Comprehensive Study Notes on Brain Lateralisation and Language Processing

Defining Lateralisation and Structural Differences in the Brain

Lateralisation as a concept is defined as the situation where the brain area responsible for a specific function is located in only one of the two cerebral hemispheres. Lateralisation implies that this asymmetrical, one-sided processing is a characteristic found in the brains of the majority of human beings, though exceptions to this rule exist. Within the field of neuropsychology, researchers use structural magnetic resonance imaging (MRI) to identify physical asymmetries that may correlate with functional lateralisation. For instance, observations have shown that the secondary auditory cortex is often larger on the left side of the brain compared to the right side. This structural observation leads to the functional hypothesis: is this difference in structure tied to the function of "language"? Specifically, researchers investigate whether language is processed more effectively or predominantly in the left hemisphere than in the right.

Methodologies for Establishing Language Lateralisation

One of the primary methods used to determine language lateralisation is the Dichotic Listening Test. In this procedure, two different auditory stimuli, such as the syllables "ga" and "ba," are presented simultaneously to each ear. Processing in the brain for auditory signals is bilateral but is significantly stronger on the side contralateral to the ear receiving the sound. If a subject perceives the syllable presented to the right ear (e.g., "ga") before the syllable presented to the left ear (e.g., "ba"), this is designated as a "right-ear advantage." This result serves as an indicator that the left hemisphere is the dominant processor for syllables and language.

Another significant development in the study of lateralisation is the Wada Test, established by Juhn A. Wada in 1949. This method involves the injection of sodium amobarbital into either the left or right internal carotid artery (Arteria carotis interna). The effect of this injection is the temporary suppression of function in the corresponding hemisphere. Observations from these tests show that speech failure (aphasia) typically occurs only following a left-sided injection, rather than a right-sided one. Additionally, the test can induce short-term hemiparesis (half-sided paralysis of the arms and legs) on the side of the body opposite to the injection; for example, a left-sided injection leads to right-sided paralysis.

Transcranial Magnetic Stimulation (TMS) is also used to study language by creating an artificial "short-term lesion." By using a magnetic coil to stimulate specific areas, researchers can temporarily disrupt the electrical activation of the underlying neurons, leading to a brief loss of the associated brain function. A temporary loss of speech production is typically only observed when TMS is applied over the inferior frontal gyrus (IFG) of the left hemisphere, specifically within the region known as the Broca-Areal. However, researchers must be cautious as this pattern can be reversed in left-handed individuals, where the Broca-Areal may be located in the right hemisphere.

Insights from Split-Brain Patients and Hemispheric Communication

The study of split-brain patients provides profound insights into lateralisation. The two hemispheres are normally connected by the Corpus callosum (the corpus callosum or "Balken"). When this structure is severed, the patient is referred to as a "split-brain." It is important to note that the Chiasma opticum (optic chiasm), where the optic nerves from the eyes cross, is not affected by split-brain surgery. Because of this crossing, visual information from a right-sided image is transported to the left visual cortex, and a left-sided image is transported to the right visual cortex.

In a healthy brain, visual information can be passed across the corpus callosum to the left hemisphere for naming and language processing. However, in a split-brain patient, visual information presented to the left visual field (which goes to the right hemisphere) cannot be communicated to the left hemisphere for naming. Consequently, the patient can see the object but is unable to verbally state or name what it is. The patient perceives the object, yet the disconnect between the visual processing in the right hemisphere and the language center in the left prevents verbalization.

Language Disorders: Types and Characteristics of Aphasia

Aphasia refers to the failure or loss of language, which most commonly occurs following a stroke in the left hemisphere. Aphasia can impact language understanding and production to varying degrees, depending on which brain area—specifically the Broca or Wernicke area—is affected. There are three primary types of aphasia: Broca, Wernicke, and Global.

Broca-Aphasia results from a defect in the Broca-Areal, located in the inferior frontal cortex, usually on the left side. In this condition, language comprehension remains largely intact, but language production is severely disturbed. Patients tend to string together short, simple words or sentences. This same syntactic disturbance is also evident in their writing, reflecting the same issues found in their spontaneous speech.

Wernicke-Aphasia occurs when the Wernicke-Areal, or the secondary auditory cortex (usually in the left hemisphere), is damaged. These patients suffer from impaired language comprehension and may not understand questions asked of them. However, their language production is preserved, often resulting in long, complex, and grammatically correct yet nonsensical sentences. This condition also affects reading and writing, showing paraphasic distortions (word substitutions), and the understanding of meaning while reading is significantly impaired.

