SMA/PreSMA and Free Will PDF

22 February 2024 12:53 Main Ideas Notes The Problem of Volition Volition involves the exercise of will to perform voluntary actions, distinguishing between mere reflexes and actions taken w ith intention. Philosophical debates question whether volition undermines or enhances human dignity, with pers...

22 February 2024 12:53 Main Ideas Notes The Problem of Volition Volition involves the exercise of will to perform voluntary actions, distinguishing between mere reflexes and actions taken w ith intention. Philosophical debates question whether volition undermines or enhances human dignity, with perspectives ranging from autonomy to environmental determinism. Neuroscientific approaches aim to identify the neural substrates and pathways enabling volition, often focusing on the prefro ntal and motor cortical areas. What is a Voluntary Action? Voluntary actions are characterized by initiation in the absence of external stimuli, reliance on internal goals or motivatio ns, and the involvement of specific brain regions such as the preSMA and SMA. These actions contrast with reflexive or automatic responses, being under conscious control and typically directed towards ac hieving a particular outcome. The neural basis of what constitutes a "voluntary" action remains a subject of ongoing research and debate. Lateral Route Parietal-premotor-M1 This neural pathway is crucial for the execution of movements that are externally triggered, linking sensory input directly t o motor output. It involves the integration of sensory information from the parietal cortex to the premotor areas and finally to the primary motor cortex (M1) for action execution. This route supports actions that are responsive to immediate environmental demands. Medial Route preSMA-SMA?-M1 The medial pathway emphasizes internally generated actions, involving areas like the preSMA and SMA, which are linked to the planning and initiation of movements without direct external cues. It plays a key role in complex, sequenced actions and may contribute to the sense of agency or the feeling of initiating an a ction voluntarily. The preSMA, in particular, is implicated in higher-order decision-making processes related to action initiation. Contrasting Views of SMA/pre-SMA Function The SMA and pre-SMA are central to debates on their precise roles in voluntary action, with studies suggesting functions ranging from action initiation, sequence planning, to cognitive control and inhibition. Some views highlight the SMA's role in motor preparation and execution, while the pre -SMA is seen as more involved in decision-making and the integration of cognitive with motor processes. Internal Generation of Action Internally generated actions are initiated without external prompts, relying on internal decision-making processes. This involves a complex interplay between memory, motivation, and motor planning areas. The distinction between internally vs. externally triggered actions underpins much of the research on volition, highlighting the brain's capacity for autonomous action initiation. Two Cortical Systems for Directing Movement The dual-systems model proposes that the brain has separate but interconnected pathways for handling externally triggered (lateral rou te) and internally generated (medial route) actions. This conceptual framework helps explain how humans navigate between responding to environmental demands and acting on interna l intentions. Jenkins et al. (2000) PET Study This study employed PET imaging to explore the neural mechanisms underlying voluntary movements, distinguishing between movem ents made in response to external cues vs. those initiated internally. Findings emphasized the differential roles of cortical areas in supporting these distinct types of movements, contributing to our understanding of the neural basis of voluntary action. Cells Preferring IG vs ET Movements in Medial and Lateral Frontal Cortex Research involving cell-counting approaches has identified specific neuronal populations within the medial and lateral frontal cortex that preferentially respond to internally generated vs. externally triggered movements. These findings provide cellular-level evidence for the differential processing of movement types, offering insights into the neuroanatomical underpinnings of voluntary action. Summary PSYC0032 The Brain in Action Page 1 Notes Philosophical Interlude The philosophical examination of volition touches on fundamental questions about free will, consciousness, and the nature of human agency. Debates often center around whether our sense of volition is a true reflection of autonomous control over actions or an illus ion arising from brain processes. Advanced Topics Advanced discussions delve into critiques of traditional models of volition, exploring alternative theories such as the stoch astic decision model, which suggests that what appears as volitional action may arise from random fluctuations in brain activity. These topics challenge conventional understandings of conscious intention and free will, pushing the boundaries of neuroscien ce and philosophy. Tanji (2001) Annual Review of Neuroscience - SMA Neurons Chain Together the Different Movements within the Overall Chunk Tanji's review emphasizes the role of the preSMA and SMA in coding and chaining together action sequences, respectively. The preSMA is involved in coding an entire action sequence as a single chunk, highlighting its role in complex action plannin g. SMA neurons are responsible for chaining together the different movements within this overall chunk, facilitating the smooth execution of sequenced actions. The study suggests four separate cognitive mechanisms for action generation, ranging from specific action routines from memor y to muscle contraction. This pattern of neural activity provides a nuanced understanding of how internally generated movements are planned and executed, distinguishing between the contributions of the pre -SMA and SMA proper. Cognitive-motor Frontal Hierarchy The concept of a cognitive-motor frontal hierarchy integrates the roles of various frontal regions in action planning and execution. This hierarchy reflects the flow of decision-making processes from more abstract, cognitive areas (like the preSMA) to those directly involved in motor execution (such as the SMA and M1). It underscores the complexity of voluntary actions, which involve not just the execution of movements but also their initiation, planning, and the selection among alternatives. Inhibitory Functions of SMA/preSMA The SMA and preSMA play significant roles in the inhibition of actions, a critical aspect of voluntary behavior allowing individuals to choose not to perform an action or to stop an ongoing action. The stop-signal task is a commonly used experimental paradigm to study this inhibitory control, where the preSMA's involvement is crucial for successful stopping. Inhibition by the SMA/preSMA is essential for the suppression of inappropriate or unwanted actions, reflecting a higher levelof control over behavior. Neuropsychological Syndromes of Medial Frontal Inhibitory Function Disturbances in the inhibitory functions of the SMA/preSMA can lead to neuropsychological syndromes such as anarchic hand syndrome, where a person's hand performs actions without their intention. Another example is utilization behavior, where individuals automatically use objects in their environment without a deliberate intention to do so, suggesting a failure in the inhibitory control over actions triggered by environmental cues. Initiation of Voluntary Action and ‘Conscious Free Will’ - Kornhuber/Deecke (1966) The Kornhuber and Deecke study in 1966 introduced the concept of the readiness potential, a neural precursor to voluntary movements, suggesting that the brain prepares for action before an individual becomes aware of the intention to act. This research has profound implications for understanding conscious free will, suggesting that the sense of volition may emerge from unconscious brain processes that precede conscious decision-making. Brain Imaging of Intention Brain imaging studies on intention focus on identifying the neural correlates of the intention to perform an action before its execution. These studies have highlighted the importance of areas such as the preSMA and parietal cortex in forming intentions, showingthat specific neural patterns can predict the decision to act before the individual becomes consciously aware of it. Notes ‘Free Will’ The concept of free will is central to discussions of volition, with debates focusing on whether individuals have control ove r their actions or whether such decisions are predetermined by brain activity. Neuroscientific findings, such as those from readiness potential studies, challenge traditional views of free will by suggest ing that conscious decisions may be the outcome of unconscious neural processes. Feelings of Volition: Frontal vs Parietal Controversy (Desmurget et al., 2009) Desmurget and colleagues' work contributes to the debate on the neural basis of the feeling of volition, comparing the roles of frontal and parietal regions. Their findings suggest a division of labor, where frontal areas may initiate actions and parietal areas monitor these actions , highlighting the complex interplay between different brain regions in generating the sensation of voluntary control. Advanced Topic: Single Neuron Evidence for Accumulating Stochastic Activity in Voluntary Action Murakami et al. (2014) identified "Act Now" neurons, which show a preference for a specific time of action. This discovery su pports the idea that specific neuronal populations may play a pivotal role in the timing of voluntary actions. These neurons seem to collectively 'vote' on when to initiate an action, suggesting a form of neural consensus or accumulatio n of activity leading up to the decision point. This mechanism can be conceptualized as an integrator, where the accumulation of neural activity eventually crosses a thresho ld leading to action initiation. This process implies a stochastic, rather than deterministic, underpinning for the timing of voluntary act ions. Conscious Intention: Genuine Experience or Just an Illusion? The findings raise profound questions about the nature of conscious intention. Reconstructionism, as proposed by Wegner (2004 ), suggests that conscious actions are not genuinely initiated by us but rather happen to us, challenging traditional notions of free wil l. Constructionism argues that the feeling of conscious will is a consequence of motor preparation, as shown in the work by Frie d et al. (1991). This perspective posits that conscious will does not cause action; rather, it is a byproduct of the brain's preparation for m ovement. This debate underscores the complexity of understanding volition and the sense of agency. It suggests that what we experience as conscious intention might be an interpretative process of the brain, highlighting the intricate relationship between consciousness, bra in activity, and the initiation of voluntary actions. Advanced Topic: A Radical Critique The readiness potential (RP), traditionally thought to signify a volitional act's neural precursors, is argued by Schurger et al. (2012) to be an artifact arising from background brain activity fluctuations. This suggests that what has been interpreted as a marker of intentional action initiation may instead reflect a statistical t hreshold being crossed rather than a distinct decision-making process. Contrasting Views of Action Decisions The classical model views the RP as an internal signal indicating volitional intent, leading to action. However, the stochastic fluctuation model presents a challenge, proposing that apparent RPs could be the result of random flu ctuations in neural activity that reach a threshold, prompting action without a traditional 'volitional' decision. Notes PSYC0032 The Brain in Action Page 2