Functions of the Dominant and Non-Dominant Hemispheres
Dominant Hemisphere (Usually Left)
The dominant hemisphere, typically the left in approximately 95% of right-handed individuals and 70% of left-handed individuals, is primarily responsible for language functions including speech production, comprehension, reading, and writing. 1
Language and Verbal Functions
- Language lateralization: All right-handed people lateralize language functions to the left hemisphere 1
- Speech production and comprehension: The left hemisphere controls expressive language (Broca's area) and receptive language (Wernicke's area) 1
- Reading and writing abilities: Written language processing is predominantly left-lateralized 1
- Verbal memory: Language-based memory systems are housed in the dominant hemisphere 1
Important Clinical Considerations for Handedness
- Only 70% of left-handed individuals have left hemisphere language dominance 1
- 15% of left-handed people have bilateral language representation 1
- 15% of left-handed people demonstrate right hemisphere language superiority 1
- Handedness assessment should extend beyond writing preference to include throwing a ball, using scissors, knife and fork to accurately determine cerebral dominance 1
Cognitive Functions
- Lexical processing and word retrieval: The left hemisphere manages lexical activation and selection during speech production 2
- Morphosyntactic processing: Grammar and sentence structure are left-lateralized 1
- Sequential and analytical processing: The dominant hemisphere excels at linear, logical tasks 3
Non-Dominant Hemisphere (Usually Right)
The non-dominant hemisphere, typically the right, controls visuospatial processing, attention (particularly to the contralateral left side), prosody, non-verbal communication, and emotional/social cognition—functions that are equally critical for quality of life despite historical underestimation. 4, 5
Visuospatial and Attention Functions
- Spatial cognition and perception: The right hemisphere processes visual-spatial relationships and organization 1, 3, 6
- Contralateral attention: The right hemisphere maintains awareness of the left side of body and environment 4, 3
- Unilateral spatial neglect occurs with right hemisphere damage: Patients ignore the left side of their world, failing to notice food on the left side of their plate or shaving only the right side of their face 4, 3
- Neglect is far more severe with right hemisphere lesions than the mild right-sided neglect occasionally seen with left hemisphere damage 3
Communication and Social Functions
- Prosody: The right hemisphere controls the emotional tone, rhythm, and intonation of speech 1, 6
- Non-verbal communication: Interpretation of facial expressions, body language, and gestures 6
- Pragmatic language skills: Adapting communication complexity to social context, interpreting humor and figurative language 1
- Social cognition: Mentalizing (understanding others' mental states) and emotion recognition 5
Executive and Cognitive Control
- Attention networks: The right hemisphere houses the dorsal attention network (DAN) critical for cognitive control 1
- Executive functions: Planning, organization, and cognitive flexibility 1, 5
- Visual processing: Complex visual analysis and pattern recognition 5
Motor Control
- Movement execution and control: The right hemisphere contributes to bilateral motor coordination beyond simple contralateral control 5
Critical Clinical Pitfalls
The "Non-Dominant" Misnomer
The term "non-dominant" is misleading and clinically dangerous—the right hemisphere performs functions equally essential for quality of life, and damage produces devastating cognitive and behavioral deficits that are often underrecognized. 5
- Right hemisphere deficits are frequently mistaken for psychiatric illness or generalized dementia rather than focal neurological injury 3
- Patients with right hemisphere lesions may deny illness (anosognosia) or even deny ownership of their left body parts, creating profound safety risks 4, 3
- Cognitive deficits after right hemisphere surgery are common when objective neuropsychological testing is performed, despite historical assumptions of minimal impact 5
Hemispheric Lateralization is Regional, Not Global
Language lateralization varies at the subhemispheric scale—different language functions (phonological, semantic, syntactic) show different patterns of lateralization across cortical regions, subcortical structures, and even the cerebellum. 7
- Approximately 15-50% of focal epilepsy patients show atypical lateralization patterns across different brain regions 7
- One healthy control in a recent study was left-lateralized anteriorly but right-lateralized posteriorly for language 7
- The concept of "hemispheric dominance" is oversimplified and potentially misleading for surgical planning, particularly for focal resections 7
Right Hemisphere Language Contributions
- The right hemisphere participates in diverse language functions including non-verbal semantic processing 5
- Following extensive left hemisphere stroke, the right hemisphere can compensate for some language functions, particularly domain-general processes like lexical selection 2
- High-level language disorders from right hemisphere stroke (executive dysfunction affecting language) may go unnoticed in structured settings but become apparent in less controlled home environments 1