Biological Psychology
Unit Two AD Mr. Cline Marshall High School Psychology * The Brain
• Sensory and Perception
• Though each sense works a little differently to do this, psychologists have developed principles to describe overarching ways in which the body deals with sensation and perception.
• Gustav Fechner, a psychologist in the nineteenth century, called the study of how external stimuli affect us psychophysics.
• He was interested in the point at which we become aware that we're sensing something.
• There could be low music playing in the background at work, and you'd never notice it if you weren't paying attention to it; if you were bored and the room were silent, you might hear the same volume of music playing as soon as it started.
• Psychologists talk mainly about two different kinds of threshold for sensation and perception: the absolute threshold and the difference threshold. * The Brain
• Sensory and Perception
• The absolute threshold, also known as the detection threshold, refers to the weakest possible stimulus that a person can still perceive.
• Since perception at these low levels can be a little unreliable, it's defined as the lowest intensity at which people perceive the stimulus 50% of the time.
• Those hearing tests you'd take in school are based on the idea of absolute threshold; they're testing that your hearing is in the normal range by playing softer and softer beeps until you stop being able to hear them.
• These kinds of tests are known as signal detection analysis, and test your ability to distinguish real sounds from background noise.
• Your accuracy--your ability to hear real signals and correctly say when there aren't signals--is known as your sensitivity. * The Brain
• Sensory and Perception
• Stimuli that are below your absolute threshold can still affect you.
• If you've heard of subliminal advertising, that's what's going on; images that flash by too quickly for us to notice can still influence what we think and feel.
• For a while, people thought that these subliminal messages could have a real dramatic effect on behavior; their use in marketing was extremely controversial.
• Traditional advertising tries to persuade you, but ultimately lets you work through the pros and cons and make an ultimate decision yourself.
• Theoretically, briefly flashing 'DRINK COKE' images could bypass our decision-making and give us an overwhelming desire to drink Coke.
* The Brain
• Sensory and Perception
• But most psychologists now believe that while subliminal messages do produce some effect on thinking, it's fleeting at best and should not be much of a concern for advertising.
* The Brain
• Sensory and Perception
• The second kind of threshold, the difference threshold, refers to our ability to detect a change in a stimulus' intensity.
• This could be the loudness of a noise or the brightness of a light.
• The just noticeable difference refers to the smallest amount two stimuli can differ so that a person can still tell them apart 50% of the time.
• Ernst Weber developed a principle, called Weber's Law, which states that the 'just noticeable difference' of a stimulus is proportional to the original intensity of the stimulus.
• This means that the same increase in strength of a stimulus may be noticeable if the original strength is low, but less noticeable if it's high.
• If you're trying to read by candlelight, and you add another candle to the table, you're likely to notice a big difference in brightness. * The Brain
• Sensory and Perception
• But if you're reading using a light bulb and you add a candle's light, you won't notice the difference as much because the reading lamp is a much brighter base stimulus than a single candle.
• Though Weber's Law does not hold true for every kind of stimulus (and therefore shouldn't really be called a law) it's accurate in a lot of situations, and also represents an important attempt to try to quantify something as seemingly subjective as a noticeable difference.
• There are a few more concepts related to attention that affect our abilities to sense and perceive.
• Just because the sound of rainfall is above our absolute threshold for perception, we're typically able to ignore it after a while.
• This is because of sensory adaptation; when we're exposed for a long time to any given stimulus, we stop perceiving it. * The Brain
• Sensory and Perception
• Our brains can then pay attention to stimuli that are changing, which are theoretically more important than those staying the same.
• We also apply selective attention when perceiving the world around us, choosing to focus on certain sensations over others.
• If you're concentrating deeply on work, you might not even hear your coworkers' conversation; but if it's near the end of the day, you hear everything and want to jump in too.
• Vision
• How do your eyes and brain work together to turn light into images?
• We humans rely on our senses of sight more than a lot of other animals. * The Brain
• Vision
• A cat can easily find its way in the dark thanks to its sensitive whiskers, but if you try closing your eyes and trying to find your way from your desk to the office kitchen, you'll probably hurt yourself (or others).
• The smell of microwavable entrees doesn't guide you with precision, nor can you pinpoint the sounds of conversation accurately enough to keep you from running into the wall outside.
