Frontal lobe 2

Anatomy of the Orbitomedial and Prefrontal Cortex

• Overview of anatomical structures in the orbitomedial and prefrontal cortex.

  • Main gyri and sulci relevant to functional zones.

  • Identification of brain areas on MRI.

  • Functional anatomy of the Prefrontal Cortex (PFC).

Medial Frontal Region

• Anatomy:

  • The cingulate gyrus lies adjacent to the corpus callosum and below the superior frontal gyrus.

  • Boundaries:

  • Cingulate sulcus separates the superior frontal gyrus from the cingulate gyrus.

  • Callosal sulcus separates the cingulate gyrus from the corpus callosum.

  • The cingulate sulcus is deflected posteriorly to meet the superior margin of the cerebral hemisphere, termed the pars marginalis or marginal ramus of the cingulate sulcus.

  • The pars marginalis (PM) helps identify the central sulcus on mid-sagittal sections.

  • Central sulcus is located just anterior to the PM and exhibits a T-shaped formation at the pars marginalis.

Paracentral Lobule

• Function: Responsible for sensation and movement of contralateral lower extremities, comprising the:

  • Primary Motor Cortex (M1) located on the precentral gyrus.

  • Primary Somatosensory Cortex (S1) located on the postcentral gyrus.

Supplementary Motor Area (SMA)

• Features:

  • The premotor cortex exists in the lateral hemispheric convexity, while a smaller premotor region on the medial side is referred to as SMA (Broadmann Area 6).

  • Both lateral and medial premotor areas share similar histological structures and are classified within BA 6.

• Function: Participates in a basal ganglia loop which is crucial for the selection and execution of voluntary motor acts.

• Body Mapping:

  • Each hemisphere's SMA contains a complete map of the body and activates synchronously, contrasting with the lateral premotor area (LPMA) which can work independently.

• Action Preparation:

  • SMA primarily influences proximal and axial musculature, establishing the "postural context" for voluntary actions (e.g., hands).

  • Involved in:

  • Internally-generated acts (e.g., spontaneous voluntary actions, self-initiated actions from memory, bilateral tasks).

  • Example: SMA involvement in throwing a ball vs. LPMA in catching a ball.

• Volitional Movement Hierarchy:

  • The SMA converts the desire to act into physical movement:
    $ext{SMA (M2)} <br>ightarrow ext{M1}$
    $ext{(M3, M4)} <br>ightarrow ext{SMA (M2)} <br>ightarrow ext{M1}$

  • The SMA serves as a crucial intermediary in this sequence.

  • Underactivity results in akinetic states such as seen in Idiopathic Parkinson's disease, which impacts voluntary actions more than reactive ones.

Pre-Supplementary Motor Area (Pre-SMA)

• Located anterior to the SMA and more strongly connected to the PFC.

• Function: Higher cognitive function regarding decision-making among alternatives.

Medial Prefrontal Cortex (mPFC)

• Composition: Encompasses a larger area of the PFC including Dorsolateral Prefrontal Cortex (DLPFC) and Orbitofrontal Cortex (OFC).

• Characterization: Exhibits high metabolic activity, part of the default mode network, implicated in:

  • Introspection and continuous self-awareness.

  • Significance of subdivisions:

  • Dorsal (cognitive-executive functions).

  • Ventral (emotional-affective functions).

• Lesion Implications:

  • Apathy and abulia (passivity and lack of drive).

  • Akinetic mutism.

Topography of the Orbital Cortex

• Orbital Frontal Cortex (OFC): Located on the ventral aspect over the eye sockets.

  • Key components:

  • Gyrus rectus: Thin cortical strip in the medial orbital region.

  • Lateral Orbital Cortex: Comprising four orbital gyri defined by an H-shaped sulcus.

    • Anterior Orbital Gyrus (frontal pole).

    • Posterior Orbital Gyrus (higher-order sensory processing).

    • Medial and Lateral Orbital Gyri (both sides of the H-shape).

• Historical Context: Brodmann's Cytoarchitectonic map represented the entire region as Area 11, discussing subdivisions.

• Connection to Emotion: Meso cortex in the posterior region linked to emotional and behavioral processes.

Frontal Pole and Area Contributions

• Frontal Pole: Involves BA 10, associated with divided attention, concentration, and task allocation.

• Overview of Brodmann Areas:

  • Anterior half: Represents 6-layered neocortex.

  • Posterior half: Involves non-neocortex meso cortex.

• Orbitofrontal Cortex Functions:

  • Implicated in behavioral restraint and inhibition of impulse control.

  • Example: Managing cravings to avoid negative long-term outcomes like obesity.

Clinical Observations

• Effects of OFC dysfunction:

  • Loss of inhibition leading to impulsivity and poor decision-making.

