Michael J. Berry, Iman H. Brivanlou, Thomas A. Jordan, Markus Meister
Harvard University, Cambridge MA, USA
A flash of light evokes neural activity in the brain with a delay of 30-100 milliseconds, much of which is due to the slow process of visual transduction in photoreceptors. A moving object can cover a considerable distance in this time, and should therefore be seen noticeably behind its actual location. As this conflicts with everyday experience, it has been suggested that the visual cortex uses the delayed visual data from the eye to extrapolate the trajectory of a moving object, so that it is perceived at its actual location. Here we report that such anticipation of moving stimuli begins in the retina. A moving bar elicits a moving wave of spiking activity in the population of retinal ganglion cells. Rather than lagging behind the visual image, the population activity travels near the leading edge of the moving bar. This response is observed over a wide range of speeds and apparently compensates for the visual response latency. We show how this anticipation follows from known mechanisms of retinal processing.
Raij T
Helsinki University of Technology. tommi@neuro.hut.fi
Cortical signals associated with visual imagery of letters were recorded from 10 healthy adults with a whole-scalp 122-channel neuromagnetometer. The auditory stimulus sequence consisted of 20 different phonemes corresponding to single letters of the Roman alphabet and of tone pips (17%), delivered once every 1.5 sec in a random order. The subjects were instructed to visually imagine the letter corresponding to the auditory stimulus and to examine its visuospatial properties: The associated brain activity was compared with activity evoked by the same stimuli when the subjects just detected the intervening tones. All subjects produced broad imagery-related responses over multiple cortical regions. After initial activation of the auditory cortices, the earliest imagery-related responses originated in the left prerolandic area 320 msec after the voice onset. They were followed within 70 msec by signals originating in the posterior parietal lobe close to midline (precuneus) and, 100 msec later, in the posterior superior temporal areas, predominantly in the left hemisphere. The activations were sustained and partially overlapping in time. Imagery-related activity in the left lateral occipital cortex was observed in two subjects, and weak late activity in the calcarine cortex in one subject. Real audiovisually presented letters activated multiple brain regions, and task-induced visuospatial processing of these stimuli further increased activity in some of these regions and activated additional areas: Some of these areas were activated during imagery as well. The results suggest that certain brain areas involved in high-level visual perception are activated during visual imagery and that the extent of imagery-related activity is dictated by the requirements of the stimuli and the task.
Ogiso T, Kobayashi K, Sugishita M
Department of Cognitive Neuroscience, Faculty of Medicine, The University of Tokyo, Japan.
Magnetoencephalography was applied to subjects who imagined themselves hurdling in self-centered space. In three of six subjects all 300 trials in the motor imagery condition revealed the precuneus dipole. When we divided the 300 trials into four overlapping blocks (one block = 150 trials), all six subjects showed precuneus activity. The latency of the precuneus dipole was about 220 ms. We suggest that the precuneus activity during motor imagery involves retrieval of spatial information and/or setting up spatial attributes. Only in one subject but twice, the current dipole located in the supplementary motor area was observed 60 ms after activation of the precuneus, which suggests that the signal from the precuneus for motor imagery is transferred to the supplementary motor area.
Rosburg T, Kreitschmann-Andermahr I, Nowak H, Sauer H
Department of Psychiatry, University of Jena, Philosophenweg 3, D-07740, Jena, Germany. i5tiro@rz.uni-jena.de
The auditory evoked field (AEF) component N100m represents the most prominent and stable peak of the AEF, and its alterations in patients with schizophrenia are an extensive topic of neuropsychiatric research. In our current study, the degree of N100m habituation was investigated in 20 male schizophrenics and 19 healthy male controls. Participants were stimulated monaurally with 270 trials of 1000 Hz tones separated by an interstimulus interval between 800-1800 ms. The trial sample of the bilaterally recorded AEF was separated into three consecutive blocks of 90 trials and these blocks were compared with each other. The mean global field power (MGFP) of the N100m decreased on average 9.1% from the first to the third trial block, while the N100m latency was increasing. The analyses of the influence of habituation revealed a systematic change of dipole location in inferior-superior direction, mainly in the left hemisphere. This habituation effect was found to be the same for both groups. The groups also did not differ in the N100m latency increase and MGFP decrease, except for one parameter. The right-hemispheric MGFP decrease from the first to second block was found to be more pronounced in patients compared to controls. However, this difference was related to medication with clozapine. Overall, the habituation behaviour of the N100m seems to
Tendolkar I, Rugg M, Fell J, Vogt H, Scholz M, Hinrichs H, Heinze HJ
Department of Neurology II, University of Magdeburg, Germany. ain24@rrz.uni-koeln.de
Neural activity associated with recognition memory was investigated using magnetencephalography (MEG) in healthy young subjects. At sensor sites overlying frontal and temporoparietal cortices, magnetic evoked fields (MEFs) revealed a difference between studied and unstudied stimuli, which onset about 400 ms following stimulus onset and lasted about 600 ms. MEG yielded reliable source information revealing the activity of three independent dipoles, located in the right medial temporal lobe (MTL), the right inferior frontal and the left inferior parietal cortices. Our findings suggest that neural activity underlying recognition memory from both superficial and deep brain structures can be monitored by MEG.
