Attentional modulations of conscious perceptionThe causal role of the frontal lobe and its white matter connections

Resumen

At any given moment, we receive much more information that our cognitive system can consciously process. Therefore, a mechanism is necessary to select the information that is relevant for our goals or for survival, and to filter out the irrelevant one. Attention is proposed as one of these mechanisms, preparing the system to allocate processing resources, selecting relevant information –filtering out irrelevant distractors–, and allowing for the control of such processing resources. These heterogeneous functions of attention have led to the conception of attention as a system composed of three networks: alerting, spatial orienting, and executive control (Petersen & Posner, 2012). Some attentional systems have also demonstrated their influence over conscious perception by modulating perceptual sensitivity and/or by biasing responses. This has led to propose a thigh relationship between attention and conscious perception, and to the search of common neural substrates. Attentional processes rely, to a great extent, in fronto-parietal networks (Fan et al., 2005). Meanwhile, some theories propose that conscious perception emerges when information is represented in a large-scale network of frontoparietal regions (Dehaene et al., 2006). Although the involvement of parietal regions in conscious perception is widely accepted, there is an extensive debate about a causal involvement of the frontal lobe (Boly et al., 2017; Odegaard et al., 2017). In this theoretical and empirical context, the main goal of the present doctoral thesis was to delineate the role of frontal regions, and long-range white matter tracts connecting them with other regions in the brain (i.e. the dorsal, middle, and ventral branches of the superior longitudinal fasciculus, SLF I, II, and III), in the relationship between attention and conscious perception. Specifically, we explored the role of the executive control network, which in this context has been less explored. This network is involved in the voluntary control of processing in novel or complex situations. To accomplish this aim, two different studies were conducted: In the first study, we searched for common neural substrates of executive control and conscious perception, employing a design of functional magnetic resonance imaging (fMRI). Participants had to detect nearthreshold Gabor stimuli, while resolving a Stroop task in which incongruent trials were infrequent. In the second study, we explored the causal involvement of a frontal region – the right SMA – in the interaction between executive control and conscious perception by using online transcranial magnetic stimulation (TMS). Additionally, a control (vertex stimulation) and an active condition (right frontal eye field [FEF] stimulation) were included. Participants performed a similar task that the one employed in the previous study. A common aim of both studies was to explore the involvement of the SLF I, II, and III in executive control modulations of conscious perception by using diffusion-weighted imaging (DWI) tractography. Results of the fMRI study demonstrated an interaction between executive control and conscious perception in the functional coupling between some frontoparietal regions, which were more strongly coupled for seen as compared to unseen reports, but only when trials were Stroop-congruent. Additionally, the microstructural properties of the left SLF II significantly correlated with the neural interaction between executive control and conscious perception. The follow up TMS experiment demonstrated that two of such reported frontal regions, the right SMA and the right FEF, were causally involved in the interaction between executive control and conscious perception, but only when white matter properties of the right SLF III or a frontal association tract (i.e. the left frontal aslant tract) were taken into account. Participants with a greater integrity of the tracts were sheltered from the TMS modulation. This suggest, at least, partially shared frontal neural substrates for executive control and conscious perception. In conclusion, this doctoral thesis provides new data for the understanding of how executive control interacts with conscious perception and about the neural mechanisms underlying these interactions. They support that the frontal lobe may be a common neural substrate for executive attention and conscious perception. They also emphasize the role of long-range white matter tracts, reinforcing models based on distributed networks. Finally, results highlight the importance of white matter in individual variability in cognitive measures and neuromodulation effects, in healthy participants.