  • Associated Conditions:

  • Phineas Gage case highlighted severe personality change due to bilateral OFC damage after a brain injury.

• Summary:

  • PFC constitutes 30% of cortical surface area, mainly anteriorly positioned in the frontal lobe.

  • Orbital and medial regions correlate with behavioral and personality functions, while the dorsolateral area focuses on cognitive abilities.

Regions of the Prefrontal Cortex

Dorsolateral Prefrontal Cortex (DLPFC)

• Located in the superior and middle frontal gyri.

• Mapped as BA 8, 9, and 46 (key for working memory).

Ventrolateral Prefrontal Cortex (VLPFC)

• Comprising Broca's area within the inferior frontal gyrus and the frontal operculum.

• Areas designated as BA 44, 45, and 47/12.

Frontal Pole (Frontopolar Region)

• Defined as BA 10.

PFC Syndromes

Dorsolateral Pathology

• Associated cognitive deficits:

  • Literal interpretation of metaphors, loss of abstract thinking, forgetfulness, and indecisiveness.

  • Difficulty in planning and multitasking.

Orbitofrontal Pathology

• Behavioral alterations:

  • Disinhibition and inappropriate behavior, impulsiveness, rash decision-making, and lack of insight.

Medial Frontal Dysfunction

• Emotional and motivational changes:

  • Dominant hemisphere lesions lead to apathy and depression.

  • Non-dominant hemisphere lesions result in pleasant indifference or elation.

  • Loss of empathy and lack of initiative (abulia).

Prefrontal Cortex Functions

• Decision Making and Behavior:

  • Helps organize purposeful and goal-directed behavior (short, medium, long-term).

  • Incorporates social interactions, personal commitments, and overcoming obstacles.

• Moral Judgement:

  • Reflects ethical decision-making reliant on mPFC function.

• Attention:

  • Each PFC region has a role in selective attention.

  • mPFC provides emotional drive; DLPFC strategizes and focuses attention, inhibiting distractions.

Case Study: Maureen O'Connor

• Former San Diego Mayor with a brain tumor affecting medial and orbital regions.

  • Highlights impact of omPFC damage on moral decision-making.

Attention and Dysfunction

• Attention regulated by the inhibition of competing stimuli.

• Dysfunction can stem from various issues across PFC regions.

• Attention deficit hyperactivity disorder (ADHD) may indicate underlying psychological or sensory issues.

Summary of PFC Roles

• Prefrontal region comprises 30% of cerebral cortex.

• Key contributions:

  • mPFC: Emotional motivation.

  • DLPFC: Strategic planning.

  • OFC: Behavioral restraint against short-term gratification.

Anatomy of the Orbitomedial and Prefrontal Cortex

• Overview of Anatomical Structures:

  • Focus on the orbitomedial and prefrontal cortex, which collectively guide complex behavior, personality, and executive control.

  • Integration of gyri and sulci landmarks to define functional zones: medial, lateral, and orbital surfaces.

  • Identification of critical brain areas via MRI and histological classification (Brodmann Areas).

Medial Frontal Region

• Cingulate Gyrus and Sulci:

  • The cingulate gyrus ($BA 24, 32, 33$) lies adjacent to the corpus callosum, forming a core part of the limbic system.

  • Boundaries:

  • Cingulate sulcus: Separates the superior frontal gyrus from the cingulate gyrus.

  • Callosal sulcus: Separates the cingulate gyrus from the corpus callosum.

  • Pars Marginalis (PM): The posterior deflection of the cingulate sulcus. It serves as a landmark for the Central Sulcus, which is consistently located just anterior to it, often forming a T-junction.

Paracentral Lobule

• Function & Location:

  • Encompasses the medial extensions of the precentral and postcentral gyri.

  • Primary Motor Cortex (M1) ($BA 4$): Specifically the medial portion controlling the contralateral lower limb and sphincters.

  • Primary Somatosensory Cortex (S1) ($BA 3, 1, 2$): Processing sensory feedback from the contralateral lower extremity.

Supplementary Motor Area (SMA) and Premotor Regions

• Lateral vs. Medial Premotor Cortex:

  • Both are classified as Brodmann Area 6 ($BA 6$).

  • Lateral Premotor Area (LPMA): Primarily involved in externally-cued movements (e.g., catching a ball in response to visual stimuli).

  • SMA (Medial BA 6): Primarily involved in internally-generated movements (e.g., deciding to get up or performing a sequence from memory).

• Functional Dynamics of the SMA:

  • Participates in a Basal Ganglia-Thalamo-Cortical loop, crucial for suppressing unwanted movements and executing planned ones.

