Pascal Halder

Coarse neural tuning for print peaks when children learn to read

Adult readers exhibit increased fast N1 activity to wordlike strings in their event-related brain potential. This increase has been linked to visual expertise for print, implying a protracted monotonic development. We investigated the development of coarse neural tuning for print by studying children longitudinally before and after learning to read, and comparing them to skilled adults. The coarse N1 tuning, which had been absent in nonreading kindergarten children, emerged in less than 2 years after the same children had mastered basic reading skills in 2nd grade. The N1 became larger for words than symbol strings in every child, and this coarse tuning was stronger for faster readers. Fast brain processes thus specialize rapidly for print when children learn to read, and play an important functional role in the fluency of early reading. Comparing 2nd graders with adults revealed a further decrease of the coarse N1 tuning in adults, presumably reflecting further reading practice. This constitutes a prominent nonlinear development of coarse neurophysiological specialization for print. The maximum tuning in novice readers possibly reflects the high sensitivity of their neural network for visual aspects of print, and a more selective tuning in expert adult readers.

Evidence for developmental changes in the visual word processing network beyond adolescence

Late development of specialization in the visual word processing system was examined using event-related potentials (ERP) and functional magnetic resonance imaging (fMRI) of word and symbol string processing in groups of adolescents (15.2-17.3 years) and adults (19.8-30.8 years). We focused our ERP analyses on fast visual activity: the occipital P1 (82-131 ms) modulated by physical stimulus characteristics and the occipito-temporal N1 (132-256 ms) reflecting visual tuning for print. Our fMRI analyses concentrated on basal occipito-temporal activations in the visual word form area VWFA. For words, the correlation of fMRI activation in the VWFA and N1 amplitude confirmed the close relationship of the electrophysiological N1 with metabolic activity in the VWFA. Further support for this relationship came from low resolution electromagnetic tomography localizing the word-specific N1 near the VWFA. Both imaging techniques revealed age-independent differences between words and symbol strings. Late development, however, was preferentially detected with ERPs. Decreases of P1 and N1 amplitudes with age were not limited to words and suggested further maturation of the underlying brain microstructure and function. Following adolescence, decreasing N1 latencies specific to words point to continued specialization of the visual word processing system. Both N1 and fMRI measures correlated with reading performance. In summary, the similarity of global fMRI activation patterns between groups suggests a fully established distribution of the reading network in adolescence, while the decreasing N1 latencies for words indicate protracted fine tuning after adolescence.

Synchronization facilitates removal of MRI artefacts from concurrent EEG recordings and increases usable bandwidth

Investigating human brain function non-invasively by simultaneous EEG and fMRI measurements is gaining in popularity as more and better solutions to the inherent technical challenges emerge. We demonstrate the use of a commercially available frequency divider and phase-locking device for the purpose of synchronizing an MRI acquisition with a simultaneous recording of the EEG. Synchronization hugely improves the effectiveness of MRI artefact removal from the EEG signal by the common mean template subtraction method. It complements or substitutes post-processing techniques like filtering, thereby increasing the usable bandwidth of the EEG signal to about 150 Hz. This is important for covering the full range of human Gamma band activity. Similarly, synchronization eliminates the necessity for over-sampling of the EEG signal.

Tuning of the visual word processing system: distinct developmental ERP and fMRI effects.

Visual tuning for words vs. symbol strings yields complementary increases of fast occipito‐temporal activity (N1 or N170) in the event‐related potential (ERP), and posterior–anterior gradients of increasing word‐specific activity with functional magnetic resonance imaging (fMRI) in the visual word form system (VWFS). However, correlation of these coarse ERP and fMRI tuning responses seems limited to the most anterior part of the VWFS in adult and adolescent readers (Brem et al. [ 2006 ]: Neuroimage 29:822–837). We thus focused on fMRI tuning gradients of young readers with their more pronounced ERP print tuning, and compared developmental aspects of ERP and fMRI response tuning in the VWFS. Children (10.3 y, n = 19), adolescents (16.2 y, n = 13) and adults (25.2 y, n = 18) were tested with the same implicit reading paradigm using counterbalanced ERP and fMRI imaging. The word‐specific occipito‐temporal N1 specialization, its corresponding source activity, as well as the integrated source activity (0–700 ms) were most prominent in children and showed a marked decrease with age. The posterior–anterior fMRI gradient of word‐specific activity instead which was fully established in children did not develop further, but exhibited a dependence on reading skills independent of age. To conclude, prominent developmental dissociation of the ERP and fMRI tuning patterns emerged despite convergent VWFS localization. The ERP response may selectively reflect fast visual aspects of print specialization, which become less important with age, while the fMRI response seems dominated by integrated task‐ and reading‐related activations in the same regions. Hum Brain Mapp, 2009.

Neurophysiological signs of rapidly emerging visual expertise for symbol strings

The current study examined whether the repeated visual presentation of novel, meaningless symbol strings triggers rapid changes in event related potentials (ERP). Adult participants performed three versions of a word and symbol string repetition detection task in the same experimental session. Analyses focussed on the occipito-temporal N1 (∼150 ms) known to reflect early word-specific processing and stimulus categorisation. While the N1 to words did not change, the occipito-temporal negativity to symbol strings increased over runs and converged with the word N1. Later (∼220 ms) more positive occipito-temporal amplitudes to repeated words in the third compared to the first run implied a repetition priming effect. This suggests that symbol string processing changed over time due to visual learning and increased perceptual expertise.

