Understanding Sensation and Perception

I'm Dr. Mark Atala, and I want to welcome you to the fifth chapter of the OpenStack Psychology textbook. Today we'll be discussing sensation and perception. So we'll be discussing things like waves and wavelengths, vision, hearing, the other senses, and gestalt principles of perception. But let's get started by defining what sensation is, and that's when sensory information is detected by a sensory receptor. Transduction is the conversion from sensory stimulus energy to an action potential. And our five senses are vision, hearing, which we call audition, smell, which is called olfaction, taste, which is gustation, and touch, which is somatosensation. Now we can talk about things like an absolute threshold, and that's the minimum amount of stimulus energy that must be present for the stimulus to be detected. detected 50% of the time. So you can ask yourself, how dim can a light be, or how soft can a sound be, and still be detected half of the time. And it's been estimated that an eye can detect a candle flame at 30 miles on a clear night. So you can see some of these other ones here too. So a drop of perfume in a six-room house. That must be one pungent perfume. Subliminal messages are messages below the threshold for conscious awareness, which means that we receive the message, but we aren't conscious of it. Now, lab research has shown that people can process and respond to information which is outside of their awareness, but hidden messages have little effect on behavior outside the laboratory. The just noticeable difference, or JND, or difference threshold, asks the question, how much of a change in stimuli is required to detect that a change has occurred? And this goes back to Ernst Weber, and this changes depending on the stimulus intensity. So, for example, if you take your phone out in a movie theater, and when a movie's on, then it's amazingly bright. But if you take it out, let's say, at a baseball game, a daytime baseball game, then you might struggle to see the screen. And so that shows that there can be a difference there. Now, Weber's law is that the difference threshold is a constant fraction of the original stimulus. And I always find this fascinating in terms of marketing and just noticeable differences, too. Now, when they lower the prices for a sale, they always lower the price. more than the just noticeable difference. So people notice that there's a lower price. But when they raise the price, they try to raise it less than the just noticeable difference. But what they usually do is they just don't raise the price. They instead shrink the product in such a way that people don't notice that it's gotten smaller. Happens all the time. Very upsetting to me. Perception is the way sensory information is organized, interpreted, and consciously experienced, and it involves both bottom-up and top-down processing. So bottom-up processing is when perceptions are built from, that should be built from sensory input, and top-down processing is how we interpret sensations, is influenced by our available knowledge, our experiences, and our thoughts. So basically, our preconceived notions that we're bringing to perception. Sensation is a physical process, and perception is a psychological process. And not all sensations result in perceptions. So you might hear a clock ticking when you enter a room, but once you start doing something, you tune it out. And that's what's known as sensory adaptation. You don't perceive stimuli that remain relatively constant over prolonged periods of time. So attention affects sensation and perception, and it influences what is sensed versus what is perceived. So this was kind of an internet phenomena, this idea of inattentional blindness, and that's a failure to notice something that's completely visible because of a lack of attention. So what they did was they had, you were supposed to monitor how often the white team passes the ball, and a gorilla walked right through the... visual field and it was clearly visible for nine seconds but a number of participants failed to notice the gorilla because they were so focused on the number of times the white team was passing the ball. Signal detection theory is the ability to identify a stimulus when it's embedded in a distracting background. And this might explain why a mother is awakened when her baby makes a sound, but not when she hears other sounds while she's asleep. Perceptions also impacted, as we said earlier, by beliefs, values, prejudices, expectations, and life experiences. So that's top-down information. Research has found that individuals from Western cultures are more prone to experience the Mueller-Lyer illusion, which is off to the right there, than individuals from non-Western cultures. And the thought is that people in Western cultures have a perceptual context of buildings with straight lines, but people from some non-Western cultures are less susceptible because they have villages made up of round huts arranged in circles. So the Zulu people of South Africa are less susceptible to the illusion. Okay, let's get some technical terms down. So the two physical characteristics of a wave are amplitude and wavelength. And amplitude is the height of a wave as measured from the highest point on a wave, its peak or crest, to the lowest point on the wave, which is the trough. A wavelength is the length of a wave from one peak to the next. So a wavelength is directly related to the frequency, which is the number of waves that pass a given point in a given time. given time period, and that's often expressed in terms of hertz, which is cycles per second. The longer wavelengths have lower frequencies, and shorter wavelengths have higher frequencies. The visible spectrum is the portion of the larger electromagnetic spectrum that we can see as humans, and so for us, it's wavelengths between 380 and 740 nanometers. So this light wavelength in humans is also associated with the perception of color. I guess I should say, too, that other species like honeybees can see light in the ultraviolet range, but we cannot, at least not without special glasses or goggles. Going back to color, though, red is associated with longer wavelengths and violets with shorter wavelengths. And a way to remember the visible spectrum is with the mnemonic