|Electrical Stimulation of localised areas of cortex.|
Scientists have stimulated different parts of the brain with weak electrical currents and observed movements in the musculature or changes in behaviour.
In humans, a Canadian neurosurgeon, Wilbur Penfield, applied electrical stimuli to different areas of the brain are known to induce movements, sensations, emotions, memories, etc.
It is now routine to expose the brain under general anaesthesia, then let the patient wake up while maintaining a pain-free state using local anaesthesia to sensitive structures while the brain is exposed; in this situation, the patient can report the effects of electrical stimulation using electrodes that have been placed stereotactically into desired areas of the brain.
If the motor area is stimulated, the patient makes an involuntary movement, and reports that the movement has occurred. If the visual area is stimulated, they may see a flash of colour; if the motor cortex is stimulated there is a movement of some muscles.
Deep brain stimulation is sometimes used as a treatment of some neurological conditions.
In patients with some movement disorders, electrical stimuli applied to parts of the brain can be used to guide neurosurgeons who treat the condition by creating lesions in regions that are reponsible for the disorder
|Transcranial Magnetic Stimulation|
Transcranial magnetic stimulation (TMS) is a procedure that uses magnetic fields to stimulate nerve cells in the brain. An electromagnet is used to create electric currents that stimulate nerve cells in the region of the brain beneath the magnet.
TMS can excite cells in the motor cortex, and the time taken for conduction of nerve impulses from the motor cortex to the spinal cord and musculature, such as the small muscles of the hand can be measured.
Modern imaging methods such as MRI (Magnetic Resonance Imaging) or PET (Positron Emission Tomography) scans provide images of brain structure and function in great detail.
If humans perform a specific task during a scan, scientists can identify which parts of the brain are active when the task is carried out.
Recent advances in imaging techniques allow radiologists to monitor local blood flow (fMRI) and metabolism (PET scanning), both of which change when a part of the brain increases its activity.
These are powerful techniques that have changed our understanding of the brain.
The example opposite shows the changes in metabolic activity on parts of the brain during a painful thermal stimulus.
Note the activity in the secons somatosensory area and insula, as well as the sites of descending nociceptive control in the midbrain, during the painful stimulus.
There is normally a positive potential at around 100 msec latency (called the P100 wave), and this wave is delayed if part of the visual pathway is affected by demyelination, as in multiple sclerosis (see the recordings below).
Visual stimuli directed to the eyes separately can reveal defects in one of the optic nerves. This is shown in the right hand diagram below, and the P100 wave is delayed in the left eye, because of optic neuritis, a common condition in multiple sclerosis, associated with transient loss of sight in one part of the visual field.
Evoked potentials are recordings of the changes in electrical activity on the surface of the scalp (or the underlying cortex) following an electrical stimulus to a sensory nerve or natural stimulus to a sensory organ.
These recordings provide information about the time take for action potentials in a selected pathway to initiate electrical activity in the underlying cortex.
Evoked potentials in the respective areas of the cortex can be observed following electrical stimulation of sensory nerves (Somatosensory Evoked Potentials), following clicks in the ear (Auditory Evoked Potentials), or light stimulation to the eyes (Visual Evoked Potentials).
Because of the spontaneous electrical activity of the cerebral cortex (EEG), the potentials generated from the visual cortex when there are changes in the pattern of light falling on the eye are superimposed on the EEG. Averaging computers are used to filter out the background 'noise' generated by the EEG; the computer can identify cortical waves of electrical activity synchronised with the visual stimulus, and the time taken for conduction of the impulses from the retina to the visual cortex can be calculated.
The latency of the visual evoked potential is increased when there is damage to the myelin in the visual pathway, and these changes in the latency are used to identify some of the effects of multiple sclerosis on the visual system.