Tesche CD, Karhu J
Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, FIN-02015 HUT, Espoo, Finland. tesche@neuro.hut.fi
Working memory (WM) is the ability to retain and associate information over brief time intervals. Functional imaging studies demonstrate that WM is mediated by a distributed network including frontal and posterior cortices, hippocampus, and cerebellum. In rodents, the presentation of stimuli in a WM task is followed by a reset of the phase of hippocampal theta. In this paper we report the observation of a similar phenomenon in normal human subjects. Neuromagnetic responses were recorded during presentation of a set of digits and a subsequent probe of the retained items. All stimuli were presented with a fixed temporal pattern. We observed phase reset of approximately 7 Hz theta in left hippocampus approximately 120 ms after probe stimuli, whereas reset of theta in right hippocampus was visible approximately 80 ms prior to these anticipated stimuli. The duration of stimulus-locked theta increased with memory load, with a limiting value of approximately 600 ms for 5-7 retained items. We suggest that, as in rats, stimulus-locked theta may index involvement of human hippocampal networks in the cognitive processing of sensory input. The anticipatory phase reset of theta indicates involvement of hippocampus in right hemisphere and cerebellar timing networks. Hippocampal structures are essential for orientation to perturbations in the sensory scene, a function that requires use of a context established by a constellation of stimuli. We suggest that the initiation and maintenance of stimulus-locked hippocampal theta observed here may facilitate processing of potentially salient and/or novel input with respect to a context established by the contents of WM.
Korzyukov O, Alho K, Kujala A, Gumenyuk V, Ilmoniemi RJ, Virtanen J, Kropotov J, Naatanen R
Department of Psychology, University of Helsinki, Finland. oleg.korzyukov@helsinki.fi
Event-related brain potentials (ERPs) and magnetoencephalographic (MEG) responses to infrequent ('deviant') tones occurring among frequent ('standard') tones of different pitch were compared with responses to rare tones presented alone. The subjects were to ignore the tones. Deviant tones elicited the mismatch negativity (MMN) and its MEG counterpart (MMNm), while the rare tones delivered alone elicited a larger N1 and its MEG counterpart (N1m) than did standard tones. Source modeling of MEG responses indicated a difference in auditory-cortex source locations between the MMNm to deviant tones and the enhanced N1m to the rare tones presented alone. Thus, the MMN/MMNm is elicited by infrequent sounds only when they occur among frequent sounds. This supports the idea that a sensory-memory trace formed in the auditory cortex by preceding repetitive sounds is a necessary precondition for MMN/MMNm elicitation.
Giannakopoulos X, Karhunen J, Oja E
Helsinki University of Technology, Laboratory of Computer and Information Science, Espoo, Finland. xavier@idsia.ch
In this paper, we compare the performance of five prominent neural or adaptive algorithms designed for Independent Component Analysis (ICA) and blind source separation (BSS). In the first part of the study, we use artificial data for comparing the accuracy, convergence speed, computational load, and other relevant properties of the algorithms. In the second part, the algorithms are applied to three different real-world data sets. The task is either blind source separation or finding interesting directions in the data for visualisation purposes. We develop criteria for selecting the most meaningful basis vectors of ICA and measuring the quality of the results. The comparison reveals characteristic differences between the studied ICA algorithms. The most important conclusions of our comparison are robustness of the ICA algorithms with respect to modest modeling imperfections, and the superiority of fixed-point algorithms with respect to the computational load
Okada YC, Salenius S
Department of Neurology, University of New Mexico School of Medicine, Albuquerque 87131, USA.
