Language : Researchers discovered previously hidden connections between brain layers during reading

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Language involves many different regions of the brain.

Researchers from the Max Planck Institute for Psycholinguistics and the Donders Institute at Radboud University discovered previously hidden connections between brain layers during reading, in a neuroimaging study reported in PNAS.

The team used laminar Functional Magnetic Resonance Imaging (lfMRI) to investigate what happens when people read Dutch words like “zalm” (salmon) compared to pseudowords (“rorf”), revealing top-down influences on deep brain layers for the first time.

How is a language represented in the brain?

This question is challenging, as language involves many regions throughout the brain that interact in a dynamic way.

For instance, when people read a word, they combine ‘bottom-up’ (lower level) visual information to recognise the letters, and ‘top-down’ (higher level) cognitive information to recognise the word and retrieve its meaning from memory.

Such top-down and bottom-up information streams are notoriously difficult to measure noninvasively (without having to open up the brain).

A research team led by Daniel Sharoh from the Donders Center for Cognitive Neuroimaging at Radboud University Nijmegen, Kirsten Weber (Radboud University, MPI), David Norris (Radboud University, MPI), and Peter Hagoort (Radboud University, MPI) wanted to investigate the brain’s reading network at a more fine-grained level.

They used the 7 Tesla MRI at the Erwin L Hahn Institute in Essen for laminar functional Magnetic Resonance Imaging (lfMRI), to measure brain activation at different depths or ‘layers’ of the brain–typically right next to each other and smaller than a millimetre.

Measuring at this level is important, as the layers can be related to the direction of the signals.

Deep layers are associated with top-down information, whereas middle layers are associated with bottom-up information.

Only laminar fMRI is sensitive enough to detect the deeper layers of the brain. With this new technique, would the investigators be able to find a top-down flow of information to the deeper layers of the brain for word reading?

To answer this question, the researchers created pseudowords such as “rorf” and “bofgieneer”, to be compared with real words such as “zalm” (salmon) and “batterij” (battery).

Pseudowords are ‘possible’ words that happen not to exist; they are pronounceable and therefore ‘readable’.

Twenty-two native Dutch speakers were asked to silently read the words and pseudowords as their brains were being scanned.

The participants also viewed ‘unreadable’ sequences of invented ‘false font’ characters that resembled existing letters. The task was to only press a button when the items were real words.

By comparing the brain activation for ‘readable’ items (words and pseudowords) and ‘unreadable’ items (false font), the investigators could isolate the ‘reading area’ of the brain.

This area is also known as the ‘visual word form area’ (VWFA) and is situated in the temporal lobe (the left occipitotemporal cortex).

As a next step, the researchers compared words directly to pseudowords, to further explore the VWFA.

Bottom-up sensory information is needed for both types of items, to recognise the strings as letters. But would top-down information from language areas be visible as well, needed to distinguish words from pseudowords?

The researchers found stronger activation for words than pseudo-words in the deep layers of the VWFA. This activation was caused by top-down projections from higher language areas of the brain (the left Middle Temporal Gyrus (lMTG) and left posterior middle temporal gyrus (lpMTG)).

These are well-known language areas involved in retrieving words and their meaning. In contrast, the researchers found decreased activation in the middle layer of the reading area, indicating that the deep layer ‘suppresses’ activation of the middle layer during word reading.

A conventional fMRI would have missed this nuance, as only laminar fMRI is sensitive to layer-specific activation.

The participants also viewed ‘unreadable’ sequences of invented ‘false font’ characters that resembled existing letters.

“This is a breakthrough finding for all imaging research”, says Peter Hagoort, director at the Max Planck Institute for Psycholinguistics in Nijmegen and co-author of the study.

“For the first time we have established different activation profiles at different layers of cortex, and figured out how this pattern is related to top-down influence in cortical processing. This has far-reaching consequences for cognitive neuroimaging, expanding our knowledge of brain networks.”


The cerebral cortex develops from the most anterior part, the forebrain region, of the neural tube.

