<h2>Demilitarized zone</h2>
I hope my answer helps you :)
Quentin is experiencing life regrets.
Quentin feels regretful that he didn't follow his dreams of when he was younger - he wanted to become a social worker, however, his wife wanted him to change careers and do something more profitable, so he did. Many years later, he feels remorse about his decision and wishes he had continued his initial plan.
Depth perception
Eleanor Gibson and Richard Walk created the visual cliff in the 1960’s to measure depth perception. This apparatus enabled them to adjust optical stimulation with a cliff simulation while keeping their subjects safe from harm. The visual cliff is constructed of checkerboard, a sheet of plexiglass and a cloth.