Event-Related Potentials Reveal Rapid Verification of Predicted Visual Input

Human information processing depends critically on continuous predictions about upcoming events, but the temporal convergence of expectancy-based top-down and input-driven bottom-up streams is poorly understood. We show that, during reading, event-related potentials differ between exposure to highly predictable and unpredictable words no later than 90 ms after visual input. This result suggests an extremely rapid comparison of expected and incoming visual information and gives an upper temporal bound for theories of top-down and bottom-up interactions in object recognition

Retinotopic mapping of the primary visual cortex – a challenge for MEG imaging of the human cortex

Magnetoencephalography (MEG) can be used to reconstruct neuronal activity with high spatial and temporal resolution. However, this reconstruction problem is ill-posed, and requires the use of prior constraints in order to produce a unique solution. At present there are a multitude of inversion algorithms, each employing different assumptions, but one major problem when comparing the accuracy of these different approaches is that often the true underlying electrical state of the brain is unknown. In this study, we explore one paradigm, retinotopic mapping in the primary visual cortex (V1), for which the ground truth is known to a reasonable degree of accuracy, enabling the comparison of MEG source reconstructions with the true electrical state of the brain. Specifically, we attempted to localize, using a beanforming method, the induced responses in the visual cortex generated by a high contrast, retinotopically varying stimulus. Although well described in primate studies, it has been an open question whether the induced gamma power in humans due to high contrast gratings derives from V1 rather than the prestriate cortex (V2). We show that the beanformer source estimate in the gamma and theta bands does vary in a manner consistent with the known retinotopy of V1. However, these peak locations, although retinotopically organized, did not accurately localize to the cortical surface. We considered possible causes for this discrepancy and suggest that improved MEG/magnetic resonance imaging co-registration and the use of more accurate source models that take into account the spatial extent and shape of the active cortex may, in future, improve the accuracy of the source reconstructions

Word frequency in fast priming: Evidence for immediate cognitive control of eye movements during reading

Numerous studies have demonstrated effects of word frequency on eye movements during reading, but the precise timing of this influence has remained unclear. The fast priming paradigm (Sereno & Rayner, 1992) was previously used to study influences of related versus unrelated primes on the target word. Here, we used this procedure to investigate whether the frequency of the prime word has a direct influence on eye movements during reading when the prime-target relation is not manipulated. We found that with average prime intervals of 32 ms readers made longer single fixation durations on the target word in the low than in the high frequency prime condition. Distributional analyses demonstrated that the effect of prime frequency on single fixation durations occurred very early, supporting theories of immediate cognitive control of eye movements. Finding prime frequency effects only 207 ms after visibility of the prime and for prime durations of 32 ms yields new time constraints for cognitive processes controlling eye movements during reading. Our variant of the fast priming paradigm provides a new approach to test early influences of word processing on eye movement control during reading

High-Frequency Oscillations in Distributed Neural Networks Reveal the Dynamics of Human Decision Making

We examine the relative timing of numerous brain regions involved in human decisions that are based on external criteria, learned information, personal preferences, or unconstrained internal considerations. Using magnetoencephalography (MEG) and advanced signal analysis techniques, we were able to non-invasively reconstruct oscillations of distributed neural networks in the high-gamma frequency band (60–150 Hz). The time course of the observed neural activity suggested that two-alternative forced choice tasks are processed in four overlapping stages: processing of sensory input, option evaluation, intention formation, and action execution. Visual areas are activated first, and show recurring activations throughout the entire decision process. The temporo-occipital junction and the intraparietal sulcus are active during evaluation of external values of the options, 250–500 ms after stimulus presentation. Simultaneously, personal preference is mediated by cortical midline structures. Subsequently, the posterior parietal and superior occipital cortices appear to encode intention, with different subregions being responsible for different types of choice. The cerebellum and inferior parietal cortex are recruited for internal generation of decisions and actions, when all options have the same value. Action execution was accompanied by activation peaks in the contralateral motor cortex. These results suggest that high-gamma oscillations as recorded by MEG allow a reliable reconstruction of decision processes with excellent spatiotemporal resolution

Emotion Separation Is Completed Early and It Depends on Visual Field Presentation

