Corpus callosum

Disconnection syndromes and the role of connecting fibers

Normal brain function is impossible without adequate signal transmission between different areas. Association, projection, and commissural fibers play a key role in this. Damage to these pathways causes so-called disconnection syndromes.

The most critical feature of this pathogenetic mechanism is that the neurons of the cerebral cortex themselves remain completely intact. Only the "routing" of signals is disrupted. The sources of incoming information (afferentation) for these polyfunctional networks are the primary sensory and motor areas, the mediodorsal and caudolateral parts of the thalamic pulvinar, as well as connections from the opposite hemisphere.

Higher cortical functions and lateralization

To implement speech, perception, planning of complex movements, and control of social behavior, the functioning of tertiary (polyfunctional) association areas, which occupy more than 50% of the cortical surface, is necessary.

Of particular importance is the lateralization of speech function:

• In the vast majority of people (about 95%), the main speech centers are located in the left hemisphere (frontal and temporoparietal areas). As a rule, this is the side opposite to the dominant right hand.
• The right hemisphere does not remain uninvolved: it provides the crucial affective (emotional) component of our speech.

Topography of speech centers and speech physiology

Speech centers are strictly separated from the primary motor and sensory areas of the cortex. Hearing is processed in the transverse temporal gyri of Heschl, vision in the visual cortex, and muscle innervation for sounds originates from the primary motor cortex. Two main centers are distinguished:

1. Broca's area (Brodmann area 44) is located in the basal part of the left frontal lobe. It is activated during speech and inner speech.
2. Wernicke's area (Brodmann area 22) is located at the junction of the temporal and parietal lobes. It is responsible for analyzing heard sounds and assigning them to meaningful words.

Speech formation can proceed via two pathways. "Automatic speech" is formed through the chain: primary visual/auditory cortex $\rightarrow$ insular cortex (activated during simple repetition) $\rightarrow$ motor cortex. "Non-automatic speech" requires the immediate activation of Broca's area.

Speech disorders: aphasias

A consequence of damage to cortical areas or the pathways connecting them is aphasia—a speech disorder. There are also isolated deficits: dysgraphia/agraphia (writing impairment) and dyslexia/alexia (reading impairment).

According to speech tempo, aphasias are divided into:

• Fluent (sensory) — speech is fast and easy, but meaningless.
• Non-fluent (motor) — speech is hesitant, produced with immense effort.

It is important to distinguish aphasia from articulation disorders (dysarthria or anarthria). Dysarthria occurs when the pyramidal tract, cerebellum, or brainstem nuclei are damaged. In this condition, only pronunciation and phonation are impaired, while grammar, syntax, and morphology (per se) remain completely normal.

Comparative characteristics of main types of aphasia

Each type of aphasia has a characteristic pattern of impairment:

• Broca's aphasia: Spontaneous speech is significantly reduced. Sentence structure is impaired (agrammatism, phonetic errors). However, comprehension of meaning is preserved. Often accompanied by right-sided hemiparesis.
• Wernicke's aphasia: Spontaneous speech is not impaired (fluent), prosody is normal. However, speech comprehension is severely impaired. Semantic paraphasias (word substitutions), neologisms, and paragrammatism are characteristic. In extreme cases, "word salad" (jargon aphasia) occurs. Accompanied by right-sided hemianopsia.
• Conduction aphasia: A classical example of a partial disconnection syndrome. Spontaneous speech and comprehension are preserved, but mild impairment is detected during word repetition and object naming (phonetic paraphasias are allowed).
• Global aphasia: Severe impairment of all components—from comprehension to spontaneous speech and word repetition.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Features of transcortical motor and amnestic aphasias
• The role of the insular cortex in the formation of automatic speech pathways
• Differential diagnosis of dyslexia and alexia
• Anatomy and physiology of the auditory cortex (Heschl's gyri)
• Topographic features of primary sensory cortical areas

Other topics in the section "DeMyer's The Neurologic Examination" / "Duus' Topical Diagnosis in Neurology"

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