INTRODUCTION TO SENSATION AND PERCEPTION

• To study sensation is to study the relationship between physical stimulation and its psychological effects.

• Sensation is the process of taking in information from the environment.

• Perception refers to the way in which we recognize, interpret, and organize our sensations.

THRESHOLDS

• In psychophysics, the branch of psychology that deals with the effects of physical stimuli on sensory response, researchers determine the smallest amount of sound, pressure, taste, or other stimuli that an individual can detect.

  • Psychologists conducting this type of experiment are attempting to determine the absolute threshold—the minimum amount of stimulation needed to detect a stimulus and cause the neuron to fire 50 percent of the time.

• Gustav Fechner (1801–1887), the founder of psychophysics, in addition to contributing to Weber’s Law determined that the perceived brightness/loudness of a sensation is proportional to the logarithm of its actual intensity.

• Another approach to measuring detection thresholds involves signal detection theory (SDT).

  • Detection thresholds are the levels of a signal or measurement that must be met before being considered valid.

  • This theory takes into consideration that there are four possible outcomes on each trial in a detection experiment: the signal (stimulus) is either present or it is not, and the participants respond that they can detect a signal or they cannot.

    • Hit—the signal was present, and the participant reported sensing it.

    • Miss—the signal was present, but the participant did not sense it.

    • False alarm—the signal was absent, but the participant reported sensing it.

    • Correct rejection—the signal was absent, and the participant did not report sensing it.

• Another type of threshold is the discrimination threshold, which is the point at which one can distinguish the difference between two stimuli.

  • The minimum amount of distance between two stimuli that can be detected as distinct is called the just noticeable difference (JND) or difference threshold.

• Subliminal perception is a form of preconscious processing that occurs when we are presented with stimuli so rapidly that we are not consciously aware of them.

  • There was some preconscious processing, known as priming, occurring even if we were not aware of it.

• Another Example of preconscious information processing can be seen in the tip-of-the-tongue phenomenon, in which we try to recall something that we already know is available but is not easily available for conscious awareness

RECEPTOR PROCESSES

• Sensory organs have specialized cells, known as receptor cells, which are designed to detect specific types of energy.

• The area from which our receptor cells receive input is the receptive field.

  • Receptive field refers to the area of an image or representation that a neuron can respond to.

• Through a process called transduction, the receptors convert the input, or stimulus, into neural impulses, which are sent to the brain.

  • Transduction is a process in which energy from one form is transformed into another.

• Transduction takes place at the level of the receptor cells, and then the neural message is passed to the nervous system.

• Olfaction, or the sense of smell, travels in a more direct path to the cerebral cortex, without stopping at or being relayed by the thalamus.

  • It is mediated by specialized sensory cells, located in a small patch of tissue in the nose called "the olfactory epithelium".

  • These cells can detect specific molecules in air, such as those from food or flowers, and send signals to our brain that allow us to perceive different smells.

SENSORY MECHANISMS

• Sensory coding is the process by which receptors convey such a range of information to the brain.

• Every stimulus has two dimensions: what it is (its qualitative dimension) and how much of it there is (its quantitative dimension).

  • The qualitative dimension is coded and expressed by which neurons are firing.

  • Quantitative dimension is a way of measuring an object or phenomenon by counting, measuring, or estimating its size.

• The quantitative information is coded by the number of cells firing.

  • Bright lights and loud noises involve the excitation of more neurons than those brought on by dim lights and quiet noises.

• Single-cell recording is a technique by which the firing rate and pattern of a single receptor cell can be measured in response to varying sensory input.

Visual Mechanisms

• Visual sensation occurs when the eye receives light input from the outside world.

  • Note that the object as it exists in the environment is known as the distal stimulus, whereas the image of that object on the retina is called the proximal stimulus.

• First, light passes through the cornea, which is a protective layer on the outside of the eye.

  • Just under the cornea is the lens.

    • The curvature of the lens changes to accommodate for distance.

      • These changes are called, logically, accommodations.

• The retina is at the back of the eye and serves as the screen onto which the proximal stimulus is projected.

  • The retina is the innermost layer of the eye, located just behind the lens and in front of the choroid layer.

    • The retina is covered with receptors known as rods and cones.

• After light stimulates the receptors, this information passes through horizontal cells to bipolar and amacrine cells.

