Anatomical substrate of speech
Speech as a higher mental function relies on the activity of the cerebral cortex. Secondary unimodal associative areas (e.g., area 22) and tertiary heteromodal areas (areas 39 and 40 at the junction of the parietal, temporal, and occipital lobes) play a key role in processing speech information. Damage to these areas leads to the breakdown of the speech system, as this is where sensory signals are integrated and complex mental acts are formed.
Interhemispheric asymmetry
The speech function is not distributed symmetrically. The dominant hemisphere (usually the left) specializes in the sequential analysis of abstract symbols: letters, words, and numbers. While the subdominant hemisphere is responsible for simultaneous analysis of holistic images, the dominant hemisphere provides the logical structure of language. Aphasia occurs precisely when structures of the dominant hemisphere are damaged, disrupting the brain's ability to process information sequentially.
Connection with other functions
Aphasia rarely occurs in isolation. Because speech areas are closely connected with regions responsible for gnosis (recognition) and praxis (goal-directed actions), the clinical picture is frequently supplemented by:
- Agnosias — impairment in recognizing objects.
- Apraxias — inability to perform complex motor acts.
- Impairments of the prefrontal cortex, which affect behavioral regulation and motivation.
Briefly: answers to topical questions
Where is the motor speech center (Broca's area) anatomically localized?
The motor speech center (Broca's area) is anatomically localized in the posterior part of the inferior frontal gyrus of the left hemisphere (in right-handed individuals).
- Localization — the basal part of the frontal lobe, in close proximity to the lower division of the motor area.
- Cytoarchitectonics — corresponds to area 44 according to Brodmann's classification.
This area is spatially separated from the primary motor areas of the cortex and is responsible for forming the vocalization program (the motor schema of speech).
Where is the sensory speech center (Wernicke's area) localized?
The sensory speech center (Wernicke's area) is localized in the posterior part of the superior temporal gyrus of the left hemisphere.
- Anatomical location — in the region where the temporal and parietal lobes meet, including the posterior half of the superior temporal gyrus and adjacent gyri of the inferior parietal lobule.
- Cytoarchitectonics — corresponds to Brodmann areas 22, 39, and 40.
This region is a secondary associative area of the auditory analyzer; it decodes information and gives rise to the perception of words.
What is the fundamental difference between aphasia and dysarthria?
The fundamental difference lies in the nature of the impairment:
| Characteristic | Aphasia | Dysarthria |
|---|---|---|
| Nature of the disorder | Partial or complete loss of the ability to speak | Impairment of articulation and phonation |
| Speech structure | Speech comprehension and/or the ability to produce coherent, logically structured speech may be impaired | Speech production per se is preserved, including grammar, morphology, and syntax |
| Causes of dysarthria/anarthria | — | Damage to the pyramidal tract, cerebellar pathways, brainstem motor nuclei, or the muscles themselves |
Which artery's basin supplies blood to the cortical speech centers?
The cortical speech centers (Broca's and Wernicke's areas) are supplied with blood from the middle cerebral artery basin (a. cerebri media).
- Broca's area — supplied from the central part of the middle cerebral artery basin (specifically, from the precentral sulcal artery).
- Wernicke's area — supplied from the posterior (parieto-occipital) part of the middle cerebral artery basin (predominantly from the angular and posterior parietal arteries).
The middle cerebral artery is the largest branch of the internal carotid artery and supplies blood to extensive areas of the frontal, temporal, and parietal lobes.
Aphasia is "A" (denial/absence) + "Phasis" (speech). Remember: "Left is logic and lexicon," which is why speech is specifically impaired when the left hemisphere is damaged.
Class and exam questions
Why is writing often impaired in aphasia as well?
Writing and reading are higher cognitive functions supported by the same association cortical areas as spoken language.
Is aphasia always associated with frontal lobe damage?
No, aphasia can occur when secondary and tertiary areas in the parietal, temporal, and occipital lobes are damaged, which is where the integration of speech functions takes place.
Does the level of arousal affect the manifestations of aphasia?
Yes, for higher mental functions to work effectively, an optimal level of arousal is required, which is maintained by the reticular formation and the thalamus.
What else to study on this topic
- Brodmann cytoarchitectonic areas and their role in speech.
- Differences between primary, secondary, and tertiary cortical areas.
- The role of the thalamus and subcortical structures in supporting cognitive functions.
- The relationship between aphasia and impairments of praxis and gnosis.
- Neurotransmitter systems (dopaminergic, cholinergic) in the regulation of cortical activity.
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