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 stimulation of the sense organs. For example, vision involves light striking the retinas of the eyes, smell is mediated by odor molecules and hearing involves pressure waves. Perception is not the passive receipt of these signals, but can be shaped by learning, memory and expectation. Perception involves these “top-down” effects as well as the “bottom-up” process of processing sensory input. The “bottom-up” processing is basically low-level information that’s used to build up higher-level information (i.e. – shapes for object recognition).
The “top-down” processing refers to a person’s concept and expectations (knowledge) that influence perception. Perception depends on complex functions of the nervous system, but subjectively seems mostly effortless because this processing happens outside conscious awareness. The process by which people translate sensory impressions into a coherent and unified view of the world around them. Though necessarily based on incomplete and unverified (or unreliable) information, perception is equated with reality for most practical purposes and guides human behavior in general.
The perceptual process allows us to experience the world around us. Take a moment to think of all the things you perceive on a daily basis. At any given moment, you might see familiar objects in your environment, feel the touch of objects and people against your skin, smell the aroma of a home-cooked meal and hear the sound of music playing in your next door neighbor’s apartment. All of these things help make up our conscious experience and allow us to interact with the people and objects around us. The perceptual process is a sequence of steps that starts with the stimuli that happen in our surroundings and leads through nerve transmission through peripheral and central nerves and the brain to our perception of what is going on. It also includes our resulting action to the original stimulus. Eight Steps of the Perceptual Process
* The first step in the process involves the sender of a message thinking of an idea that he wants to communicate. For example, when you want to tell someone to pass the salt, you first have to think that idea. Encoding
* The second step involves encoding the message into a language that can be understood. This means that you translate the desire for the taste of salt on your food into the words required to get the salt. Choosing a Medium
* Once you have encoded the message, you must choose the medium by which to send the message. When you are in the same room with someone, you will usually talk. If you are away from the person you want to communicate with, you could call on the phone or send an email. Output of Message
* After you have successfully encoded the thought into a language that can be understood, you then communicate the message to the other individual. For example, at this point, you would say, “Can you please pass me the salt?” Decoding of Message
* The receiver must then decode the message that was sent. At this point, the recipient of the message accepts the message and converts it into a form that can be interpreted. Create a Meaning
* Once the recipient receives the message, she will create meaning out of it. She will hear the message and try to understand what is being said. Once she hears, “Can you please pass the salt?,” she will understand that you want the salt. Dealing With Noise
* During the process of communication, noise can interfere. Noise is anything that can distract the recipient of the message from hearing it clearly. For example, if a radio is on in the background, the recipient of a message may not hear you clearly when you ask a question. She may also be thinking about something that happened earlier and may not be paying attention to the question. Feedback
* Once the message is clearly received and understood by the recipient, feedback occurs. The recipient of the message could give feedback by
grabbing the salt and passing it to you. He could also respond by saying, “No” or “In a minute.” A monocular cue is a visual cue for depth perception that only requires one eye. People with vision loss in one eye can still rely on these cues to navigate the world, although their depth perception will be impaired. Some examples include motion parallax, interposition, and linear perspective. Many of these cues can be seen in works of art, where artists rely on visual tricks to add depth and texture to visual scenes so viewers feel like they are looking at a three dimensional environment. Types of Monocular Depth Cues
Motion Parallax and Kinetic Depth Perception
* Motion Parallax occurs when an object travels across the retina of a moving person. The moving person focuses on one object while noting the relative movement of other objects. Parallax causes objects in the distance to appear to move more slowly than objects that are closer. Kinetic depth perception allows us to gauge the velocity of moving objects. When an object moves away, it appears to grow smaller. When an object approaches, it appears to become larger. We constantly judge changes in our positions to others using kinetic depth perception cues. Linear and Aerial Perspectives
* Linear perspective frequently occurs with the observation of parallel lines. Such lines will appear to recede and converge at the horizon. The horizon will appear to rise. This cue is related to the relative size and texture gradient, and often all three work together. Aerial perspective is also referred to as relative height. This cue notes that objects closer to the line of horizon will appear farther away. This phenomenon relates to the way light scatters in the air, causing objects on the horizon to appear in faded colors or washed in lower light luminance and contrast, while objects that are near will appear to have vibrant or intense colors with strong contrast.
* Most objects have a textured surface. When an object is farther away from the viewing point, the texture appears finer and smoother, and may appear to be closer. Familiar and Relative Size
* Previous experience with objects allows us to know many objects’ relative size. This experience informs our interpretation of distance. The familiar size cue tells us that the visual angle of objects becomes smaller with distance, allowing us to calculate the probable depth or distance of objects. Known size, together with perspective and texture effects, are strong depth cues. When we know one object is similar in size to another object and both objects are within our plane of vision, the relative size cue allows us to understand that the object with the larger visual angle on the retina is closer than the other object. Shadow and Occlusion
* The shadow monocular depth cue has several rules: If an object is solid, it will cast a shadow. If there is only one light source, then all shadows will fall in the same direction and the shadow will be opposite from the source of light. Objects with shadows falling on them are farther away than objects casting the shadow. If the object is lower than the “ground plane” (like a well), the shadow will appear on the same side as the source of light. Occlusion, also referred to as overlapping or interposition, occurs when one object blocks another object. The object that is blocked is understood, by this cue, to be farther away than the object blocking it. Peripheral Vision and Accommodation
* The peripheral vision cue occurs due to the curvature of the eyeball. This curvature causes the visual field to distort or appear to bend at its extreme edges. This visual distortion is accommodated for when we interpret an image or scene. Often we will ignore that the lines of objects, which our prior knowledge understands are straight, will appear to be curved. The effect of this curvature can be seen in some photographs where no accommodation has taken place, which is partially why photographs often do not capture the image we think we have seen. The accommodation cue occurs when the dioptic power of the lens increases and allows close objects to be focused clearly on the retina. How this cue informs the understanding of distance is not yet clearly understood by science. We perceive monocular depth cues just as easily with one eye as with two, reflecting how powerful and important these depth cues are to our vision.