John J. Foxe

Multisensory contributions to low-level, `unisensory¿ processing

Neurobiologists have traditionally assumed that multisensory integration is a higher order process that occurs after sensory signals have undergone extensive processing through a hierarchy of unisensory subcortical and cortical regions. Recent findings, however, question this assumption. Studies in humans, nonhuman primates and other species demonstrate multisensory convergence in low level cortical structures that were generally believed to be unisensory in function. In addition to enriching current models of multisensory processing and perceptual functions, these new findings require a revision in our thinking about unisensory processing in low level cortical areas.

Parieto-occipital ∼1 0Hz activity reflects anticipatory state of visual attention mechanisms

High-density eeg recordings revealed sensory specific modulation of anticipatory parieto-occipital approximately 10 Hz oscillatory activity when visually presented word cues instructed subjects in an intermodal selective attention paradigm. Cueing attention to the auditory features of imminent compound audio-visual stimuli resulted in significantly higher approximately 10 Hz amplitude in the period preceding onset of this stimulus than when attention was cued to the visual features. We propose that this parieto-occipital approximately 10 Hz activity reflects a disengaged visual attentional system in preparation for anticipated auditory input that is attentionally more relevant. Conversely, lower approximately 10 Hz activity during the attend-visual condition may reflect active engagement of parieto-occipital areas in the anticipatory period. These results support models implicating parieto-occipital areas in the directing and maintenance of visual attention.

The Role of Alpha-Band Brain Oscillations as a Sensory Suppression Mechanism during Selective Attention

Evidence has amassed from both animal intracranial recordings and human electrophysiology that neural oscillatory mechanisms play a critical role in a number of cognitive functions such as learning, memory, feature binding and sensory gating. The wide availability of high-density electrical and magnetic recordings (64–256 channels) over the past two decades has allowed for renewed efforts in the characterization and localization of these rhythms. A variety of cognitive effects that are associated with specific brain oscillations have been reported, which range in spectral, temporal, and spatial characteristics depending on the context. Our laboratory has focused on investigating the role of alpha-band oscillatory activity (8–14 Hz) as a potential attentional suppression mechanism, and this particular oscillatory attention mechanism will be the focus of the current review. We discuss findings in the context of intersensory selective attention as well as intrasensory spatial and feature-based attention in the visual, auditory, and tactile domains. The weight of evidence suggests that alpha-band oscillations can be actively invoked within cortical regions across multiple sensory systems, particularly when these regions are involved in processing irrelevant or distracting information. That is, a central role for alpha seems to be as an attentional suppression mechanism when objects or features need to be specifically ignored or selected against.

Multisensory auditory–somatosensory interactions in early cortical processing revealed by high-density electrical mapping.

We investigated the time-course and scalp topography of multisensory interactions between simultaneous auditory and somatosensory stimulation in humans. Event-related potentials (ERPs) were recorded from 64 scalp electrodes while subjects were presented with auditory-alone stimulation (1000-Hz tones), somatosensory-alone stimulation (median nerve electrical pulses), and simultaneous auditory-somatosensory (AS) combined stimulation. Interaction effects were assessed by comparing the responses to combined stimulation with the algebraic sum of responses to the constituent auditory and somatosensory stimuli when they were presented alone. Spatiotemporal analysis of ERPs and scalp current density (SCD) topographies revealed AS interaction over the central/postcentral scalp which onset at approximately 50 ms post-stimulus presentation. Both the topography and timing of these interactions are consistent with multisensory integration early in the cortical processing hierarchy, in brain regions traditionally held to be unisensory.

The case for feedforward multisensory convergence during early cortical processing.

The prevailing hierarchical model of sensory processing in the brain holds that different modalities of sensory information emanating from a single object are analyzed extensively during passage through their respective unisensory processing streams before they are combined in higher-order ‘multisensory’ regions of the cortex. Because of this view, multisensory interactions that have been found at early, putatively ‘unisensory’ cortical processing stages during hemodynamic imaging studies have been assumed to reflect feedback modulations that occur subsequent to multisensory processing in the higher-order multisensory areas. In this paper, we consider findings that challenge an exclusively feedback interpretation of early multisensory integration effects. First, high-density electrical mapping studies in humans have shown that multisensory convergence and integration effects can occur so early in the time course of sensory processing that purely feedback mediation becomes extremely unlikely. Second, direct neural recordings in monkeys show that, in some cases, convergent inputs at early cortical stages have physiological profiles characteristic of feedforward rather than feedback inputs. Third, damage to higher-order integrative regions in humans often spares the ability to integrate across sensory modalities. Finally, recent anatomic tracer studies have reported direct anatomical connections between primary visual and auditory cortex. These findings make it clear that multisensory convergence at early stages of cortical processing results from feedforward as well as feedback and lateral connections, thus using the full range of anatomical connections available in brain circuitry.

