This study will assess learning bias (the tendency to learn more from negative relative to positive outcomes) at baseline in 3 groups of participants (currently depressed, previously depressed and never depressed). It will then try to modify this bias using a simple computerised training task completed at home daily for 2 weeks. Outcome measures include symptoms of depression, cognitive measures (i.e. accuracy and reaction time during completion of tasks), pupillometry measures and salivary cortisol.

Depressed patients tend to focus on negative, at the expense of positive, events. This bias of attention is thought to be one factor which causes some people to be at higher risk of developing depression. One reason depressed people might pay more attention to negative events is that they think those events are more useful to them when they learn about the future. In this study the investigators will compare the learning styles (i.e. whether participants focus more on negative or positive information when learning about the future), cognitive biases and salivary cortisol levels (the levels of the hormone cortisol can be measured in the saliva and is known to be raised in those at risk of depression) of currently depressed (N=40), previously depressed (N=40) and never depressed (N=40) participants. The study will then randomly assign participants to complete one of two version of a learning task daily over two weeks. One version of the task will be designed to encourage an optimistic learning style, whereas the other will encourage neither an optimistic nor pessimistic learning style. The study will then retest participants' learning styles, cognitive biases, salivary cortisol and mood (using questionnaire measures) after completion of the task and again after 1 month. Study activities for participants will be: Screening visit (1 hours), baseline test visit (2 hours), task completion at home (10 mins per day for 2 weeks), test visit 2 (2 hours, completed 2 weeks after baseline visit) and test visit 3 (2 hours, completed 1 month after test visit 2). Participants will be asked to collect saliva samples, to measure cortisol, immediately after they wake up on the 3 testing days. Note that testing sessions involve pupillometry (measurement of area of pupil of the eye). This is done using a specialised camera pointed at the eye and is not invasive.

Inclusion Criteria:

• Participant is willing and able to give informed consent for participation in the study.
• Male or Female, aged 18 to 60 years.
• Fluent in English.
• Not currently taking any psychoactive medications (except hormone contraceptives).
• Currently meets SCID (Structured Clinical Interview for Diagnosis:DSMV) criteria for MDD--Major Depressive Disorder-- (currently depressed group only).
• Meets SCID criteria for 2 or more previous episodes of MDD, has been in remission for at least 3 months (previously depressed group only).
• No previous or current axis I diagnosis (control group only).

Exclusion Criteria:

• Previous or current diagnosis of bipolar disorder or psychotic illness.
• Previous or current anxiety disorder (control group only, anxiety disorder is permitted in current and previously depressed group as long as primary diagnosis of MDD (current or previous) is present).
• Significant suicidal ideation.
• Use of drug of abuse within the last 3 months.
• Receiving treatment (either pharmacological or psychological) for psychiatric condition.
• Medical or surgical treatment which prevents dilation/constriction of pupil.
• Prior experience of the tasks used in the study.

Anonymised data will be shared with researchers who contact the PI after study completion.