ROYGB. bib. So that would be red, orange, yellow, green, blue, indigo, and violet. The frequency of a sound wave is associated with the perception of pitch. So high frequency sounds are perceived as high pitch and low frequency sounds as low pitch. The audible range for humans is between 20 and 20,000 hertz, which with the greatest sensitivity to frequencies falling in the middle there. And as you can see on the chart to the right, you can see different animals too. So dogs here between 70 and 45,000 hertz. Mice between 1,000 and 91,000 hertz. You can see the other ones for yourself. I just wanted to comment to your book talks about the beluga whale. Here's between 1,000 hertz and 123,000 hertz. It's quite a range. The loudness of a given sound is closely associated with the amplitude of the sound wave. Higher amplitudes are associated with louder sounds, and loudness is measured in terms of decibels, which is a logarithmic unit of sound intensity. A typical conversation is about 60 decibels, and a rock concert might be 120 decibels, although I've been to louder ones. The potential for hearing damage is between 80 to 130 decibels, and that would be like a lawnmower or a jackhammer. And the threshold for pain is about 130 decibels, and that would be like a jet taking off or a revolver firing at close range, which is probably why people don't look like living by airports or gun ranges. Timber! This refers to a sound's purity. And it's affected by the complex interplay of frequency, amplitude, and the timing of sound waves. So for instance, different musical instruments can play the same musical note at the same level of loudness, and yet they sound different. And that's what timbre is. Let's switch to vision. So the anatomy of the visual system. So the cornea is a transparent covering over the eye. and it serves as a barrier between the inner eye and the outside world, and it's involved in focusing light waves that enter the eye. And so these light waves are transmitted across the cornea and enter the eye. through the pupil, which is a small opening in the eye through which light passes. The size of the pupil can change as a function of light levels as well as emotional arousal. So in low light conditions, like a candlelit dinner, the pupil is dilated or expands to allow more light to enter the eye. But pupil dilation is also a mark of emotional arousal, so there's a lot going on at that dinner. The pupil size is controlled by muscles connected to the iris, and that's the colored portion of the eye. And so, for example, I have green eyes. After passing through the pupil, the light crosses the lens, which is a curved transparent structure that serves to provide additional focus. In a normal-sighted individual, the lens will focus images perfectly onto the fovea, And the fovea is a small indentation at the back of the eye, and it's part of the retina, which is the light-sensitive lining of the eye. The fovea contains densely packed, specialized photoreceptor cells, known as cones, that are light-detecting cells, which means that they're photosensitive. Now, cones are a specialized type of photoreceptor cell that works best in bright light, and they're directly involved in our perception of color. which we'll talk about in a moment. Rods are specialized photoreceptors that work well in low light conditions. Although they lack the spadial resolution and color function of the cones, they are involved in our vision in dimly lit environments, as well as in our perception of movement on the periphery of our visual field. So that's why if you want to see something in low light conditions, it's best not to look directly at it. You're best... looking at it out of the side of your eye. The rods and cones are connected to retinal ganglion cells, which converge and exit through the back of the eye to form the optic nerve, and then that carries the visual information from the retina to the brain. There's a point in the visual field called the blind spot where the optic nerves connect to the eyes. Now, we don't notice this blind spot for two reasons. Number one, each eye gets a slightly different view of the visual field. so the blind spots don't align perfectly, so that's good. And number two, our brain fills in the information missing from the blind spot, from information from the other eye. The optic nerve from each eye merges just below the brain at a point called the optic chiasm. At the point of the optic chiasm, information from the right visual field, which comes from both eyes, is sent to the left side of the brain. And information from the left visual field, also coming from both eyes, is sent to the right side of the brain. Theories of color vision. And people actually, I kid you not, used to argue about which one of these was right. The trichromatic theory of color vision says that all colors in the spectrum can be produced by combining red, green, and blue. And that there's three types of cones that correspond or are receptive to each of those colors. The opponent process theory says that color is coded in opponent pairs, so black and white, yellow and blue, and red and green. So that a cell that's excited by wavelengths associated with green would be inhibited by wavelengths associated with red and vice versa. So you see in terms of color opposites. And that leads to negative after images, which is the continuation of a visual sensation after the removal of a stimulus. So you can look at a figure or a picture that doesn't look right and you stare at it and then you look at like a whiteboard and all of a sudden you see an American flag. The trichromatic theory of color vision and the opponent process theory are not mutually exclusive. They just apply to different levels of the nervous system. So for visual processing on the retina, the trichromatic theory applies. The cones are responsive to three different wavelengths that represent red, blue, and green. But once the signal moves past the retina on its way to the brain, the cells respond in a way consistent with the opponent process theory. Now off to the right there, that doesn't actually involve, well, it sort of involves color vision. Those are what you would use to see if you're colorblind. So if you see the number 12 and the number 42, you're not colorblind, but your book doesn't talk about