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Biomedical subjects

O Hauk

Publications and source records attributed to O Hauk.

11 recordsLinked to original sources
PubMed2006

The time course of visual word recognition as revealed by linear regression analysis of ERP data.

EEG correlates of a range of psycholinguistic word properties were used to investigate the time course of access to psycholinguistic information during visual word recognition. Neurophysiological responses recorded in a visual lexical decision task were submitted to linear regression analysis. First, 10 psycholinguistic features of each of 300 stimulus words were submitted to a principal component analysis, which yielded four orthogonal variables likely to reflect separable processes in visual word recognition: Word length, Letter n-gram frequency, Lexical frequency and Semantic coherence of a word's morphological family. Since the lexical decision task required subjects to distinguish between words and pseudowords, the binary variable Lexicality was also investigated using a factorial design. Word-pseudoword differences in the event-related potential first appeared at 160 ms after word onset. However, regression analysis of EEG data documented a much earlier effect of both Word length and Letter n-gram frequency around 90 ms. Lexical frequency showed its earliest effect slightly later, at 110 ms, and Semantic coherence significantly correlated with neurophysiological measures around 160 ms, simultaneously with the lexicality effect. Source estimates indicated parieto-temporo-occipital generators for the factors Length, Letter n-gram frequency and Word frequency, but widespread activation with foci in left anterior temporal lobe and inferior frontal cortex related to Semantic coherence. At later stages (>200 ms), all variables exhibited simultaneous EEG correlates. These results indicate that information about surface form and meaning of a lexical item is first accessed at different times in different brain systems and then processed simultaneously, thus supporting cascaded interactive processing models.

Adult↗
PubMed2006

The sound of actions as reflected by mismatch negativity: rapid activation of cortical sensory-motor networks by sounds associated with finger and tongue movements.

In order to explore the activation dynamics of the human action recognition system, we investigated electrophysiological distinctions between the brain responses to sounds produced by human finger and tongue movements. Of special interest were the questions of how early these differences may occur, and whether the neural activation at the early stages of processing involves cortical motor representations related to the generation of these sounds. For this purpose we employed a high-density EEG set-up and recorded mismatch negativity (MMN) using a recently developed novel multideviant paradigm which allows acquisition of a high number of trials within a given time period. Deviant stimuli were naturally recorded finger and tongue clicks, as well as control stimuli with similar physical features but without the clear action associations (this was tested in a separate behavioural experiment). Both natural stimuli produced larger MMNs than their respective control stimuli at approximately 100 ms, indicating activation of memory traces for familiar action-related sounds. Furthermore, MMN topography at this latency differed between the brain responses to the natural finger and natural tongue sounds. Source estimation revealed the strongest sources for finger sounds in centrolateral areas of the left hemisphere, suggesting that hearing a sound related to finger actions evokes activity in motor areas associated with the dominant hand. Furthermore, tongue sounds produced activation in more inferior brain areas. Our data suggest that motor areas in the human brain are part of neural systems subserving the early automatic recognition of action-related sounds.

Acoustic Stimulation↗
PubMed2006

[Q:] When would you prefer a SOSSAGE to a SAUSAGE? [A:] At about 100 msec. ERP correlates of orthographic typicality and lexicality in written word recognition.

Using a speeded lexical decision task, event-related potentials (ERPs), and minimum norm current source estimates, we investigated early spatiotemporal aspects of cortical activation elicited by words and pseudo-words that varied in their orthographic typicality, that is, in the frequency of their component letter pairs (bi-grams) and triplets (tri-grams). At around 100 msec after stimulus onset, the ERP pattern revealed a significant typicality effect, where words and pseudo-words with atypical orthography (e.g., yacht, cacht) elicited stronger brain activation than items characterized by typical spelling patterns (cart, yart). At approximately 200 msec, the ERP pattern revealed a significant lexicality effect, with pseudo-words eliciting stronger brain activity than words. The two main factors interacted significantly at around 160 msec, where words showed a typicality effect but pseudo-words did not. The principal cortical sources of the effects of both typicality and lexicality were localized in the inferior temporal cortex. Around 160 msec, atypical words elicited the stronger source currents in the left anterior inferior temporal cortex, whereas the left perisylvian cortex was the site of greater activation to typical words. Our data support distinct but interactive processing stages in word recognition, with surface features of the stimulus being processed before the word as a meaningful lexical entry. The interaction of typicality and lexicality can be explained by integration of information from the early form-based system and lexicosemantic processes.