Neuronal activity of the human brain was studied with magnetoencephalography (MEG) in a spatial working memory task similar to those commonly used with nonhuman primates. The subject was required to remember target positions for 3 s and make a same-different judgement with a finger lift comparing the position of the probed target with the probe or to execute a memory-guided saccade to the probed target. In this type of task single-unit studies have shown attention- and memory-related activities independent of movement type during the retention interval in a large number of cortical areas of the primates, including the parietal and prefrontal areas. Consistent with these results, there were strong stimulus-driven transient and sustained responses and modulations of oscillatory activity during the retention period. Although we did not determine the source locations, coarse estimates of the currents responsible for the MEG signals showed activity over a wide area of the cortex, most prominently over the Rolandic, parietal and occipital areas, but also over the frontal area. Some of the activities in these cortical areas reflect processes that may be identified with attention and memory, while others were related to preparation of the overt movements.
Vigario R, Sarela J, Jousmaki V, Hamalainen M, Oja E
Laboratory of Computer and Information Science, Helsinki University of Technology, HUT, Finland. Ricardo.Vigario@hut.fi
Multichannel recordings of the electromagnetic fields emerging from neural currents in the brain generate large amounts of data. Suitable feature extraction methods are, therefore, useful to facilitate the representation and interpretation of the data. Recently developed independent component analysis (ICA) has been shown to be an efficient tool for artifact identification and extraction from electroencephalographic (EEG) and magnetoencephalographic (MEG) recordings. In addition, ICA has been applied to the analysis of brain signals evoked by sensory stimuli. This paper reviews our recent results in this field.
Ziemus B, Huonker R, Haueisen J, Liepert J, Spengler F, Weiller C
FSU Jena, Department of Neurology, Germany.
In animals simple passive co-activation causes a fusion and expansion of the involved cortical representations. We used passive tactile finger co-activation for 40 min to investigate cortical representational changes in the human somatosensory cortex. Magnetic source imaging revealed that the euclidean distance between median and ulnar nerve somatosensory evoked fields (SEF) was significantly reduced after application of 600 synchronous airpuff stimuli to the fingertips of four fingers. In the control experiment without co-activation no significant change in distance was observed. Perception threshold and spatial two-point discrimination were not affected by the synchronous stimulation. This is in contrast to blind three-finger Braille readers who frequently mislocalize stimuli applied to the reading fingers. This points to a lack of behavioural relevance or the short duration of co-activation.
Tesche CD, Karhu JJ
Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, Espoo, Finland. tesche@neuro.hut.fi
The traditional view of cerebellum is a structure that modifies and synchronizes elements of motor performance. Recent evidence indicates that human cerebellum is involved in a wide range of nonmotor sensory and cognitive functions. A common feature in these diverse motor and nonmotor tasks may be the capacity of cerebellar neuronal circuits to process and anticipate sensory input with high temporal acuity. We present evidence supporting this hypothesis from measurements of the magnetic field at the scalp evoked by neuronal population activity in human cerebellum. Intermittent electrical stimulation of the finger and the median nerve elicited stimulus-locked cerebellar responses with oscillatory components at 6-12 Hz and 25-35 Hz. Sustained oscillatory activity followed random stimulus omissions, with initiation of cerebellar responses prior to the next overt stimulus. These responses indexed processing of temporal features of somatosensory input independent of motor performance or response. The refractory behavior of the responses suggested that a neuronal trace of the temporal pattern of somatosensory stimulation remained in cerebellar circuits for 2-4 s. The cerebellar activity elicited by violation of an established temporal pattern was enhanced when attention was directed to somatosensory stimuli, in concordance with recent imaging studies suggesting participation of cerebellum in attentional networks. The attentional enhancement of the cerebellar responses supports the salience of cerebellar activity in the processing of purely somatosensory input. The short-term maintenance of cerebellar templates for predictable sensory input may reflect a physiological substrate for fine-grained temporal tuning and optimization of performance in large-scale sensory and integrative systems.
