6. Chapter 6: General discussion
6.2. Summary of background and aims
Observing, recognising, understanding, learning, and imitating others’ bodily actions are fundamental processes in human interaction. To implement any of those cognitive processes, one perceives another person’s action and transforms such percept into a representation that can be later updated, retained, and accessed. The transformation of body-related information into an associative form that can be later recalled in favour of prospective behaviour requires memory mechanisms.
Most of the studies in the memory domain have used arbitrary stimuli such as sequences of numbers or letters, coloured squares, lines, and shapes, allowing us to identify non- overlapping memory systems to store semantic and visuo-spatial information in WM (Baddeley and Hitch, 1974). A memory system to hold more socially meaningful stimuli such as bodies and actions was later proposed by Smyth and colleagues, who provided behavioural evidence of a system to encode, recall, and maintain others’ actions in one’s memory (Smyth et al., 1988,1989). Importantly, these and other studies suggested that
postures) is underpinned by that neural circuitry that allow us to move and feel our own body, that is, our own body representation in the brain. Investigating these brain regions during encoding of visually perceived body stimuli is the main aim of the present thesis. With the advent of neuroimaging and novel electrophysiological techniques, two important discoveries in the fields of perception of actions and WM have guided my PhD work. Briefly, different studies in perception of actions and bodies showed that (1) a number of brain regions are active during action observation and action execution (di Pellegrino et al., 1992; Gallese et al., 1996). Therefore, part of the cortical areas that represent our own body and actions in the brain, allowing us to move and feel our body, play a key role in perception of others’ actions and bodies. Secondly, in the WM field, several studies showed that (2) brain regions contribute to perceiving and maintaining stimuli to-be- remembered in WM. Accordingly, the brain regions contributing to perception also underpin the maintenance of the percept in memory (Fuster and Alexander, 1971; Harris et al., 2002; Vogel and Machizawa, 2004; Serences et al., 2009). Those two findings suggest that brain regions with a role in perception may contribute to storing percepts beyond the perceptual stage. In the case of perceiving bodies and actions, somatosensory and motor cortices may underpin encoding and maintenance of bodily percepts beyond online perception. The examination of these brain regions during encoding of visual body- related stimuli is one of the main purposes of this thesis. In the next sections I revisit in more detail the aims and background information of the different chapters.
Background and aims of Chapter 2: Revealing hidden representations of the body in the brain. Overall, there is a good understanding on how to design experiments examining WM for arbitrary stimuli and on the perception of bodies through neuroimaging techniques. However, there is no clear approach to develop EEG studies on WM for visually perceived body stimuli.
The aim of Chapter 2 was to develop a combination of method and guideline to study perception and memory encoding of body-related images. This method is based on the ERP-EEG technique. Importantly, there are two constrains that need to be considered when applying such technique: i) ERP-EEG possesses magnificent temporal resolution but low spatial resolution. ii) Encoding body images elicits a visual response that spreads from posterior/visual to more anterior and body-related cortices (i.e., the neural candidates to process bodies in memory). This visual-evoked potential (VEP) masks brain responses that are also responsible for the processing of body-related information. Therefore, encoding and other processes linked to the transformation of bodily information onto our own cortical body representation (i.e., sensorimotor and somatosensory cortices) are difficult to dissociate from the VEPs generated at the sight of body stimuli. To solve this issue, we proposed the elicitation of a time-locked neural response in somatosensory cortex during encoding of visual information. This can be accomplished by delivering task- irrelevant tactile taps to the participants’ index fingers while they encode and hold in memory different numbers of body images. The tactile tap elicits a somatosensory-evoked potential (SEP) that allows measurement of somatosensory processing, which is modulated by the type of visual information ‘delivered’ by the VEP. Then, by means of the subtractive method detailed in Chapter 2 and 3, it is possible to dissociate SEPs from concurrent VEPS. Hence, it is possible to explore the involvement of body-related cortices when seeing and encoding bodily-related information with high temporal resolution.
Background and aims of Chapter 3: Neural dissociation for visual and sensorimotor WM - somatosensory brain areas.