colorblindness, so we'll pass over that in silence, as Wittgenstein would say. Depth perception is is our ability to perceive spadial relations in three dimensions. And that allows us to say that things are in front of others, or behind, or above, or below, or whatever else you want to say. Binocular cues rely on both eyes. And, for example, binocular disparity is, because each eye sees a slightly different view, you can see three dimensions. And so, for example, 3D movies exploit this. by projecting slightly different images onto the screen to be seen separately by your right and left eye, and that gives the illusion of depth, and so that's kind of interesting. Now there's far more monocular cues than there are binocular cues, and monocular just involves or requires one eye. Linear perspective, for example, is when you perceive the depth when two lines converge, but there's a number of other monocular cues like interposition, the the partial overlap of objects, so you can see that one thing's in front of another thing, so that means it's closer, and the relative size and closeness of images to the horizon. Let's talk about the auditory system. So you have your outer ear, which is the pinna, which is the visible part. You have an auditory canal and a tympanic membrane, which we also call your eardrum. Now, I can't pronounce the things. in your middle ear, but there's three tiny bones called the ossicles, and that's the malus, I'm going to try to pronounce them, which is hammer, the incus, which is anvil, and the stapes, which is stirrup. I'm just going to say stirrup though. The inner ear is made up of the cochlea, which is a snail-shaped fluid filled, and it contains the sensory receptor cells. And these are hair cells that are auditory receptor cells of the inner ear embedded in the basilar membrane. So, the way this works is that sound waves travel down the auditory canal to the tympanic membrane, causing it to vibrate, which results in music. Music. That results in movement of the ossicles, which makes the stirrup press on an area of the cochlea known as the oval window. And that moves fluid in the cochlea that stimulates hair cells, which are embedded in the... basilar membrane. So basically the stimulation of the hair cells leads to the activation of hearing, so the experience of hearing. And if you want to chart this out, auditory information goes to the inferior colliculus, which goes to the medial geniculate nucleus of the thalamus, and then onto the auditory cortex in the temporal lobe for further processing. What about pitch perception? Well, low frequency sounds are low pitched and high frequency sounds tend to be higher pitched. And this is how the auditory system differentiates between various pitches. Two different theories here. The temporal theory is that frequency is coded by the activity level of a sensory neuron. So hair cells fire only for particular frequencies. And this is good. So that's hair cells in the cochlea, just to remind you. But it can't explain the whole range of hearing. So we also have place theory. And that's that different portions of the basilar membrane, and that's also in the cochlea, are sensitive to different frequencies. So hair cells in the base portion are high-pitched receptors, and those in the tip of the basilar membrane are low-pitched. Both of these theories are correct, but they explain different aspects of pitch perception. So low frequencies are really explained by the temporal theory, and that's up to 4,000 hertz, but higher frequencies can only be encoded using place theory. Sound localization. How do we know where sounds are coming from? We have monaural, which are one-eared, and binaural, two-eared cues, and both are used. So an example of a monaural cue would be that each pinna, which is the outside of your ear, interacts with sound waves differently, and depending on the source relative to our bodies, that's how we can localize the sound. Intraoral level difference means that sounds coming from the right side, let's say the sound is coming from the right side of your body, it's more intense in your right ear than your left because your head is blocking the sound wave. It's the attenuation of the sound wave as it passes through your head. Intraoral timing differences is are that small differences in time that it takes for the sound waves to reach each ear. And so that can give you an idea about localization too. What about hearing loss? Well, deafness is a partial. or complete inability to hear. And people who are born deaf have what's called congenital deafness. A conductive hearing loss is deafness due to age, genetic predisposition, or environmental effects such as extreme noise, certain illnesses, or toxins. And sensorineural hearing loss is a failure to transmit neural signals from the cochlea to the brain. And there's a disease that is a degeneration of your inner ear structures. A way to bring hearing back is through cochlear implants, and these are electronic devices that include a microphone, a speech processor, and an electrode array, and they receive sound information and directly stimulate the auditory nerve. So taste and smell are called the chemical senses. because they both have sensory receptors that respond to molecules in the food we eat or the air we breathe. There is a pronounced interaction between these two chemical senses. So that's why, again, you can't taste things very well when you have a cold. Taste, which we call gustation, has four basic groupings, sweet, sour, salty, and bitter. But research shows there are at least six taste groupings. So the fifth is known as umami, and that's a Japanese word that roughly translates to yummy, and it's associated with a taste for monosodium glutamate, which is MSG. And there's also evidence suggesting that there's a taste for fatty content. Your taste buds are formed by groupings of taste receptor cells with hair-like extensions that protrude into the central pore of the taste bud. They have a life cycle of about 10 days to two weeks. So even destroying some of them by burning your tongue won't have any long-term effect. They grow right back. Your taste information is transmitted to the medulla, the thalamus, and the limbic system, and also to the gustatory cortex. Smell or olfaction, and I want to say this, that I have a good sense