  • Somatotopy: Contains a complete body map; characteristically, bilateral activation occurs even during unilateral tasks, aiding in bimanual coordination.

  • Postural Context: Influences proximal and axial muscles to stabilize the body during distal limb movement.

• Volitional Movement Hierarchy:

  • M3/M4 (Cingulate Motor Areas) $\rightarrow$ SMA (M2) $\rightarrow$ M1 (Primary Motor).

  • Dysfunction in this hierarchy (e.g., in Parkinson’s Disease) leads to akinesia or difficulty initiating spontaneous actions.

Pre-Supplementary Motor Area (Pre-SMA)

• Localization: Situated anterior to the SMA.

• Connectivity: Higher connectivity with the Prefrontal Cortex compared to the motor regions.

• Function: Governs the "cognitive" side of motor control, such as switching between tasks or resolving conflict between competing motor plans.

Medial Prefrontal Cortex (mPFC)

• Composition: Includes the medial portions of $BA 9, 10, 11, 12$ and the anterior cingulate ($BA 24, 32$).

• Default Mode Network (DMN):

  • The mPFC is a central hub of the DMN, showing high metabolic activity during "rest" and introspection.

• Functional Subdivisions:

  • Dorsal mPFC: Connected to the DLPFC; involved in social cognition and monitoring others' intentions.

  • Ventral mPFC: Connected closely to the amygdala and hypothalamus; involved in emotional regulation and visceral responses.

• Lesion Clinical Correlates:

  • Apathy: Lack of feeling or interest.

  • Abulia: Pathological inability to make decisions or exercise willpower.

  • Akinetic Mutism: A state where the patient is awake but neither speaks nor moves.

Topography of the Orbital Cortex (OFC)

• Ventral Anatomy:

  • Gyrus Rectus: The most medial strip, separated from the medial orbital gyrus by the olfactory sulcus (housing the olfactory bulb/tract).

  • H-Shaped Sulcus: Divides the lateral orbital surface into four quadrants:

  1. Anterior Orbital Gyrus: Cognitive evaluation of rewards.

  2. Posterior Orbital Gyrus: Integrates sensory information (taste, smell, touch).

  3. Medial Orbital Gyrus: Links to emotional processing.

  4. Lateral Orbital Gyrus: Linked to suppressing behavior when rewards are no longer present.

• Cytoarchitecture:

  • Transition from agranular/dysgranular (posterior/limbic) to granular (anterior/neocortical) cortex.

  • Higher "limbic" connectivity in the posterior sections ($BA 13, 14$ in some classifications).

Frontal Pole and Specific BA Roles

• Frontal Pole (BA 10):

  • The largest cytoarchitectonic area in the human brain; essential for metacognition, multitasking, and holding long-term goals while performing sub-tasks.

• OFC Functions:

  • Impulse Control: Managing the conflict between immediate gratification (limbic) and long-term consequences (cortical).

  • Clinical Case (Phineas Gage): Landmark case of bilateral OFC/mPFC damage resulting in a transformation from a responsible foreman to an irreverent, impulsive individual.

Regions of the Prefrontal Cortex (PFC)

• Dorsolateral PFC (DLPFC) ($BA 8, 9, 46$):

  • The "Executive Center": responsible for working memory, cognitive flexibility (set-shifting), and planning.

• Ventrolateral PFC (VLPFC) ($BA 44, 45, 47$):

  • Includes Broca’s Area (left hemisphere $BA 44, 45$) for motor speech and syntactic processing.

  • $BA 47/12$: Involved in behavioral inhibition and processing of language semantics.

PFC Syndromes & Clinical Observations

• Dorsolateral Syndrome (Dysexecutive):

  • Deficits in the Wisconsin Card Sorting Test (perseveration).

  • Concrete thinking; inability to grasp metaphors or abstract concepts.

• Orbitofrontal Syndrome (Disinhibited):

  • "Environmental Dependency Syndrome": patients might impulsively use objects just because they are there.

  • Lack of social tact, hypersexuality, and emotional lability.

• Medial Frontal Syndrome (Apathetic):

  • Pseudo-depression: specifically if dominant hemisphere is affected.

  • Sudden loss of empathy or the "spark" of personality.

Summary of Integrated Functions

• Decision Making: PFC allows for the simulation of future outcomes without physical risk.

• Moral Judgment: The mPFC integrates social norms with emotional weights to produce ethical decisions.

• Attention:

  • DLPFC: Top-down control of focus/filtering noise.

  • mPFC: Provides the motivational drive to attend to a task.

• Case Study: Maureen O'Connor's brain tumor (meningioma) pressing on the orbital and medial surfaces led to a loss of financial judgment and gambling $\$1$ billion, illustrating that "moral" centers are biologically grounded.