Biomedical subjects

K Uutela

Publications and source records attributed to K Uutela.

11 recordsLinked to original sources

PubMed2001

Coinciding early activation of the human primary visual cortex and anteromedial cuneus.

Proper understanding of processes underlying visual perception requires information on the activation order of distinct brain areas. We measured dynamics of cortical signals with magnetoencephalography while human subjects viewed stimuli at four visual quadrants. The signals were analyzed with minimum current estimates at the individual and group level. Activation emerged 55-70 ms after stimulus onset both in the primary posterior visual areas and in the anteromedial part of the cuneus. Other cortical areas were active after this initial dual activation. Comparison of data between species suggests that the anteromedial cuneus either comprises a homologue of the monkey area V6 or is an area unique to humans. Our results show that visual stimuli activate two cortical areas right from the beginning of the cortical response. The anteromedial cuneus has the temporal position needed to interact with the primary visual cortex V1 and thereby to modify information transferred via V1 to extrastriate cortices.

Adult↗

PubMed2000

Audiovisual integration of letters in the human brain.

Letters of the alphabet have auditory (phonemic) and visual (graphemic) qualities. To investigate the neural representations of such audiovisual objects, we recorded neuromagnetic cortical responses to auditorily, visually, and audiovisually presented single letters. The auditory and visual brain activations first converged around 225 ms after stimulus onset and then interacted predominantly in the right temporo-occipito-parietal junction (280345 ms) and the left (380-540 ms) and right (450-535 ms) superior temporal sulci. These multisensory brain areas, playing a role in audiovisual integration of phonemes and graphemes, participate in the neural network supporting the supramodal concept of a "letter." The dynamics of these functions bring new insight into the interplay between sensory and association cortices during object recognition.

Acoustic Stimulation↗

PubMed2000

Foveal attention modulates responses to peripheral stimuli.

When attending to a visual object, peripheral stimuli must be monitored for appropriate redirection of attention and gaze. Earlier work has revealed precentral and posterior parietal activation when attention has been directed to peripheral vision. We wanted to find out whether similar cortical areas are active when stimuli are presented in nonattended regions of the visual field. The timing and distribution of neuromagnetic responses to a peripheral luminance stimulus were studied in human subjects with and without attention to fixation. Cortical current distribution was analyzed with a minimum L1-norm estimate. Attention enhanced responses 100-160 ms after the stimulus onset in the right precentral cortex, close to the known location of the right frontal eye field. In subjects whose right precentral region was not distinctly active before 160 ms, focused attention commonly enhanced right inferior parietal responses between 180 and 240 ms, whereas in the subjects with clear earlier precentral response no parietal enhancement was detected. In control studies both attended and nonattended stimuli in the peripheral visual field evoked the right precentral response, whereas during auditory attention the visual stimuli failed to evoke such response. These results show that during focused visual attention the right precentral cortex is sensitive to stimuli in all parts of the visual field. A rapid response suggests bypassing of elaborate analysis of stimulus features, possibly to encode target location for a saccade or redirection of attention. In addition, load for frontal and parietal nodi of the attentional network seem to vary between individuals.

Adult↗

PubMed1999

Prolonged attentional dwell time in dyslexic adults.

Dyslexic adults have been shown to be slow in processing rapid sequences of stimuli in all sensory modalities. We now demonstrate, by means of an attentional blink task, that the attentional dwell time is prolonged by approximately 30% in dyslexic adults compared with normal readers. Thus a target captures attentional resources for considerably longer time in dyslexics than control subjects. The observed prolongation could significantly contribute to the sluggish temporal processing of dyslexic adults.

Adult↗

PubMed1999

Non-impaired auditory phase locking in dyslexic adults.

Dyslexic adults have profound difficulties in discriminating rapidly presented sound sequences. To test whether these deficits might be caused by impaired neuronal phase locking to the envelopes of the sound stimuli, 20 normal-reading and 13 dyslexic adults discriminated pitches of pure tones at approximately 1 kHz (producing spectral pitch due to place coding in the cochlea) and of approximately 80 Hz amplitude modulations of white noise (producing periodicity pitch based on temporal information only). We proposed that a specific deficit in phase locking would result in a worse ability to discriminate periodicity than spectral pitch. The dyslexics were significantly less accurate than the control subjects in discriminating both spectral and periodicity pitch stimuli but their performance was not disproportionally worse in the periodicity pitch task. Thus it seems that impaired neuronal phase-locking cannot explain the problems dyslexics face in processing of rapid sound sequences.

Acoustic Stimulation↗

PubMed1999

Cortical visuomotor integration during eye pursuit and eye-finger pursuit.