of smell, but I grew up in an olfactory town. That's a terrible joke. Olfactory receptor cells are located in a mucus membrane at the top of the nose. Small hair-like extensions from these receptors serve as the sites for odor molecules dissolved in the mucus to interact with chemical receptors located on these extensions. Chemical changes within the cell result in signals being sent to the olfactory bulb, and that's a bulb-like structure, hence olfactory bulb. at the tip of the frontal lobe where the olfactory nerves begin. There's a lot of variation in the sensitivity of the olfactory system between species. So, for example, dogs have a much more sensitive olfactory system than humans do. And there's some evidence that dogs can even smell dangerous drops in people's blood, glucose levels, and cancerous tumors. So they can be trained. to be able to do that. Many species also respond to chemical messages known as pheromones sent by other individuals of their species. These messages often involve information about reproductive status, and there's been a lot of research and controversy about the importance of pheromones in humans, because sometimes they're put into perfumes. There are many types of receptors in your skin that respond to different kinds of touch-related stimuli. So let's talk about touch. So Meissner's corpuscles respond to pressure and lower frequency vibrations. I'm sorry, the Pashinian corpuscles detect transient pressure and higher frequency vibrations. Merkel's discs respond to light pressure and Ruffini corpuscles detect stretch. I think that's actually pronounced pacinian corpuscles, so my apologies there. In addition to the receptors in the skin, there are also many free nerve endings that respond to a variety of different types of touch-related stimuli, as well as serving as receptors for both thermoception, which is temperature perception, and nociception, which is a signal indicating potential harm and maybe pain. So let's talk about pain perception. It's, well this is obvious, but it's an unpleasant experience that involves both physical and psychological components. And basically, pain can be considered inflammatory or neuropathic in nature. Inflammatory pain is pain that signals some type of tissue damage, and neuropathic pain is pain that results from damage to neurons of either the peripheral or central nervous system, and sometimes that involves exaggerated pain signals sent to the brain. Some people are born without the ability to feel pain, and we would say they have a congenital insensitivity to pain or congenital analgesia. And that's a rare genetic disorder resulting in an individual being born without the ability to feel pain. I'm being redundant here. My apologies. People with this disorder can detect temperature and pressure differences, but they often suffer severe injuries. And this may come as no surprise to you, but people with congenital analgesia also have much shorter lifespans than most other people. The vestibular sense contributes to our ability to maintain balance in body posture. The major sensory organs, these are in the semicircular canals of this system, are located next to the cochlea in the inner ear. The vestibular system also collects information for controlling movement and the reflexes that move various parts of our bodies to compensate for changes in body position. Therefore, both proprioception which is the perception of your body position, and kinesthesia, which is the perception of the body's movement through space, interact with information provided by the vestibular system. Well, let's finish up by talking about gestalt principles of perception. And this starts with Max Wertheimer, and he published a paper demonstrating that individuals perceive motion in these rapidly flickering static images. And so the motion is not in the stimuli, it's in our perception of it. And that's actually known as the five phenomena. And this was an insight that came to him as he used a child's toy tachystoscope. I've actually heard this also explained as something he noticed one time when he was riding on a train. So gestalt psychology is a movement within psychology. Gestalt literally means form or pattern. And so you can think of it like that. The idea is that the brain creates a perception that is more than simply the sum of the available sensory inputs, and it does so in predictable ways. What are those ways? Well, one Gestalt principle is the figure-ground relationship. And according to this, we tend to segment our visual world into figure and ground, where figure is the object of our focus and ground is the background. So off to the right, you'll see these were quite popular in... the late 1800s, but you can still buy them today. They're vases with faces, and so you can see the vase as the figure, or you can see the faces as the figure, and then one's always acting as the background for the other. Another Gestalt principle for organizing sensory stimuli is proximity, and this asserts that things that are close to one another tend to be grouped together. So you can see in B... that you're going to see three columns because we take those dots and based on their proximity we say, well, those aren't lines, those are columns. The principle of similarity is another way to group. And off to the right there you can see a line of white and black dots as opposed to a column of alternating black and white. The law of continuity suggests that we're more likely to perceive continuous, smooth, flowing lines rather than jagged, broken lines. And so that's to the bottom right there, where it's basically connect the dots. You see those as continuous. And the principle of closure states that we organize our perception into complete objects rather than as a series of parts. And so by putting together a series of circles with cutouts in them, we can perceive. a necker cube. And so that's kind of interesting too. Well, that's the end of our sensation and perception chapter. But I want to remind you that all your problems, at least all your APA style problems, can be solved with my Learn APA Style book. So when you want to learn to write correctly or write right, consult my book and videos on Learn APA Style, which are about writing and psychology. in the social sciences. Have a great day, and thanks for listening.

