What is the function of the temporal lobe and what are the consequences of its dysfunction?

Temporal Lobe Function and Dysfunction

The temporal lobe plays critical roles in language processing, auditory function, memory formation, and emotional regulation, with dysfunction resulting in specific deficits including language impairment, memory loss, and seizures depending on the affected region.

Anatomical Structure and Organization

• The temporal lobe constitutes approximately 17% of the human cerebral cortex and is uniquely developed in primates, with the largest size in humans 1
• It encompasses both six-layered isocortex and allocortex regions that support auditory, visual, language processing, emotions, and memory functions 2
• The temporal lobe contains several key structures:
  • Lateral surface: Superior, middle, and inferior temporal gyri
  • Medial structures: Hippocampal formation (parahippocampal gyrus, subiculum, hippocampus, dentate gyrus) and amygdala 1
• The temporal lobe connects with other brain regions through multiple white matter tracts including the arcuate fasciculus, inferior longitudinal fasciculus, and uncinate fasciculus 1, 3

Functional Roles

Language Processing

• The left temporal lobe contains critical language areas including Wernicke's area in the superior temporal gyrus, essential for language comprehension 4
• Word comprehension deficits strongly correlate with blood flow within Wernicke's area, with lexical processing more strongly related to the volume of hypoperfused tissue than lesion volume 4
• The left posterior middle temporal/fusiform gyrus is crucial for naming functions 4
• The left temporal pole is involved in lexical and semantic retrieval of knowledge 4

Auditory Processing

• The primary auditory cortex is located in Heschl's gyri within the temporal lobe 2
• Auditory processing follows a hierarchical organization with increasingly complex sound processing occurring as information moves from primary to association areas 4

Memory Function

• The hippocampal formation and associated structures are critical for memory formation and consolidation 5
• Different temporal lobe structures contribute to specific memory functions:
  • Middle temporal gyrus in the dominant hemisphere generates N400 potentials that predict immediate recall performance 6
  • Left anterior medial temporal lobe generates N400 potentials that predict delayed recall performance 6
• The dentate gyrus (part of the hippocampus) plays a crucial role in memory formation, with abnormalities found in 41% of infants who died unexpectedly 4

Emotional Processing

• The amygdala receives input from the olfactory bulb and association cortex for other sensory modalities 1
• It projects to the septal area and prefrontal cortex, mediating emotional responses to sensory stimuli 1
• The temporal lobe has important reciprocal connections to the limbic system, including the limbic cortex, hypothalamus, and amygdala, forming a key network in controlling autonomic function 4

Consequences of Dysfunction

Seizures and Epilepsy

• Temporal lobe epilepsy presents with distinctive symptoms including:
  • Auras (rising epigastric sensations, unusual smells)
  • Emotional or psychic symptoms (fear, anxiety, déjà vu)
  • Automatisms and post-ictal confusion 7
• FDG-PET shows temporal lobe hypometabolism in epilepsy patients with 79-95% sensitivity and specificity 4, 7
• Temporal lobe hypometabolic regions often extend beyond the presumed epileptogenic zone, reflecting broader cerebral dysfunction 4, 7

Language Impairment

• Damage to the left superior temporal gyrus can result in Wernicke's aphasia, characterized by fluent but meaningless speech with poor comprehension 4
• Posterior temporal-parietal lesions and damage to the dorsal superior longitudinal and arcuate fasciculus are associated with impaired repetition 4
• Damage to the left temporo-occipital areas can cause deficits in word finding 4
• Increased resting state functional connectivity between left and right superior temporal gyri correlates with poorer language function in stroke patients 4

Memory Deficits

• Bilateral temporal lobe damage can result in severe anterograde amnesia (inability to form new memories) 5
• Temporal lobe epilepsy is characterized by cognitive comorbidities affecting both spatial and non-spatial memory 8
• Bitemporal glucose hypometabolism is associated with memory deficits and higher risk of postoperative memory decline following temporal lobe surgery 4

Emotional and Behavioral Changes

• Damage to the amygdala can impair emotional processing, particularly fear recognition 1
• Hypometabolism associated with the ipsilateral insular cortex may correlate with emotional or somesthetic symptoms 4
• Temporal lobe dysfunction can contribute to psychiatric symptoms including anxiety, depression, and psychosis 7

Clinical Assessment and Imaging

• MRI is the preferred imaging modality for temporal lobe assessment, with high-resolution protocols including coronal T1-weighted imaging and fluid-attenuated inversion recovery sequences 7
• FDG-PET can detect hypometabolism in epileptogenic zones with high sensitivity, particularly valuable when MRI is negative 7, 5
• Functional MRI activity in the early subacute phase following stroke shows promise as a predictor of long-term language recovery 4
• Resting state functional connectivity is a promising candidate biomarker for cognitive and language function assessment 4

Treatment Approaches for Temporal Lobe Dysfunction

• Medical management is typically attempted before considering surgical intervention for conditions like temporal lobe epilepsy 5
• Complete surgical resection of the epileptogenic region is the treatment of choice for medically refractory mesial temporal sclerosis 5
• Concordance between multiple diagnostic modalities (MRI, EEG, PET, SPECT) is associated with better surgical outcomes 5
• Contralateral thalamic hypometabolism on imaging is associated with poorer surgical outcomes compared to ipsilateral thalamic hypometabolism 5