The sensitivity of a given sensory system to the relevant stimuli can be expressed as an absolute threshold. Another way to think about this is by asking how dim can a light be or how soft can a sound be and still be detected half of the time. The sensitivity of our sensory receptors can be quite amazing. Under quiet conditions, the hair cells the receptor cells of the inner ear can detect the tick of a clock 20 feet away Galanter, It is also possible for us to get messages that are presented below the threshold for conscious awareness—these are called subliminal messages.
A stimulus reaches a physiological threshold when it is strong enough to excite sensory receptors and send nerve impulses to the brain: this is an absolute threshold. A message below that threshold is said to be subliminal: we receive it, but we are not consciously aware of it. Therefore, the message is sensed, but for whatever reason, it has not been selected for processing in working or short-term memory. Over the years there has been a great deal of speculation about the use of subliminal messages in advertising, rock music, and self-help audio programs.
Research evidence shows that in laboratory settings, people can process and respond to information outside of awareness. Figure 2. Priming can be used to improve intellectual test performance. Research subjects primed with the stereotype of a professor — a sort of intellectual role model — outperformed those primed with an anti-intellectual stereotype.
These days, most scientific research on unconscious processes is aimed at showing that people do not need consciousness for certain psychological processes or behaviors. One such example is attitude formation. The most basic process of attitude formation is through mere exposure Zajonc, Merely perceiving a stimulus repeatedly, such as a brand on a billboard one passes every day or a song that is played on the radio frequently, renders it more positive.
Interestingly, mere exposure does not require conscious awareness of the object of an attitude. In fact, mere-exposure effects occur even when novel stimuli are presented subliminally for extremely brief durations e.
Intriguingly, in such subliminal mere-exposure experiments, participants indicate a preference for, or a positive attitude towards, stimuli they do not consciously remember being exposed to. Another example of modern research on unconscious processes is research on priming. Priming generally relies on supraliminal stimuli, which means that the messaging may occur out of awareness, but it is still perceived, unlike subliminal messaging.
Supraliminal messages are be perceived by the conscious mind. For example, in one study, shoppers listened to either French or German music the supraliminal messaging while buying wine, and sales originating from either country were higher when music from that same country was played overhead.
These lists contained words commonly associated with the elderly e. The remaining participants received a language task in which the critical words were replaced by words not related to the elderly.
After participants had finished they were told the experiment was over, but they were secretly monitored to see how long they took to walk to the nearest elevator. The primed participants took significantly longer.
That is, after being exposed to words typically associated with being old, they behaved in line with the stereotype of old people: being slow. Such priming effects have been shown in other domains as well. For example, Dijksterhuis and van Knippenberg demonstrated that priming can improve intellectual performance.
They asked their participants to answer 42 general knowledge questions taken from the game Trivial Pursuit. Imagine, for instance, that you were asked to take a hearing test. The signals are purposefully made to be very faint, making accurate judgments difficult. The problem for you is that the very faint signals create uncertainty.
Because our ears are constantly sending background information to the brain, you will sometimes think that you heard a sound when none was there, and you will sometimes fail to detect a sound that is there.
Your task is to determine whether the neural activity that you are experiencing is due to the background noise alone or is the result of a signal within the noise. The responses that you give on the hearing test can be analyzed using signal detection analysis. As you can see in Figure 5. The analysis of the data from a psychophysics experiment creates two measures.
One measure, known as sensitivity , refers to the true ability of the individual to detect the presence or absence of signals. People who have better hearing will have higher sensitivity than will those with poorer hearing. Imagine, for instance, that rather than taking a hearing test, you are a soldier on guard duty, and your job is to detect the very faint sound of the breaking of a branch that indicates that an enemy is nearby. You can see that in this case making a false alarm by alerting the other soldiers to the sound might not be as costly as a miss a failure to report the sound , which could be deadly.
Therefore, you might well adopt a very lenient response bias in which whenever you are at all unsure, you send a warning signal. In this case your responses may not be very accurate your sensitivity may be low because you are making a lot of false alarms and yet the extreme response bias can save lives.
Another application of signal detection occurs when medical technicians study body images for the presence of cancerous tumours. Again, a miss in which the technician incorrectly determines that there is no tumour can be very costly, but false alarms referring patients who do not have tumours to further testing also have costs.
The ultimate decisions that the technicians make are based on the quality of the signal clarity of the image , their experience and training the ability to recognize certain shapes and textures of tumours , and their best guesses about the relative costs of misses versus false alarms.
Although we have focused to this point on the absolute threshold, a second important criterion concerns the ability to assess differences between stimuli. The difference threshold or just noticeable difference [JND] , refers to the change in a stimulus that can just barely be detected by the organism. The German physiologist Ernst Weber made an important discovery about the JND — namely, that the ability to detect differences depends not so much on the size of the difference but on the size of the difference in relation to the absolute size of the stimulus.
