CHAPTER 5: SENSATION & PERCEPTION

Sensation

• the detection of physical stimuli and transmission of that infomation to the brain

• its essence is detection

• integrated into experience

Perception

• Perception is the brains further processing, organization, and interpretation of sensory information

• its essence is construction of useful and meaningful info about a particular sensation

• integrated into experience

Top-down & Bottom-up processing

• Bottom-up processing: based on physical features of a stimulus

• Top-down processing: how knowledge, expectations, or past experiences shape the interpretation of sensory information

Transduction

• translation of a stimuli

• sensory receptors receive physical or chemical stimulation → sends information to brain in form of neural impulses (except smell) → info goes through thalamus → projected to specific regions in cerebral cortex for each sense

Quality vs. Quantity

• Qualitative info consists of the most basic qualities of a stimulus

  • difference between salty and sweet taste

• Quantitative info: consists of degree, or magnitude, of those qualities

  • relative saltiness or sweetness

Sensory Thresholds

• absolute threshold is the minimum intensity of stimulation that must occur before experience a sensation.

  • can detect 50% of the time

• difference threshold (noticeable) is the smallest difference between two stimuli that you can notice.

  • Weber’s law: states that this difference is based on the proportion of the original stimulus rather than a fixed amount of difference. more intense stimulus, bigger change needed for you to notice

Signal Detection Theory

• Theory of perception based on the idea that the detection of a stimulus requires a judgment. Consists of two main components:

  • sensitivity to stimulus in the presence of noise

  • criteria used to make judgment from ambiguous info

• an example is second guessing yourself when someone asks if you heard a sound even though you didn’t.

Sensory Adaptation

• decrease in sensitivity to a constant level of stimulation

  • an example is getting used to a smell

Synesthesia

• Happens when a person perceives that a visual image has a taste

• in others, colors evoke smells, sights evoke sounds, and numbers come in colors.

Parts of the Eye

• Cornea: the eye’s thick, transparent outer layer

• Lens: clear, curved structure located behind the iris and pupil

• Retina: thin inner surface of the back of the eyeball

• Pupil: dark circle at center of the eye that is a small opening in front of the lens. determines how much light enters the eye

• Iris: a circular muscle that determines eye color and controls the pupil’s size.

• Fovea: center of the retina, where cones are densely packed

• ganglion cells: first neurons in the visual pathway and first to generate action potentials

• optic nerve: bundle of axons that exits the eye at the back of the retina

How do we see?

1. light passes through cornea.

2. cornea focuses the light which then enters the lens

3. the light is then bent further inward and focused to form an image on the retina.

4. retina contains sensory receptors that transduce light into neural signals

Accommodation

• Behind the iris, muscles change the shape of the lens. They flatten it to focus on distant objects and thicken it to focus on closer objects.

• lens and cornea work together to collect and focus light rays reflected from an object

Rods and Cones

• these are the two receptor cells located in the retina

• Rods:

  • respond at extremely low levels of light.

  • responsible for night vision hence why they do not support color vision.

  • They are poor at fine detail

  • each retina holds 120 million rods

  • concentrated at the retina’s edges

• Cones:

  • less sensitive to low levels of light

  • responsible primarily for vision under brighter conditions

  • responsible for seeing color and detail

  • each retina holds 6 million cones

  • densely packed in small region called fovea and become scarce near the outer edge

Transmission from Eye to Brain

1. Generation of electrical signals by the sensory receptors in the retina

2. receptors contain photopigments (protein molecules that split apart when exposed to light)

3. decomposition of photopigments alters the membrane potential of the photoreceptors and trigger action potentials in downstream neurons

4. after light is transduced by rods and cones, other cells in middle layer of the retina perform series of sophisticated computations

5. outputs of these cells cenverge on the retinal ganglion cells

6. ganglion cells send their signals along their axons from inside of the eye to the thalamus

7. these axons gather in a bundle, the optic nerve. this part of the eye has no rods or cones, producing a blind spot in each eye.

8. at the optic chiasm, half of the axons in optic nerves cross, particularly the ones that start from the portion of the retina nearest to the nose. (left vision to right hemisphere and vice versa)

9. info reaches visual areas of thalamus and then travels to primary visual cortex in the occipital lobes.

