Perceiving Depth, Distance, and Size
Perception of depth, size, and distance is achieved using both monocular and binocular cues.
Relate the factors involved in depth perception
- Cues about the size and distance of objects are determined relative to the size and distance of other objects.
- Monocular cues about size and shape are used in perceiving depth. Binocular vision compares the input from both eyes to create the perception of depth, or stereopsis.
- Shape constancy allows the individual to see an object as having a constant shape from different angles, so that each eye is recognizing a single shape and not two distinct images. Input from both eyes is compared, and stereopsis—the impression of depth—occurs.
- Depth is perceived when the visual stimuli (such as distance, size, or shape) from each eye are compared binocularly, or using both eyes.
- convergence: The act of moving toward union.
- stereopsis: In vision, the impression of depth that is perceived when a scene is viewed with both eyes.
- binocular: Using two eyes or viewpoints; especially using two eyes or viewpoints to ascertain distance.
- monocular: Of or with one eye.
How do we perceive images? Are the images we see directly mapped onto our brain like a projector? In reality, perception and vision are far more complicated than that. Visual stimuli enter as light through the photoreceptors in the retina, where they are changed into neural impulses. These impulses travel through the central nervous system, stop at the sensory way-station of the thalamus, and then are routed to the visual cortex. From the visual cortex, the information goes to the parietal lobe and the temporal lobe. Approximately one-third of the cerebral cortex plays a role in processing visual stimuli.
Depth perception is the visual ability to perceive the world in three dimensions, coupled with the ability to gauge how far away an object is. Depth perception, size, and distance are ascertained through both monocular (one eye) and binocular (two eyes) cues. Monocular vision is poor at determining depth. When an image is projected onto a single retina, cues about the relative size of the object compared to other objects are obtained. In binocular vision, these relative sizes are compared, since each individual eye is seeing a slightly different image from a different angle.
Convergence: The train tracks look as though they come to a single point in the distance, illustrating the concept of convergence.
Depth perception relies on the convergence of both eyes upon a single object, the relative differences between the shape and size of the images on each retina, the relative size of objects in relation to each other, and other cues such as texture and constancy. For example, shape constancy allows the individual to see an object as a constant shape from different angles, so that each eye is recognizing a single shape and not two distinct images. When the input from both eyes is compared, stereopsis, or the impression of depth, occurs.
Relative Size of Objects
Ebbinghaus illusion: The Ebbinghaus illusion illustrates how the perception of size is altered by the relative sizes of other objects. The two center circles are the same size, though they may be perceived to be different sizes.
Size and distance of objects are also determined in relation to each other. Visual cues (for instance, far-away objects appearing smaller and near objects appearing larger) develop in the early years of life. Convergence upon a single point is another visual cue that provides information about distance. As objects move farther away into the distance, they converge into a single point. An example of this can be seen in the image of train tracks disappearing into the distance.
Optical illusions, such as the Ebbinghaus illusion, show how our perception of size is altered by the relative sizes of other objects around us. In the Ebbinghaus illusion, the size of the center circles is the same, but looks different due to the size of the surrounding circles.
Depth from Motion
When an object moves toward an observer, the retinal projection of the object expands over a period of time, which leads to the perception of movement in a line toward the observer. This change in stimulus enables the observer not only to see the object as moving, but to perceive the distance of the moving object. This is useful when you cross the street: as you watch a car come toward you, your brain uses the change in size projected on your retina to determine how far away it is.
Motion is perceived when two different retinal pathways, which rely on specific features and luminance, converge together.
Differentiate between first-order and second-order motion perception
- Both monocular and binocular vision can detect motion, but binocular vision is better at detecting motion because of its superior depth perception.
- First-order motion perception occurs through specialized neurons located in the retina that track motion through luminance. Luminance tracking sees images as continual movement.
- Second-order motion perception occurs through examining the changes in an object's position over time through feature tracking on the retina. Feature tracking is able to separate motion from blank intervals in which no motion is occurring.
- Optical illusions such as the phi phenomenon and the barber pole illusion demonstrate how motion is perceived.
- luminance: The amount of light that passes through or that is emitted from a particular area and that falls within a given solid angle.
- stroboscopic: Studying or observing periodic movement by rendering a moving body visible only at regular intervals.
- motion perception: The process of inferring the speed and direction of objects based on visual input.
Motion perception is the process of inferring the speed and direction of elements in a scene based on visual input. Monocular vision, or vision from one eye, can detect nearby motion; however, this type of vision is poor at depth perception. For this reason, binocular vision is better at perceiving motion from a distance. In monocular vision, the eye sees a two-dimensional image in motion, which is sufficient at near distances but not from farther away. In binocular vision, both eyes are used together to perceive motion of an object by tracking the differences in size, location, and angle of the object between the two eyes. Motion perception happens in two ways that are generally referred to as first-order motion perception and second-order motion perception.
