Lightness constancy is the ability of the visual system to perceive an object's lightness or color as remaining stable despite significant changes in the intensity of the light reflecting off it. This mechanism allows us to recognize objects reliably under varying environmental lighting conditions.

The phi-phenomenon is a perceptual illusion where the brain perceives continuous motion from a series of static images or lights presented in rapid succession. This effect is fundamental to how we perceive movement in film, television, and animated signage, as the brain fills in the gaps between the stationary stimuli.

The Gestalt principles describe how humans organize visual elements into coherent groups. While proximity, closure, and continuity are core principles, 'similarity' is actually a recognized Gestalt principle, making the provided answer key factually incorrect based on standard psychological theory. The question asks for what is NOT a principle, but the provided answer 'similarity' is a fundamental principle of Gestalt psychology.

Perceptual selectivity is influenced by both internal (personal) and external (stimulus) factors. Motivation, past experience, and expectations are internal psychological states that shape how we attend to stimuli. In contrast, size is an external physical attribute of the stimulus itself, rather than an internal state of the observer, making it an external factor in perceptual selectivity.

This phenomenon illustrates context effects, where the perception of a stimulus is heavily dependent on the surrounding environment. By using a tube, the observer removes the contextual cues (such as buildings or trees) that the brain normally uses to estimate distance and size. Without these reference points, the brain's ability to scale the object correctly is impaired, leading to a change in perceived size.

Perceptual constancy is the ability of the brain to perceive objects as having stable characteristics, such as size, shape, and color, even when the sensory information reaching the eyes changes. For example, as an object moves further away, its retinal image shrinks, but we still perceive it as the same size. This mechanism is essential for navigating the environment effectively, as it allows for a stable and predictable interpretation of the world.

The figure-ground relationship is the primary principle of perceptual organization, describing how our visual system automatically separates a scene into a main object (the figure) and its surroundings (the ground). This distinction is essential for recognizing objects in our environment. While other options like the law of closure are important Gestalt principles, figure-ground is the foundational step in organizing visual input into meaningful perceptions.

Figure-ground perception is the ability of the visual system to organize the visual field into objects that stand out (the figure) and the background against which they are set (the ground). This is a foundational aspect of perception that allows us to focus on specific objects while ignoring the surrounding context, enabling clear identification of items in our environment.

Color constancy is the tendency to perceive an object as having the same color despite changes in lighting conditions. This phenomenon relies heavily on the brain comparing the light reflected from an object with the light reflected from its surroundings, rather than relying solely on the absolute wavelength of light reaching the eye.

The Lamaze method is a childbirth preparation technique that emphasizes natural labor through various coping mechanisms. Distraction is a primary focus, where the mother uses breathing patterns, focal points, and mental imagery to divert attention away from the intensity of uterine contractions. By shifting cognitive focus, the perception of pain is significantly reduced, allowing the individual to manage the physiological stress of labor more effectively.