Neuronal correlates of full and partial visual conscious perception

Stimuli may induce only partial consciousness—an intermediate between null and full consciousness—where the presence but not identity of an object can be reported. The differences in the neuronal basis of full and partial consciousness are poorly understood. We investigated if evoked and oscillatory activity could dissociate full from partial conscious perception. We recorded human cortical activity with magnetoencephalography (MEG) during a visual perception task in which stimulus could be either partially or fully perceived. Partial consciousness was associated with an early increase in evoked activity and theta/low-alpha-band oscillations while full consciousness was also associated with late evoked activity and beta-band oscillations. Full from partial consciousness was dissociated by stronger evoked activity and late increase in theta oscillations that were localized to higher-order visual regions and posterior parietal and prefrontal cortices. Our results reveal both evoked activity and theta oscillations dissociate partial and full consciousness.     

Learning new names for new objects: cortical effects as measured by magnetoencephalography

We tracked the evolvement of naming-related cortical dynamics with magnetoencephalography when five normal adults successfully learned names and/or meanings of unfamiliar objects. In all subjects, the learning of new names was associated with pronounced cortical effects. The learning effect was of long latency and emerged as a change of activation in the same cortical network that was active during naming of familiar items. In four out of five subjects, the cortical learning effect occurred in the inferior parietal lobe. In three of these subjects, the cortical effect was left-sided. These results suggest that the inferior parietal lobe plays an important role in the acquisition of novel words, presumably as a part of working memory systems.     

Processing events: behavioral and neuromagnetic correlates of Aspectual Coercion

Much recent psycho- and neuro-linguistic work has aimed to elucidate the mechanisms by which sentence meanings are composed by investigating the processing of semantic mismatch. One controversial case for theories of semantic composition is expressions such as the clown jumped for ten minutes, in which the aspectual properties of a punctual verb clash with those of a durative modifier. Such sentences have been proposed to involve a coercion operation which shifts the punctual meaning of the verb to an iterative one. However, processing studies addressing this hypothesis have yielded mixed results. In this study, we tested four hypotheses of how aspectual mismatch is resolved with self-paced reading and magnetoencephalography. Using a set of verbs normed for punctuality, we identified an immediate behavioral cost of mismatch. The neural correlates of this processing were found to match effects in midline prefrontal regions previously implicated in the resolution of complement coercion. We also identified earlier effects in right-lateral frontal and temporal sites. We suggest that of the representational hypotheses currently in the literature, these data are most consistent with an account where aspectual mismatch initially involves the composition of an anomalous meaning that is later repaired via coercion.     

Sensitivity to syntax in visual cortex

One of the most intriguing findings on language comprehension is that violations of syntactic predictions can affect event-related potentials as early as 120 ms, in the same time-window as early sensory processing. This effect, the so-called early left-anterior negativity (ELAN), has been argued to reflect word category access and initial syntactic structure building (Friederici, 2002). In two experiments, we used magnetoencephalography to investigate whether (a) rapid word category identification relies on overt category-marking closed-class morphemes and (b) whether violations of word category predictions affect modality-specific sensory responses. Participants read sentences containing violations of word category predictions. Unexpected items varied in whether or not their word category was marked by an overt function morpheme. In Experiment 1, the amplitude of the visual evoked M100 component was increased for unexpected items, but only when word category was overtly marked by a function morpheme. Dipole modeling localized the generator of this effect to the occipital cortex. Experiment 2 replicated the main results of Experiment 1 and eliminated two non-morphology-related explanations of the M100 contrast we observed between targets containing overt category-marking and targets that lacked such morphology. Our results show that during reading, syntactically relevant cues in the input can affect activity in occipital regions at around 125 ms, a finding that may shed new light on the remarkable rapidity of language processing.     

Effects of unilateral hippocampus-amygdala-partial temporal lobe resection on auditory EEG/MEG responses: a case study

Auditory orienting and discrimination were studied with combined multi-channel EEG and MEG recordings in a patient with unilateral amygdala-hippocampus-partial temporal lobe resection of the right hemisphere. The results revealed abnormalities of habituation in alerting- and orienting-related responses, and discrimination-related responses, elicited by auditory stimulation contralateral to the resected cerebral hemisphere. These results give support to the notions about the role of the amygdala and hippocampus in alerting and orienting, respectively, and of the temporal cortex in auditory discrimination.     