First-Order Motion Perception
First-order motion perception occurs through specialized neurons located in the retina, which track motion through luminance. However, this type of motion perception is limited. An object must be directly in front of the retina, with motion perpendicular to the retina, in order to be perceived as moving. The motion-sensing neurons detect a change in luminance at one point on the retina and correlate it with a change in luminance at a neighboring point on the retina after a short delay.
Second-Order Motion Perception
Second-order motion perception occurs by examining the changes in an objects' position over time through feature tracking on the retina. This method detects motion through changes in size, texture, contrast, and other features. One advantage to feature-tracking is that motion can be separated both by motion and by blank intervals where no motion is occurring. This type of motion perception can be used to figure out how fast something is moving toward you—TTC, or "time to contact."
Visual illusions offer insight into how motion is perceived. The phi phenomenon is an illusion involving a regular sequence of luminous impulses. Due to first-order motion perception, the luminous impulses are seen as a continual movement. The phi phenomenon explains how early animation worked: it involves taking a series of still images that change slightly, and moving through them very quickly so that the image appears to be moving, rather than the series of still images that it is.
Another visual illusion is the barber pole illusion. In the barber pole illusion, a barber pole is rotated along the x-axis, but the diagonal stripes appear to move along the pole in a vertical fashion (y-axis) that is inconsistent with the actual direction the pole is turning in. The barber pole illusion also demonstrates how motion is perceived through first-order perception, which only sees movement as continual. The feature-tracking aspect of second-order perception does not perceive the aftereffects of a motion; it perceives movement as stroboscopic
, or as a series of still images.
Barber pole illusion: In the barber pole illusion, a barber pole is rotated along the x-axis, but the diagonal stripes appear to move down the pole's y-axis in a way that is inconsistent with the actual direction the pole is turning in.
We encounter more stimuli than we can attend to; unconscious perception helps the brain process all stimuli, not just those we take in consciously.
Describe the relationship between priming and subliminal stimulation
- Unconscious perception involves the processing of sensory inputs that have not been selected for conscious perception.
- Unconsciously, the brain processes all the stimuli we encounter, not just those we consciously attend to. The brain takes in these signals and interprets them in ways that influence how we respond to our environment.
- Priming is an unconscious process whereby neural networks are activated and strengthened, which influences perception of future stimuli.
- Priming allows for the brain to quickly and efficiently process stimuli from the environment.
- stimulus: In psychology, any energy pattern (e.g., light or sound) that is registered by the senses.
- priming: The implicit memory effect in which exposure to a stimulus influences response to a subsequent stimulus.
- Perception: The organization, identification, and interpretation of sensory information.
Individuals take in more stimuli from their environment than they can consciously attend to at any given moment. The brain is constantly processing all the stimuli it is exposed to, not just those that it consciously attends to. Unconscious perception involves the processing of sensory inputs that are not selected for conscious perception. The brain takes in these unnoticed signals and interprets them in ways that influence how individuals respond to their environment.
The perceptual learning of unconscious processing occurs through priming. Priming occurs when an unconscious response to an initial stimulus affects responses to future stimuli. One of the classic examples is word recognition, thanks to some of the earliest experiments on priming in the early 1970s: the work of David Meyer and Roger Schvaneveldt showed that people decided that a string of letters was a word when the letters followed an associatively or semantically related word. For example, NURSE was recognized more quickly when it followed DOCTOR than when it followed BREAD. This is one of the simplest examples of priming. When information from an initial stimulus enters the brain, neural pathways associated with that stimulus are activated, and a second stimulus is interpreted through that specific context.
Experience affects the activation of neural networks: When information from an initial stimulus enters the brain, neural pathways associated with that stimulus are activated, and the stimulus is interpreted in a specific manner.
One example of priming is in the childhood game Simon Says. Simon is able to trick the players because of priming. By saying "Simon says touch your nose," "Simon says touch your ear," and so on, participants are primed to follow the "Simon says" direction and are likely to slip up when that phrase is omitted because they expect it to be there.
In another example, individuals in a study were primed with neutral, polite, or rude words prior to an interview with an investigator. Priming the participants with words prior to the interview activated the neural circuits associated with reactions to those words. The participants who had been primed with rude words interrupted the investigator most often, and those primed with polite words did so the least often.
The presentation of an unattended stimulus can prime our brains for a future response to that stimulus. This process is known as subliminal stimulation. A number of studies have examined how unconscious stimuli influence human perception. Researchers, for example, have demonstrated how the type of music that is played in supermarkets can influence the buying habits of consumers. In another study, researchers discovered that holding a cold or hot beverage prior to an interview can influence how the individual perceives the interviewer. While subliminal stimulation appears to have a temporary effect, there is no evidence yet that it produces an enduring effect on behavior.
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