But let's get started by defining what sensation is, and that's when sensory information is detected by a sensory receptor. Transduction is the conversion from sensory stimulus energy to an action potential. And our five senses are vision, hearing, which we call audition, smell, which is called olfaction, taste, which is gustation, and touch, which is somatosensation.

Now we can talk about things like an absolute threshold, and that's the minimum amount of stimulus energy that must be present for the stimulus to be detected. detected 50% of the time. So you can ask yourself, how dim can a light be, or how soft can a sound be, and still be detected half of the time.

And it's been estimated that an eye can detect a candle flame at 30 miles on a clear night. So you can see some of these other ones here too. So a drop of perfume in a six-room house.

That must be one pungent perfume. Subliminal messages are messages below the threshold for conscious awareness, which means that we receive the message, but we aren't conscious of it. Now, lab research has shown that people can process and respond to information which is outside of their awareness, but hidden messages have little effect on behavior outside the laboratory.

The just noticeable difference, or JND, or difference threshold, asks the question, how much of a change in stimuli is required to detect that a change has occurred? And this goes back to Ernst Weber, and this changes depending on the stimulus intensity. So, for example, if you take your phone out in a movie theater, and when a movie's on, then it's amazingly bright.

But if you take it out, let's say, at a baseball game, a daytime baseball game, then you might struggle to see the screen. And so that shows that there can be a difference there. Now, Weber's law is that the difference threshold is a constant fraction of the original stimulus.

And I always find this fascinating in terms of marketing and just noticeable differences, too. Now, when they lower the prices for a sale, they always lower the price. more than the just noticeable difference.

So people notice that there's a lower price. But when they raise the price, they try to raise it less than the just noticeable difference. But what they usually do is they just don't raise the price. They instead shrink the product in such a way that people don't notice that it's gotten smaller. Happens all the time.

Very upsetting to me. Perception is the way sensory information is organized, interpreted, and consciously experienced, and it involves both bottom-up and top-down processing. So bottom-up processing is when perceptions are built from, that should be built from sensory input, and top-down processing is how we interpret sensations, is influenced by our available knowledge, our experiences, and our thoughts.

So basically, our preconceived notions that we're bringing to perception. Sensation is a physical process, and perception is a psychological process. And not all sensations result in perceptions.

So you might hear a clock ticking when you enter a room, but once you start doing something, you tune it out. And that's what's known as sensory adaptation. You don't perceive stimuli that remain relatively constant over prolonged periods of time.

So attention affects sensation and perception, and it influences what is sensed versus what is perceived. So this was kind of an internet phenomena, this idea of inattentional blindness, and that's a failure to notice something that's completely visible because of a lack of attention. So what they did was they had, you were supposed to monitor how often the white team passes the ball, and a gorilla walked right through the... visual field and it was clearly visible for nine seconds but a number of participants failed to notice the gorilla because they were so focused on the number of times the white team was passing the ball.

Signal detection theory is the ability to identify a stimulus when it's embedded in a distracting background. And this might explain why a mother is awakened when her baby makes a sound, but not when she hears other sounds while she's asleep. Perceptions also impacted, as we said earlier, by beliefs, values, prejudices, expectations, and life experiences. So that's top-down information.

Research has found that individuals from Western cultures are more prone to experience the Mueller-Lyer illusion, which is off to the right there, than individuals from non-Western cultures. And the thought is that people in Western cultures have a perceptual context of buildings with straight lines, but people from some non-Western cultures are less susceptible because they have villages made up of round huts arranged in circles. So the Zulu people of South Africa are less susceptible to the illusion. Okay, let's get some technical terms down. So the two physical characteristics of a wave are amplitude and wavelength.

And amplitude is the height of a wave as measured from the highest point on a wave, its peak or crest, to the lowest point on the wave, which is the trough. A wavelength is the length of a wave from one peak to the next. So a wavelength is directly related to the frequency, which is the number of waves that pass a given point in a given time. given time period, and that's often expressed in terms of hertz, which is cycles per second. The longer wavelengths have lower frequencies, and shorter wavelengths have higher frequencies.

The visible spectrum is the portion of the larger electromagnetic spectrum that we can see as humans, and so for us, it's wavelengths between 380 and 740 nanometers. So this light wavelength in humans is also associated with the perception of color. I guess I should say, too, that other species like honeybees can see light in the ultraviolet range, but we cannot, at least not without special glasses or goggles.

Going back to color, though, red is associated with longer wavelengths and violets with shorter wavelengths. And a way to remember the visible spectrum is with the mnemonic ROYGB. bib. So that would be red, orange, yellow, green, blue, indigo, and violet. The frequency of a sound wave is associated with the perception of pitch.

