Neurobiology of language

1. Understanding the Brain: The Neurobiology of Everyday Life
Neuroanatomy of speech: an example with aphasia
By Johanna Nicolle

2. The peripheral auditory system, with an
illustration of the outer, middle and inner ear.
Neuroanatomy of speech perception
The acoustic signal is a mechanical waveform, which passes
through the ear canal and reaches the tympanic membrane of the
middle ear in the form of variations in air pressure. It then
travels through the malleus, the incus and the stapes, the three
ossicles at the level of the middle ear that amplify the pressure
of the sound vibrations and transmit it to the oval window. The
latter is a membrane that separates the middle ear from the
cochlea, which is composed by fluid-filled sections. In this way
the sound is conveyed from an air to a fluid environment, still in
the form of a mechanical wave. It is in the cochlea that the
signal is transformed from a mechanical to a neural code by the
inner hair cells, whose mechanoreceptors capture the
movements of the endolymph, the fluid of the scala media, the
specific region of the cochlea where the wave firstly enters via
the oval window, and transform it into neural signals,
stimulating the neural dendrites of the auditory nerve.

3. Neuroanatomy of speech perception
The soundwave, now transformed into a neural signal, is
transmitted to the primary auditory cortex via several synapses
at the level of the brainstem and midbrain. It first stops at the
cochlear nucleus, at the level of the brainstem, and there some
fibers cross to the contralateral side while others continue
ipsilaterally, both reaching the superior olivary complex, at the
level of the pons. There again through contralateral or ipsilateral
projections the signal reaches the inferior colliculus, a structure
located at the level of the midbrain. From there, the signal is
transmitted to the thalamic medial geniculate nucleus, which is
the last relay before reaching the primary auditory cortex, at the
level of the temporal plane.

4. Neuroanatomy of speech perception
Now the question is: how to give rise to language expression and
production, what are neural processes involved, and basically how is it
support by the brain?

5. Neuroanatomy of speech perception
Aphasias are good examples to give a first idea of speech processing:
Pierre Paul Broca was a French surgeon and an anatomist who describes two important cases of aphasia
1. Leborgne: he was unable to produce any words or phrases expect “tan”.
2. Lelong had severely reduced ability to produce speech preventing him to not produce significant or complicated speech
Upon autopsy Broca examined their brains and noticed that in both patients was damaged in the posterior inferior frontal
gyrus which is now known as the Broca's area.
These cases demonstrated that patients with aphasia can understand words and simple sentences, they know what they
want to say but they are unable to generate fluent speech with correct grammatical structure of a sentence. Such disorders
impact though expression.

6. Neuroanatomy of speech perception
Aphasias are good examples to give a first idea of speech processing:
Another aphasia case was demonstrated by Carl Wernicke, a German psychiatrist and neuropathologist who detected
damaged in the Wernicke’s area, a posterior section of the superior temporal gyrus. Patients with Wernicke's aphasia
have an impaired comprehension of spoken and written word. Nevertheless, they are able to generate basic language,
but they have significant difficulty understanding either spoken or written language.
 network of brain areas that is responsible for both receptive and expressive language.
1. Productive language are therefore known to depend on the broca’s area which can be either in silent, or in writing,
or speaking.
2. Receptive language therefore depends on an area defined by Wernicke.