Crossmodal binding through neural coherence: implications for multisensory processing

Picture yourself on a crowded sideway with people milling about. The acoustic and visual signals generated by the crowd provide you with complementary information about their locations and motion which needs to be integrated. It is not well understood how such inputs from different sensory channels are combined into unified perceptual states. Coherence of oscillatory neural signals might be an essential mechanism supporting multisensory perception. Evidence is now emerging which indicates that coupled oscillatory activity might serve to link neural signals across uni- and multisensory regions and to express the degree of crossmodal matching of stimulus-related information. These results argue for a new view on multisensory processing which considers the dynamic interplay of neural populations as a key to crossmodal integration.

Neural mechanisms involved in error processing: A comparison of errors made with and without awareness

The ability to detect an error in ones own performance and then to improve ongoing performance based on this error processing is critical for effective behaviour. In our event-related fMRI experiment, we show that explicit awareness of a response inhibition commission error and subsequent post-error behaviour were associated with bilateral prefrontal and parietal brain activation. Activity in the anterior cingulate region, typically associated with error detection, was equivalent for both errors subjects were aware of and those they were not aware of making. While anterior cingulate activation has repeatedly been associated with error-related processing, these results suggest that, in isolation, it is not sufficient for conscious awareness of errors or post-error adaptation of response strategies. Instead, it appears, irrespective of awareness, to detect information about stimuli/responses that requires interpretation in other brain regions for strategic implementation of post-error adjustments of behaviour.

Activation Timecourse of Ventral Visual Stream Object-recognition Areas: High Density Electrical Mapping of Perceptual Closure Processes

Object recognition is achieved even in circumstances when only partial information is available to the observer. Perceptual closure processes are essential in enabling such recognitions to occur. We presented successively less fragmented images while recording high-density event-related potentials (ERPs), which permitted us to monitor brain activity during the perceptual closure processes leading up to object recognition. We reveal a bilateral ERP component (Ncl) that tracks these processes (onsets 230 msec, maximal at 290 msec). Scalp-current density mapping of the Ncl revealed bilateral occipito-temporal scalp foci, which are consistent with generators in the human ventral visual stream, and specifically the lateral-occipital or LO complex as defined by hemodynamic studies of object recognition.

Visual spatial attention tracking using high-density SSVEP data for independent brain-computer communication

The steady-state visual evoked potential (SSVEP) has been employed successfully in brain-computer interface (BCI) research, but its use in a design entirely independent of eye movement has until recently not been reported. This paper presents strong evidence suggesting that the SSVEP can be used as an electrophysiological correlate of visual spatial attention that may be harnessed on its own or in conjunction with other correlates to achieve control in an independent BCI. In this study, 64-channel electroencephalography data were recorded from subjects who covertly attended to one of two bilateral flicker stimuli with superimposed letter sequences. Offline classification of left/right spatial attention was attempted by extracting SSVEPs at optimal channels selected for each subject on the basis of the scalp distribution of SSVEP magnitudes. This yielded an average accuracy of approximately 71% across ten subjects (highest 86%) comparable across two separate cases in which flicker frequencies were set within and outside the alpha range respectively. Further, combining SSVEP features with attention-dependent parieto-occipital alpha band modulations resulted in an average accuracy of 79% (highest 87%).

Prefrontal-subcortical dissociations underlying inhibitory control revealed by event-related fMRI

Using event‐related fMRI, this study investigated the neural dynamics of response inhibition under fluctuating task demands. Fourteen participants performed a GO/NOGO task requiring inhibition of a prepotent motor response to NOGO events that occurred as part of either a Fast or Slow presentation stream of GO stimuli. We compared functional activations associated with correct withholds (Stops) required during the Fast presentation stream of stimuli to Stops required during the Slow presentation stream. A predominantly right hemispheric network was activated across conditions, consistent with previous studies. Furthermore, a functional dissociation of activations between conditions was observed. Slow Stops elicited additional activation in anterior dorsal and polar prefrontal cortex and left inferior parietal cortex. Fast Stops showed additional activation in a network that included right dorsolateral prefrontal cortex, insula and dorsal striatum. These results are discussed in terms of our understanding of the impact of preparation on the distributed network underlying response inhibition and the contribution of subcortical areas, such as the basal ganglia, to executive control processes.