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Transcript of Chp 4
Human perception is very complex and hard to be replicated.
There are two processes going on in our effort to understand the world around us.
- Detecting external stimulus (such as lights, air vibrations, or odours) by our sensory organs, and transmiting this info to our brain. This process is sensation.
-Organizing and interpreting that information relying on our prior experiences. This process is perception.
Our interpretation of the world can be wrong and even distorted, but it allows us to adapt to the environment.
The sensory system has been shaped over the course of evolution to solve adaptive problems, such as providing information on potential dangers or identifying food.
Since each animal species faced different adaptive challenges, each is sensitive to different types of physical energy.
The first experimental psychologists were interested in understanding sensations.
They were working on psychophysics tested the limits of human sensory systems.
Psychophysics is an area that focuses on how physical stimuli is translated into psychological experience.
Q1: “Can we detect every stimulus?”
Q2: “Can we detect every change in the amount of stimulation?”
Q3: “How do we make judgments when stimuli are ambiguous?”
Q4:“What is the role of awareness and attention in our sensation?”
Q5:“Does our sensitivity to stimuli
change over prolonged stimulation?”
We have certain thresholds. We cannot detect a stimulus if it is under certain energy level.
For each specific type of sensory inputs, we have absolute thresholds.
Taste: One teaspoon (5 ml) of sugar in 7.5 litres of water
Smell: One drop of perfume diffused into the entire volume of
Touch: The wing of a fly falling on your cheek from a distance of
Hearing: The tick of a clock at 20 feet (6 meters) under quite
Vision: A candle flame can be seen at 50 kilometers on a dark,
We also have a threshold for change in the amount of stimulus.
The minimum amount of change required in order for us to detect a difference is called as just noticeable difference.
You are reading and TV is on, Volume at level 6-->level 7 (You notice the difference)
Watching TV, Volume at level 15--->level 16 (You may not notice the difference. You might need a three-unit change in the volume to notice the difference this time.)
Just noticeable difference increases as the stimulus become more intense.
The size of a just noticeable difference is based on a relative proportion of a difference rather than a fixed amount of difference. This principle is known as Weber’s law.
We are not 100% correct in detecting a stimulus, especially, in ambiguous situations.
People’s expectations often influence the extent to which they are biased.
Signal-detection theory proposes that detecting a stimulus requires making a judgment based on a subjective interpretation of ambiguous information.
Awareness and attention play important role in the sensation and perception.
First, you may not notice a stimulus if you are not aware or paying attention.
Second, you may also detect a stimulus unconsciously even though you are not aware of it. This is called as subliminal perception.
With continued exposure, your sensitivity will decrease. This is called as sensory adaptation.
The reason is that we are tuned to detect changes in the environment. It is more important to keep track of changes in the environment, because these might signal a threat to our safety. On the other hand, it is less critical to keep responding to unchanging stimuli.
The job of our taste sense, the gustatory system, is to keep poisons out of our digestive system while allowing good food in.
Chemical substances from food dissolve in saliva. Taste receptors on the taste buds turn this chemical information into neural impulses that are sent to thalamus and cortex.
There are different kinds of taste receptors and each responds most strongly to one of four “primary” taste sensations: sweet, sour, salty, and bitter.
If we all have the same taste system, why do our taste for food and eating preferences change?
-Some people have more taste buds than others as result of genetic inheritance.
-Taste preferences are largely learned and heavily influenced by social processes. Social influence contributes greatly to the striking ethnic and cultural disparities found in taste preferences.
-Although taste and smell are distinct sensory systems, the ability to identify flavours declines noticeably when odour cues are absent.
Smell system, the olfactory system, involves sensing chemicals from outside the body.
These chemical substances can evaporate and are carried in the air.
They reach our nasal passage; smell receptors at the upper portion of our nasal passage turn this chemical information into neural impulses that are sent to cortex.
Unlike other sensory information, smell signals go directly to cortex, olfactory bulb, just below the frontal lobes, and it bypasses the thalamus.
Regions in the prefrontal cortex process information about whether a smell is pleasant or aversive, while the intensity of smell is processed by the amygdala.
Since these regions are also responsible from emotions and memory formation, it is not surprising that different smells can evoke powerful memories and feelings.
Compared to many animals, our sense of smell is inferior, because we rely most heavily on vision and other senses.
Touch conveys sensations of pain, temperature, and pressure.
Sensory receptors terminate in the outer layer of our skin. Anything (mechanical, thermal or chemical) that makes contact with our skin provides stimulation.
