Human Affective and Social Neuroscience

Delving into Human Affective and Social Neuroscience offers a fascinating journey into how our brains process emotions and navigate the complexities of social interaction. This field explores the neural circuits underlying feelings like joy, sadness, and fear, as well as the mechanisms that drive our motivations and social connections. Understanding these brain processes is crucial for comprehending human behavior and mental health.

Unraveling Human Affective and Social Neuroscience: An Overview

Human affective and social neuroscience is an interdisciplinary field that combines insights from psychology, biology, and neuroscience. It seeks to understand the biological foundations of emotions and social behavior. Key areas of study include brain evolution, various neuroscientific methods, the intricate social brain, and specific neural circuits related to fear, sadness, and motivation.

The Essentials of the Brain: Structure and Function

The human brain is a complex organ divided into major regions, each with specialized functions. These divisions include the Telencephalon, Diencephalon, Brain stem, and Cerebellum.

• Telencephalon: The largest part of the brain, comprising the cerebral cortex (dorsal portion), basal ganglia, amygdala, caudate, putamen, globus pallidus, substantia innominata (basal portion), and medial portion (hippocampal formation, cingulate gyrus, amygdala).
• Cerebral Cortex: The outer layer involved in higher-order functions like thought, language, and memory, organized into lobes (frontal, parietal, occipital, temporal) and specialized functional areas (motor, sensory, auditory, visual projection/association areas).
• Hippocampal Formation: Critical for memory transfer from short-term to long-term and learning mechanisms.
• Septum: Connects hippocampus and diencephalon; damage can cause hyperactivity and aggression, while stimulation can induce pleasure.
• Basal Ganglia (Corpus Striatum): Consists of the Caudate and Putamen, vital for motor and action planning, decision-making, motivation, reinforcement, and reward perception.
• Cingulate Gyrus: Involved in emotion formation and processing, learning, memory, and reward processing.
• Amygdala: A complex of nuclei in the temporal lobe crucial for emotional learning, emotional reactions, reward processing, and emotional memory.
• Diencephalon: Located between the telencephalon and brain stem, it includes the thalamus and hypothalamus, playing roles in sensory relay and autonomic control.
• Brain Stem: Connects the cerebrum to the spinal cord, controlling vital functions like breathing, heart rate, and sleep.
• Cerebellum: Primarily involved in motor control, coordination, and balance.

Neurons and Synapses: The Brain's Communication Network

Neurons are the fundamental units of the brain, communicating via electrical and chemical signals across synapses.

• Synapses: Connections between neurons.
• Electrical Synapses: Rapid and synchronous, common in reflexes.
• Chemical Synapses: Plastic, can amplify signals, and involve neurotransmitters.
• Neurotransmitters: Chemical messengers.
• Synthesized in presynaptic terminal: Biogenic amines (DOPAMINE, SEROTONIN, NORADRENALINE, ACETYLCHOLINE) and amino acids (GLUTAMATE – excitatory, GABA – inhibitory).
• Synthesized in neuronal cell body: Neuropeptides (SUBSTANCE P, ENDORPHINE) and gas substances (NITRATE OXIDE).

Brain Evolution: How Our Brains Grew

Brain size has significantly increased over hominid evolution, with accelerated growth starting around 1 million years ago. Chimpanzee and human DNA are 99% equal, yet human brains are substantially larger.

• Cost of Big Brains: Consumes 20% of the body's energy despite being only 2% of body weight.
• Theories for Big Brains:
• Ecological Theories: Environmental factors, like climate change, selected for enhanced cognitive problem-solving skills.
• Social Theories: The complexity of interactions within larger social groups drove brain size (larger social groups correlate with bigger neocortex size ratios).
• Cultural Theories: Cultural practices, such as milk consumption leading to lactose tolerance, selected for gene variants that could influence brain size.

Understanding Emotions: Fear, Sadness, and Motivation in the Brain

Emotions are central to human experience, and neuroscience provides insights into their underlying mechanisms.

Fear-Related Neural Circuits and Responses to Threat

Fear is a primal emotion with specific neural pathways. The amygdala, often called the “alarm center,” plays a crucial role.