“WHAT” and “WHERE” Pathways

• visual areas beyond the primary visual cortex from to parallel processing streams.

• Ventral or “WHAT“ stream

  • projects from occipital lobe to temporal lobe

  • specialized in perception and recognition of objects, such as their color or shape

• Dorsal or “WHERE“ stream

  • projects from occipital lobe to parietal lobe

  • specialized in spatial perception

  • determining where an object is and relating it to other objects in a scene

Color of light determined by wavelength

• color is not a property of an object. It appears to be a particular color because of wavelengths of light it reflects

• color does not exist, it is simply a product of our visual system

Trichromatic Theory

• color vision results from activity in 3 types of cones sensitive to different wavelengths

• S cone: most sensitve to short wavelengths (blue-violet light)

• M cone: most sensitive to medium wavelengths (yellow-green light)

• L cone: most sensitive to long wavelengths (red-orange light)

Opponent - Process Theory

• red and green are opponent colors, as are blue and yellow.

• Ex: when we stare at a red image for some time, we see a green afterimage when we look away. The receptors for red become fatigued but the green receptors are not, hence, afterimage looks green

• describes the second stage of visual processing which occurs in the ganglion cells. one type of g-cell receives excitatory input from L cones but inhibited by M cones and another type of g-cell is excited by input from S cones but inhibited by L and M.

Hue, Saturation, and Lightness

• Hue: consists of the distintive characteristics that place a particular color in the spectrum. These depend primarily on the lights dominant wavelength when it reaches the eye.

  • greeness or orangeness

• Saturation: purity of a color. It varies according to the mixture of wavelengths in a stimulus.

  • basic colors such as red or blue have only one wavelength

  • Pastels such as baby blue or pink have a mixture of wavelengths so they are less pure

• Lightness: color’s perceived intensity determined by the total amount of light reaching the eye. also depends on the background

Organization fo Visual Info

• Optical illusions: help visual systems determine identity of objects, sizes and distances in the environment.

• Figure and Ground: we automatically divide visual scenes into objects and background, not both at the same time

• Gestalt Principles of Perceptual Organization: Postulated that the brain uses innate principles to group sensory info into organized wholes.

  • Proximity: the closer 2 figures are to each other, more likely we are to group them as part of the same object

  • Similarity: Group figures according to how closely they resemble each other

  • Good Continuation: group together edges or contours that are smooth as opposed to abrupt or shape edges

  • Closure: complete figures that have gaps.

  • Common fate: tend to see things that move together as the same group

Object Constancy

• leads us to perceive the object as unchanging despite changes in sensory data that compose the object (changes to object angle, distance or illumination)

• Shape constancy: angles we are seeing the object from

• Color constancy: wavelengths of light reflected from the object compared with those reflected from its background

• Lightness constancy: how much light is being reflected from the object and its background

Face Perception

• Humans are able to notice the subtle differences in facial features that differentiate unique individuals and interpret slight changes in facial expressions

Depth Perception

• Binocular depth cues: available from both eyes together and are present only when viewing the three-dimensional world. They provide internal cues about how far away something is.

• Monocular depth cues: available from each eye alone and provide organizational info that can be used to infer depth

• Motion depth cues: emerge when we move through space and depend on relative changes to visual input with motion

• Binocular disparity: caused by the distance between humans’ two eyes. Each eye has a slightly different view of the world, so the brain has access to two different but overlapping retinal images.

  • the brain uses this disparity to compute distances to nearby objects.

• Convergence: the eye muscles turn the eyes inward when we view nearby objects.

Size Perception

• size of an object’s retinal image depends on that object’s distance from the observer. Farther away the object is, the smaller its retinal image.