Nishitani N, Hari R
Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, P.O. Box 2200, FIN-02015 HUT, Espoo, Finland. nobu@rehab.go.jp
Brain-imaging studies have shown that the human Broca's region and precentral motor cortex are activated both during execution of hand actions and during observation of similar actions performed by other individuals. We aimed to clarify the temporal dynamics of this cortical activation by neuromagnetic recordings during execution, on-line imitation, and observation of right-hand reaching movements that ended with a precision pinch of the tip of a manipulandum. During execution, the left inferior frontal cortex [Brodmann's area (BA) 44] was activated first (peak approximately 250 ms before the pinching); this activation was followed within 100-200 ms by activation in the left primary motor area (BA4) and 150-250 ms later in the right BA4. During imitation and observation, the sequence was otherwise similar, but it started from the left occipital cortex (BA19). Activation was always strongest during action imitation. Only the occipital activation was detected when the subject observed the experimenter reaching his hand without pinching. These results suggest that the left BA44 is the orchestrator of the human "mirror neuron system" and is strongly involved in action imitation. The mirror system matches action observation and execution and probably contributes to our understanding of actions made by others.
Hari R, Imada T
Low Temperature Laboratory, Helsinki University of Technology, Espoo, FIN-02015 HUT, Finland. hari@neuro.hut.fi
The generation mechanism of movement-evoked fields (MEFs) is poorly known and the existence of ipsilateral MEFs is still in dispute. We recorded whole-scalp neuromagnetic activity from eight subjects who were pressing response keys alternately with the right 2nd and 4th digits, while keeping the left palm on the table containing the keys. Clear ipsilateral MEFs peaked 58 +/- 2 ms after the key touch, with sources in the hand area of the right primary somatosensory cortex. The ipsilateral MEFs decreased to half size when the resting left hand was palm up on the table. However, very similar responses were obtained when another person operated the response keys and the subjects just kept their left palm on the table. No signals were elicited when the subjects only viewed these actions with no hand contact to the table. The results indicate that the MEFs receive a strong contribution from tactile input. In our experiment the ipsilateral sensorimotor activation was triggered by the movement-related vibrations transmitted to the resting hand.
Simoes C, Hari R
Low Temperature Laboratory, Helsinki University of Technology, Espoo, FIN-02015 HUT, Finland. cristina@neuro.hut.fi
We studied the interaction between responses to contra- and ipsilateral stimuli in the human second somatosensory cortex SII by recording somatosensory evoked magnetic fields (SEFs) from 8 healthy subjects with a 122-channel whole-scalp SQUID magnetometer. Right (R) and left (L) median nerves were electrically stimulated at the wrists at intensities exceeding the motor threshold. In each stimulus sequence, the four equiprobable pairs (L-L, R-R, L-R, R-L) were presented in a random order once every 2 s, with a 300-ms interstimulus interval within the pair. The responses were modelled with a four-dipole model, with current dipoles located in the SI and SII cortices of both hemispheres. The SII responses peaked around 85-120 ms and responses to the 1st (2nd) stimulus on the pair were on average 2 (12) ms earlier and about 3 (2.5) times stronger for contralateral than ipsilateral stimuli. Independently of the condition, the 2nd response always peaked later than the 1st; the mean delay was 16 ms. The responses to the 2nd stimulus depended only slightly on the type of the 1st: the latency increased more and the amplitude decreased less after different than identical 1st stimuli. These results suggest that neuronal activations due to contra- and ipsilateral stimuli overlap strongly in the human SII cortex.
Hari R, Forss N
Brain Research Unit, Helsinki University of Technology, Espoo, Finland. hari@neuro.hut.fi
Magnetoencephalography (MEG) is a totally non-invasive research method which provides information about cortical dynamics on a millisecond time-scale. Whole-scalp magnetic field patterns following stimulation of different peripheral nerves indicate activation of an extensive cortical network. At the SI cortex, the responses reflect mainly the activity of area 3b, with clearly somatotopical representations of different body parts. The SII cortex is activated bilaterally and it also receives, besides tactile input, nociceptive afference. Somatically evoked MEG signals may also be detected from the posterior parietal cortex, central mesial cortex and the frontal lobe. The serial versus parallel processing in the cortical somatosensory network is still under debate.