So high frequency sounds are perceived as high pitch and low frequency sounds as low pitch. The audible range for humans is between 20 and 20,000 hertz, which with the greatest sensitivity to frequencies falling in the middle there. And as you can see on the chart to the right, you can see different animals too. So dogs here between 70 and 45,000 hertz. Mice between 1,000 and 91,000 hertz.

You can see the other ones for yourself. I just wanted to comment to your book talks about the beluga whale. Here's between 1,000 hertz and 123,000 hertz. It's quite a range. The loudness of a given sound is closely associated with the amplitude of the sound wave.

Higher amplitudes are associated with louder sounds, and loudness is measured in terms of decibels, which is a logarithmic unit of sound intensity. A typical conversation is about 60 decibels, and a rock concert might be 120 decibels, although I've been to louder ones. The potential for hearing damage is between 80 to 130 decibels, and that would be like a lawnmower or a jackhammer. And the threshold for pain is about 130 decibels, and that would be like a jet taking off or a revolver firing at close range, which is probably why people don't look like living by airports or gun ranges.

Timber! This refers to a sound's purity. And it's affected by the complex interplay of frequency, amplitude, and the timing of sound waves. So for instance, different musical instruments can play the same musical note at the same level of loudness, and yet they sound different. And that's what timbre is.

Let's switch to vision. So the anatomy of the visual system. So the cornea is a transparent covering over the eye.

and it serves as a barrier between the inner eye and the outside world, and it's involved in focusing light waves that enter the eye. And so these light waves are transmitted across the cornea and enter the eye. through the pupil, which is a small opening in the eye through which light passes.

The size of the pupil can change as a function of light levels as well as emotional arousal. So in low light conditions, like a candlelit dinner, the pupil is dilated or expands to allow more light to enter the eye. But pupil dilation is also a mark of emotional arousal, so there's a lot going on at that dinner. The pupil size is controlled by muscles connected to the iris, and that's the colored portion of the eye. And so, for example, I have green eyes.

After passing through the pupil, the light crosses the lens, which is a curved transparent structure that serves to provide additional focus. In a normal-sighted individual, the lens will focus images perfectly onto the fovea, And the fovea is a small indentation at the back of the eye, and it's part of the retina, which is the light-sensitive lining of the eye. The fovea contains densely packed, specialized photoreceptor cells, known as cones, that are light-detecting cells, which means that they're photosensitive. Now, cones are a specialized type of photoreceptor cell that works best in bright light, and they're directly involved in our perception of color. which we'll talk about in a moment.

Rods are specialized photoreceptors that work well in low light conditions. Although they lack the spadial resolution and color function of the cones, they are involved in our vision in dimly lit environments, as well as in our perception of movement on the periphery of our visual field. So that's why if you want to see something in low light conditions, it's best not to look directly at it. You're best... looking at it out of the side of your eye.

The rods and cones are connected to retinal ganglion cells, which converge and exit through the back of the eye to form the optic nerve, and then that carries the visual information from the retina to the brain. There's a point in the visual field called the blind spot where the optic nerves connect to the eyes. Now, we don't notice this blind spot for two reasons. Number one, each eye gets a slightly different view of the visual field. so the blind spots don't align perfectly, so that's good.

And number two, our brain fills in the information missing from the blind spot, from information from the other eye. The optic nerve from each eye merges just below the brain at a point called the optic chiasm. At the point of the optic chiasm, information from the right visual field, which comes from both eyes, is sent to the left side of the brain.

And information from the left visual field, also coming from both eyes, is sent to the right side of the brain. Theories of color vision. And people actually, I kid you not, used to argue about which one of these was right. The trichromatic theory of color vision says that all colors in the spectrum can be produced by combining red, green, and blue. And that there's three types of cones that correspond or are receptive to each of those colors.

The opponent process theory says that color is coded in opponent pairs, so black and white, yellow and blue, and red and green. So that a cell that's excited by wavelengths associated with green would be inhibited by wavelengths associated with red and vice versa. So you see in terms of color opposites.

And that leads to negative after images, which is the continuation of a visual sensation after the removal of a stimulus. So you can look at a figure or a picture that doesn't look right and you stare at it and then you look at like a whiteboard and all of a sudden you see an American flag. The trichromatic theory of color vision and the opponent process theory are not mutually exclusive.

They just apply to different levels of the nervous system. So for visual processing on the retina, the trichromatic theory applies. The cones are responsive to three different wavelengths that represent red, blue, and green. But once the signal moves past the retina on its way to the brain, the cells respond in a way consistent with the opponent process theory. Now off to the right there, that doesn't actually involve, well, it sort of involves color vision.

Those are what you would use to see if you're colorblind. So if you see the number 12 and the number 42, you're not colorblind, but your book doesn't talk about colorblindness, so we'll pass over that in silence, as Wittgenstein would say. Depth perception is is our ability to perceive spadial relations in three dimensions.

