these measures, PH's timing was therefore still veridical. Age-matched control participants showed similar discrepancies. Indeed, normal individual differences in TOJ and McGurk timing correlated negatively: subjects needing an auditory lag for subjective simultaneity needed an auditory lead for maximal McGurk, and vice versa. This generalised to the Stream-Bounce illusion. Such surprising antagonism seems opposed to good sensory resynchronisation, yet average timing across tasks was still near-veridical. Our findings reveal remarkable disunity of audiovisual timing within and between subjects. To explain this we propose that the timing of audiovisual signals within different brain mechanisms is perceived relative to the average timing across mechanisms. Such renormalisation fully explains the curious antagonistic relationship between disparate timing estimates in PH and healthy participants, and how they can still perceive the timing of external events correctly, on average.
- Dissociable fronto-striatal effects of dopamine D2 receptor stimulation on cognitive versus motor flexibility. [JOURNAL ARTICLE]
- Cortex 2013 Apr 11.
Genetic and pharmacological studies suggest an important role of the dopamine D2 receptor (DRD2) in flexible behavioral adaptation, mostly shown in reward-based learning paradigms. Recent evidence from imaging genetics indicates that also intentional cognitive flexibility, associated with lateral frontal cortex, is affected by variations in DRD2 signaling. In the present functional magnetic resonance imaging (MRI) study, we tested the effects of a direct pharmacological manipulation of DRD2 stimulation on intentional flexibility in a task-switching context, requiring switches between cognitive task rules and between response hands. In a double blind, counterbalanced design, participants received either a low dose of the DRD2 agonist bromocriptine or a placebo in two separate sessions. Bromocriptine modulated the blood-oxygen-level-dependent (BOLD) signal during rule switching: rule-switching-related activity in the left posterior lateral frontal cortex and in the striatum was increased compared to placebo, at comparable performance levels. Fronto-striatal connectivity under bromocriptine was slightly increased for rule switches compared to rule repetitions. Hand-switching-related activity, in contrast, was reduced under bromocriptine in sensorimotor regions. Our results provide converging evidence for an involvement of DRD2 signaling in fronto-striatal mechanisms underlying intentional flexibility, and indicate that the neural mechanisms underlying different types of flexibility (cognitive vs motor) are affected differently by increased dopaminergic stimulation.
- Schemas reveal spatial relations to a patient with simultanagnosia. [JOURNAL ARTICLE]
- Cortex 2013 Mar 27.
Maps, graphs, and diagrams use simplified graphic forms, like lines and blobs, to represent basic spatial relations, like boundaries and enclosures. A schema is an iconic representation where perceptual detail has been abstracted away from reality in order to provide a more flexible structure for cognition. Unlike truly symbolic representations of spatial relations (i.e., prepositions) a schema preserves some analog spatial qualities of the relation it stands in for. We tested the efficacy of schemas in facilitating the perception and comprehension of spatial relations in a patient with bilateral occipitoparietal damage and resulting simultanagnosia. Patient E.E. performed six matching tasks involving WORDS (in, on, above, below), photographic PICTURES of objects, and/or SCHEMAS depicting the same spatial relations. E.E. was instructed to match a single spatial relation to a corresponding image from an array of four choices. On the two tasks that did not include matching to or from schemas, E.E. performed at chance levels. On tasks with schemas, performance was significantly better, indicating that schematic representations make spatial relations visible in a manner that symbols and complex images do not. The results provide general insight as to how schemas facilitate spatial reasoning when used in graphic depictions, and how such theoretically intermediate representational structures could serve to link perceptual and verbal representations of spatial relations in the brain.
- Right-sided representational neglect after left brain damage in a case without visuospatial working memory deficits. [JOURNAL ARTICLE]
- Cortex 2013 Mar 27.
Brain damaged patients suffering from representational neglect (RN) fail to report, orient to, or verbally describe contra-lesional elements of imagined environments or objects. So far this disorder has only been reported after right brain damage, leading to the idea that only the right hemisphere is involved in this deficit. A widely accepted account attributes RN to a lateralized impairment in the visuospatial component of working memory. So far, however, this hypothesis has not been tested in detail. In the present paper, we describe, for the first time, the case of a left brain damaged patient suffering from right-sided RN while imagining both known and new environments and objects. An in-depth evaluation of her visuospatial working memory abilities, with special focus on the presence of a lateralized deficit, did not reveal any abnormality. In sharp contrast, her ability to memorize visual information was severely compromised. The implications of these results are discussed in the light of recent insights in the neglect syndrome.
- Hemispheric dissociation of reward processing in humans: Insights from deep brain stimulation. [JOURNAL ARTICLE]
- Cortex 2013 Mar 29.
Rewards have various effects on human behavior and multiple representations in the human brain. Behaviorally, rewards notably enhance response vigor in incentive motivation paradigms and bias subsequent choices in instrumental learning paradigms. Neurally, rewards affect activity in different fronto-striatal regions attached to different motor effectors, for instance in left and right hemispheres for the two hands. Here we address the question of whether manipulating reward-related brain activity has local or general effects, with respect to behavioral paradigms and motor effectors. Neuronal activity was manipulated in a single hemisphere using unilateral deep brain stimulation (DBS) in patients with Parkinson's disease. Results suggest that DBS amplifies the representation of reward magnitude within the targeted hemisphere, so as to affect the behavior of the contralateral hand specifically. These unilateral DBS effects on behavior include both boosting incentive motivation and biasing instrumental choices. Furthermore, using computational modeling we show that DBS effects on incentive motivation can predict DBS effects on instrumental learning (or vice versa). Thus, we demonstrate the feasibility of causally manipulating reward-related neuronal activity in humans, in a manner that is specific to a class of motor effectors but that generalizes to different computational processes. As these findings proved independent from therapeutic effects on parkinsonian motor symptoms, they might provide insight into DBS impact on non-motor disorders, such as apathy or hypomania.
- Increased cortical surface area of the left planum temporale in musicians facilitates the categorization of phonetic and temporal speech sounds. [JOURNAL ARTICLE]
- Cortex 2013 Apr 1.
We measured musicians and non-musicians by using structural magnetic resonance imaging to investigate relationships between cortical features of the left planum temporale (PT) and the categorization of consonant-vowel (CV) syllables and their reduced-spectrum analogues. The present work is based on previous functional studies consistently showing that the left PT is particularly responsive to transient acoustic features in CV syllables and their reduced-spectrum analogues, and on striking evidence pointing to structural alterations of the left PT as a function of musicianship. By combining these two observations, we hypothesized to find that differences in cortical surface area (SA) and cortical thickness (CT) of the left PT in musicians may facilitate the categorization of fast-changing phonetic cues. Behavioural results indicated that musicians and non-musicians achieved a comparable performance in the categorization of CV syllables, whereas the musicians performed significantly better than the controls in the more demanding reduced-spectrum condition. This better behavioural performance corresponds to an increased cortical SA of the left PT in musicians compared to non-musicians. No differences in CT of the left PT were found between groups. In line with our predictions, we revealed a positive correlation between cortical SA of the left PT in musicians and the behavioural performance during the acoustically more demanding reduced-spectrum condition. Hence, we provide first evidence for a relationship between musical expertise, cortical SA of the left PT, and the processing of fast-changing phonetic cues.
- Aphasia rehabilitation: Does generalisation from anomia therapy occur and is it predictable? A case series study. [JOURNAL ARTICLE]
- Cortex 2013 Feb 4.