To elucidate cortical mechanisms of visuomotor integration, we recorded whole-scalp neuromagnetic signals from six normal volunteers while they were viewing a black dot moving linearly at the speed of 4 degrees /sec within a virtual rectangle. The dot changed its direction randomly once every 0.3-2 sec. The subject either (1) fixated a cross in the center of the screen (eye fixation task), (2) followed the moving dot with the eyes (eye pursuit task), or (3) followed the dot with both the eyes and the right index finger (eye-finger pursuit task). Prominent magnetic signals, triggered by the changes of the direction of the dot, were seen in all conditions, but they were clearly enhanced by the tasks and were strongest during the eye-finger pursuit task and over the anterior inferior parietal lobule (aIPL). Source modeling indicated activation of aIPL [Brodmann's area (BA) 40], the posterosuperior parietal lobule (SPL; BA 7), the dorsolateral frontal cortex (DLF; BA 6), and the occipital cortex (BA 18/19). The activation first peaked in the occipital areas, then in the aIPL and DLF, and some 50 msec later in the SPL. Our results suggest that all these areas are involved in visuomotor transformation, with aIPL playing a crucial role in this process.

Adult↗

PubMed1999

Visualization of magnetoencephalographic data using minimum current estimates.

The locations of active brain areas can be estimated from the magnetic field the neural current sources produce. In this work we study a visualization method of magnetoencephalographic data that is based on minimum[symbol: see text] (1)-norm estimates. The method can represent several local or distributed sources and does not need explicit a priori information. We evaluated the performance of the method using simulation studies. In a situation resembling typical magnetoencephalographic measurement, the mean estimated source strength exceeded baseline level up to 2 cm from the simulated point-like source. The method can also visualize several sources, activated simultaneously or in a sequence, which we demonstrated by analyzing magnetic responses associated with sensory stimulation and a picture naming task.

Algorithms↗

PubMed1999

Auditory stream segregation in dyslexic adults.

Developmental dyslexia is often associated with problems in phonological processing based on, or accompanied by, deficits in the perception of rapid auditory changes. Thirteen dyslexic adults and 18 control subjects were tested on sequences of alternating tones of high (1000 Hz) and low (400 Hz) pitch, which at short stimulus onset asynchronies (SOAs) led to perceptual separation of the sound sequence into high- and low-pitched streams. The control subjects perceived the tone sequence as connected down to SOAs of 130 ms, with segregation of the streams at shorter SOAs; in dyslexic subjects the segregation occurred already at 210 ms. Auditory stream segregation has previously been shown to impair the detection of phoneme order in segments of speech sounds. The observed aberrant segregation of sound streams in dyslexic subjects might thus contribute to their difficulties in achieving awareness of phonemes or phoneme order and in the acquisition of literacy.

Acoustic Stimulation↗

PubMed1998

Global optimization in the localization of neuromagnetic sources.

The locations of active brain areas can be estimated from the magnetic field produced by the neural current sources. In many cases, the actual current distribution can be modeled with a set of stationary current dipoles with time-varying amplitudes. This work studies global optimization methods that find the minimum of the least-squares error function of the current dipole estimation problem. Three different global optimization methods were investigated: clustering method, simulated annealing, and genetic algorithms. In simulation studies, the genetic algorithm was the most effective method. The methods were also applied to analysis of actual measurement data.

Algorithms↗

PubMed1997

Apparent asynchrony between interictal electric and magnetic spikes.

We recorded simultaneous multichannel electroencephalogram (EEG) and magnetoencephalogram (MEG) in four children with partial epilepsy. Sources of averaged spikes were modelled with current dipoles. Of 10 spike averages obtained, three peaked simultaneously in MEG and EEG, and in seven averages, the MEG peak preceded the main EEG peak by 9-40 ms. A small positive early EEG signal coincided with the MEG peak in six asynchronous spikes. The simultaneous MEG and EEG spikes originated within 5-23 mm, while sources of asynchronous peaks were 12-67 mm apart. We conclude that non-identical neurone currents underlie the MEG and EEG signals, and emphasize the importance of modelling early phases of EEG spikes when localizing interictal epileptic zones.

Adolescent↗

PubMed1996

Impaired visual word processing in dyslexia revealed with magnetoencephalography.

Dyslexia is most often attributed to phonological impairments, manifested in abnormal activation of the left temporal and temporoparietal cortex in response to auditorily presented language and possibly associated with anomalies in the cytoarchitecture and hemispheric symmetry of the plana temporale. The immediate cortical correlate of the severely impaired reading process has, however, remained obscure. Here we report on the distinct time courses of cortical activation in dyslexic and control subjects during passive viewing of single words, tracked with whole-head magnetoencephalography. A striking difference was found in the left inferior temporo-occipital region where intracranial recordings have recently identified word-specific responses within 200 msec after stimulus onset: controls showed a sharp activation at about 180 msec after word presentation, whereas dyslexics failed to activate this area entirely, or showed a slowly increasing late response. Perception of words as specific units thus seems to be impaired in dyslexics.

Adolescent↗