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Sensation and Perception
Sensation
What is Sensation? Sensation โ occurs when special receptors in the sense organsโthe eyes, ears, nose, skin, and taste budsโare activated, allowing various forms of outside stimuli to become neural signals in the brain. โ is the process by which we receive information from the environment. Transduction โ process of converting outside stimuli, such as light, into neural activity. โ is like the brain's translator for information from the outside world. Sensory Receptors โ are specialized forms of neurons, the cells that make up the nervous system. โ Instead of receiving neurotransmitters from other cells, these receptor cells are stimulated by different kinds of energy โ Each receptor type transduces the physical information into electrical information in different ways, which then either depolarizes or hyperpolarizes the cell, causing it to fire more or to fire less based on the timing and intensity of information it is detecting from the environment Synesthesia โ Condition in which the signals from the various sensory organs are processed differently, resulting in the sense information being interpreted as more than one sensation
Sensory Thresholds Ernst Weber (1795โ1878) โ His research led to the formulation known as Weberโs law of just noticeable differences (JND, or the difference threshold). โ Weberโs law โ simply means that whatever the difference between stimuli might be, it is always a constant โ the noticeable difference between the two things is a constant proportion , not a fixed amount. โ describes how our perception of differences in stimuli is relative to the size or intensity of the original stimulus. โ JND โ is the smallest difference between two stimuli that are detectable 50 percent of the time, โ is the practical application of Weber's Law. โ tells us precisely how much a stimulus must change for us to perceive a difference. โ is a specific measure that tells us the exact size of the smallest noticeable difference between two stimuli, based on Weber's Law. โ It tells you exactly how much you need to change something for you to notice the difference. Gustav Fechner (1801โ1887) โ expanded on Weberโs work by studying something he called the absolute threshold โ Absolute Threshold โ is the lowest level of stimulation that a person can consciously detect 50 percent of the time the stimulation is present. โ way to measure how sensitive your senses are to picking up on very faint or subtle things in your environment. โ is about detecting something from nothing Subliminal Stimuli โ Stimuli that are below the level of conscious awareness. โ (The word limin means โthreshold,โ so sublimin means โbelow the threshold.โ) โ These stimuli are just strong enough to activate the sensory receptors but not strong enough for people to be consciously aware of them. Subliminal Perception โ these stimuli act upon the unconscious mind, influencing behavior in a process
โ (or hue) is largely determined by the length of the wave. Long wavelengths (measured in nanometers) are found at the red end of the visible spectrum, whereas shorter wavelengths are found at the blue end. โ Long wavelengths = REDDISH โ Short wavelengths = BLUEISH โ Saturation โ refers to the purity (intensity) of the color people perceive: โ A highly saturated red, for example, would contain only red wavelengths, whereas a less-saturated red might contain a mixture of wavelengths.
Structure of the Eye
โ Rods โ Visual sensory receptors found at the back of the retina โ Responsible for non-color sensitivity to levels of light โ scotopic vision: (poor acuity) , good for low luminance, primarily peripheral; very sensitive to light; little information about color โ Cones โ Visual sensory receptors found at the back of the retina โ Responsible for color and vision sharpness โ primarily at Fovea ( good acuity ), require substantial luminance ( not very sensitive to light ), and Photopic vision ( color vision ). (two theories: Trichromatic theory and Opponent- Process theory) โ Both rods and cones synapse with bipolar cells, which synapse with ganglion cells, which form the optic nerve. โ Fovea โ The central area of the retina; greatest density of photoreceptors โ Made up of cones โ Optic nerve โ Sends visual information to the brain โ Both rods and cones synapse with bipolar cells, which synapse with ganglion cells, which form the optic nerve. โ Bundle of axons โ Blind spot (optic disc) โ Where the optic nerve leaves the eye; there are no photoreceptor cells here โ where the optic nerve connects to the eye and contains neither rods nor cones. Light enters the eye through the cornea and pupil. The iris controls the size of the pupil. From the pupil, light passes through the lens to the retina, where it is transformed into nerve impulses. The nerve impulses travel to the brain along the optic nerve. Path of incoming light: Cornea, (aqueous humor), pupil (the hole in iris), lens, (vitreous fluid), past blood vessels & vision neuron support structures, then to receptor at retina. (info ultimately through the support cells then out axon at optic disk (blindspot) into the optic nerveโฆ)
The Visual Pathway
- Light from the Right Visual Field : a. When light from the right side of what you're looking at enters both your eyes, it falls on the left side of each eye's retina.
- Stimulating Neural Message: a. This light stimulates a message in each eye that travels along the optic nerve.
- To the Thalamus: a. These messages then go to the thalamus, which is like a relay center in your brain.