It is now apparent that the visual system reacts to stimuli very fast, with many brain areas activated within 100 ms. It is, however, unclear how much detail is extracted about stimulus properties in the early stages of visual processing. Here, using magnetoencephalography we show that the visual system separates different facial expressions of emotion well within 100 ms after image onset, and that this separation is processed differently depending on where in the visual field the stimulus is presented. Seven right-handed males participated in a face affect recognition experiment in which they viewed happy, fearful and neutral faces. Blocks of images were shown either at the center or in one of the four quadrants of the visual field. For centrally presented faces, the emotions were separated fast, first in the right superior temporal sulcus (STS; 35–48 ms), followed by the right amygdala (57–64 ms) and medial pre-frontal cortex (83–96 ms). For faces presented in the periphery, the emotions were separated first in the ipsilateral amygdala and contralateral STS. We conclude that amygdala and STS likely play a different role in early visual processing, recruiting distinct neural networks for action: the amygdala alerts sub-cortical centers for appropriate autonomic system response for fight or flight decisions, while the STS facilitates more cognitive appraisal of situations and links appropriate cortical sites together. It is then likely that different problems may arise when either network fails to initiate or function properly

Assessment of whole-head magnetoencephalography during transcranial electric entrainment of brain oscillations

Application of non-invasive brain stimulation for perturbing brain activity is well established. Various forms of brain stimulation protocols have been effectively demonstrated to modulate behavior associated with the perturbed brain activity. However, the interaction of brain stimulation with ongoing brain activity has been challenging to characterize as the stimulation artifacts in the recordings of brain activity impedes such characterization. The proposed amplitude modulated transcranial alternating current stimulation (tACSAM) attenuates possible stimulation artifacts at the frequency of interest. This is possible by modulating the amplitude of high frequency transcranial alternating current (tACS) signal at a lower physiological frequency of interest to generate the tACSAM signal. Furthermore, application of tACSAM allows localization of the perturbed brain activity with millimeter precision by applying spatial filters on magnetoencephalography (MEG) recordings. For characterization of the tACSAM-perturbed brain activity, conventional spectral analysis may not be sufficient. Thus, power and PLV were compared between tACS and tACSAM in a phantom model and MEG data recorded from healthy human volunteers. The synchronization estimate, phase lock value (PLV), is a measure of circular variance between two signals calculated as a function of instantaneous phase difference between the ongoing brain activity and the applied stimulation signal. Even though, systematic linear phase shifts due to the applied tES signal occur in MEG sensors, mathematically such systematic linear phase shifts nullify while calculating PLV. Systematic evaluation of the MEG data acquired during tACSAM showed increased PLV compared to tACS indicating increased demodulation in such paradigm. Upon observing tACSAM-related increased demodulation, it was still unclear whether such perturbations of brain activity could modulate behavior. To address this question, twenty volunteers while engaging in a working memory paradigm received tACSAM or no stimulation. Working memory is associated with transient storage and processing of information. Increasing the difficulty of working memory paradigm increases the amplitude of brain activity in the theta band (4 – 8 Hz), while encoding the temporal order of the transient information in the phase of the theta activity. Thus, by targeting individual’s theta peak frequency using tACSAM, it was possible to modulate the accuracy in the working memory paradigm. The accuracy on a working memory parading of volunteers receiving tACSAM deteriorated compared to the participants who did not receive brain stimulation. Therefore, targeting brain activity in theta band using tACSAM interferes with execution of normal working memory processes, probably by interfering with the maintenance of temporal order of the transient information. Furthermore, tACSAM but not sham stimulation inhibited the increase in amplitude of theta activity during the n-back task, which is essential for working memory processes. Even though, it is possible to assess the brain activity recorded during tACSAM, presence of stimulation artifacts in the assessed brain activity cannot be excluded. However, it was possible to gather evidence that tACSAM is associated with demodulation. TACSAM-induced phase synchrony at the modulation frequency was larger compared to tACS even though the power during tACS is larger compared to tACSAM. This observation is in favor of possible functional interaction of tACSAM signal with neurons in the brain. However, currently it is not possible to distinguish between the contribution towards demodulation of tACSAM signal by non-linearities of the stimulation setup and functional interactions with neurons in the brain. In conclusion, tACSAM can alter cognitive function, such as working memory performance, possibly through entrainment. The results obtained from such investigations must be interpreted with great care, as the extent by which possible stimulation artifacts impact the MEG recordings is not entirely clear. Further investigations are necessary to develop quantitative assessment techniques for characterizing artifacts of the stimulation and eventually develop brain state dependent stimulation paradigms in real time as a research tool and therapeutic intervention