  • ames box

  • ponzo illusion

Motion Perception

• motion is generally detected by the relative movement of visual info

Process of Hearing

1. process of hearing begins with movements and vibrations of objects that cause displacement of air molecules

2. These produce a change in air pressure, and that change travels through the air

3. The changes in air pressure during a period of time is called a sound wave

   • sound wave’s amplitude determines its loudness

   • wave’s frequency determines its pitch. measured in vibrations per second called hertz (Hz)

4. those sound waves arrive at the person’s outer ear and travel down the auditory canal to the eardrum

5. The sound waves make the eardrum vibrate.

6. These vibrations are transferred to ossicles, three tiny bones commonly called the hammer, anvil, and stirrup

7. the ossicles transfer the eardrum’s vibrations to the oval window, a membrane located within the cochlea in the inner ear, which is a fluid-filled tube that curls into a snail-like shape.

8. Running through the center of the cochlea is the thin basilar membrane

9. The oval windows vibrations create pressure waves in the cochlear fluid, which prompt the basilar membrane to oscillate

10. Movement of the basilar membrane stimulates hair cells to bend and to send info to the auditory nerve

11. These hair cells are the primary auditory receptors. Thus sound waves, which are mechanical signals, hit the eardrum and are converted to neural signals that travel to the brain along the auditory nerve.

12. This conversion of sound waves to brain activity produces the sensation of sound

13. Auditory neurons in the thalamus extend their axons to the primary auditory cortex, which is located in the temporal lobe

Vestibular System

• vestibular sense uses info from receptors in the semicircular canals of the inner ear

• These contain a liquid that moves when the head moves, bending hair cells at the ends of the canal.

• The bending generates nerve impulses that inform us of the head’s rotation. important for maintaining balance

Cochlear Implants

• a small electronic device that ca help provide sense of sound to a person with severe hearing impairment

• stimulates auditory nerve

• sound is picked up by a tiny microphone behind the ear, sent through a computer processor and then transmitted to the implants electrodes inside the cochlea

Encoding mechanisms

• Temporal coding: used to encode relatively low frequencies

  • firing rate of the hair cells can occur only for relatively low frequencies

  • higher frequencies, temporal coding can be maintained only if hair cells fire in volleys, in which different groups of cells take turns firing.

• place coding: basilar membrane stiffness decreases along its length, higher frequencies vibrate better at its base, while lower frequencies vibrate more towards the tip.

• thus, hair cells at the base of the cochlea are activated by high frequency sounds and at the tip by low frequency sounds

Sound Localization

• locating origin of a sound is an important part of auditory perception, but the sensory receptors cannot code where events occur

• Instead, the brain integrates the different sensory info coming from each ear

Taste

• external signals triggering both taste and smell are chemical in nature

• stimuli for taste are chemical substances from food that dissolve in saliva

• taste receptors are part of the taste buds, sensory organs on the tongue (also mouth and throat)

• when food or fluid stimulate the taste buds, they send signals to the thalamus.

• these signals are then routed to the insula and frontal lobe, which produce experience of taste.

• every taste experience is composed of a mixture of 5 basic qualities: sweet, sour, salty, bitter, and umami (savory)

• taste relies heavily on smell as well as texture of the food.

Smell

• has the most direct route to the brain

• we smell something that passes through the nose, into the nasal cavitys upper and back portions

• in the nose and nasal cavity, a warm, moist environment helps the odorant molecules come into contact with the olfactory epithelium. This thin layer of tissue is embedded with thousands of smell receptors, which are responsive to different odors

  • unclear how

• smell signals bypass the thalamus. Instead the smell receptors transmit info directly to the olfactory bulb

  • bulb is located just under the frontal lobes and it is the brain center for smell

  • from here, smell info goes to other brain areas

• whether a smell is pleasant or unpleasant is processed in the brains prefrontal cortex and people can make that distinction

Pheromones

• chemicals released by animals that trigger physiological or behavioral reactions in other animals and insects

• play a major role in sexual signaling in many animal species

Touch

• conveys sensations of temperature, pressure, and pain perceived through the skin

• linked to release of endorphins, chemical signals that promote a sense of pleasure or well-being

• also linked to pain, important to avoid injury

Process of touch

• haptic receptors for both temp and pressure are sensory neurons that reach to the skins outer layer

  • there are receptors for cold and warm

  • receptors for pressure are nerve fibers at the bases of hair follicles that respond to movement of the hair

• touch info goes to thalamus which sends it to the primary somatosensory cortex in the parietal lobe.