• We perceive the world largely through our eyes.
• Still, our eyes are still limited in the grand scheme of things; they can perceive only a tiny fraction of the light that makes up the full electromagnetic spectrum.
• Light can have many different wavelengths and different properties at these different wavelengths.
• X-rays have really short wavelengths and can be used in medicine to 'see through' the body. * The Brain
• Vision
• Microwaves have much longer wavelengths and heat up your food when you're feeling lazy.
• We can't see x-rays or microwaves; we can only see light that has wavelengths within what's known as the visible spectrum.
• The longer wavelengths of the visible spectrum produce red light, and the shorter wavelengths produce purple light.
• If you've ever seen a rainbow, or light that's been passed through a prism, you've seen the full range of the visible spectrum in order from longest to shortest wavelength.
* The Brain
• Vision
• The light in the visible spectrum really isn't any different than the other kinds of light.
• We only call it the visible spectrum because it's the range that human eyes have evolved to be able to perceive.
• If birds had defined a visible spectrum, it would include ultraviolet light, or wavelengths that are just a little shorter than what humans can see.
• Birds see ultraviolet light just like any other color, because their eyes have evolved a little differently than ours.
• So we've talked a lot about what our eyes perceive; now let's take a closer look at how our eyes take in light and give us the world of images and color we're used to seeing
* The Brain
• Vision
• Light first passes through the cornea, the outermost part of our eyeball, where it begins to be focused.
• Then it enters the pupil, a small opening that leads to the lens.
• Take a look in the mirror; the pupil is the black part at the center of your eye.
• The colored part around it is called the iris, and it grows and shrinks to protect the pupil and make sure the right amount of light gets in.
• On a really bright day, your pupils appear to shrink as your irises block more of them; in a dark room, your pupils appear to grow as your irises retract to allow in as much light as possible. * The Brain
• Vision
• You've probably noticed that when you turn out your light to go to bed, your room seems really dark; eventually, your eyes adjust and you can see enough to stumble to the bathroom without running into your desk.
• Your irises retracting to allow more light into your pupil allows you to do this.
• Once the light makes it past the pupil, it hits the lens, a surface where it is further focused through a process called accommodation.
• If you need glasses, it's probably due to problems with accommodation; if your lenses won't do it naturally, light needs to first pass through artificial lenses--either glasses or contacts--in order to be properly focused.
• Finally, the light hits the retina, a layer of tissue that lines the inner part of the eye. * The Brain
• Vision
• The retina begins the process of turning the light into an image.
• The retina contains two kinds of photoreceptor neurons, known as rods and cones.
• Rods are located more at the edge of the retina and process black and white; cones are in the interior and process color and image details.
• Exactly how color is processed has gone through some debate.
* The Brain
• Vision
• Exactly how color is processed has gone through some debate.
• 19th century scientists Thomas Young and Hermann von Helmholtz developed the Young-Helmholtz theory of color perception, which proposed that there are three different kinds of cones that process different colors of light--one for blue, one for green and one for red.
• Other colors are made up of mixtures of these.
• While this theory elegantly explains color-blindness (people who are red-green colorblind, for example, would be missing those kinds of cones), it does not quite explain why these people are able to see yellow, for example, which would need to be processed by both these kinds of cones.
• Another more recent theory is the opponent-process theory, which holds that we don't process colors as a mix of three primary colors but instead as three sets of opponent colors: red-green, yellow-blue and white-black. * The Brain
• Vision
• Evidence for this model can be found in the phenomenon of after-image.
* The Brain
• Vision • If you stare for a long time at something red then look at a white surface, you'll see a green after-image.
• Signals from the rods and cones pass through a bundle of neurons called the optic nerve on their way to the brain.
• The retina's process of converting electromagnetic light waves to electronic stimuli is called phototransduction.
• The brain processes and merges the signals from both eyes to produce a properly oriented, three-dimensional image.
• Certain feature detector neurons in the visual cortex can isolate things like lines and shapes in order to help us quickly interpret what we're seeing. • In all, vision, humans' most important sense, involves a complicated process of converting light signals into images in the brain.
• Light passes through the lens, where it is focused, to the retina where photoreceptors called rods and cones convert the information to electrical impulses that can be interpreted by the brain.