As an example, if you have a cup of coffee that has only a very little bit of sugar in it say one teaspoon , adding another teaspoon of sugar will make a big difference in taste. Our tendency to perceive cost differences between products is dependent not only on the amount of money we will spend or save, but also on the amount of money saved relative to the price of the purchase.
If you study Figure 5. But can subliminal stimuli events that occur below the absolute threshold and of which we are not conscious have an influence on our behaviour? A variety of research programs have found that subliminal stimuli can influence our judgments and behaviour, at least in the short term Dijksterhuis, But whether the presentation of subliminal stimuli can influence the products that we buy has been a more controversial topic in psychology.
To be sure they paid attention to the display, the students were asked to note whether the strings contained a small b. However, immediately before each of the letter strings, the researchers presented either the name of a drink that is popular in Holland Lipton Ice or a control string containing the same letters as Lipton Ice NpeicTol.
Perception involves the processes of sensory interaction, selective attention, sensory adaptation, and perceptual constancy.
Although our perception is very accurate, it is not perfect. Our expectations and emotions colour our perceptions and may result in illusions. Skip to content Chapter 5. Sensing and Perceiving. Previous: 5. Next: 6. Humans have the ability to adapt to changes in light conditions. As mentioned before, rods are primarily involved in our ability to see in dim light.
They are the photoreceptors responsible for allowing us to see in a dark room. You might notice that this night vision ability takes around 10 minutes to turn on, a process called dark adaptation. This is because our rods become bleached in normal light conditions and require time to recover.
We experience the opposite effect when we leave a dark movie theatre and head out into the afternoon sun. During light adaptation , a large number of rods and cones are bleached at once, causing us to be blinded for a few seconds.
Light adaptation happens almost instantly compared with dark adaptation. Interestingly, some people think pirates wore a patch over one eye in order to keep it adapted to the dark while the other was adapted to the light.
Our cones allow us to see details in normal light conditions, as well as color. We have cones that respond preferentially, not exclusively, for red, green and blue Svaetichin, This trichromatic theory is not new; it dates back to the early 19th century Young, ; Von Helmholtz, This theory, however, does not explain the odd effect that occurs when we look at a white wall after staring at a picture for around 30 seconds.
Try this: stare at the image of the flag in Figure 3 for 30 seconds and then immediately look at a sheet of white paper or a wall. According to the trichromatic theory of color vision, you should see white when you do that. Is that what you experienced? This is where the opponent-process theory comes in Hering, This theory states that our cones send information to retinal ganglion cells that respond to pairs of colors red-green, blue-yellow, black-white.
These specialized cells take information from the cones and compute the difference between the two colors—a process that explains why we cannot see reddish-green or bluish-yellow, as well as why we see afterimages.
Color deficient vision can result from issues with the cones or retinal ganglion cells involved in color vision. Some of the most well-known celebrities and top earners in the world are musicians. Our worship of musicians may seem silly when you consider that all they are doing is vibrating the air a certain way to create sound waves , the physical stimulus for audition.
People are capable of getting a large amount of information from the basic qualities of sound waves. The amplitude or intensity of a sound wave codes for the loudness of a stimulus; higher amplitude sound waves result in louder sounds.
The pitch of a stimulus is coded in the frequency of a sound wave; higher frequency sounds are higher pitched. We can also gauge the quality, or timbre , of a sound by the complexity of the sound wave. In order for us to sense sound waves from our environment they must reach our inner ear. Lucky for us, we have evolved tools that allow those waves to be funneled and amplified during this journey.
Initially, sound waves are funneled by your pinna the external part of your ear that you can actually see into your auditory canal the hole you stick Q-tips into despite the box advising against it. During their journey, sound waves eventually reach a thin, stretched membrane called the tympanic membrane eardrum , which vibrates against the three smallest bones in the body—the malleus hammer , the incus anvil , and the stapes stirrup —collectively called the ossicles. Both the tympanic membrane and the ossicles amplify the sound waves before they enter the fluid-filled cochlea , a snail-shell-like bone structure containing auditory hair cells arranged on the basilar membrane see Figure 4 according to the frequency they respond to called tonotopic organization.
Depending on age, humans can normally detect sounds between 20 Hz and 20 kHz. It is inside the cochlea that sound waves are converted into an electrical message. Because we have an ear on each side of our head, we are capable of localizing sound in 3D space pretty well in the same way that having two eyes produces 3D vision. Have you ever dropped something on the floor without seeing where it went?
Did you notice that you were somewhat capable of locating this object based on the sound it made when it hit the ground?
We can reliably locate something based on which ear receives the sound first. What about the height of a sound? If both ears receive a sound at the same time, how are we capable of localizing sound vertically? After being processed by auditory hair cells, electrical signals are sent through the cochlear nerve a division of the vestibulocochlear nerve to the thalamus, and then the primary auditory cortex of the temporal lobe.
Information from the vestibular system is sent through the vestibular nerve the other division of the vestibulocochlear nerve to muscles involved in the movement of our eyes, neck, and other parts of our body. This information allows us to maintain our gaze on an object while we are in motion.
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