These stimulations are carried by the nerve fibres to spinal cord and then to the brainstem. There fibres from each side of the body cross over to the opposite side of the brain. Then, they reach thalamus and to the somatosensory cortex in the brain’s parietal lobe.
Researchers especially focused on the study of pain, because it an important warning system for us.
Like other sensory experiences, the actual experience of pain is created in the brain. And like all other perceptions, it is likely to be misinterpreted or distorted.
There are two kinds of nerve fibres that convey pain information: fast fibres for sharp, immediate pain, and slow fibres for chronic, steady pain.
The difference is a result of the structure of the axons. Fast fibres are covered with a substance called as myelin. This substance help electrical impulse move faster.
Pain signals reach spinal cord, midbrain, thalamus and cortex. The neural activity in the midbrain region was found to affect whether we feel the pain or not. Endorphin and morphine-like painkillers were also found to be effective on this neural path and so blocking the pain.
The stimulus for hearing is a sound wave.
Sound wave is the displacement of air molecule by changes in the air pressure.
A sound is defined by its certain characteristics.
-Loudness: Amplitude (magnitude) of the wave. The louder the sound, the more
energy it has and the larger its amplitude will be.
Very loud sounds can threaten the quality of your hearing.
-Pitch (highness or lowness of a sound): Determined by its frequency. The
frequency of a sound is measured in vibrations per second, called Hertz (Hz).
Humans can detect sound waves with frequencies from about 20 Hz to 20,000.
Our ability to hear is based on the interactions of various regions of the ear.
Sound waves arrive at the outer ear and travel down the auditory canal to the eardrum.
Eardrum --> Ossicles (middle ear)-->Oval Window (inner ear)--> Cochlea
Cochlea is a fluid-filled tube that curls into a snake-like shape. The vibrations of the oval window create pressure waves in the fluid of the inner ear. These waves stimulate the hair cells located on the surface of the basilar membrane. These hair cells are primary auditory receptors and they generate action potentials that travel to thalamus and auditory cortex, most of which is located in temporal lobe.
Our brains also process the source of the sound, localization of the sound.
The fact that our ears are set apart contributes to auditory localization.
The brain integrates the different sensory information coming from each of our ear.
The brain uses two cues to locate the sound: the difference in timing between in its arrival in each ear; and the difference in its intensity in the ears. The difference will be larger if a sound locates from one side of the body.
Vision is our most important source of knowledge.
The stimuli for vision are light waves.
The eye works like a camera and focuses light to form an image on the retina. Light first passes through the cornea, transparent, outer layer of the eye.
Lights are bent farther inward by lens.
After light passes through cornea, lens and pupil an inverted image is formed on the retina.
Retina is the inner surface of the back of the eyeball. It contains the receptors that convert light into neural signals. The retina has two types of receptor cells: rods and cones.
-Rods play a key role in night vision and peripheral vision.
-Cones, on the other hand, play a key role in daylight vision and they are sensitive to colour and detail.
Light that arrived at these receptors triggers neural signals and these impulses are sent to the brain through the optic nerve.
Optic nerve is a collection of axons and it exits the eye at the back of each retina.
Optic nerve from each eye splits into two parts and cross at the optic chiasm.
At this point the information coming from both eyes is combined and then splits according to the visual field.
The right side of primary visual cortex deals with the left half of the field of view from both eyes, and similarly for the left brain.
Sensory neurons in the primary visual cortex, located in the occipital lobe, are specialized to respond to different stimuli.
Some respond best to colours, some to shape and others to direction of motion.
Visual sensation and perception rely on parallel processing.
Since the amount of information is too much, our brain divides up the task into components and run them simultaneously
Another characteristic of our visual perception is the top-down processing and our need to organize sensory information.
The idea of the organization of sensory info in mind was first proposed by Gestalt psychologists.
They suggested a series of principles to explain how perceived features of a visual scene are grouped into organized wholes.
Figure and ground: Dividing visual displays into figure and ground is a fundamental way in which we organize visual perceptions.
Proximity and similarity: we tend to group things that are close to each other. Also, we tend to group objects that resemble each other.
Closure: We tend to complete figures that have gaps in them.
We are also able to construct three-dimensional mental representations of the visual world on the basis of two-dimensional retinal input.
Our brain drives that information from the rules of spatial relations, using depth cues.
One important depth cue is binocular disparity. Since each eye has a slightly different view, by comparing these two images we can understand the distance of an object. The closer it is, the disparity between the images seen by each eye increases.
We also use monocular cues, cues about distance based on the image in either eye. For example: linear perspective, lines converge in the distance.
Although these cues are very helpful for us to understand place of objects in the space, they can also lead deceptions. For example; The Muller-Lyer Illusion