And that allows us to say that things are in front of others, or behind, or above, or below, or whatever else you want to say. Binocular cues rely on both eyes. And, for example, binocular disparity is, because each eye sees a slightly different view, you can see three dimensions.

And so, for example, 3D movies exploit this. by projecting slightly different images onto the screen to be seen separately by your right and left eye, and that gives the illusion of depth, and so that's kind of interesting. Now there's far more monocular cues than there are binocular cues, and monocular just involves or requires one eye. Linear perspective, for example, is when you perceive the depth when two lines converge, but there's a number of other monocular cues like interposition, the the partial overlap of objects, so you can see that one thing's in front of another thing, so that means it's closer, and the relative size and closeness of images to the horizon. Let's talk about the auditory system.

So you have your outer ear, which is the pinna, which is the visible part. You have an auditory canal and a tympanic membrane, which we also call your eardrum. Now, I can't pronounce the things. in your middle ear, but there's three tiny bones called the ossicles, and that's the malus, I'm going to try to pronounce them, which is hammer, the incus, which is anvil, and the stapes, which is stirrup. I'm just going to say stirrup though.

The inner ear is made up of the cochlea, which is a snail-shaped fluid filled, and it contains the sensory receptor cells. And these are hair cells that are auditory receptor cells of the inner ear embedded in the basilar membrane. So, the way this works is that sound waves travel down the auditory canal to the tympanic membrane, causing it to vibrate, which results in music.

Music. That results in movement of the ossicles, which makes the stirrup press on an area of the cochlea known as the oval window. And that moves fluid in the cochlea that stimulates hair cells, which are embedded in the...

basilar membrane. So basically the stimulation of the hair cells leads to the activation of hearing, so the experience of hearing. And if you want to chart this out, auditory information goes to the inferior colliculus, which goes to the medial geniculate nucleus of the thalamus, and then onto the auditory cortex in the temporal lobe for further processing.

What about pitch perception? Well, low frequency sounds are low pitched and high frequency sounds tend to be higher pitched. And this is how the auditory system differentiates between various pitches.

Two different theories here. The temporal theory is that frequency is coded by the activity level of a sensory neuron. So hair cells fire only for particular frequencies.

And this is good. So that's hair cells in the cochlea, just to remind you. But it can't explain the whole range of hearing. So we also have place theory.

And that's that different portions of the basilar membrane, and that's also in the cochlea, are sensitive to different frequencies. So hair cells in the base portion are high-pitched receptors, and those in the tip of the basilar membrane are low-pitched. Both of these theories are correct, but they explain different aspects of pitch perception.

So low frequencies are really explained by the temporal theory, and that's up to 4,000 hertz, but higher frequencies can only be encoded using place theory. Sound localization. How do we know where sounds are coming from?

We have monaural, which are one-eared, and binaural, two-eared cues, and both are used. So an example of a monaural cue would be that each pinna, which is the outside of your ear, interacts with sound waves differently, and depending on the source relative to our bodies, that's how we can localize the sound. Intraoral level difference means that sounds coming from the right side, let's say the sound is coming from the right side of your body, it's more intense in your right ear than your left because your head is blocking the sound wave.

It's the attenuation of the sound wave as it passes through your head. Intraoral timing differences is are that small differences in time that it takes for the sound waves to reach each ear. And so that can give you an idea about localization too. What about hearing loss?

Well, deafness is a partial. or complete inability to hear. And people who are born deaf have what's called congenital deafness. A conductive hearing loss is deafness due to age, genetic predisposition, or environmental effects such as extreme noise, certain illnesses, or toxins.

And sensorineural hearing loss is a failure to transmit neural signals from the cochlea to the brain. And there's a disease that is a degeneration of your inner ear structures. A way to bring hearing back is through cochlear implants, and these are electronic devices that include a microphone, a speech processor, and an electrode array, and they receive sound information and directly stimulate the auditory nerve.

So taste and smell are called the chemical senses. because they both have sensory receptors that respond to molecules in the food we eat or the air we breathe. There is a pronounced interaction between these two chemical senses. So that's why, again, you can't taste things very well when you have a cold. Taste, which we call gustation, has four basic groupings, sweet, sour, salty, and bitter.

But research shows there are at least six taste groupings. So the fifth is known as umami, and that's a Japanese word that roughly translates to yummy, and it's associated with a taste for monosodium glutamate, which is MSG. And there's also evidence suggesting that there's a taste for fatty content. Your taste buds are formed by groupings of taste receptor cells with hair-like extensions that protrude into the central pore of the taste bud. They have a life cycle of about 10 days to two weeks.

So even destroying some of them by burning your tongue won't have any long-term effect. They grow right back. Your taste information is transmitted to the medulla, the thalamus, and the limbic system, and also to the gustatory cortex. Smell or olfaction, and I want to say this, that I have a good sense of smell, but I grew up in an olfactory town.

