The insular cortex, or insula, is a part of the human brain that, despite making up only about two percent of the total cortical surface area, plays a significant role in our inner world and interactions with the environment. Its hidden position and extensive connections suggest a multifaceted nature. This region continually reveals new insights into how we experience ourselves and the world, underscoring its importance in understanding typical brain function and various neurological and psychiatric conditions.
Locating the Insular Cortex in the Brain
The insular cortex is a distinct lobe of the cerebral cortex, often called the “island” (from the Latin “insula”) due to its concealed location. It is nestled deep within the lateral sulcus, also known as the Sylvian fissure, a prominent groove separating the frontal and parietal lobes from the temporal lobe. This hidden placement means it is not visible from the brain’s surface.
The insula is divided into anterior and posterior regions by the central insular sulcus, and it is surrounded by the peri-insular sulcus. The anterior portion consists of three short gyri, while the posterior part contains two longer gyri. Bordered laterally by the frontal, parietal, and temporal lobes, its position allows it to act as a central hub, facilitating communication and integration between various brain areas.
The Insula’s Diverse Roles
The insula integrates various types of information to create subjective experiences. Its functions span from basic bodily sensations to complex emotional and cognitive processes, connecting the mind and body.
Interoception
A primary function of the insular cortex is interoception, the sense and interpretation of internal bodily states. This includes awareness of physiological signals such as hunger, thirst, heart rate, and body temperature. The anterior insular cortex (AIC) is involved in mediating interoceptive attention, contributing to self and body awareness. The posterior insula specializes in sensory processing, including the perception of pain and temperature, with information flowing from posterior to anterior regions for complex integration.
Emotions
The insula is involved in processing emotions, particularly disgust. Its role extends beyond disgust, influencing emotions like fear, anger, happiness, and surprise. The anterior insula, with strong links to limbic structures, integrates emotional information with physiological signals, contributing to emotional awareness and empathy.
Decision-Making
The insula contributes to decision-making, especially choices involving risk and uncertainty. Activity in the insula, alongside the nucleus accumbens and anterior cingulate cortex, occurs during decision-making, reward anticipation, and feedback processing. The anterior insular cortex influences risk preference, with its inactivation potentially reducing risk-taking behavior. It is involved in anticipating outcomes based on internal bodily feelings and evaluating potential negative consequences.
Consciousness and Self-Awareness
The insula is involved in consciousness and self-awareness by integrating internal and external information. The posterior insula integrates homeostatic information, contributing to a basic physical sense of self-awareness. As information moves to the anterior insula, it becomes more complex, incorporating emotional and cognitive information, contributing to a coherent sense of self and conscious experience.
Pain Perception
The insular cortex contributes to the subjective experience of pain, distinguishing it from mere sensory input. Both the anterior and posterior insula are involved in different pain circuits, mediating sensory-discriminative and affective-motivational aspects. Electrical stimulation of the insula in humans can elicit pain, and its activation correlates with the perceived magnitude of pain.
The Insula’s Role in Health and Disease
Dysfunction within the insular cortex is linked to various health conditions, affecting how individuals perceive internal states, process emotions, and make decisions. These links offer insights into the underlying mechanisms of these disorders.
Addiction
In addiction, the insula plays a role in craving, drug-seeking behavior, and relapse. Damage to the insula is linked to a disruption in smoking addiction, leading to easier cessation without cravings or relapse. The insula represents the interoceptive effects of drug use and is activated during drug urges, correlating with conscious feelings of craving. It is involved in the subjective “feeling” of addiction and the motivation to seek drugs.
Anxiety and Mood Disorders
The insula is implicated in anxiety and mood disorders, including panic attacks, generalized anxiety, and depression. Patients with these conditions often show altered insular activity, related to changes in interoception or emotional processing. Increased insula activation is observed during aversive, anxiety-related processes, and individuals with panic disorder may have fewer inhibitory receptors in the insula. This altered processing can contribute to the distorted interpretation of bodily sensations.
Chronic Pain
Insular dysfunction contributes to the subjective suffering and persistence of chronic pain. The insula integrates information to influence both sensory and affective dimensions of pain. Changes in insular activity and connectivity, along with alterations in gray matter volume, are observed in various chronic pain conditions, suggesting its involvement in the transition from acute to persistent pain.
Eating Disorders
Eating disorders, such as anorexia nervosa and bulimia nervosa, are linked to insular function. Altered insular activation in response to food cues, and differences in interoceptive awareness of hunger and satiety signals, are observed in individuals with these conditions. This can contribute to body image distortions and altered perceptions of internal bodily states related to food intake.
Neurological Conditions
The insula is involved in certain neurological conditions. Strokes affecting the insular cortex can lead to symptoms including motor and somatosensory deficits, dysarthria, and different types of aphasia, depending on the specific area affected. In epilepsy, the insula can be a source of seizures, often due to structural abnormalities like neoplastic lesions, malformations of cortical development, or vascular malformations.