detecting-pre-stimulus-source-level-effects-on-object-perception-with

Detecting Pre-Stimulus Source-Level Effects on Object Perception with Magnetoencephalography

Pre-stimulus oscillatory brain activity influences upcoming perception. The characteristics of this pre-stimulus activity can predict whether a ...

Paris-Lodron Universität Salzburg

Visual rhythmic stimulation evokes a robust power increase exactly at the stimulation frequency, the so-called steady-state response (SSR). Localization of visual SSRs normally shows a very focal modulation of power in visual cortex and led to the treatment and interpretation of SSRs as a local phenomenon. Given the brain network dynamics, we hypothesized that SSRs have additional large-scale effects on the brain functional network that can be revealed by means of graph theory. We used rhythmic visual stimulation at a range of frequencies (4-30 Hz), recorded MEG and investigated source level connectivity across the whole brain. Using graph theoretical measures we observed a frequency-unspecific reduction of global density in the alpha band "disconnecting" visual cortex from the rest of the network. Also, a frequency-specific increase of connectivity between occipital cortex and precuneus was found at the stimulation frequency that exhibited the highest resonance (30 Hz). In conclusion, we showed that SSRs dynamically re-organized the brain functional network. These large-scale effects should be taken into account not only when attempting to explain the nature of SSRs, but also when used in various experimental designs.

Large-scale network-level processes during entrainment.

We recently proposed that besides levels of local cortical excitability, also distinct pre-stimulus network states (windows to consciousness) determine whether a near-threshold stimulus will be consciously perceived. In the present magnetoencephalography study, we scrutinised these pre-stimulus network states with a focus on the primary somatosensory cortex. For this purpose participants performed a simple near-threshold tactile detection task. Confirming previous studies, we found reduced alpha and beta power in the somatosensory region contralateral to stimulation prior to correct stimulus detection as compared to undetected stimuli, and stronger event-related responses following successful stimulus detection. As expected, using graph theoretical measures, we also observed modulated pre-stimulus network level integration. Specifically, the right primary somatosensory cortex contralateral to stimulation showed an increased integration in the theta band, and additionally, a decreased integration in the beta band. Overall, these results underline the importance of network states for enabling conscious perception. Moreover, they indicate that also a reduction of irrelevant functional connections contributes to the window to consciousness by tuning pre-stimulus pathways of information flow. 

The Tactile Window to Consciousness is Characterized by Frequency-Specific Integration and Segregation of the Primary Somatosensory Cortex.

The partial awareness hypothesis is a theoretical proposal that recently provided a reconciling solution to graded and dichotomous accounts of consciousness. It suggests that we can become conscious of distinct properties of an object independently, ranging from low-level features to complex forms of representation. We investigated this hypothesis using classic visual word masking adapted to a near-threshold paradigm. The masking intensity was adjusted to the individual perception threshold, at which individual alphabetical letters, but not words, could be perceived in approximately half of the trials. We confined perception to a pre-lexical stage of word processing that corresponded to a clear condition of partial awareness. At this level of representation, the stimulus properties began to emerge within consciousness, yet they did not escalate to full stimulus awareness. In other words, participants were able to perceive individual letters, while remaining unaware of the whole letter strings presented. Cortical activity measured with MEG was compared between physically identical trials that differed in perception (perceived, not perceived). We found that compared to no awareness, partial awareness of words was characterized by suppression of oscillatory alpha power in left temporal and parietal cortices. The analysis of functional connectivity with seeds based on the power effect in these two regions revealed sparse connections for the parietal seed, and strong connections between the temporal seed and other regions of the language network. We suggest that the engagement of language regions indexed by alpha power suppression is responsible for establishing and maintaining conscious representations of individual pre-lexical units.

Alpha suppression and connectivity modulations in left temporal and parietal cortices index partial awareness of words.

Periodic modulations of an acoustic feature, such as amplitude over a certain frequency range, leads to phase locking of neural responses to the envelope of the modulation. Using electrophysiological methods this neural activity pattern, also called the auditory steady-state response (aSSR), is visible following frequency transformation of the evoked response as a clear spectral peak at the modulation frequency. Despite several studies employing the aSSR that show, for example, strongest responses for ∼40 Hz and an overall right-hemispheric dominance, it has not been investigated so far to what extent within auditory cortex different modulation frequencies elicit aSSRs at a homogenous source or whether the localization of the aSSR is topographically organized in a systematic manner. The latter would be suggested by previous neuroimaging works in monkeys and humans showing a periodotopic organization within and across distinct auditory fields. However, the sluggishness of the signal from these neuroimaging works prohibit inferences with regards to the fine-temporal features of the neural response. In the present study, we employed amplitude-modulated (AM) sounds over a range between 4 and 85 Hz to elicit aSSRs while recording brain activity via magnetoencephalography (MEG). Using beamforming and a fine spatially resolved grid restricted to auditory cortical processing regions, our study revealed a topographic representation of the aSSR that depends on AM rate, in particular in the medial-lateral (bilateral) and posterior-anterior (right auditory cortex) direction. In summary, our findings confirm previous studies that showing different AM rates to elicit maximal response in distinct neural populations. They extend these findings however by also showing that these respective neural ensembles in auditory cortex actually phase lock their activity over a wide modulation frequency range.

Amplitude modulation rate dependent topographic organization of the auditory steady-state response in human auditory cortex.

Successful lip-reading requires a mapping from visual to phonological information [1]. Recently, visual and motor cortices have been implicated in tracking lip movements (e.g., [2]). It remains unclear, however, whether visuo-phonological mapping occurs already at the level of the visual cortex-that is, whether this structure tracks the acoustic signal in a functionally relevant manner. To elucidate this, we investigated how the cortex tracks (i.e., entrains to) absent acoustic speech signals carried by silent lip movements. Crucially, we contrasted the entrainment to unheard forward (intelligible) and backward (unintelligible) acoustic speech. We observed that the visual cortex exhibited stronger entrainment to the unheard forward acoustic speech envelope compared to the unheard backward acoustic speech envelope. Supporting the notion of a visuo-phonological mapping process, this forward-backward difference of occipital entrainment was not present for actually observed lip movements. Importantly, the respective occipital region received more top-down input, especially from left premotor, primary motor, and somatosensory regions and, to a lesser extent, also from posterior temporal cortex. Strikingly, across participants, the extent of top-down modulation of the visual cortex stemming from these regions partially correlated with the strength of entrainment to absent acoustic forward speech envelope, but not to present forward lip movements. Our findings demonstrate that a distributed cortical network, including key dorsal stream auditory regions [3-5], influences how the visual cortex shows sensitivity to the intelligibility of speech while tracking silent lip movements.

Nathan Weisz

Center for Cognitive Neuroscience

Paris-Lodron Universität Salzburg

Detecting Pre-Stimulus Source-Level Effects on Object Perception with Magnetoencephalography