• Key Brain Areas: Amygdala, hippocampus (emotional memories), prefrontal cortex (emotional regulation), cingulate cortex (emotional processing/modulation), and downstream targets of the amygdala for reflexive defense behaviors.
• Pathways of Threat Response:
• Direct Pathway: Sensory info → Thalamus → Amygdala (lateral nucleus) → Rapid physiological responses (heart rate, blood pressure, muscle activation).
• Indirect Pathway: Sensory info → Thalamus → Visual Cortex → Amygdala (lateral nucleus) → More detailed processing.
• HPA Axis Activation: The Hypothalamic-Pituitary-Adrenal axis mediates autonomous threat responses, involving the amygdala, hypothalamus, pituitary, and adrenal cortex.
• Fear vs. Sadness Neurobiology: Fear is minimally reduced by opiates but effectively by benzodiazepines. Its circuits involve the amygdala, anterior/medial hypothalamus, and dorsal PAG.
• Psychopathology: Fear circuits are implicated in specific phobias, social anxiety disorder, generalized anxiety disorder, and PTSD.
• Therapeutic Approaches:
• Exposure Therapy: A CBT approach based on the extinction model, where gradual exposure to feared situations reduces sensitivity over time.
• Pharmacotherapy: Benzodiazepines (e.g., diazepam) enhance GABA's inhibitory effect, reducing fear and anxiety symptoms. They bind to GABA-A receptors, concentrated in the ancestral fear circuit (central amygdala, PAG, reticular nucleus) and neocortex.
• Patient S.M.: A patient with bilateral amygdala lesions showed a lack of avoidant behaviors, autonomic responses to fear, and subjective fear ratings, yet could remember/describe fear and panic from CO₂ inhalation. This suggests fear can be dissociated from other emotions, and panic/fear may have separate neural pathways.

Sadness and the Brain: Attachment-Based Distress

Sadness, particularly separation distress, is deeply rooted in our neurobiology, often linked to early attachment experiences.

• Attachment: Seeking contact with an attachment figure during danger or stress, forming from birth and providing safety. Early childhood attachment quality influences physiological threat reactions.
• Separation Distress: Starts around 6 months, causing distress vocalizations (DVs). Severe early-age separation distress can permanently alter the HPA axis and CRF secretion.
• Brain Areas for Separation Distress (in animal models): Anterior cingulate cortex (ACC), dorsomedial thalamus, periaqueductal gray (PAG), regions within the oldest areas of the cerebellum, ventral septal area, dorsal preoptical region, and bed nucleus of stria terminalis (BNST).
• Sadness and Separation Distress: Recalling sad events activates similar areas to DV-related areas in animals, plus the insular and orbitofrontal cortex. The PAG is key, providing an affective component to physical pain and inducing DVs when stimulated dorsally.
• Neurotransmitters Influencing Separation Distress:
• Endogenous Opioids (Mu-opioids): Alleviate physical pain and DVs. Intense sadness is accompanied by decreased activity in mu-opioid circuits (rostral anterior cingulate, ventral pallidum, amygdala, temporal cortex).
• Modulation: Gentle touch stimulates opioid secretion, reducing DVs. Opioid receptor blockers (naltrexone) reduce this effect, while opiate administration reduces the need for touch.
• Oxytocin: Soothing touch stimulates oxytocin secretion, significantly inhibiting separation distress. Intranasally administered oxytocin reduces cortisol levels in social stress tests.
• Prolactine.
• Sadness vs. Fear Neurobiology: Sadness is effectively attenuated by small opiate dosages, with minimal effect from benzodiazepines. Its circuits involve the anterior cingulate cortex, dorsomedial thalamus, dorsal PAG, ancient cerebellar areas, BNST, and preoptic areas.
• Psychopathology: Sadness circuits are potentially related to panic disorder and depression.
• Panic Disorder: Benzodiazepines are ineffective, but antidepressant imipramine (effective in relieving animal separation distress) helps. Physiological aspects of panic attacks may stem from reduced opioid activity.
• Depressive Syndrome: Viewing sad film clips increases activity in the medial prefrontal cortex and cingulate gyrus. Increased resting state activity is found in the thalamus and subgenual cingulate (area 25), which shows volume reduction and fewer glial cells in depression. Increased activity in area 25 is associated with refractory depression, and its activity reduction correlates with antidepressant efficacy. Deep brain stimulation of area 25 can treat depressive states.
• Social Loss: Can activate the separation-distress circuit (HPA axis activation, DVs), potentially leading to depression, especially with reduced mu-opioids, serotonin, noradrenaline, and dopamine, and suppressed active separation distress.

The Major Motivation Circuit: Seeking and Anticipation

The brain's motivation system is primarily about