Global Aphasia occurs when both the Broca and Wernicke areas are defective. In these cases, both language production and speech comprehension are virtually non-existent. These patients often speak only single syllables or words, or repeat the same phrases (stereotypies). Their ability to understand speech is also drastically reduced.

The Wernicke-Geschwind Model for Language Processing

The Wernicke-Geschwind Model outlines the phases of language processing and production, mapping them to specific brain regions that are, in most cases, restricted to the left hemisphere. The process begins in the primary auditory cortex (both left and right), which performs the initial sound analysis, such as determining if a sound is high or low-pitched. Next, the secondary auditory cortex, referred to as the Wernicke Areal when it handles language, determines if the sound is speech and identifies the specific word, thus enabling comprehension. The information then proceeds to the inferior frontal cortex, known as the Broca-Areal for its role in production, which prepares the motor plan for speech. Finally, the motor cortex executes the pronunciation.

The Relationship Between Handedness and Lateralisation

There is a documented correlation between handedness and language lateralisation. Among right-handed individuals, nearly all have their Broca and Wernicke areas located in the left hemisphere. In contrast, findings for left-handed individuals are quite heterogeneous. Studies (such as those by Jäncke cited in Karnath/Thier) show that approximately $23-78\%$ of left-handers have left-sided Broca and Wernicke areas. Between $9-66\%$ exhibit bilateral (both left and right) language areas, and $11-19\%$ have right-sided language dominance.

One theory proposed to explain the origin of handedness and lateralisation is by Fred Previc (1991). He suggests that handedness is determined by the position of the fetus in the womb. According to this theory, a fetus with the right ear facing outward and the head positioned downward experiences stimulation that promotes the right-sided balance system. Consequently, after birth, the right side of the body is already trained for balance, allowing the right side of the body to take over other complex manual tasks, leading to right-handedness. This remains just one of many theories, as no single cause has been definitively proven.

Spatial Perception and Neglect Syndrome

Neglect patients exhibit a specific behavioral pattern where they focus their attention toward the right, even if the space is empty or if a person they are meant to address is standing directly in front of them. Neglect usually follows damage to the right hemisphere of the brain, particularly in areas critical for spatial attention. These areas include the inferior parietal cortex, the superior and medial temporal cortex, and the ventral prefrontal cortex. It is crucial to distinguish neglect from a vision problem; the visual cortex itself is usually undamaged, and the patient's basic sight functions normally. Instead, neglect is fundamentally a disorder of spatial attention.

In practical terms, neglect patients do not perceive objects or parts of objects located on their left side because of a rightward shift in their attention. This can be observed in tasks such as asking a patient to cross out lines on a page, where they will only cross the lines on the right half. Eye-movement tracking while observing a drawing shows a distinct lack of movement toward the left. This was famously documented in self-portraits by artist Anton Räderscheidt, who suffered from neglect after a stroke and gradually showed recovery over time.

Neglect is not limited to physical sight but also affects spatial imagination in the mind's eye. Based on research by Bisiach & Luzzatti (1978), neglect patients describing a familiar location will omit details on the left side relative to their imagined perspective. If they are asked to imagine looking at the location from the opposite direction, they will then omit the details that were previously described but now fall on their "new" left, and report details that were previously omitted.

Review Questions on Lateralisation

1) How is lateralisation defined? 2) Why is looking at an MRI image insufficient for concluding the lateralisation of language? 3) How is the dichotic listening test conducted, and how does it demonstrate language lateralisation? 4) How is the Wada test conducted, and how does it demonstrate language lateralisation? 5) How is a TMS test for language production conducted, and what does it show about lateralisation? 6) Which brain structures are defective or severed in a split-brain patient? 7) Why is the sense of sight not entirely lost in split-brain cases? 8) How is language lateralisation shown in split-brain patients, and what is the experimental setup? 9) Which types of aphasia are there, which brain areas are involved, and what are their key traits? 10) Describe the Wernicke-Geschwind model for language processing and production, and can you point out the areas on a brain model? 11) What is the connection between language and handedness, and do all left-handers have right-hemisphere language processing? 12) Name one possible theory regarding handedness and lateralisation. 13) What characterizes the behavior of a neglect patient? Provide examples. 14) Which brain areas are disturbed or remain intact in neglect? Can you label them on a brain model? 15) Why is neglect considered an attention disorder rather than a visual disorder?