Timing of human cortical functions during cognition: role of MEG

Understanding of sensory and cognitive brain processes requires information about activation timing within and between different brain sites. Such data can be obtained by magnetoencephalography (MEG) that tracks cortical activation sequences with a millisecond temporal accuracy. MEG is gaining a well-established role in human neuroscience, complementing with its excellent temporal resolution the spatially more focused brain imaging methods. As examples of MEG's role in cognitive neuroscience, we discuss time windows related to cortical processing of sensory and multisensory stimuli, effects of the subject's own voice on the activity of their auditory cortex, timing of brain activation in reading, and cortical dynamics of the human mirror-neuron system activated when the subject views another person's movements.     

The precise time course of lexical activation: MEG measurements of the effects of frequency, probability, and density in lexical decision

Visually presented letter strings consistently yield three MEG response components: the M170, associated with letter-string processing (Tarkiainen, Helenius, Hansen, Cornelissen, & Salmelin, 1999); the M250, affected by phonotactic probability, (Pylkk?nen, Stringfellow, & Marantz, 2002); and the M350, responsive to lexical frequency (Embick, Hackl, Schaeffer, Kelepir, & Marantz, 2001). Pylkk?nen et al. found evidence that the M350 reflects lexical activation prior to competition among phonologically similar words. We investigate the effects of lexical and sublexical frequency and neighborhood density on the M250 and M350 through orthogonal manipulation of phonotactic probability, density, and frequency. The results confirm that probability but not density affects the latency of the M250 and M350; however, an interaction between probability and density on M350 latencies suggests an earlier influence of neighborhoods than previously reported.     

Word-specific repetition effects revealed by MEG and the implications for lexical access

This magnetoencephalography (MEG) study investigated the early stages of lexical access in reading, with the goal of establishing when initial contact with lexical information takes place. We identified two candidate evoked responses that could reflect this processing stage: the occipitotemporal N170/M170 and the frontocentral P2. Using a repetition priming paradigm in which long and variable lags were used to reduce the predictability of each repetition, we found that (i) repetition of words, but not pseudowords, evoked a differential bilateral frontal response in the 150–250 ms window, (ii) a differential repetition N400m effect was observed between words and pseudowords. We argue that this frontal response, an MEG correlate of the P2 identified in ERP studies, reflects early access to long-term memory representations, which we tentatively characterize as being modality-specific.     

An extended motor network generates beta and gamma oscillatory perturbations during development

This study examines the time course and neural generators of oscillatory beta and gamma motor responses in typically-developing children. Participants completed a unilateral flexion–extension task using each index finger as whole-head magnetoencephalography (MEG) data were acquired. These MEG data were imaged in the frequency-domain using spatial filtering and the resulting event-related synchronizations and desynchronizations (ERS/ERD) were subjected to voxel-wise statistical analyses to illuminate time–frequency specific activation patterns. Consistent with adult data, these children exhibited a pre-movement ERD that was strongest over the contralateral post-central gyrus, and a post-movement ERS response with the most prominent peak being in the contralateral precentral gyrus near premotor cortices. We also observed a high-frequency (∼80 Hz) ERS response that coincided with movement onset and was centered on the contralateral precentral gyrus, slightly superior and posterior to the beta ERS. In addition to pre- and post-central gyri activations, these children exhibited beta and gamma activity in supplementary motor areas (SMA) before and during movement, and beta activation in cerebellar cortices before and after movement. We believe the gamma synchronization may be an excellent candidate signal of basic cortical motor control, as the spatiotemporal dynamics indicate the primary motor cortex generates this response (and not the beta oscillations) which is closely yoked to the initial muscle activation. Lastly, these data suggest several additional neural regions including the SMA and cerebellum are involved in basic movements during development.     

Lexical access in early stages of visual word processing: A single-trial correlational MEG study of heteronym recognition

We present an MEG study of heteronym recognition, aiming to distinguish between two theories of lexical access: the ‘early access’ theory, which entails that lexical access occurs at early (pre 200 ms) stages of processing, and the ‘late access’ theory, which interprets this early activity as orthographic word-form identification rather than genuine lexical access. A correlational analysis method was employed to examine effects of the heteronyms’ form and lexical properties on brain activity. We find support for the ‘late access’ view, in that lexical properties did not affect processing until after 300 ms, while earlier activation was primarily modulated by orthographic form.