An overview of the neural correlates of meditation
A key question in relation to the neural correlates of meditation is whether there is a single area or only a few areas of the brain crucially involved in meditation experiences, associated with attention, awareness, and compassion (localistic view), or whether there is a large-scale brain activity pattern (holistic view) involved in meditation. Several findings about the neural correlates of meditation (Lutz et al. 2007, 2008b; Manna et al. 2010; Raffone and Srinivasan 2010) suggest a “middle way” as the most plausible response to such a question, which goes beyond the dichotomy of localistic and holistic views. Indeed, it seems plausible that meditation leads to changes in activity and structure of specific sets of interacting brain areas, in terms of networks or functional systems, rather than of a single key area or a large, undifferentiated set of brain regions. In particular, it seems that meditation influences brain networks involved in cognitive control and conscious access (e.g., Raffone and Srinivasan 2009), feeling and emotional awareness (e.g., Craig 2009; Lutz et al. 2008b), and processing related to the self (e.g., Pagnoni et al. 2008).
Another question is whether such neural activities and structures involve more “archaic” brain regions, such as those of the limbic system connected to emotions and body regulation, or whether more cognitive and “refined” areas acquired more recently in evolution, such as the prefrontal cortex, are involved. The latter, more than any other brain regions, differentiate Homo sapiens from other species, and can be related to fluid intelligence, planning, and cognitive control. Furthermore, one may wonder whether meditation affects “deep” regions of the brain, for more basal functions, such as thalamic nuclei or brain stem structures involved in regulating vigilance and the sleep-wake cycle, and whether there is a larger involvement of the right brain hemisphere or of the left hemisphere in meditation. There might also be interest in which brain rhythms (electroencephalographic or EEG) are mainly involved in meditation, such as the alpha or theta rhythms. Finally, one may wonder whether there are universal neural correlates for all types of meditation, or whether there are specific correlates of each type or form of meditation. Below I will attempt to provide tentative answers to such challenging questions based on neuroscientific findings.
A panoramic overview of neuroscientific studies of meditation suggests that attention (“concentration”) and awareness (mindfulness) during meditation are associated with a set of areas at different functional levels in the brain, at both cortical and subcortical levels. These include sensory areas such as the somatosensory cortex and posterior insula, which are involved in processing different types of body inputs, such as those related to touch, pain, and the so-called interoception, that is sensory inputs from the inside of the body such as visceral inputs (e.g., Craig 2009). Areas implicated in rapid emotional responses, such as the amygdala, can also be associated with meditation states and meditation-based mental training (Hoelzel et al. 2011; Lutz et al. 2008b). The amygdala is an area of the so-called limbic system for emotional coloring of perceptual and memory inputs linked to the survival of the organism, and is over-activated in stress and anxiety, and altered in mood disorders. Remarkably it has been found that meditation (mindfulness) based training does not only change the function (activity) of the amygdala, but also its structure (Hoelzel et al. 2011).
Meditation-based mental training also influences areas linked to episodic memory, such as the hippocampus, and thus not only responses to perceptual inputs. In particular it reduces the occurrence of over-general memories in autobiographical retrieval. This influence therefore increases the specificity and vividness of autobiographical memory, such as in depression, which is often characterized by over-general memories (e.g., Williams et al. 2000).
Areas linked to the control of attention and cognitive processing, such as the anterior cingulate cortex and dorsolateral prefrontal cortex, are also markedly associated to meditation states and traits (Cahn and Polich 2006; Raffone and Srinivasan 2009). These areas are involved in several aspects of cognitive functioning, including the control of attention and conscious access to perceptual information, as well as in so-called cognitive flexibility, i.e. the ability to respond to stimuli in flexible ways depending on the current task context and demand (Moore and Malinowski 2009). Meditation-related brain regions also include those associated with monitoring and stimulus-independent thinking, such as the anterior prefrontal cortex, the brain region that more than any other differentiates Homo sapiens from other animals (including primates), and other areas linked to body (interoceptive) awareness, feelings, and mental states, such as the anterior insula (in particular in the right hemisphere) (see Hoelzel et al. 2011; Lazar et al. 2005; Lutz et al. 2008b; Manna et al. 2010; Siegel 2007).
Moreover, several studies suggest that meditation and mindfulness modulate the brain network involved in self-representation and self-referential processing, the so-called “default mode network” (Raichle et al. 2001). In other words, meditation affects those areas of the brain that are responsible for shaping our experience of“self ” and our thinking about this “self.” This network also appears to be involved in mind wandering during resting and task performance, in self-projection in the past and in the future, and in several types of identifications linked to one’s own mental processes and representations of the mental states of others. It is not by chance that mindfulness meditation makes the operation of such a network more flexible and regulated, and reduces the conditioning of mind wandering on cognition, mood, and mental states, as well as the identification with mental processes and aspects of the flow of experience (Dor-Ziderman et al. 2013; Pagnoni et al. 2008). Therefore, meditation appears to change not only brain responses to stimuli, but also the intrinsic or spontaneous activity of the brain during wakefulness. More research needs to be conducted to explore whether spontaneous mentation and brain activity during dreams is also affected by meditation training.