That's a terrible joke. Olfactory receptor cells are located in a mucus membrane at the top of the nose. Small hair-like extensions from these receptors serve as the sites for odor molecules dissolved in the mucus to interact with chemical receptors located on these extensions. Chemical changes within the cell result in signals being sent to the olfactory bulb, and that's a bulb-like structure, hence olfactory bulb.

at the tip of the frontal lobe where the olfactory nerves begin. There's a lot of variation in the sensitivity of the olfactory system between species. So, for example, dogs have a much more sensitive olfactory system than humans do.

And there's some evidence that dogs can even smell dangerous drops in people's blood, glucose levels, and cancerous tumors. So they can be trained. to be able to do that. Many species also respond to chemical messages known as pheromones sent by other individuals of their species. These messages often involve information about reproductive status, and there's been a lot of research and controversy about the importance of pheromones in humans, because sometimes they're put into perfumes.

There are many types of receptors in your skin that respond to different kinds of touch-related stimuli. So let's talk about touch. So Meissner's corpuscles respond to pressure and lower frequency vibrations. I'm sorry, the Pashinian corpuscles detect transient pressure and higher frequency vibrations. Merkel's discs respond to light pressure and Ruffini corpuscles detect stretch.

I think that's actually pronounced pacinian corpuscles, so my apologies there. In addition to the receptors in the skin, there are also many free nerve endings that respond to a variety of different types of touch-related stimuli, as well as serving as receptors for both thermoception, which is temperature perception, and nociception, which is a signal indicating potential harm and maybe pain. So let's talk about pain perception.

It's, well this is obvious, but it's an unpleasant experience that involves both physical and psychological components. And basically, pain can be considered inflammatory or neuropathic in nature. Inflammatory pain is pain that signals some type of tissue damage, and neuropathic pain is pain that results from damage to neurons of either the peripheral or central nervous system, and sometimes that involves exaggerated pain signals sent to the brain. Some people are born without the ability to feel pain, and we would say they have a congenital insensitivity to pain or congenital analgesia. And that's a rare genetic disorder resulting in an individual being born without the ability to feel pain.

I'm being redundant here. My apologies. People with this disorder can detect temperature and pressure differences, but they often suffer severe injuries. And this may come as no surprise to you, but people with congenital analgesia also have much shorter lifespans than most other people.

The vestibular sense contributes to our ability to maintain balance in body posture. The major sensory organs, these are in the semicircular canals of this system, are located next to the cochlea in the inner ear. The vestibular system also collects information for controlling movement and the reflexes that move various parts of our bodies to compensate for changes in body position.

Therefore, both proprioception which is the perception of your body position, and kinesthesia, which is the perception of the body's movement through space, interact with information provided by the vestibular system. Well, let's finish up by talking about gestalt principles of perception. And this starts with Max Wertheimer, and he published a paper demonstrating that individuals perceive motion in these rapidly flickering static images. And so the motion is not in the stimuli, it's in our perception of it.

And that's actually known as the five phenomena. And this was an insight that came to him as he used a child's toy tachystoscope. I've actually heard this also explained as something he noticed one time when he was riding on a train.

So gestalt psychology is a movement within psychology. Gestalt literally means form or pattern. And so you can think of it like that.

The idea is that the brain creates a perception that is more than simply the sum of the available sensory inputs, and it does so in predictable ways. What are those ways? Well, one Gestalt principle is the figure-ground relationship.

And according to this, we tend to segment our visual world into figure and ground, where figure is the object of our focus and ground is the background. So off to the right, you'll see these were quite popular in... the late 1800s, but you can still buy them today.

They're vases with faces, and so you can see the vase as the figure, or you can see the faces as the figure, and then one's always acting as the background for the other. Another Gestalt principle for organizing sensory stimuli is proximity, and this asserts that things that are close to one another tend to be grouped together. So you can see in B...

that you're going to see three columns because we take those dots and based on their proximity we say, well, those aren't lines, those are columns. The principle of similarity is another way to group. And off to the right there you can see a line of white and black dots as opposed to a column of alternating black and white.

The law of continuity suggests that we're more likely to perceive continuous, smooth, flowing lines rather than jagged, broken lines. And so that's to the bottom right there, where it's basically connect the dots. You see those as continuous. And the principle of closure states that we organize our perception into complete objects rather than as a series of parts.

And so by putting together a series of circles with cutouts in them, we can perceive. a necker cube. And so that's kind of interesting too.

Well, that's the end of our sensation and perception chapter. But I want to remind you that all your problems, at least all your APA style problems, can be solved with my Learn APA Style book. So when you want to learn to write correctly or write right, consult my book and videos on Learn APA Style, which are about writing and psychology.

in the social sciences. Have a great day, and thanks for listening.

Transcript for:Understanding Sensation and Perception

Transcript for:
Understanding Sensation and Perception