Modulación espacial de la percepción y la atención visual

Abstract

The world around us has a spatial organization that is also present in the representation that our brain builds. However, the cognitive resources employed to process each specific location of the visual field are different. Consequently, the spatial dimension might modulate cognitive processes such as visual perception and attention. Although this modulation has already been studied, there is no previous research systematically describing both the visual field and the perceptual and attentional neural mechanisms. Therefore, the main aim of this doctoral thesis was to investigate the spatial organization of the mechanisms underlying human perception and attention. Thus, the existence of optimal spatial locations in both processes could be revealed. To this aim, we conducted five studies combining stimulation at multiple locations of the visual field with high-density electroencephalographic (EEG) recordings and behavioural measures. Study I focused on investigating the spatial organization of perception. Stimuli were presented in 60 locations of the visual field according to the retinotopic organization and the cortical magnification of the visual cortex. Event-Related Potential (ERP) analysis revealed five components with specific spatial patterns: C1, C2, P1, N1 and P2. Specifically, C1 component was larger for peripheral stimuli. The optimal locations of C2 changed across the horizontal meridian, being peripheral for upper hemifield stimulation and foveal for lower hemifield. P1 and N1 were larger when the contralateral lower field was stimulated. Finally, P2 component was maximum for peripheral upper field stimuli. Once we defined the spatial differences in the perceptual process, we carried out Study II to describe whether this modulation was also present in the attentional process. Based on the previous results, the visual field division was reduced to 24 locations. We applied ERP and time-frequency analyses since alpha oscillations have been described as the best neural correlate for visuospatial attention. However, contrary to our expectations, non-attended stimuli elicited higher activity than attended stimuli, pointing to an exogenous attentional capture and/or a habituation effect we had not predicted in our experimental design. Participants were asked to respond to target stimuli in both attended and non-attended location, which could lead to the exogenous attentional capture. The habituation effect may have resulted from the stimulation of a single location on each trial, where the attended location was constant while non-attended location varied. Having identified the problematic factors in our previous design, we carried out Study III with the same purpose. Thus, we avoid the exogenous capture generated by non-attended stimuli asking our participants to respond exclusively to cued target stimuli. In addition, to mitigate the habituation effect, both attended and non-attended stimuli were presented together in each trial and the non-attended stimulus location was the same within each block. Our results showed that both P1 and N1 components generally benefited by attention across the visual field. Regarding alpha oscillations, attentional modulation exhibited a facilitation effect on the contralateral hemifield and an inhibitory effect on the ipsilateral hemifield. Based on the previous behavioural results about periphery regions of the visual field in this doctoral thesis and the lack of evidence in the previous literature, we design the Study IV. Here, we investigated the behavioural modulation of high eccentricity locations in attentional facilitation. Reaction times showed a facilitator effect, although the pattern of its spatial distribution was unspecific across the visual field. In addition, there was no attentional effect on correct responses, but discrimination ability was better for the right versus the left hemifield and for the peripheral versus parafoveal locations. Finally, Study V focused on the spatio-temporal modulation of attention during the orienting period, i.e., from cue to stimulus onset. We combined multivariate and time-frequency analyses to decode and describe the pattern generated by valid cues presented on either the left or the right hemifield. Our results showed two states in the orienting period, during which attention allocation could always be tracked by visual regions. First, attention rhythmically alternated between hemifields at a rate of 10 Hz, as in an exploration-exploitation state. Then, it settled onto the cued location, apparently triggered by frontal regions, entering into a stage of exploitation. To conclude, the current doctoral thesis shows the relevance of the spatial location in the spatio-temporal dynamics underlying visuospatial perception and attention, whose implications are crucial in research employing visual stimulation.