In sum, neuroscientific findings suggest that meditation enhances the abilities of cognitive and affective regulation, strengthening the ability to regulate dysfunctional mental states and conditioning such as stress, anxiety, and negative mood. And, in more subtle ways, meditation regulates processes of identification and attachment. Such neuroscientific outcomes are consistent with results from several studies into the clinical effectiveness of mindfulness-based programs for stress and anxiety reduction, for preventing relapse into depression (through mindfulness-based cognitive therapy—MBCT—Segal et al. 2002), and in focused applications such as for treatment of eating disorders, addictions, and post-traumatic stress disorder (see Keng et al. 2011).
It is also remarkable that meditative practices appear to be linked to brain areas that are involved in the different functions of cognitive, affective, and body state regulation. Such areas can be regarded as interfaces between emotion and cognition, and between the regulation of mental and body states. In particular, such areas include the anterior cingulate cortex and the anterior insula, both regions that were acquired remotely in evolution, but with important re-adaptations in humans. For example, in humans such areas incorporate the so-called von Economo neurons, which are capable of establishing long- range connections in the brain and can thus mediate influences of emotions, motives, and mental states over long distances in the brain, such as related to consciousness and decision making.
According to the perspectives of influential researchers (Allman et al. 2001; Craig 2009), such neurons play a key role in mediating the influences of mental states on thoughts and actions: for example, the influence of negative and unwholesome mental states such as anger. In other words, meditation and mindfulness might be expected to influence the ability to reduce the impact of anger by regulating the “broadcasting” of anger-related signals in the brain. In combination with reducing the influence of negative mental states on consciousness, positive mental states generated with meditation, such as serenity and joy, may spontaneously broadcast in the brain and thus reduce mind wandering and support sustained attention, receptive awareness, and meditative insight. Interestingly, such neurons appear altered in autistic people (Allman et al. 2005), who often have heightened anxiety, and can also be found, though in less refined patterns, in other species that display refined social interactions, such as other superior primates, dolphins, and elephants (e.g., Butti et al. 2009; Hakeem et al. 2009; Nimchinsky et al. 1999). In this respect, it is noteworthy that such species are capable of mirror self-recognition, suggesting some enhanced self-awareness in comparison with other species.
Thus, it seems that such key areas, which provide interfaces between mind and body, and emotion and cognition, and mediate the influence of mental states on thinking, consciousness, and action, are modified by meditation. For example, a recent study (Hasenkamp et al. 2012) has shown that such areas are activated when there is awareness of distraction during meditation, while other areas, such as the dorsolateral prefrontal cortex, are activated in the process of refocusing and sustaining attention on the intended meditation object.
Moreover, both the anterior insula and anterior cingulate cortex are involved in pain experience, particularly in the subjective or secondary aspect of pain experience (Baliki et al. 2009; Vogt and Sikes 2000). Other regions, such as the somatosensory cortex and posterior insula, are involved in the direct (primary) sensory experience of pain, and others, such as the amygdala, in the unpleasant feeling of the pain experience. In a recent study (Zeidan et al. 2011) it was found that a meditative exercise focused on breathing (for 20 minutes per day for four days) led to modified activities of the anterior insula and the anterior cingulate cortex, as related to subjective reports of the pain experience. However, the responses of brain areas associated with the sensory experience of pain were not modified. This is consistent with the accounts of meditators managing pain described by Vidyamala Burch in Chapter 7 of this volume.
These findings help to counteract a common and inappropriate stereotype about meditation: that it is a means for withdrawal, isolation, abstraction, or anesthesia from the surrounding world. Rather, the neuroscientific evidence suggests that meditation changes the interpretation of sensory inputs, which are, however, openly received in the field of perceptual awareness, rather than being gated (except for visual inputs when meditating with closed eyes). In the same vein, it has been found that compassion meditation does not lead to a suppression of sensory inputs related to the pain of another person, but rather intensifies them, while activating higher-level areas related to empathy and emotion sharing (which is different from emotional contagion) (Lutz et al. 2008b). Remarkably, such areas include the anterior insula, which also appears to be a key brain area linked to mindfulness (see also Lazar et al. 2005).
This research suggests that brain areas that are plausible substrates of mindfulness are also implicated in empathy and compassion. Such evidence is consistent with the suggestion that the ability to connect and understand our own emotions and mental states, which is developed through mindfulness (insight) meditation, is related to the ability to empathize with others. Psychotherapy research suggests that therapist difficulty in seeing and recognizing their own emotions and experiences makes it difficult for them to recognize the same emotions and experiences in others (e.g., patients) (Stedmon and Dallos 2009).
Now we turn to the question of whether meditation practice is associated with hemispheric lateralization of brain activity patterns. Even though the function of sustained attention in meditation appears linked to the right brain hemisphere, neuroscientific findings show the involvement of both hemispheres in meditation, although not necessarily with a bilateral involvement of the different regions. Such involvement also depends on the specific task, subjects, and type of meditation (e.g., see Cahn and Polich 2006; Lutz et al. 2008a).
Finally, regarding the brain rhythms related to meditation, taking together different studies it seems that multiple rhythms can be related to meditation states and traits, including the so-called “slow” delta, alpha, and theta rhythms and the “fast” beta and gamma rhythms. Again, this depends on the tasks, subjects (their expertise), forms of meditation, and observed brain regions (see, for example, Cahn and Polich 2006).