- To the Visual Cortex: a. From the thalamus, the messages continue on to the visual cortex in the back part of the left hemisphere of your brain.
- Different Parts of the Retina: a. Notice that the message from the temporal half (outer side) of the left retina goes to the left occipital lobe. b. Meanwhile, the message from the nasal half (inner side) of the right retina crosses over to the left hemisphere. This crossover point is called the "optic chiasm."
- Combining Both Eyes: a. The optic nerve tissue from both eyes joins together to form the left optic tract before heading to the thalamus and then the left occipital lobe.
- For the Left Visual Field: a. Now, if you look at things on the left side, like something in your left peripheral vision, messages from the right side of both your retinas will travel along the right optic tract to the right visual cortex in the same way. Dark Adaptation โ occurs as the eye recovers its ability to see when going from a brightly lit state to a dark state. Light adaptation โ is the adjustment of the eyes when we move from darkness into an area that is illuminated.
Common problems with vision โ Cataracts: โ clouding of the lens of the eye; affects acuity and color vision โ Retinopathy: โ damage to the small blood vessels; begins to leak and may cause blurred vision, blind spots, or floaters โ Glaucoma: โ fluid pressure builds up inside the eye, damaging the optic nerve; blurred vision and loss of peripheral vision โ Macular degeneration: โ inability to see objects clearly; distorted vision and dark spots in the center of vision โ Hyperopia (farsightedness): โ focusing the image behind the retina; difficulty in seeing objects close up โ The focus point is beyond the retina โ Myopia (nearsightedness): โ focusing the image in front of the retina; difficulty in seeing objects far away โ The focal point falls short of the retina
The Science of Hearing: Can You Hear Me Now? Sound Waves and The Ear Sound โ is mechanical energy typically caused by vibrating objects. Sound Waves โ The vibration of molecules of air that surround us โ Wavelengths โ are interpreted by the brain as frequency or pitch (high, medium, or low). โ Hertz(Hz) โ How Frequency is measured in cycles (waves) per second โ Human limits are between 20 and 20,000 Hz , with the most sensitivity from about 2000 to 4000 Hz , very important for conversational speech. โ Amplitude โ is interpreted as volume , how soft or loud a sound is. โ corresponds to the perceptual term loudness โ Decibel โ is a unit of measure for loudness. โ Purity (timbre) โ a richness in the tone of the sound. โ Determined by the complexity of the wavelength
Middle Ear Amplifies sound waves Ossicles: โ three bones in the middle ear (malleus/incus/stapes or hammer/anvil/stirrup) set in motion by ear drum that transmit sound vibrations to the cochlea โ The smallest bones in the body โ Vibration of these bones amplifies vibrations from the eardrum โ Stirrup โ The last bone in the chain โ causes a membrane covering the opening of the inner ear to vibrate called the oval window โ Oval Window โ Its vibrations set off another chain reaction within the ear Inner Ear Where sound is transduced into neural impulses Cochlea: โ a part of the inner ear contains fluid and receptors โ When the oval window vibrates, it causes the fluid in the cochlea to vibrate Basilar membrane: โ subject to pressure changes in cochlear fluid; contains the organ of Corti, โ Resting place of the organ of Corti โ Organ of Corti โ An organ that contains auditory sensory (hair) cells Hair cells: โ Hair cells of the organ of Corti deflected by fluid movement trigger neural impulses to the brain via the auditory nerve. โ Receptors for sound/ sound receptors Auditory Nerve โ which contains the axons of all the receptor neurons Auditory Cortex โ interpret the sounds โ Transduces sounds
Perceiving Pitch Theories Place theory: โ Differences in pitch result from stimulation of different areas (places) of the basilar membrane. โ the pitch a person hears depends on where the hair cells that are stimulated are located on the organ of Corti. โ high-pitched sound, all of the hair cells near the oval window will be stimulated โ low pitched, all of the hair cells that are stimulated will be located farther away on the organ of Corti. โ For place theory to be correct, the basilar membrane has to vibrate unevenlyโ which it does when the frequency of the sound is above 1000 Hz โ For higher pitch sounds Frequency theory: โ Differences in pitch are due to rate of neural impulses traveling up the auditory nerve. โ related to how fast the basilar membrane vibrates. โ The faster this membrane vibrates, the higher the pitch. โ The slower it vibrates, the lower the pitch. โ For the frequency theory to be correct, the neurons associated with the hair cells would have to fire as fast as the basilar membrane vibrates. โ For lower pitch sounds โ developed by Ernest Rutherford in 1886 Volley Principle โ Volleying โ Process of groups of auditory neurons taking turns firing โ developed by Ernest Wever and Charles Bray โ account for pitches from about 400 Hz up to about 4000