Neuropathic pain in spinal cord injury: significance of clinical and electrophysiological measures

A large percentage of spinal cord‐injured subjects suffer from neuropathic pain below the level of the lesion (bNP). The neural mechanisms underlying this condition are not clear. The aim of this study was to elucidate the general effects of spinal deafferentiation and of bNP on electroencephalographic (EEG) activity. In addition, the relationship between the presence of bNP and impaired function of the spinothalamic tract was studied. Measurements were performed in complete and incomplete spinal cord‐injured subjects with and without bNP as well as in a healthy control group. Spinothalamic tract function, assessed by contact heat evoked potentials, did not differ between subjects with and without bNP; nevertheless, it was impaired in 94% of subjects suffering from bNP. In the EEG recordings, the degree of deafferentiation was reflected in a slowing of EEG peak frequency in the 6–12‐Hz band. Taking into account this unspecific effect, spinal cord‐injured subjects with bNP showed significantly slower EEG activity than subjects without bNP. A discrimination analysis in the subjects with spinothalamic tract dysfunction correctly classified 84% of subjects as belonging to either the group with bNP or the group without bNP, according to their EEG peak frequency. These findings could be helpful for both the development of an objective diagnosis of bNP and for testing the effectiveness of new therapeutic agents.

Electrophysiological and hemodynamic evidence for late maturation of hand power grip and force control under visual feedback

Several human imaging studies have described the neural network involved in power grip under visual control and the subset of cortical areas within this network that are sensitive to force modulation. As there is behavioral evidence for late maturation in even simple hand motor tasks involving visual feedback, we aimed at identifying the neural correlates of these developmental changes. Subjects from three developmental age groups (9–11, 15–17, and adults) performed the same power grip task in both a functional magnetic resonance imaging and an event‐related potential (ERP) session. Trials started with a visual target indicating whether to squeeze at 20%, 40%, or 75% of their maximum and online visual feedback on the actual amount of force was provided. Longer reaction times and more shallow slopes of the force curve characterized the behavior of the younger age groups, especially the children. Both neurophysiological methods detected both general as well as force modulation‐specific maturational changes. General development was characterized by decreasing ERP amplitudes and increasing deactivation of an extended network, closely resembling the so‐called “default” network. The most pronounced developmental changes specific for force control were observed in an ERP component and brain regions involved in feedback processing. In contrast to adult subjects, we found evidence for a stronger dependency on visual feedback information in the younger age groups. Our results also suggest that the ability to deactivate task‐irrelevant networks might be a late developmental achievement. Hum Brain Mapp, 2007.

Electrophysiological evidence for cortical plasticity with movement repetition

The role of movement repetition and practice has been extensively studied as an aspect of motor skill learning but has rarely been investigated in its own right. As practice is considered a prerequisite for motor learning we expected that even the repetitive execution of a simple movement would rapidly induce changes in neural activations without changing performance. We used 64‐channel event‐related potential mapping to investigate these effects of movement repetition on corresponding brain activity in humans. Ten healthy right‐handed young adults performed a power grip task under visual force control to ensure constant behaviour during the experimental session. The session consisted of two parts intersected by a break. For analysis each part was subdivided into two runs to control for potential attention or fatigue effects, which would be expected to disappear during the break. Microstate analysis revealed that distinct topographies and source configurations during movement preparation, movement execution and feedback integration are responsive to repetition. The observed patterns of changes differed for the three microstates, suggesting that different, repetition‐sensitive neural mechanisms are involved. Moreover, this study clearly confirms that movement repetition, in the absence of skill learning, is capable of inducing changes in neural networks.

Maturation of luminance- and motion-defined form perception beyond adolescence: A combined ERP and fMRI study

Abilities to discriminate forms defined by motion continue to develop throughout childhood. To investigate late development of the visual motion system, we measured brain activity with event-related EEG potentials (ERPs) and functional magnetic resonance imaging (fMRI) in groups of adolescents (15-17 years) and adults (20-30 years) during a visual form discrimination task--with forms being either defined by motion or luminance contrast. We further explored whether possible developmental changes varied with the degree of motion coherence reflecting maturation specific to global motion processing. Both the fMRI activation patterns and ERP topographies were very similar between adolescents and adults, suggesting that the basic visual networks for processing motion and form are established by the age of 15-17. The ERP response to luminance- and motion-defined forms was dominated by a posterior negativity (N1: 120-270 ms). The N1 of the motion contrast was delayed in adolescents, whereas the N1 of the static condition did not differ between groups. Since the motion-evoked N1 is thought to arise in the middle temporal area MT/V5, our results indicate that visual motion processing in MT continues to get faster, becoming still more efficient during late development. Neither the ERP nor the fMRI results revealed maturation effects specific to motion coherence. This indicates that the specific mechanisms to process global dot motion are already mature in adolescence. The present findings support the view that static perception matures earlier than dynamic perception, and that these visual systems have different developmental courses.

Enhanced recovery of human spinothalamic function is associated with central neuropathic pain after SCI

Spinothalamic tract (STT) dysfunction seems to be crucially involved in the development of central neuropathic pain (NP) after spinal cord injury (SCI). However, previous attempts to identify differences in the extent or location of STT damage between subjects with and without NP failed. Here we show that the spontaneous recovery of human STT function (within the first year after SCI) in subjects suffering NP is enhanced compared to those not affected. Furthermore, the correlation between current pain intensity (assessed on average 5 years after SCI) and extent of functional recovery substantiates the close relationship between recovery of STT function and the occurrence of NP. These findings contribute to a better understanding of mechanisms involved in the generation of NP after SCI.