By Catherine A. Sanderson, Amherst College
How we perceive the world is often driven less by our objective reality than by psychological factors that influence our interpretation of the world. After all, we don’t just sense various sights, sounds, tastes, and smell, we perceive them in a particular way. We don’t just take in all the sensory data as individual parts. Instead, we organize these sensations into meaningful perceptual units.
Our perceptions are usually in line with our sensations. However, in the case of perceptual illusions, errors in the perceptual process lead to false or misleading impressions.
In the Ponzo illusion, two horizontal lines of the same size are stacked one above the other inside two vertical lines that converge at the top of the image. We perceive the top horizontal line as appearing larger than the bottom line due to the effect of the converging vertical lines.
In the Müller-Lyer illusion, two identical horizontal lines have two shorter lines at both ends—in one case extending outward and in the other extending inward. We perceive the one with the lines extending outward as longer than the one with the lines extending inward.
One of the most famous real-world optical illusions is that of the desert mirage, what appears to be a pool of water appearing on the horizon. A mirage is a naturally occurring illusion that happens when the ground is very hot and the air is very cool. When light rays move through cold air and into the hot air just above the ground, the waves are refracted or bent back toward the sky before reaching our eye. So, we are actually seeing a distorted image of the blue sky and that is what appears to be water. Our brain assumes the light has traveled in a straight line, as it normally does, and that we are therefore seeing what appears to be water on the ground.
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Process of Sensory Reduction
Perception begins whenever we select something we are going to pay attention to in a given environment because in all situations we are surrounded by way more sensory information than we can possibly turn into perceptions.
This process of sensory reduction—also known as selective attention—involves filtering out and attending only to our highest-priority sensory messages. So, if we’re at a cocktail party, we may choose to pay attention on a conversation we’re having and ignore the other sensations around us.
What exactly the brain attends to is influenced by psychological factors, such as needs, desires, and expectations.
If we’re really hungry, we might pay less attention to the conversation we’re having and focus instead on trying to catch the eye of the person passing around a tray of appetizers.
This process of selective attention is made easier by specialized neurons in the brain that respond only to specific characteristics of visual stimuli: angles, shapes, edges, or movements.
Organizing Sensory Information
After we’ve figured out what we want to pay attention to in a given situation, either at a conscious or unconscious level, we also have to organize that information in some meaningful way. We have to perceive objects as separate from other stimuli and as having a distinct and meaningful form.
Early in the 20th century, so-called Gestalt psychologists were among the first to study how the mind organizes sensations into a perceptual gestalt—the German word meaning form or shape. They recognized that we automatically organize isolated objects we see in the world to form more cohesive wholes. The most fundamental Gestalt principle of organization is our ability to distinguish an object from its surroundings—the figure from the ground.
Black-and-white images are often used to illustrate surprising switches between what is perceived as foreground and what is perceived as background.
One classic illustration of the figure-ground problem can be perceived as two black silhouettes of people facing each other with a white background in between them. But this same image can alternatively be seen as a white vase with the black on either side appearing as the background. Images that allow for more than one possibility show how our perception organizes what we see.
A more difficult version of this perceptual switching occurs in images cleverly constructed to allow you to perceive two different figures within the same region, while the background region remains unchanged. One famous example portrays one image of a young woman, neck exposed, looking away, while the other is of an old woman in profile, with a larger face, and a much larger nose and chin.
In both of these examples, the brain is sorting out how to define the figure (the thing to focus on) and how to define the ground (the things to ignore).
Grouping Based on Features
Other Gestalt principles of perceptual organization focus on how we automatically group stimuli together based on various features. These features can include similarity, proximity, continuity, and closure.
One fundamental question is whether our ability to organize sensory information is innate, or whether this ability develops over time, with experience in the world.
People who are blind from birth but later undergo surgery and gain their ability to see suggest that some visual abilities, such as the ability to see color, result from fundamental biological processes. But other abilities, such as recognizing objects, seem to require experience in the world.
Common Questions about Perception
A desert mirage is a real-world optical illusion. It occurs when the ground is very hot and the air is very cool. When light rays move through cold air and into the hot air just above the ground, the waves are refracted or bent back towards the sky before reaching our eye. So, we are actually seeing a distorted image of the blue sky and that is what appears to be water. Our brain assumes the light has traveled in a straight line, as it normally does, and that we are therefore seeing what appears to be water on the ground.
The process of sensory reduction—also known as selective attention—involves filtering out and attending only to our highest-priority sensory messages.
We automatically organize isolated objects we see in the world to form more cohesive wholes. We group stimuli together based on various features. These features can include similarity, proximity, continuity, and closure.