Optogenetics refers to using light to control cell activation (e.g: neurons) in living tissue. The cognitive approach to psychology uses two key concepts: information processing depends on internal representations (1) and these mental representations undergo transformations (2). People derive multiple representations from stimuli. Physical representations are activated first, phonetic representations next and category representations last. The chronometric method refers to measuring reaction time. Information perception is biased by memory and attention. People engage in four primary mental operations: encode (1), compare (2), decide (3) and respond (4). The word superiority effect refers to the fact that participants are most accurate in identifying the target letter when the stimuli are words. Letter and word representation are activated in parallel. There are four broad groups of study populations. Neurologically intact animals and humans and neurologically abnormal animals and humans. There are several types of brain disorders:Vascular disordersCerebral vascular accidents (stroke) occur when blood flow to the brain is suddenly disrupted. The cause is occlusion of the normal passage of blood by a foreign substance. Ischemia is a restriction in blood supply to tissues and is the result of partial occlusion. Cerebral arteriosclerosis is a chronic condition in which cerebral blood vessels become narrow because of thickening and hardening of the arteries.TumoursA tumour is a mass of tissue that grows abnormally and has no physiological function. Brain lesions can result from tumours. Benign tumours are non-recurring tumours and tumours that don’t spread. Malignant tumours are recurring tumours and likely to distribute over several areas. Degenerative and infectious disordersMany neurological disorders result from progressive disease (e.g: Alzheimer’s). This can be the result of genetic or environmental factors but also viruses....