2a. Effects of brain damage

The problem of causality…

Brain imaging makes it possible to examine the brain activity related to psychological processes, however neuroimaging techniques, such as fMRI and PET suffer from one serious limitation - the fact that some brain activity is associated with a task/hypothetical psychological process does not mean that the activity causes the observed behaviour or hypothesised psychological process

Ways to determine causality in animals

In animals, lesions can be produced experimentaly

Ways to determine causality in humans

• Neurosurgery: removal if brain tissue for treatment of neurological or psychiatric disorders, most often epilepsy

• Stroke: cerebrovascular accident resulting in the disruption of blood circulation in the brain and/or brain haemorhage

• Brain trauma or tumours

• Neurodegeneration: degeneration of brain tissue in dementia

• Infection of brian tissue

Neuropsychology

The area of psychology that examines the effects of brain damage on abilities and behaviour. If damage to a particular brain region/structure is systematically associated with a certain cognitive impairment, that region/structure is NECESSARY for the cognitive process to function

Clinical neuropsychology

It is the applied clinical variant of neuropsychology. Clinical neuropsychologists assess the ffect of brain damage in patients and diganose neural disorders and help patients and family adjust

Broca’s area and speech

• Broca studied brains of patients who had impaired speech (aphasia)

• One of his patients (Mr Leborgne) was nicknamed Tan for his inability to utter anythign other than “tan”

• in 1861, through post-mortem autopsy, Broca determined that tan had a lesion caused by syphilis in the left inferior frontal lobe

• Subsequent research ahs confirmed that lesions to this area indeed often result in language impairments

Wernicke’s aphasia

• The ability to comprehend the meaning of words is highly impaired. Reading and writing often is as well

• They often use sentences but with the wrong words or non existent words

Corpus Callosum

White mater tracts connecting the two hemispheres

Wada test findings

• The localisation of language is heavily biased towards the left hemisphere

• Split-brain studies and Wada test studies have shown that the linguistic competence of the right hemisphere is very limited

Patient H.M.

Most famous clinical case in human memory literature → to treat his epilepsy, he received bilateral, medial temporal lobe resection. After the surgery, the epilepsy was greaty improved but H.M. showed a nearly total, profound amnesia that persisted for his entire life. H.M. had profound anterograde amnesia and partial retrograde amnesia

Anterograde amnesia

The inability to form new long term memories after a brain injury, disease or psychological trauma, while retaining memories before the event

Retrograde amnesia

The inability to recall memories or infromation from before a brain injury or event

Hemispatial neglect

• Inattention to parts of the visual field

• Affects up to 2/3 of right hemispheric stroke patients

• Can differ from very mild to almost complete

• Can strongly affect one’s independence

• Crucially different from blindness and cortical blindness

Neglect symptoms

• Only attent to things on the right

• Move in the opposite direction from you if you come from the neglected side

• Problems reading

• Ignoring objects in their environment

• Problems navigating space

• Not using particular limbs

• Lack of insight

Neglect: damaged areas

The parietal lobe therefore seems crucially involved in regulating attending to things

Neuroimaging: key approaches

1. To see where a certain task/function is localised in the brain

2. To see something about how it works

Logic of dissociations

Neuopsychological data can be used to test theories about the architecture of psychological processes even without knwoing the exact location of the damage

Strength of neuropsychology

It enables causal inference unlike electrophysiology and neuroimaging

Limitations of neuropsychology

Lesions resulting from trauma or neurological degeneration are rarely anatomically selective - they tend to affect multiple brain regions/structures. Also brain damage is always associated with general cognitive, emotional and personality changes whose effect on cognitive performance is very considerable and difficult to separate from the effects of damage to a specific region/structure

Transcranial Magnetic Stimulation (TMS)

A non-invasive magnetic stimulation of human motor cortex

The neurophysiology of TMS

• A large current is briefly discharged into a coil of wirse held on the subject’s head

• The current generates a rapidly changing (increasing) magnetic field around the coil of wire and this field passes into the brain

• In the cortex, the magnetic field generates electric (ionic) current through neurons’ membranes

Effects of TMS on performance

• A TMS pulse typically induces a brief chaotic increase in neural activity often followed by a more sustained reduction in excitability

• This results in a disorganisation of neural activity, typically resulting in impaired performance

• Thus, the effect is similar to that of neurological lesion, only mild, reversible and safe

• For the reasons above, TMS is often referred to as a virtual lesion technique

TMS spatial resolution

Typically 10-20mm, 5-10mm at best → influenced by distance from the scalp, connectivity between target region and adjacent regions

TMS temporal resolution

TMS was clearly capable of telling when the targeted area was involved in processing so the temporal resolution is high

TMS limitations

• A suitable control condition is needed

• The effects of TMs on the brain are limited to the cortex - TMS cannot ‘reach’ deeper cortical and subcortical regions/structures (e.g. hipoocampus, thalamus)

• The effects of TMS on behaviour/performance are much more subtle (and hence can be harder to detect) than the effects of neurological damage (in patients)

• Although it is generally very sfe, it is associated with a small risk of eliciting a seizure. To minimise the risk, low levels of stimulation is used; participants are carefully screened