Sustained activity has been associated with maintaining task-
relevant information in WM. This sustained activity seems to arise from those perceptual brain areas that participate in processing the percept in the absence of WM demands. Moreover, this form of activity increases with the number of stimuli to-be-remembered (i.e., memory load). For instance, Vogel et al. (2004, 2005) found an enhancement of EEGwaveforms arising from posterior/visual cortex that was concomitant to the number of visually depicted shapes to-be-remembered in a delayed match-to-sample task. Harris et al. (2002) and Katus and colleagues (2014, 2015) found that memorising a greater number of tactile taps encompasses increasing brain activity over somatosensory cortex. Despite the results of those studies showing enhancement of modality-specific cortices, it is unclear if sustained activity is defined by the sensory input modality or by the perceptual encoding properties of the information to-be-remembered.
Since perception of body stimuli (e.g., hand images, but not shapes) involves brain areas beyond visual cortices (i.e., parietal/somatosensory cortices; SCx), modulation of sustained activity by memory load could be found over SCx as result of visually driven processing of hand images in WM. To explore this hypothesis, in Chapter 3 we recorded different levels of sensory response during encoding of visual stimuli depicting hand images and shapes. Specifically, we elicited and later dissociated visual-evoked potentials and somatosensory-evoked potentials (VEPs, SEPs) by using the method detailed in Chapter 2. Then we examined whether or not the number of hand images to-be- remembered modulates visual cortex, as well as somatosensory regions beyond visual carry-over effects that are generated at the sight of body stimuli.
Background and aims of Chapter 4: Sensorimotor recruitment during WM for body and non-body-related images. Current models portray WM as a reestablishment of perceptual experience. Interestingly, we know from studies on action observation that body-related stimuli elicit perceptual activity beyond sensory-input streams. However, whether or not such brain regions (somatosensory and motor cortices) are also recruited during WM is still unclear.
In the previous chapter we applied a novel EEG method and developed a WM paradigm that allowed us to inspect one the neural candidates to support memory processing of
visual body images (i.e., somatosensory cortex, SCx). Here, we adapted this paradigm to explore another cortical region known for playing an important role in perception of bodies, the motor cortex. To this aim, we recorded visual and motor-cortical potentials during the active maintenance of body and non-body-related images in WM. The motor-cortical responses were elicited by a task-irrelevant key pressing that was performed during the retention interval of body and non-body-related images in WM. These motor-cortical potentials (MCPs) resemble the readiness potential (Deecke et al., 1976), an intricate component arising from motor cortices and known to expose the underlying processing of one’s forthcoming motor responses, the difficulty of an executed or imagined action (Kranczioch et al., 2009, 2010), as well as others’ observed bodily actions (van Schie et al., 2004). Remarkably, MCPs allowed us to probe the state of the motor cortex in a visual WM task by dissociating the underlying process from other on-going EEG components (see methods section Chapter 4).
Background and aims of Chapter 5: Disrupting sensorimotor processing during WM for body and non-body-related images. In the previous chapters I examined cortical potentials of somatosensory and motor cortices, two of the brain regions that could potentially underpin a WM system for visually perceived body information. Nevertheless, while our EEG studies showed contralateral involvement during encoding and maintenance of hand images in WM, earlier studies from Smyth and colleagues indicated general involvement of the sensorimotor system. Specifically, the authors showed that either hemisphere (contralateral and ipsilateral) seems to be responsible for the maintenance of visually perceived body postures and actions in WM.
In this chapter we aimed to investigate whether or not contralateral sensorimotor areas, which showed enhancement of activity during encoding and maintenance in the two EEG studies of this thesis, support memory maintenance of the stimuli in WM. In other words,
right/left hand images? To explore such somatotopic mechanisms, we created three different behavioural experiments that were based on the previous EEG studies. In each of these experiments, we interfered with the hypothetical sensorimotor processing of body images by adding a secondary task. The secondary task was performed during the maintenance of the stimuli in WM and executed by moving one hand (experiment 1 and 2) or both hands (experiment 3). The purpose of the secondary task was to interrupt sensorimotor processing through exhaustion of computational resources. If sensorimotor cortex supports memory maintenance of hand images in a somatotopic manner (i.e., following contralateral processing), memory performance for right hand images would be lower when moving the right hand than when moving the left hand (experiment 1 and 2, Chapter 5). On the contrary, if sensorimotor cortex supports memory maintenance of hand images in a more general fashion (i.e., irrespective of laterality conveyed in the hands), memory performance for right hand images would be similar when moving the right hand and left hand.