- Define sensation and perception, and understand the difference between the two
- Define the concepts of absolute threshold, subliminal stimulation, and difference threshold
- Define Signal-Detection Theory


[SLIDE 1]

<strong>Sensation</strong> is the stimulation of sensory receptors and the transmission of sensory information to the central nervous system, which consists of the spinal cord or brain. Sensory receptors are located in sensory organs such as the eyes and ears, the skin, and elsewhere in the body. Stimulation of the senses is an automatic process. It results from sources of energy, like light and sound, or from the presence of chemicals, as in smell and taste.

<strong>Perception</strong> is not mechanical, but is an active process in which sensations are organized and interpreted to form an inner representation of the world. Perception may begin with sensation, but it also reflects our experiences and expectations as it makes sense of sensory stimuli. 


[SLIDE 2]

Perception of sights, sounds, and other sources of sensory input depends largely on the so-called five senses: vision, hearing, smell, taste, and touch. But touch is just one of several skin senses, which also include pressure, warmth, cold, and pain.

There are also senses that alert us to body position without having to watch every step taken. Similar sensations may lead to different perceptions in different people, or to different situations in the same person.

The next few lessons will explore our senses, but first we will consider a number of concepts used when we talk about the relationships between sensations and perceptions: absolute threshold, subliminal stimulation, difference threshold, signal–detection theory, feature detectors, and sensory adaptation.  


[SLIDE 3]

<strong>Absolute threshold</strong> refers to the weakest level of a stimulus that is necessary to produce a sensation. For example, the absolute threshold for light would be the minimum brightness, or physical energy, required to activate the visual sensory system.

A psychophysicist looks for the absolute thresholds of the senses by exposing individuals to progressively stronger stimuli until they find the minimum stimuli that the person can detect 50% of the time. Some people are more sensitive than others, and the same person might have a slightly different response at different times.

Each kind of sensory input has different measures and can have different absolute thresholds. For example, one measure of hearing is pitch, which is the highness or lowness of a sound. This is determined by the frequency of the sound waves.  


[SLIDE 4]

Some television commercials contain words or sexual images that are flashed so briefly on the screen that we do not become conscious of them. But can they still influence us? Behaviorist John B. Watson was a pioneer in associating appealing stimuli with products in advertising.

Sensory stimulation that is below a person’s absolute threshold for conscious perception is termed <strong>subliminal stimulation</strong> and its perception is called subliminal perception. Visual stimuli can be flashed too briefly to enable us to process them. Auditory stimuli can be played at a volume too low to consciously hear or can be played backward.  


[SLIDE 5]

How much of a difference in intensity between two lights is required before you will detect one as being brighter than the other? The minimum difference in magnitude of two stimuli required to tell them apart is their <strong>difference threshold</strong>.

Just as with the absolute threshold, psychologists agree to the standard of a difference in strength that can be detected 50% of the time. Psychophysicist Ernst Weber discovered that the threshold for perceiving differences in the intensity of light is about 2%, actually closer to 1/60th, of their intensity. This fraction, 1/60th, is known as <strong>Weber’s constant for light</strong>.

A related concept is the <strong>just noticeable difference</strong>, or jnd, which is the minimum difference in stimuli that a person can detect. For example, at least 50% of the time, most people can tell if a light becomes just 1/60th brighter or dimmer.  


[SLIDE 6]

In addition to being alert to certain amounts of stimulation, people are also influenced by psychological factors.

<strong>Signal–detection theory</strong> considers these factors. According to signal–detection theory, the relationship between a physical stimulus and a sensory response is not fully mechanical. Our ability to detect stimuli such as blips on a radar screen depends not only on the intensity of the blips but also on our training or learning, motivation or desire to perceive, and psychological states such as fatigue or alertness. 
The intensity of the signal is one factor that determines whether people will perceive sensory stimuli (signals) or a difference between signals. Another is the degree to which the signal can be distinguished from background noise. The sharpness of a person’s biological sensory system is still another factor. For example, is it diminished by age? We also tend to detect stimuli we are searching for and people sometimes hear what they want to hear. Thus, one psychological factor in signal detection is focusing on signals one considers important.


[SLIDE 7]

Various neurons in the visual cortex of the brain fire in response to particular features of visual input. Many cells in the brain detect, or fire in response to, lines presented at various angles – either vertical, horizontal, and in between. Other cells fire in response to specific colors.

Because they respond to different aspects or features of a scene, these brain cells are called <strong>feature detectors</strong>. For example, consider you are waiting for a bus. Visual feature detectors respond to the bus’s edges, depth, contours, textures, shadows, speed, and kinds of motion. There are also feature detectors for other senses. Auditory feature detectors, for example, respond to the pitch, loudness, and other aspects of the sounds of the bus.


[SLIDE 8]

<strong>Sensory adaptation</strong> refers to the processes by which we become more sensitive to stimuli of low magnitude and less sensitive to stimuli that remain the same, such as the background noises. For example, consider how the visual sense adapts to lower intensities of light. When we first walk into a darkened movie theater, we see little but the images on the screen. As we search for our seats, however, we become increasingly sensitive to the faces around us and to the features of the theater.

The process of becoming more sensitive to stimulation is referred to as <strong>sensitization</strong>. This is also called <strong>positive adaptation</strong>. But we can become less sensitive to constant stimulation. For example, living in a city we become desensitized to sounds of traffic. The process of becoming less sensitive to stimulation is referred to as <strong>desensitization</strong> or <strong>negative adaptation</strong>.

Our sensitivities to stimulation provide our brains with information that we use to understand and influence the world outside. Psychologists study the ways in which we sense and perceive this information.