During a developmental screening at a primary health centre in Delhi, a 2.5-year-old girl is identified with delayed speech, poor social engagement, and repetitive hand-flapping behaviours. Her parents report no significant perinatal complications or family history of neurodevelopmental disorders. Which is the most common neurobiological abnormality associated with Autism Spectrum Disorder?

Explanation

## Neurobiological Abnormalities in Autism Spectrum Disorder ### Most Common Structural Finding **Key Point:** Reduced grey matter volume in the prefrontal cortex (PFC) and temporal regions (including superior temporal sulcus and amygdala), coupled with altered functional and structural connectivity, represents the most consistently replicated neurobiological finding in ASD across neuroimaging studies. ### Neuroimaging Findings in ASD ```mermaid flowchart TD A["ASD Brain Abnormalities"]:::outcome --> B["Structural Changes"]:::outcome A --> C["Connectivity Alterations"]:::outcome B --> D["Reduced grey matter<br/>PFC & temporal cortex"]:::action B --> E["Cerebellar changes<br/>less consistent"]:::action C --> F["Reduced long-range<br/>connectivity"]:::action C --> G["Increased local<br/>connectivity"]:::action D --> H["Most common finding"]:::urgent F --> I["Weak central coherence"]:::outcome G --> J["Enhanced local processing"]:::outcome ``` ### Detailed Neurobiological Basis | Brain Region | Abnormality | Functional Consequence | |---|---|---| | **Prefrontal cortex (PFC)** | Reduced grey matter volume | Impaired executive function, social reasoning | | **Superior temporal sulcus (STS)** | Reduced volume & altered activation | Impaired biological motion perception, social cognition | | **Amygdala** | Variable size; altered connectivity | Atypical emotional processing, social fear | | **Temporal cortex** | Reduced grey matter | Language processing deficits | | **Cerebellum** | Reduced Purkinje cells (variable) | Motor coordination, cerebellar-cognitive syndrome | | **White matter** | Reduced fractional anisotropy | Impaired inter-regional communication | **High-Yield:** The "underconnectivity hypothesis" — reduced long-range connectivity between distant brain regions combined with relatively preserved or increased local connectivity — is a leading neurobiological model of ASD and is frequently tested in NEET PG. ### Connectivity Alterations **Mnemonic: "WEAK CENTRAL COHERENCE" (WCC)** - **W**eak long-range connectivity (between prefrontal and temporal regions) - **E**nhanced local processing (within-region hyperconnectivity) - **A**ltered default mode network (DMN) function - **K**inetic deficits in biological motion perception - **C**erebral asymmetry (atypical lateralization) - **E**xcitatory-inhibitory imbalance (E/I imbalance hypothesis) ### Clinical Pearl **Warning:** While cerebellar abnormalities (Purkinje cell loss, vermis hypoplasia) have been reported in some ASD cases, these findings are neither universal nor the most common neurobiological abnormality. Corpus callosum agenesis and hydrocephalus are rare and not characteristic of idiopathic ASD. ### Neurochemical Basis The reduced grey matter volume in social brain regions (PFC, STS, amygdala) correlates with: - Altered oxytocin and vasopressin signalling - Imbalanced excitatory (glutamate) and inhibitory (GABA) neurotransmission - Disrupted mirror neuron system function These changes underpin the core deficits in social communication and restricted, repetitive behaviours.