The upper left quadrant pertains to the interior of the individual. This is about subjective experience as well as how we think and feel about these experiences. Wilber et al explain this quadrant in the following way:
Thoughts, ideas, opinions, intentions, motivations, purpose, vision, values, worldview, and life philosophy all exist within your personal interiors. All concepts from 'I think she likes me' to 'E+MC2' are in this 'I' space. (1)
In terms of academic subjects or schools of thought this quadrant has been the focus of psychologists and psychoanalysts concerned with human beings' inner life or their development. As it refers to an individual's particular perceptions or experiences, approaches, such as the Myers-Briggs Type Indicator, have made a contribution to our understanding of the upper left quadrant. Positive psychology, with its interest in emotional states and optimistic/pessimistic 'explanatory styles', also belongs in this quadrant.
Since this quadrant is also about consciousness (what it is and how it arises) philosophers have contributed enormously to our understanding of individual's interior lives.
1. Ken Wilber et al, Integral Life Practice, Integral Books, Boston, 2008, pp. 30-31.