Korvenoja A, Huttunen J, Salli E, Pohjonen H, Martinkauppi S, Palva JM, Lauronen L, Virtanen J, Ilmoniemi RJ, Aronen HJ
BioMag Laboratory, Helsinki University Central Hospital, Finland. antti.korvenoja@helsinki.fi
We combined information from functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) to assess which cortical areas and in which temporal order show macroscopic activation after right median nerve stimulation. Five healthy subjects were studied with the two imaging modalities, which both revealed significant activation in the contra- and ipsilateral primary somatosensory cortex (SI), the contra- and ipsilateral opercular areas, the walls of the contralateral postcentral sulcus (PoCS), and the contralateral supplementary motor area (SMA). In fMRI, two separate foci of activation in the opercular cortex were discerned, one posteriorly in the parietal operculum (PO), and one anteriorly near the insula or frontal operculum (anterior operculum, AO). The activation sites from fMRI were used to constrain the solution of the inverse problem of MEG, which allowed us to construct a model of the temporal sequence of activation of the different sites. According to this model, the mean onset latency for significant activation at the contralateral SI was 20 msec (range, 17-22 msec), followed by activation of PoCS at 23 msec (range, 21-25 msec). The contralateral PO was activated at 26 msec (range, 19-32 msec) and AO at 33 msec (range, 22-51 msec). The contralateral SMA became active at 36 msec (range, 24-48 msec). The ipsilateral SI, PO, and AO became activated at 54-67 msec. We conclude that fMRI provides a useful means to constrain the inverse problem of MEG, allowing the construction of spatiotemporal models of cortical activation, which may have significant implications for the understanding of cortical network functioning.
Reite M, Teale P, Rojas DC, Sheeder J, Arciniegas D
Department of Psychiatry, University of Colorado Health Sciences Center, Denver 80262, USA.
BACKGROUND: Schizoaffective disorder is one of the most severe of the affective psychoses, but its pathophysiology is poorly understood. Because cerebral lateralization may be disturbed in psychotic disorders generally, studies examining cerebral asymmetry may improve understanding of the neurobiology specific to schizoaffective disorder. This study examines cerebral lateralization in this patient population using magnetic source localization. METHODS: We studied 16 subjects with schizoaffective disorder and 16 controls. Magnetic source localization was used to identify the location of the 20 msec latency somatosensory evoked field component (M20). RESULTS: In control subjects, the source location was further anterior in the right hemisphere. The subjects with schizoaffective disorder were reverse lateralized. CONCLUSIONS: The findings of a reversed asymmetry of the M20 in patients with schizoaffective disorder suggest an anatomical shift in the placement of the post central gyrus in this disorder, compatible with a disorder of cerebral lateralization. Whether this finding converges or diverges with measurement of the M20 in other psychotic disorders will require further investigation.
Forss N, Jousmaki V
Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, P.O. Box 2200, FIN-02015 HUT Espoo, Finland. nina@neuro.hut.fi
We measured somatosensory evoked fields (SEFs) to electric median nerve stimuli from eight healthy subjects with a whole-scalp 122-channel neuromagnetometer in two different conditions: (i) 'rest', with stimuli producing clear tactile sensation without any motor movement, and (ii) 'contraction' with exactly the same stimuli as in 'rest', but with the subjects maintaining sub-maximal isometric contraction in thenar muscles of the stimulated hand. The aim was to study the role of the primary (SI) and secondary somatosensory (SII) cortices in sensorimotor integration. The amplitude of the SI response N20m did not change with coincident isometric contraction, whereas P35m was significantly reduced. On the contrary, activation of contra- and ipsilateral SII cortices was significantly enhanced during the contraction. We suggest that isometric contraction facilitates activation of SII cortices to tactile stimuli, possibly by decreasing inhibition from the SI cortex. The enhanced SII activation may be related to tuning of SII neurons towards relevant tactile input arising from the region of the body where the muscle activation occurs.
William Bialek and Fred Rieke and Rob R. de Ruyter van Stevenick and David Warland
Fred Rieke and W. Geoffrey Owen and William Bialek
David Warland and Michael Landolfa and John P. Miller and William Bialek