Function

The frontal lobe:

It is the largest lobe, located in front of the cerebral hemispheres, and has significant functions for our body, and these are:

  • Prospective memory a type of memory that involves remembering the plans that you made, from a simple daily plan to future lifelong plans.[2]
  • Speech and language 

 The frontal lobe has an area called Broca’s area located in the posterior inferior frontal gyrus that is involved in speech production. A recent study shows that the exact function of Broca’s area is to mediate sensory representations that originate in the temporal cortex and going to the motor cortex.[3]

  • Personality

During the past centuries, several researchers have described that there are personality changes that occurred after frontal lobe injuries. One of the most important cases was about Phineas Cage, who was a gentle, polite sociable young, man until a large iron rod, went through his eye-damaging his prefrontal cortex.

This injury made him emotionally insensitive, perform socially inappropriate behaviors, and was unable to make a rational judgment. A recent study suggests that when there is damage to the prefrontal cortex, there are five sub-types of personality changes occur, and these include:

  • Executive disturbances
  • Disturbed Social behavior
  • Emotional Dysregulation
  • Hypo-emotionality/De-energization
  • Distress [4]
  • Decision making:

The ability to decide on something involves reasoning, learning, and creativity. A study conducted in 2012 proposed a new model to understand how the decision-making process occurs in the frontal lobe, specifically how the brain creates a new strategy to a new-recurrent situation or an open-ended environment, they called it the PROBE model.

There are typically three possible ways to adapt to a situation:

Selecting a previously learned strategy that applies precisely to the current situation

Adjusting an already learned approach

Developing a creative behavioral method

The PROBE model illustrates that the brain can compare from three to four behavioral methods at most, then choose the best strategy for the situation.[5]

  • Movement control

The frontal lobe has the motor cortex divided into two regions: the primary motor area located posterior to the precentral sulcus and non-primary motor areas including premotor cortex, supplementary motor area, and cingulate motor areas. The exact function of each structure and its role in the movement is still an active research area.[6]

The parietal lobe:

It is located posterior to the frontal lobe and superior to the temporal lobe and classified into two functional regions.

The anterior parietal lobe contains the primary sensory cortex (SI), located in the postcentral gyrus (Broadman area BA 3, 1, 2). SI receives the majority of the sensory inputs that are coming from the thalamus, and it’s responsible for interpreting the simple somatosensory signals like (touch, position, vibration, pressure, pain, temperature).[7]

The posterior parietal lobe has two regions: the superior parietal lobule and the inferior parietal lobule.

  • The Superior parietal lobule contains the somatosensory association (BA 5, 7) cortex which is involved in higher order functions like motor planning action.
  • The Inferior parietal lobule (supramarginal gyrus BA 40, angular gyrus BA 39) has the  Secondary somatosensory cortex (SII), which receives the somatosensory inputs from the thalamus and the contralateral SII, and they integrate those inputs with other major modalities (examples: visual inputs, auditory inputs) to form a higher order complex functions like:
  • Sensorimotor planning
  • Learning
  • Language
  • Spatial recognition
  • Stereognosis: the ability to differentiate between objects regarding their size, shape, weight, and any other differences.[8]

The temporal lobe:

The second most substantial portion occupies the middle cranial fossa and lies posterior to the frontal lobe and inferior to the parietal lobe. There are two surfaces, the lateral surface, and the medial surface.[9]

The lateral surface is classified by the superior temporal sulcus and the lateral temporal sulcus into three gyri; the superior temporal gyrus and the middle temporal gyrus and the inferior temporal gyrus.

  • Superior temporal gyrus (STG) is further sub-divided into two surfaces, the dorsal surface (superior temporal plane STP) and the lateral surface of the STG. 

The STP is located deep in the Sylvain fissure. The most significant anatomical landmark in STP is Heschl’s gyrus (HG) which contains the primary auditory cortex.

It is responsible for translating and processing all sounds and tones, and it is minimally affected by task requirement. Task requirement: a test where the examiner pronounces some words and ask the participant to categorize them acoustically, or phonemically, or semantically.[10]

The STP has another important area next to the HG called Wernicke’s area. In the past, this area was thought to have a significant role in speech perception and comprehension, but recent evidence shows that this area is not involved in this process. Researchers found that this process is not a simple task, but moreover, it is a complex task that is distributed all over the brain. 