Eye movements in time : auditory influences on oculomotor timing

The dominant models of eye movement timing consider only visual factors as modulators of when gaze orients (e.g. EZ-Reader, SWIFT, CRISP, LATEST). Yet realworld perception is multimodal, and temporal information from audition can both aid the predictive orienting of gaze (to relevant audiovisual onsets in time), and inform visual orientation decisions known to modulate saccade timing, e.g. where to orient. The aim of this thesis was to further the current understanding of eye movement timing to incorporate auditory information; specifically investigating the implicit and explicit capacity for musical beats to influence (and entrain) eye movements, and to quantify the capacity and limitations of direct control when volitionally matching eye movements to auditory onsets. To achieve this, a highly-simplified gaze-contingent visual search paradigm was refined that minimised visual and task factors in order to measure auditory influence. The findings of this thesis present evidence that self-paced eye movements are impervious to implicit auditory influences. The explicit control of eye movements, as small corrections in time to align with similarly timed music, was very limited. In contrast, when visual transitions were externally timed, audiovisual correspondence systematically delayed fixation durations. The thesis also measured the extent of direct control that can be exerted on eye movements, including the role of auditory feedback, as well as modulating visual complexity to further increase inhibition and temporal precision. These studies show a predictive relationship between the level of direct volitional control that an individual can affect and how synchronised they are. Additionally, these studies quantify a large subpopulation of quick eye movements that are impervious to direct control. These findings are discussed as provocation for revised oculomotor models, future work that considers the temporal relationship between shifts of attention and gaze, and implications for wider psychological research that employs timed eye movement measures

The form pathways in the visual brain.

The perception of visual forms is crucial for humans for successful interactions with the environment. This process occurs automatically, and its outcome is reflected in the inferences and decisions we constantly make. The focus of this thesis is on how the brain handles different aspects of the perception of forms. To study this in normal human individuals, experiments were performed using functional magnetic resonance imaging (fMRI), magnetoencephalography (MEG) and psychophysical methods. This thesis first discusses experiments designed to unravel the mechanisms of form construction, i.e. those from which all the component parts of a single form are assembled. Results suggest that the construction of very simple forms occurs in intermediate visual areas in a parallel and recursive process, with an increase in brain activity with increments in form complexity. A further experiment was performed to study how regularities or known characteristics of images, and the brain responses they elicit, will contribute to explain current percepts. Results from this experiment are consistent with a model where images with learnt attributes activate more strongly anterior visual areas and images with random patterns cause higher activations in earlier visual areas, probably due to top-down signals that reduce activity when it is possible to explain the causes of the sensory stimulation. Finally, it shows differences in the evoked neural activity when forms are either detected or classified, relating these processes to the activity generated in early visual areas. Based on the results of these experiments, a mechanism of top-down and bottom-up interactions between visual areas in the human brain is discussed in the context of the perception of forms

Eye movements in time : auditory influences on oculomotor timing

The dominant models of eye movement timing consider only visual factors as modulators of when gaze orients (e.g. EZ-Reader, SWIFT, CRISP, LATEST). Yet realworld perception is multimodal, and temporal information from audition can both aid the predictive orienting of gaze (to relevant audiovisual onsets in time), and inform visual orientation decisions known to modulate saccade timing, e.g. where to orient. The aim of this thesis was to further the current understanding of eye movement timing to incorporate auditory information; specifically investigating the implicit and explicit capacity for musical beats to influence (and entrain) eye movements, and to quantify the capacity and limitations of direct control when volitionally matching eye movements to auditory onsets. To achieve this, a highly-simplified gaze-contingent visual search paradigm was refined that minimised visual and task factors in order to measure auditory influence. The findings of this thesis present evidence that self-paced eye movements are impervious to implicit auditory influences. The explicit control of eye movements, as small corrections in time to align with similarly timed music, was very limited. In contrast, when visual transitions were externally timed, audiovisual correspondence systematically delayed fixation durations. The thesis also measured the extent of direct control that can be exerted on eye movements, including the role of auditory feedback, as well as modulating visual complexity to further increase inhibition and temporal precision. These studies show a predictive relationship between the level of direct volitional control that an individual can affect and how synchronised they are. Additionally, these studies quantify a large subpopulation of quick eye movements that are impervious to direct control. These findings are discussed as provocation for revised oculomotor models, future work that considers the temporal relationship between shifts of attention and gaze, and implications for wider psychological research that employs timed eye movement measures