Perception (from the Latin perceptio, percipio) is the organization, identification, and interpretation of sensory information in order to represent and understand the environment. All perception involves signals in the nervous system, which in turn result from physical or chemical stimulation of the sense organs. For example, vision involves light striking the retina of the eye, smell is mediated by odor molecules, and hearing involves pressure waves. Perception is not the passive receipt of these signals, but is shaped by learning, memory, expectation, and attention. Perception involves these "top-down" effects as well as the "bottom-up" process of processing sensory input. The "bottom-up" processing transforms low-level information to higher-level information (e.g., extracts shapes for object recognition). The "top-down" processing refers to a person's concept and expectations (knowledge), and selective mechanisms (attention) that influence perception. Perception depends on complex functions of the nervous system, but subjectively seems mostly effortless because this processing happens outside conscious awareness.