Speech and Cortical Asymmetry Flashcards
1
Q
How did Paul Broca discover the Broca’s area in 1861? What did this mean for the study of neurology?
A
- In 1861, Paul Broca, a French Neurologist, heard of a patient, named Leborgne, who had a 21-year history of progressive loss of spoken speech but no loss of comprehension nor mental function. He was nicknamed “Tan” due to his inability to clearly speak any words other than “tan”.
- When Leborgne died Broca performed an autopsy. He determined that Leborgne had a focal lesion (probably due to a stroke) in the frontal lobe of the left cerebral hemisphere.
- For the next two years, Broca went on to find autopsy evidence from 12 more cases to show that the damage to a particular part of the left frontal lobe, just above the lateral sulcus, causes a specific deficit in spoken speech
- This region of the brain is now known as Broca’s area
- A the time this was revolutionary! It was the first neurological evidence that different parts of the cerebral cortex could have different functions
2
Q
Where is the Broca’s area (use diagram in lecture notes to help you)?
A
- This is a normal brain; the part that was damaged in Leborgne is indicated in red; it is on the lower lateral part of the frontal lobe immediately above the lateral sulcus (though varies between different brains). This region is known as Broca’s area.
3
Q
What can you see if you retract the upper and lower opercula of the brain? How does this relate to speech?
A
- If the upper and lower opercula (lips) for the lateral sulcus are retracted, an extra region of cortex is visible under the sulcus: this is the insula (or insula cortex - see diagram for location). It is now thought that some speech functions are controlled in the insula as well as Broca’s area.
- Before Broca, doctors had thought that the cerebral cortex was a unified organ like the liver, with all parts contributing to all activity. It now became clear that this was not the case; different physical locations in the cortex were involved in different actions
4
Q
What is the opercular cortex? What does it correlate to?
A
- The opercular cortex (shown in green and blue below) is the cortex on the upper and lower ‘lips’ of the lateral fissure. This cortex is thicker in the left hemisphere frontal lobe.. This is thought to correlate with language production
5
Q
What did Brodmann show in 1909 and what important question did this raise?
A
- In 1909, The anatomist Brodmann showed, using a stain for neuronal cell bodies, that there were subtle differences in neuron type and density in different parts of the cortex. He divided the cortex into over 50 anatomically different areas. This was the start of the science of cytoarchitectonics. For example, the primary motor cortex corresponds to Brodmann area 4 (N.B. you don’t have to memorised all these areas!)
- This raised the important question….
- Does each anatomically different area have a different function?
- (Probable answer is “Yes” but we don’t know all the functions)
6
Q
What did later studies of patients in the early 20th century with focal cortical lesions make it possible to identify?
A
- Later studies of patients in the early 20th century with focal cortical lesions made it possible to identify separate functional regions in the cortex for each of the main sensory modalities of touch, vision, & hearing.
- These all corresponded with one or more different Brodmann areas; for example the visual cortex includes Brodmann areas 17, 18,19 , the somatosensory (touch) cortex includes areas 1,2 & 3.
- Auditory cortex is centred on areas 41 and 42, etc.
- However, after allowing for the motor cortex and various sensory cortices, there was still most of the cortex left over, whose function was obscure*. This cortex became known as association cortex. Its function has been gradually clarified by studies of patients with strokes that affect various parts of the association cortex, but there is still much we don’t know!
- (*this might be the origin of the myth that we only use 10% of our brain)
7
Q
What did Broca’s studies show about which side of the brain language is controlled from? What are these areas known as?
A
- Broca’s original studies showed that language vocalisation is controlled by specialised cortical areas in the left hemisphere only just above the lateral sulcus. These areas are now known as Broca’s area. They includes Brodmann’s areas 44 & 45. (the areas are sometimes known as pars opercularis and pars triangularis of the frontal lobe)
8
Q
What is Wernicke’s area, where is it and what does it control?
A
- Studies by Karl Wernicke following on from Broca’s work indicated that language perception (input) is controlled by a cortical area at the upper end of the superior temporal gyrus in the temporal lobe. This is now called Wernicke’s area; it is adjacent to the primary auditory cortex, which is also on the temporal lobe
9
Q
What have modern studies with fMRI (functional MRI) shown about the area involved in speech recognition? Which side of the brain is speech heard by?
A
- Modern studies with fMRI (functional MRI) have shown that the area that is involved in speech recognition may extend beyond the classical boundary of Wernicke’s area, to include part of Broca’s area. Speech will of course be heard by the auditory cortex on both the left and right sides of the brain, as shown in the FMRI images here.
- However note the absence of signal (in yellow) in the equivalent to Wernicke’s area in the right hemisphere.
(see diagram in notes)
10
Q
What are the symptoms of Broca’s (expressive) aphasia?
A
o Halting speech o Repetitive o Disordered grammar o Disordered syntax o Disordered word order o Sense behind words - Patients with expressive aphasia use single words: they find it difficult to link words together into grammatical sentences
11
Q
What are the symptoms of Wernicke’s (receptive) aphasia?
A
o Fluent speech o No repetition o Good syntax o Grammar ok o Meaningless o Inappropriate words - Patients with receptive aphasia speak fluently but in an almost meaningless way
12
Q
How are Broca’s and Wernicke’s areas joined? What is the result of damage to this?
A
- Broca & Wernicke’s areas are joined by a bundle of cortico-cortical association fibres called the arcuate fasciculus. Damage to arcuate fasciculus produces conduction aphasia.
- Patients with Conduction Aphasia show an impaired ability to repeat back heard or written words. Speech output is characterized by word-finding difficulties. (however the condition in general is fairly subtle)
- Patients with conduction aphasia also have difficulty reading aloud but they may have relatively good comprehension.
13
Q
What is the Wernicke-Geschwind model: how does this relate to how we produce speech and check to see if it’s what we wanted to say? What does this mean for deaf children?
A
- When we wish to speak the word concepts are formed in Wernicke’s area, stored in some kind of buffer memory and then sent via the arcuate fasciculus to Broca’s area. Here they are converted into motor programs which are then sent to the motor cortex of the mouth, lips and tongue. When we speak we hear our own voice. The sounds produce patterns of neuronal activity in the auditory cortex. These patterns are decoded into perceived words or fractions of words in Wernicke’s area. The input signals (perceived words) are compared with the output in the buffer to see if our physical speech sounds matched what we wanted to say.
- Without this feedback of your own heard speech learning to speak is difficult if not impossible. This is why deaf children find speech more difficult to learn.
14
Q
Which artery supplies Broca’s and Wernicke’s area? What will be the result of a stroke affecting the proximal part of this artery?
A
- Both Broca & Wernicke’s areas are supplied by branches of the middle cerebral artery.
- A stroke that affects the proximal part of the middle cerebral artery will affect both Broca and Wernicke areas, producing ‘global’ or total aphasia
15
Q
How can the parts of the brain involved in vocalisation and hearing words be shown? What do these imaging methods support the lesion evidence for? How does this relate to the side of the brain used for speech? How many people have an anomalous pattern of specialisation, and what type of specialisation might this include?
A
- Nowadays brain activity (fMRI or PET Scans) can be used to show what parts of the brain are active during vocalisation and hearing words:
- These imaging methods support the lesion evidence that in most subjects normally only the left side of the brain is active during listening to speech or spoken speech
- Based on analysis of stroke patients, fMRI & PET scans, it is estimated that between 70% to 95% of humans have a left-hemisphere language specialization. This includes most left-handed people.
- 5% to 30% of humans have anomalous patterns of specialization. These might include: (a) having a right-hemisphere language specialization or (b) having bilateral specialization.
