language is a critical component for communication, and for many how they internally process cognitive thoughts (“inner speech”)
speech can thus be considered in various components:
unspoken “inner speech”
attempted or spoken “produced speech”
perceived speech
inner speech, perceived speech, and reading are all represented in the precentral gyrus 1)
Inner speech
inner speech is also called imagined speech, internal speech, covert speech, silent speech, self-talk, speech imagery, internal monologue, or verbal thought
is theorized to support complex cognitive processes including working memory, verbal rehearsal, logical reasoning, executive function, behavioral control, and motivation2)
is also implicated in silent reading, with many people reporting evoked auditory or motor speech imagery while reading
inner-speech can be decoded from signals recorded by intracortical microelectrode arrays in the supramarginal gyrus (SMG), revealing a shared representation across inner, produced, and perceived speech
in 2025, a brain-computer interface (BCI) was developed which could decode inner speech in real time which can decode self-paced imagined sentences from a large vocabulary (125,000 words) and even allow for the user to select a password to turn decoding of inner speech on or off - the interface 3)
inner speech appears to be a more weakly modulated version of attempted speech, although the two can be distinguished with the help of a neural “motor-intent” dimension
an attempted speech BCI can be trained to ignore inner speech with high accuracy
Physiology
Wernicke-Geschwind model of the language system in the brain
a linguistic auditory signal is first sent from the auditory cortex to Wernicke's area
the lexicon is accessed in Wernicke's area, and these words are sent via the arcuate fasciculus to Broca's area, where morphology, syntax, and instructions for articulation are generated.
this is then sent from Broca's area to the motor cortex for articulation
Interperson communication brain coupling
word-specific brain activity peaks 250ms prior to the word being spoken and in response, the word-specific brain activity in the listener's brain peaks 250ms after it is spoken 4)
Pathophysiology
receptive dysphasia
inability to understand speech
usually due to damage to Wernicke's speech area
most commonly due to an ischaemic stroke affecting the posterior temporal lobe of the dominant hemisphere of the brain due to thrombus or embolus in the middle cerebral artery
nominal dysphasia
inability to find words
expressive dysphasia
inability to express speech
usually due to damage to Broca's area which is in the lower part of the premotor cortex in the language dominant hemisphere
dysarthria
inability or impaired articulation of speech
this is often due to impaired coordination or weakness of the speech muscles required to articulate speech
eg. slurred speech due to TIA or stroke affecting facial muscles
bulbar palsy
caused by a lower motor neuron (LMN) lesion in the medulla oblongata, or from lesions to these nerves outside the brainstem, and also snake venom, botulism.
this causes impairment of function of the glossopharyngeal nerve (CN IX), the vagus nerve (CN X), the accessory nerve (CN XI), and the hypoglossal nerve (CN XII)
there is usually also dysphagia, flaccid paralysis or weakness, drooling, nasal speech, decreased jaw jerk and decreased gag reflex
there may also be LMN weakness of the limbs
sparing of the ocular muscles differentiates it from myasthenia gravis
pseudobulbar palsy
usually caused by the bilateral damage to corticobulbar pathways, which are upper motor neuron (UMN) pathways that course from the cerebral cortex to nuclei of cranial nerves in the brain stem.
there is usually also dysphagia, small/stiff/spastic tongue, brisk jaw jerk, labile affect, UMN weakness of the limbs