Adolescent↗
PubMed2004

Effects of word length and frequency on the human event-related potential.

OBJECTIVE: We investigated the influence of the length and frequency of printed words on the amplitude and peak latencies of event-related potentials (ERPs). This served two goals, namely (I) to clarify their possible effects as confounds in ERP experiments employing word-stimuli, and (II) to determine the point in time of lexical access in visual word recognition. METHODS: EEG was recorded from 64 scalp sites while subjects (n=12) performed a lexical decision task. Word length and frequency were orthogonally varied between stimulus groups, whereas variables including regularity of spelling and orthographic tri-gram frequency were kept constant. RESULTS: Long words produced the strongest brain response early on (approximately 100 ms after stimulus onset), whereas those to short words became strongest later (150-360 ms). Lower ERP amplitudes were elicited by words with high frequency compared with low frequency words in the latency ranges 150-190 ms and 320-360 ms. However, we did not find evidence for a robust alteration of peak latencies with word frequency. CONCLUSIONS: Length and frequency of word stimuli have independent and additive effects on the amplitude of the ERP. Studies on the precise time course of cognitive processes should consider their potentially confounding character. Our data support the view that lexical access takes place as early as 150 ms after onset of written word stimuli.

Adult↗
PubMed2002

Comparison of data transformation procedures to enhance topographical accuracy in time-series analysis of the human EEG.

We describe a methodology to apply current source density (CSD) and minimum norm (MN) estimation as pre-processing tools for time-series analysis of single trial EEG data. The performance of these methods is compared for the case of wavelet time-frequency analysis of simulated gamma-band activity. A reasonable comparison of CSD and MN on the single trial level requires regularization such that the corresponding transformed data sets have similar signal-to-noise ratios (SNRs). For region-of-interest approaches, it should be possible to optimize the SNR for single estimates rather than for the whole distributed solution. An effective implementation of the MN method is described. Simulated data sets were created by modulating the strengths of a radial and a tangential test dipole with wavelets in the frequency range of the gamma band, superimposed with simulated spatially uncorrelated noise. The MN and CSD transformed data sets as well as the average reference (AR) representation were subjected to wavelet frequency-domain analysis, and power spectra were mapped for relevant frequency bands. For both CSD and MN, the influence of noise can be sufficiently suppressed by regularization to yield meaningful information, but only MN represents both radial and tangential dipole sources appropriately as single peaks. Therefore, when relating wavelet power spectrum topographies to their neuronal generators, MN should be preferred.

Artifacts↗
PubMed2001

Grapheme monitoring in picture naming: an electrophysiological study of language production.

Electrocortical correlates of language production were examined in two picture naming tasks that involved grapheme monitoring. In both tasks subjects (N=12) had to detect target letters in picture names, the target letter being positioned either at the beginning or at the end of the picture name. Between tasks, the target letter was shown either before (target-picture, TP) or after (picture-target, PT) the presentation of the object pictures. In both tasks, subjects responded faster, whenever the target letter appeared at the beginning of the picture name than at its end. The EEG, recorded from 64 electrodes, was analyzed in the signal and in the source space (using the Minimum Norm estimate). Differences in the event-related potential (ERP) following the second stimulus became evident earlier in the PT (at 320 ms) than the TP (456 ms) task. This onset of diverging ERPs was called the "point of divergence" (POD). The ERP following the POD was characterized by a positive deflection in the "begin" condition in both tasks. In the "end" condition, the sources of brain activity were focused over the left hemisphere in the TP, while a bilateral distribution characterized the PT task. Performance and electrocortical indices support the hypothesis of serial "left-to-right" processing of a representation of the picture name. The left-hemispheric activity focus in the TP task is assumed to indicate the encoding of the picture name, while frontal symmetrical activity in the PT task might indicate the involvement of working memory processes.

Adult↗
PubMed2000

Linear inverse filtering improves spatial separation of nonlinear brain dynamics: a simulation study.

