Choosing to inhibit: new insights into the unconscious modulation of free-choices

In our daily activities, we all experience a certain degree of control over our behaviours and the more we feel ‘in control’ the more we are likely to describe our behaviours as self-generated or ‘intentional’. Such intentional control refers to the capacity of humans to perform actions based on internal decisions and motivations, rather than external stimulation. Within the psychological debate on free will, this evidence raised the question on how ‘free-choices’ are taken when decisions are not dictated by immediate external imperatives. The core argument concerns whether voluntary actions follow a conscious intention to act or whether the feeling of being in control is just an epiphenomenon of unconscious neural mechanisms that are the true origin of behaviour. Different components of a free-choice can be isolated and investigated. Participants can choose what action to make, when to make an action, or whether to make an action at all (Brass & Haggard, 2008). Each of these refers to a different aspect of free-choice, but all of them involve the presence of a choice between multiple available options. Studying voluntary responses in this manner and comparing them with action or inhibition in response to a specific external stimulus, allow us to obtain useful insights into the origin of endogenous decisions. Among these components, the decision about whether to act – the so called ‘intentional inhibition’ – has received less attention. Such decision can be taken at almost any stage during motor preparation, until a point of no return (Schultze-Kraft et al., 2016). Libet (1983) controversially suggested that last-moment decisions to inhibit an action involved a purely conscious form of ‘free won’t’ (Libet, Gleason, Wright, & Pearl, 1983). However, alike voluntary actions, conscious decisions to inhibit might also depend on unconscious brain processes. For this reason, to what extent intentional decisions to inhibit are necessarily based on a deliberate choice is still an open question (Parkinson & Haggard, 2014). The present thesis will examine how unperceivable – subliminal – information in the environment, physiological states of the body and ongoing pre-conscious fluctuations in brain activity contribute to generate voluntary decisions to act or to inhibit. Starting from the contemporary debate raging around free will and taking into account the most recent cognitive models of voluntary actions, the introductory section of the thesis will provide an overview on the basic concepts linked to volition (Chapter 1). Particular attention will be given to behavioural inhibition and how this component has been studied along a continuum from ‘stimulus-driven inhibition’ to ‘intentional inhibition’. For each concept introduced, I will provide a review of the current state of the art regarding both the neural and behavioural mechanisms involved. In particular, I shall focus on previous research suggesting that making free-choices activate a specific network of brain activity. Chapter 2 will review current evidence regarding how subliminal information in the environment and psychophysiological states act as modulators for free-choice mechanisms both at the behavioural and neural level. Indeed, there is consistent evidence concerned with the ability of subliminal stimuli to bias our free decisions by influencing the activity within the ‘choice network’. Similarly, psychophysiological states such as the arousal have been shown to moderate a number of cognitive tasks including response inhibition. The second part of the thesis will focus on the empirical work I have conducted to investigate some of the theoretical issues previously introduced. The experiment described in Chapter 3 exploits a ‘Go/Nogo’ paradigm assessing the effect of subliminal priming by highlighting the dramatic effect of congruent and incongruent subliminal information on reaction times and free-choices. As the first experiment validated the paradigm as a meaningful tool to disentangle between forced and free components of making choices in relation to subliminal processing, the functional magnetic resonance (fMRI) experiment described in Chapter 4 capitalizes on the same kind of manipulation. A region of interest (ROI) analysis was conducted to test whether the degree of intentionality of the response and the information provided by subliminal information might modulate the activity within the ‘free-choice network’. In Chapter 5 the effect of an increased level of arousal induced by physical exercise on the performance in the same task will be examined. The experimental section of the present thesis will end with Chapter 6 in which the neural underpinnings of the conscious generation of actions by means of multi-voxel pattern analysis (MVPA) has been investigated. The thesis will end with a general discussion (Chapter 7). Here I shall rely on the evidence presented in the preceding experimental chapters to propose that free-choices are determined by the interplay between brain, body, and sensory environment