Localisation of Function in the Brain: Flashcards
cortical functions:
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context:
-Franz Gall, a German physician, accidentally began the debate over localisation of function in the human brain in the early 1800s
-he proposed that a person’s personality was reflected in bumps on the skull that in turn reflected functions of the brain lying underneath; this theory was called phrenology
-Gall’s idea that certain functions were localised to specific regions of the brain eventually became extremly influential
-the opposing view was that the brain functions in a more holistic manner, with all or large parts of the brain involed all behaviours
Broca and Wernicke:
-in 1861, Broca, on the basis of his case studies of brain-damaged patients, has concluded that speech production was localised to an area in the frontal lobe, now known as ‘Broca’s area’
-Wernicke (1874) followed this up by showing that damage localised to a small area of the temporal lobe resulted in a loss of speech comprehension
localisation of function in the brain:
-by the end of the 19th century, other researhers had shown in cats, dogs, and monkeys, that damage could have highly specific effects on movement and perception
-overall, these studies seem to show conclusively that the brain is organised in a highly systematic way, with functions localised to specific areas
-by the middle of the 20th century, we could map out a number of functions localised in the cortex of the brain
electrical stimulation in the brain:
-electrical stimulation of these areas can produce the appropiate sensation: visual images from the visual cortex, sound sensation from the auditory cortex, the sesantion of touch or pressure from stimulation of somatosensory cortex, and movement of skeletal muscles from stimulation of the motor cortex
somatosensory cortex:
-the somatosensory cortex receives sensory input from receptors in the skin, including touch, pain, pressure, and temperature from all areas of the body surface
-the body surface is represented systematically in the somatosensory cortex
-head areas are represented at the bottom of the postcentral gyrus, and legs and feet at the top: that is, it is a map of the body surface, though upside down
motor cortex:
-the motor cortex in the precental gyrus is also organised systematically, with the muscles of the legs and feet at the top and the masculature of our vocal apparatus (muscles of the mouth and tongue, larynx and pharynx) at the bottom
-stimulation of tiny areas of the motor cortex can produce movement of individual muscle fibres in the appropiate part of the body
auditory areas of the cortex:
-the visual area receives input directly from eyes and the auditory areas from the ears, damage to them can lead to blindness and deafness
-they are known as the primary visual and auditory cortex, but visual perception, for example, requires additional processing in neighbouring cortical areas (secondary visual areas)
-it is in these areas that sensation is converted into perception, we know this because damage to these secondary visual areas does not lead to blindness, but can lead to loss of specific aspects of visual perception
-for example, propagnosia and achromatopsia
research on lateralisation of the brain:
-research on sensory and motor processes supports Gall’s original idea that functions were localised in the brain
-however, the debate on localisation was revived by the work of Lashley in the 1920s
-Lashley was interested in how learning was organised in the brain, and he studied how rats learned mazes, he found that large lesions on visual areas impaired maze learning, but that smaller lesions covering the same brain areas had no effect and it seemed that the size of the lesion was critical
2 main laws:
-law of mass action: as effects on learning were proportional to the amount of cortex damaged, Lashley concluded that behavioural functions such as learning were spread widely across cortical areas, they were not localised to specific regions
-law of equipotentiality: related to the law of mass action, this law states that different areas of cortex have similar capacities to process learning, so that one area can take over functions if another area is damaged, therefore only large lesions affect learning
Hemispheric lateralisation:
-the idea that some functions are found only in one hemisphere, e.g. language is usually lateralised to the left and Sperry demonstrated that some visual-spatial functions are lateralised to the right hemisphere
Broca’s area:
-area at the base of the left frontal involved in speech production, thought to contain the motor plans for words; first identified by Paul Broca in the 19th century
Wernicke’s area:
-area in the temporal lobe thought to contain our store of words; Wernicke showed in the 19th century that damage to Wernicke’s area resulted in receptive aphasia
Language and Hemispheric lateralisation:
-sensory and motor processes are organised in an extremely orderly way
-somatosensory and motor pathways are crossed, connecting the left hemisphere to the right side of the body and the right hemisphere to the left side of the body
-however, the cortical organisation is the same in each hemisphere
-visual and auditory systems are more complicated
-however, the organisation is perfectly orderly, and visual and auditory areas in the left hemisphere are matched by visual and auditory areas in the right hemisphere
-another way of putting is that a description of sensory and motor cortical areas in the left hemisphere can be applied equally to the right hemisphere
-the hemispheres are symmetrical, or mirror images of each other, with respect to sensory and motor cortical functions
Language:
-Broca and Wernicke also made a profound contribution to our understanding of hermispheric lateralisation of function
-this is the idea that some functions might be found only in one hemisphere rather than in both (lateralised -> to one side)
Broca and ‘Tan’:
-Broca, a French physician, was interested in the brain areas involved in language
-in 1861, he heard about a patient with a particularly striking language impairment
-this patient has suffered brain damage many years earlier that resulted in him being only able to speak one word, ‘tan’
-he could understand speech, following instructions and clearly understanding that was spoken to him
-tan, as he became known as, unfortunately died a week or so after Broca first met him, though this did mean Broca could perform an autopsy on his brain
-he found substantial damage to an area towards the base of the frontal lobe
-over the next four years, Broca accumulated a dozen or so cases where the symptoms were the same as in Tan, namely a lack of speech production but intact speech comprehension
-in all cases, autopsies revealed damage to the same area at the base of the frontal lobe, the damage in all cases was only in the left hemisphere
-Broca concluded that this area of the left hemisphere, now known as Broca’s area, was responsible for speech production
-the syndrome where speech is lost but comprehension is intact became known as ‘Broca’s aphasia’ or ‘expressive aphasia’
Wernicke:
-at around the same time as Broca, Wernicke was studying patients with the opposite syndrome to Tan’s-they could not understand speech, failing to follow instructions, but could produce some fluent speech
-autopsy findings were that these patients all had damage in an area of the left hemisphere at the top of the temporal lobe, near the auditory cortex, known as Wernicke’s area
-the syndrome of intact speech production but loss of speech comprehension became known as Wernicke’s aphasia or receptive aphasia
-an early simple model of speech saw Wernicke’s area as containing out store of words (the lexicon)
-when we want to speak, the word is located and activated in Wernicke’s area and the information is transmitted to Broca’s area
-this contains the motor plans for words: that is, patterns of muscle activation that allow us to speak a particular word
-this pattern is transmitted to the motor cortex, where the muscles of our vocal apparatus are activated and the word is spoken
Reading and writing:
-later research investigated reading and writing
-reading in particular involved the visual system
-the word we read is transmitted to the visual cortex for initial processing, then passed to the angular gyrus
-this structure then passes the information to Wernicke’s area and our internal lexicon where the word can be recognised, for writing, the word is activated in Wernicke’s area then passed to Broca’s area where the motor plan for writing the word can be transmitted to the motor cortex
-on rare occasions the angular gyrus is damaged
-as it is part of our reading system, this means that the person cannot read; this is called alexia
-however, Wernicke’s and Broca’s areas are intact, so the person cannot read, this is called alexia
-however, Wernicke’s and Broca’s areas are intact, so the person can write
-this produces a syndrome where someone can write, but cannot read what they have just written
-this is called alexia without a graphic (the inability to write), or pure word blindness
