4. EVALUACIÓN DE LA AUDIENCIA
4.1. CIERRE Y EVALUACIÓN DE LA AUDIENCIA
The Bereitschaftspotential or readiness potential (RP) (Deecke et al., 1976; Shibasaki and Hallett, 2006; Smulders and Miller, 2012) refers to a movement- preceding negativity that originates in motor cortices before a movement is implemented (Cunnington et al., 2003). The RP has been classically understood as an indicator of preparation for self-initiated and goal-directed upcoming movements (Shibasaki and Hallett, 2006). The RP is normally elicited by asking participants to perform/execute simple finger or arm movements and it is normally recorded through EEG. The RP could be considered within the family components of motor-cortical potentials (MCPs).
According to the RP’s time course and source localization, two different periods can be distinguished: first, the RP appears as a steady bilateral activity that can be source localized over SMA. Second, it lateralizes over the motor cortices of the effector in the task around 300-500ms before the actual onset of the movement (Deecke et al., 1976; Cui and Deecke, 1999; Rueda-Delgado et al., 2014) (Fig. 4-1).
These lateralized components are well known in their isolated version: the lateralized RP. In unilateral motor responses, structural and functional interhemispheric asymmetries are deducted by computing a double ERP subtraction between mean amplitudes (De Jong et al., 1988; Eimer and Coles, 2003). The subsequent lateralized readiness potential (LRP) amplitude is modulated by several factors such as the force, intention, or the complexity of forthcoming action (e.g., finger movements) (Cui et al., 2000a, 2000b) whether these are executed or imagined (Kranczioch et al., 2009, 2010).
Another component of interest is the contingent negative variation (CNV), which can be measured from the onset of a warning stimulus to a forthcoming imperative
second stimulus involving a certain response. The CNV has been associated to orienting and expectancy towards a given signal (Leuthold et al., 2004; Leuthold and Jentzsch, 2009). Even though the CNV looks very similar to the RP and it engages similar neural generators, the RP is more lateralized, it requires motor response, and it is normally observed in the absence of external imperative stimuli. Remarkably, the lateralization of the RP seems to happen when participants decide consciously to perform a body movement (Trevena and Miller, 2002). An evident difference between RP and CNV is that the latter is expected to involve neural generators contributing to additional cognitive processes. Specifically, the later part of the wave of the CNV can be movement-related, as well as related to perception and other processes such as memory and attention (Smulders and Miller, 2012; Rueda-Delgado et al., 2014). The mix of cognitive processes found in the CNV is due to processing of the upcoming warning stimulus and the preparation of the movement to be implemented. Nevertheless, if a fast response is required in a CNV task, this activity would resemble that of the late RP: a response parameter associated with motor processing, involving sensory and motor associations that can be modulated by external factors inherent to the requirements of the task at hand (Frost et al., 1988; Leuthold and Jentzsch, 2009; Brunia et al., 2012).
Relevant for the present study is the variant of both aforementioned components: the cued motor-cortical potential. Compared to the volitional RP, this motor preceding negativity is elicited by explicitly asking participants to execute a movement. In comparison to other motor-cortical potentials (MCPs), it is very similar to the RP and the late portion of the CNV. In the specific case of the RP, both the cued MCP and the traditional RP possess similar latencies but the cued MCP involves additional activation from lateral premotor cortex (Smith and Staines, 2012).
Despite MCPs having been normally associated with goal-directed bodily actions, the requirement of sensorimotor activity as arising inly prior to motor execution is debated. For instance, listening to ‘do and don’t do’ abstract or action-related sentences activates the motor regions, reflecting different modulations between positive and negative action-related sentences compared to abstract ones across fronto-parietal cortices (Tettamanti et al., 2008). An EEG study showed that volitional non-actions (i.e., choosing not to act), elicits ERPs comparable to those observed during volitional and instructed acts (Kühn et al., 2009). Another EEG study, Alexander et al. (2016) compared volitional decisions with and without motor responses, showing a similar RP regardless of the presence of the actual motor response. These authors argue in favour of the RP as a neural signature reflecting decision-related processes instead of purely motor activity. Direct recording of neural activity in the premotor cortex of the macaque monkey has shown that neurons respond to both acting and non-acting (i.e., refraining from doing it) and that some these neurons fire when particularly observing others in either one of these two motor conditions (Bonini et al., 2014).
Overall, MCPs studies suggest that: i) sensorimotor cortices do not strictly support information-representation of the forthcoming movement. Instead, they seem to reflect the motor consequences of an act whether this is executed or not (i.e., the consequences of an action not to be performed). ii) This idea is strikingly similar to Prinz’s work (Prinz, 1997; Schütz-Bosbach and Prinz, 2007), which tied perception of events to the motor consequences embedded in them. This postulate implies the use of stored associations and representations to guide future behaviour. iii) Supporting this, predicting others’ actions during action observation modulates the readiness potential (Pineda et al., 2000; Kilner et al., 2004; van Schie et al., 2004). Thus, motor cortices seem to play an important role in processing visual information related to
others’ bodies and actions. Similar to Chapter 3, this visual to sensorimotor transformation allows the possibility of measuring visually driven processing of body- related stimuli during their maintenance in cortices other than visual (i.e., motor).
Figure 4-1. Illustration of readiness potential waveform. Considering time-course, slope, and sensory generators, three periods have been generally identified: early RP develops until - 500 to -300ms before movement onset (central, bilateral). Late RP develops until movement onset, generally includes the half second before movement onset to -80 or to 0 ms (zero indicating participants’ actual motor response) (contralateral to the effector). A reaffererent potential in the form of a positive peak is generally observed around 150-200ms after movement onset. Positive ERP waveform plotted upward.