We examined topographic variations in nonlinear measures based on scalp voltages, which were generated by two simulated current dipoles each placed in a different hemisphere of a spherical volume conductor (three-shell model). Dipole dynamics were that of a three-torus and the x-component of the Lorenz-system and scalp voltage were calculated for a configuration of 29 electrode positions. Although estimates for correlation dimension D2 and Lyapunov exponent L1 were close to the theoretical values for the original time series, the simulated scalp voltage data showed almost no topographic resolution of dipole positions. In order to enhance topographic differentiation, we constructed linear inverse filters, to focus on brain activity from a specified brain region. It turned out that the nonlinear measures for the inversely filtered time series were much closer to the expected values (with respect to the location of the dipoles used in the simulation) than when using unfiltered data. Our preliminary results indicate that inverse filtering can improve the topographic resolution of nonlinear scalp EEG estimates.

Brain↗
PubMed2000

Electroencephalographic activity over temporal brain areas during phonological encoding in picture naming.

OBJECTIVES: The present study examined electroencephalographic (EEG) correlates of phonological encoding during picture naming with special emphasis on hemispheric asymmetries of these EEG correlates. We also examined whether a small set of stimuli was sufficient to study the phonological encoding, and to what extent the complexity of the produced message affects the EEG responses. METHODS: Event-related electrical brain activity during the covert and overt production of names and nominal phrases derived from 16 variants from 4 different pictures was compared with that during passive viewing of the same pictures. RESULTS: Topographical and source analyses of the differential EEG activity (naming versus passive viewing) indicated that the N1 and P2 components of the visual evoked potential resulted from the same brain areas, but were activated stronger during naming as compared to passive viewing. In contrast, the differential EEG activity from 275 to 400 ms suggested the involvement of additional brain areas during naming with more pronounced left- than right-hemispheric activation in middle and posterior temporal regions for both overt and covert naming. CONCLUSIONS: Results suggest the involvement of Wernicke's area in the phonological encoding of a message during speech production, which can even be obtained with a small set of pictures and a large number of repetitions.

Adult↗
PubMed1999

Determining working memory from ERP topography.

Event-related potentials were recorded during a delayed matching-to-sample design from 17 volunteers (5 f) using high-resolution (65 channels) EEG-recordings. In the two-stimulus paradigm, the 500-ms stimulus S1 comprised a visual pattern of two diamonds differing in size, angular rotation and location; in the delay period, Working Memory (WM) load was varied in the following way: a stimulus-free interval of 1 s was followed by a 6-s presentation either of a pattern identical to the S1 (low WM load) or of a pattern differing from S1 (high WM load). The 500-ms stimulus S2 comprised one diamond; the subject's task was to indicate by left- or right-hand (respectively) button press, whether the S2 matched the (a) left- or (b) right-positioned S1-diamond, or (c) did not match at all (NoGo). The topographical distribution of activity in the time intervals (a) following S1-offset, (b) during the WM manipulation interval and (c) prior to S2 were evaluated in the signal (scalp potential) and source (Minimum Norm) space. Following S1-offset the ERP pattern was characterised by negativity over posterior areas, slightly more so over the right hemisphere. In the subsequent 6-s interval high WM load elicited a larger negative slow ERP than low WM load, the negativity increase due to high WM load being larger over frontal than central areas. Source modelling indicated activity in anterior areas under high, and posterior activity under low WM load. Asymmetry of activity, although indicating a shift to left-hemispheric activity under high compared to low WM load, varied considerably between subjects. Results suggest that high-resolution ERP recordings allow to examine cortical activity during WM challenge.

Adolescent↗
PubMed1998

Monitoring brain activity of human subjects during delayed matching to sample tasks comparing verbal and pictorial stimuli with modal and cross-modal presentation: an event related potential study employing a source reconstruction method.

The time course of the event related potentials evoked within a delayed matching to sample task employing verbal and pictorial stimuli was analyzed with a source reconstruction method (minimum norm method). During signal stimulus presentation pictorial stimuli evoked more activity than verbal stimuli. Activity was particularly prominent in left frontal areas for the match of verbal-verbal stimulus pairs and over right posterior regions for the match of verbal-pictorial stimuli. Anticipation of the to-be-matched stimulus produced more pronounced activity for pictorial stimuli and generally stronger left and frontal activity. Results are discussed referring to a biological model of language processing.

Adult↗