Neurology Chapter 3: Aphasia and Connectivity

Chapter 3: Neurology, Connectivity, and the Nature of Aphasia

• Objectives for the chapter:

  • Identify anatomical structures crucial for speech and language processing.
  • Understand the vascular system supporting these structures and how injuries affect behavior.
  • Describe methods of studying the brain and their impact on language processing.
  • Recognize different approaches to classifying aphasia.
  • Understand brain connectivity in language function and recovery.

• Introduction:

  • The chapter provides an overview of anatomical structures important for speech and language processing.
  • It includes an introduction to the vascular system and its damage effects from stroke.
  • It focuses on the relevant structures to language processing and how disruptions lead to aphasia.
  • It covers aphasia symptoms, classification approaches, and the neural connectivity involved in recovery.

Neuroanatomy Important for Speech and Language Processing

• Neural structures involved in repeating a word (e.g. neurorehabilitation):
  • Auditory
  • Semantic
  • Phonological
  • Articulatory processes

Brain Imaging and Neuromodulation Techniques

• Structural approaches: -Computed tomography (CT) and magnetic resonance imaging (MRI): Used clinically to diagnose stroke and brain injuries. CT uses X-rays and MRI uses magnets to create images of brain tissue (including diffusion-weighted imaging/tensor imaging for white matter).

• Functional approaches:

  • Functional MRI (fMRI): Measures blood flow changes in response to stimuli, indicating neural activity.
  • Positron Emission Tomography (PET): Measures blood flow by tracing radioactive isotopes.
  • Electroencephalography (EEG): Measures voltage fluctuations on the scalp.
  • Magnetoencephalography (MEG): Measures magnetic fields generated by electrical activity in the brain.
  • Functional near-infrared spectroscopy (fNIRS): Measures blood flow changes using near-infrared light

• Neuromodulation techniques: -Repetitive transcranial magnetic stimulation (rTMS) and Transcranial direct current stimulation (tDCS): Noninvasive techniques to alter brain activity (used to either increase or decrease cortical excitability.)

Noninvasive Brain Stimulation

• rTMS: Changes magnetic fields in the brain, either increasing or decreasing cortical excitability, helping in understanding neural function, and in therapeutic potential.
• tDCS: Uses low-intensity direct currents to alter neural activity, similarly used in treating impairments,

Aphasia From a Traditional Neuroanatomical Approach

• Cortical Aphasia:

  • Classifies aphasia subtypes based on fluency, comprehension, and repetition abilities, such as Broca's and Wernicke's aphasia.
  • Classifies presentations based on characteristics in relative repetition and auditory comprehension.
  • Uses neuroanatomical correlates (Table 3.2).
  • Can be further categorized into more subtypes.

• Subcortical aphasia:

  • Often described as borderline fluent.
  • Associated with thalamic and capsular/putaminal damage.
  • Often includes some impairments in phrase length, comprehension, and repetition, with variable features.

Other Approaches to Understanding Aphasia

• Cognitive neuropsychological approach: Focuses on the various levels of language processing across all modalities (e.g., auditory, visual, tactile).
• Language performance in individual with aphasia is examined, understanding impairment through models of healthy language processing rather than brain areas affected.
• Includes elements of phonology, semantics, phonological output and articulation, and similar models such as information processing models

Network-Based Perspective to Language Processing and Aphasia

• Neurotypical language network:
  • Bilateral frontal, temporal, and parietal regions work together
• Aphasia language network: -Similar patterns of structural connections in the brain (and disruptions) are seen in stroke patients as in healthy individuals, with functional and structural connections playing a role
• Connectivity:
  • Structural (MRI/DTI): White matter tracts connecting brain regions
  • Functional (fMRI): Interaction and communication between brain regions that is temporally correlated
  • Effective (fMRI): Causation or influence between specific brain regions.

Domain-General Systems and Language Processing

• Domain-general processes (e.g. attention, memory, and executive functions): Often play a role or may be involved in post-stroke language recovery.
• Neuroimaging studies suggest involvement of additional regions, such as the midfrontal gyrus (MFG), especially for more demanding language tasks. This is an important finding about the flexibility of the brain.
• These approaches are complementary, with the traditional neuroanatomical approach remaining useful for providing a broad overview.

Summary

• Aphasia is a multidimensional impairment arising from damage to complex brain networks, impacting various language domains.
• Current methods of evaluating aphasia should incorporate a deeper examination of underlying psycholinguistic processing, including phonology, semantics, and syntax, using multimodal neuroimaging for enhanced diagnostics.
• These methods would lead to a more comprehensive characterization of the impairment and facilitate better prognosis and treatment planning.

Study Questions

• Brain regions involved in writing a word from a picture.
• Regions of the brain supplied by the middle cerebral artery.
• Function of the Circle of Willis.
• Differences in approaches to understanding language in aphasia.
• Alternative brain areas connected with language processing.
• Neuroimaging methods used in aphasia.
• Classifications of aphasia and relative strengths and weaknesses in linguistic areas (e.g. fluency, phonology, semantics, syntax, morphology).
• Network-based approach vs. traditional neuroanatomical approach.
• Relevant connectivity concepts in post-stroke aphasia.

Explore the essential anatomical structures and vascular systems involved in speech and language processing. This chapter covers the effects of brain injuries on behavior, methods of studying the brain, classifications of aphasia, and the significance of brain connectivity in recovery. Gain insights into how these factors intertwine in understanding language disorders.