The primary function of this area is the phonological representation, a process where the pronounced word is interpreted based on their tones and sound and trying to link it to a previously learned sound.[11]

The lateral surface of the STG is thought to be the secondary auditory cortex that also functions in interpreting sounds, but mostly in the activities that involve task requirement.[10] 

  • Middle temporal gyrus (MTG) has four sub-regions, the anterior, middle, posterior and sulcus MTG.[12]

The Anterior MTG is primarily involved in:

Default mode network has a specific activity that exists naturally in the brain at rest. So if you are studying or engaging in a game or doing any other action that demands you to stay focused or setting a particular goal this mode will be deactivated.

  1. Sound recognition helps the other areas that we talked about before.
  2. Semantic retrieval a process that assigns meaning to the words or sounds by trying to retrieve the previously learned concepts if they existed.

The Middle MTG has two functions:

  1. Semantic memory a type of memory involved in remembering the thoughts or objectives that are common knowledge (for example, where the bathroom is located).
  2. Semantic control network a system of connections between different areas of the brain including the middle MTG, to assign meaning to words, sounds that require both stored knowledge and mechanisms of semantic retrieval.

The Posterior MTG is thought to be part of the classical sensory language area.

The Sulcus MTG is involved in decoding gaze directions and in speech.

  • Inferior temporal gyrus (IT) is involved in visual perception and facial perception by containing the ventral visual pathway, the pathway that carries the information from the primary visual cortex to the temporal lobe, to determine the content of the vision.[13]

The Medial surface of the temporal lobe (Mesial temporal lobe) includes important structures (Hippocampus, Entorhinal, Perirhinal, Parahippocampal cortex) that are anatomically related and are mandatory for declarative memory. Declarative memory is a type of the long term memory that involves remembering the concepts or ideas and the events that happened or learned throughout your life. It is further divided into three types of memory:

  • Semantic memory we talked about it previously (see middle MTG).
  • Recognition memory the memory involved in recognizing an object, and all the other details that relate to this object. There are two forms: recollection and familiarity.
  • Recollection means you can remember the object and almost every single detail that is related to that object, such as time and place.
  • Familiarity means you remember encountering the object previously, but you don’t recall any specific detail about it. For example, when you say to a person, Your face is familiar, but I can’t remember where and when we met.
  • Episodic memory the memory that specializes in recalling an event and their associated details, and this is different from recognition memory in which you can consciously able to memorialize a specific event that happened throughout your life without being exposed to a similar situation.

The medial temporal lobe (memory system) is still an active research area, more precisely the exact function of each structure in this lobe is currently being studied.[14]

The occipital lobe:

The occipital lobe is the smallest lobe in the cerebrum cortex, and it is located in the most posterior region of the brain, posterior to the parietal lobe and temporal lobe.

The role of this lobe is visual processing and interpretation. Typically based on the function and structure, the visual cortex is divided into five areas (v1-v5). The primary visual cortex (v1, BA 17) is the first area that receives the visual information from the thalamus, and its located around the calcarine sulcus.

The visual cortex receive, process, interpret the visual information, then this processed information is sent to the other regions of the brain to be further analyzed (example: inferior temporal lobe). This visual information helps us to determine, recognize, and compare the objects to each other.[15]

Clinical Significance

Cerebral cortex dysfunction can occur due to a variety of causes (lesions) like tumors, trauma, infections, autoimmune diseases, cerebrovascular accident. The clinical features for each cause will depend on which lobe is affected. I will review some of the clinical features and their relation to each lobe.

Frontal lobe lesions can present as:[16]

  • Flaccid hemiplegia
  • Weakness
  • Apraxia
  • Personality disorders
  • Aphasia

Parietal lobe lesions can produce:[8]

  • Astereognosis
  • Aphasia
  • Apraxia
  • Loss of sensation

Temporal lobe lesions :[17]

  • Deafness
  • Phenomic paraphasia
  • Auditory or memory, visual hallucinations

Occipital lobe lesions:

Visual field deficits like complete blindness or color blindness.[15]


Source:
Max Planck Institute
Media Contacts:
Tomoko Udo – Max Planck Institute
Image Source:
The image is in the public domain.

Original Research: Closed access
“Laminar specific fMRI reveals directed interactions in distributed networks during language processing”. Sharoh, D., Van Mourik, T., Bains, L. J., Segaert, K., Weber, K., Hagoort, P., & Norris, D.
PNAS doi:10.1073/pnas.1907858116.

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