Localisation of function in the brain

• Franz accidentally began the debate over localisation of the function in the brain. He has said that a person's personality was reflected in bumps on the skull this theory was called phrenology
• Broca studied brain damaged patients and concluded that speech production was localised to an area in the frontal lobe known as brocas area
• Wenicke concluded that speech comprehension was localised to an area in the temporal lobe known as wernickes area
• By the the of the 19th century there was proof that dogs cats and monkeys with small lesions have highly specific effects on movement and perception showing that brain is organized in a highly systematic way with functions localised to specific areas
• precentral gyrus is the primary motor cortex
• Postcentral gyrus is the primary somatosensory cortex
• The somatosensory cortex receives information from the receptors in the skin. The body's surface is represented systematically in the somatosensory cortex with the head areas at the bottom off the postcentral gyrus and lega and feet at the top.
• The motor cortex in the precentral gyrus is also ordered systematically with muscles of the legs and feet at the top and muscles of mouth and tounge at bottom
• Electrical stimulation of the visual, auditory, somatosensory and motor areas of the cortex produce different sensations:

  • Visual images from visual cortex
  • Sound sensation from auditory cortex
  • Sensation of touch from somatosensory cortex
  • Movement of skeletal muscles from motor cortex
• Primary visual cortex receives information from eyes and primary auditory cortex from the ears damage to them can lead to blindess and deafness
• Visual perception requires extra processing in secondary visual areas, in these areas sensation is converted into perception. Damage to the secondary visual areas can cause loss of specific aspects of perception.

  • Loss of the ability to recognise familiar faces
  • Loss of the ability to see in colour
• Lashley was interested on how learning is organized in the brain so he studied on rats learned mazes. He found that large lesions on visual areas impaired maze learning but smaller lesions covering the same brain area had no effect, he created 2 laws:

  • Law of mass action
  • Law of equipotentiality
• Law of mass action states that effects on learning are proportional to the amount of cortex damaged. He had concluded that behavioural functions like learning were spread across cortical areas and they aren't localised to specific regions.
• Law of equipotentiality states that different areas of the cortex have similar capacities to process learning so one area can take over other functions if another area is damaged so only large lesions affect learning
• Broca has performed an autopsy on Tans brain, Tan was someone who suffered brain damage and he could only say one word which was tan but he easily understood speech since he could follow instructions and understand what was spoken to him. Broca had found damage to an area at the base if the frontal lobe in the left hemisphere. This is brocas area and its responsible for speech production. Syndrome where speech production is lost but comprehension is intact is called bricas aphasia
• Wernicke had studied people who could not understand speech, they failed to follow instructions but could produce fluent speech but the speech was all random. Autopsy had shown that they all had damage in an area of the left hemisphere at top of the temporal lobe near to auditory cortex known as wernickes area. The syndrome of intact speech production but loss of speech comprehension is known as wernickes area
• An early simple module of speech saw wernickes area as out store of words known as lexicon
• When we want to speak:

  • Word is located and activated in Wernickes area
  • Information is then transmitted to brocas area which contain motor plans for the word, patterns of muscle activation that allow us to speak the word
  • Pattern is transferred to pur motor cortex and muscles of purr vocal apparatus are activated and the word is spoken
• When we want to read:

  • Words that have been read are sent to the visual cortex for inital processing, its then passed on to angular gyrus
  • Its then passed to wernickes area and the word is recognised in our internal lexicon
• When we want to write:

  • Word activated in werrnickes area and its passed through brocas area.
  • Motor plans for writing the word are then passed to the motor cortex
• When angular gyrus is damages person can't read, this is called as alexia. But wernickes and brocas area are intact so the person can write but they can't read what they have written this is known as pure world blindness
• Epilepsy is a medical condition characterised bt uncontrolled electrical discharges in the brain. If it's severe then brain function is seriously affected and person may suffer violent convulsions and lose consciousness for a few seconds (grand mal attacks. Usually epilepsy aren't severe and can be controlled by drugs
• Sometimes they aren't obvious reasons for what causes epilepsy. It's assumed that there is an imbalance of excitatory and inhibitory processes in the brain but sometimes it can be caused by areas of scar tissue in brain as a result of accidental brain damage or previous operations on the brain. In these cases this is known as a "focus" for the epileptic attack
• If focus for epilepsy is identified ot can be removed surgically but this is only used if its severe and drugs don't work
• The 2 hemispheres of the brain are connected by corpus callous. It allows epileptic discharges to travel from one hemisphere to the other
• The operation where corpus callosum is cut is called as commissurotomy. It prevents epileptic discharges so it reduces severity or attack. Although it wasn't so successful at reducing symptoms of epilepsy but patients didn't suffer any I'll effects after the operation
• Sperry who worked on hemisphere functions in monkeys got interested in split brain patients.
• Sperry's experiment was based on visual information so even when both hemispheres are separated each eye still sends projections to both hemispheres. He devised an experimental procedure known as divided field
• In the procedure divided field, the right side of the right and left eye connect to right hemisphere and left side of left and right eye is connected to left hemisphere
• For split brain patients If there's an image/visual stimuli on the left visual field (LVF) it will be detected by the right optic nerves from both eyes and its sent to the right hemisphere
• in intact humans

  • Stimulus is present to either hemispheres and it will be sent through the corpus callosum so the other hemisphere will also get the same information
• Limitation of divided field is that there is a natural tendency fir participants to move their eyes towards stimulus, if their eyes move too much the stimulus is likely to be picked by both hemispheres. To prevent this the stimulus is only present for a brief amount of time of 200 miliseconds