  • Bipolar cells are neurons in the eye that carry signals from photoreceptors to ganglion cells.

  • Amacrine cells are interneurons in the retina that modulate visual responses and help relay information between bipolar cells, ganglion cells, and other retinal neurons.

• The stimulation then travels to the ganglion cells of the optic nerves.

  • Optic nerves are the neural pathways that connect the eye to the brain.

    • Where the optic nerve exits the retina, humans have a blind spot because there are no photoreceptors there.

• The optic nerves cross at the optic chiasm, sending half of the information from each visual field to the opposite side of the brain.

• Serial processing occurs when the brain computes information step-by-step in a methodical and linear matter, while parallel processing happens when the brain computes multiple pieces of information simultaneously.

• Feature detector neurons “see” different parts of the pattern, such as a line set at a specific angle to the background.

• Information becomes more complex as it travels through the sensory system, is known as convergence and occurs across all sensory systems.

• David Hubel (1926–2013) and Torsten Wiesel (1924–), through experiments with cats, determined that mammals, including humans, will develop normal vision along these lines so long as any impairments are corrected during the critical period, the first months after birth.

• Young-Helmholtz or trichromatic theory - According to this theory, the cones in the retina of the eye are activated by light waves associated with blue, red, and green.

• Another theory, known as opponent process theory, contends that cells within the thalamus respond to opponent pairs of receptor sets—namely, black/white, red/green, and blue/yellow.

• Afterimage is an optical illusion in which an image continues to appear in one's vision after the exposure to the original image has ceased.

  • It is usually caused by the eye's continued stimulation by the color and brightness of the original image.

• Dichromats are people who cannot distinguish along the red/green or blue/yellow continuums.

• Monochromats see only in shades of black and white (this is much more rare).

Auditory Mechanisms

• Auditory input, in the form of sound waves, enters the ear by passing through the outer ear, the part of the ear that is on the outside of your head, and into the ear canal.

• The outer ear collects and magnifies sound waves.

• The vibrations then enter the middle ear, first vibrating the tympanic membrane.

  • This membrane abuts the ossicles, the three tiny bones that comprise the middle ear.

  • The last of the three ossicles is the stapes, which vibrates against the oval window.

• The inner ear is also responsible for balance and contains vestibular sacs, which have receptors sensitive to tilting.

• Place theory asserts that sound waves generate activity at different places along the basilar membrane.

• Frequency theory in hearing states that we sense pitch because the rate of neural impulses is equal to the frequency of a particular sound.

• Deafness can occur from damage to the ear structure or the neural pathway.

• Conductive deafness refers to injury to the outer or middle ear structures, such as the eardrum.

• Impairment of some structure or structures from the cochlea to the auditory cortex results in sensorineural, or nerve, deafness.

Other Sensory Mechanisms

• Olfaction (smell) is a chemical sense.Deep in the nose, scent molecules reach the olfactory epithelium.

  • Scent molecules touch receptor cells here.

  • These receptors' axons reach the brain's olfactory bulbs.

  • The limbic system and olfactory cortex receive information.

  • Smells evoke memories because the amygdala and hippocampus are connected to olfactory nerves.

• Gustation (taste) is also a chemical sense.

  • Papillae cover the tongue.

  • Taste buds are on the papillae.

    • Sweet, salty, bitter, sour, and umami (savory). Reasons may explain these five flavors.

    • Sweetness frequently comes with calories.

    • Most toxic plants taste bitter, which we dislike.

    • Taste buds provide information to the medulla oblongata, then the pons and thalamus.

    • The hypothalamus, limbic system, and gustatory cortex receive this information.

• The skin has cutaneous and tactile receptors that provide information about pressure, pain, and temperature.

  • Cutaneous receptors are those that detect sensations of touch, pressure and temperature.

  • Tactile receptors are specialized nerve endings in the skin that detect light touches, vibrations and even pain.

• Other senses include the vestibular sense, which involves the sensation of balance.

  • This sense is located in the semicircular canals of the inner ear.

• Kinesthesis, found in the joints and ligaments, transmits information about the location and position of the limbs and body parts.

• Synesthesia is a neurological condition in which stimulation of one sense leads to automatic activation of another sense; for example, one might “hear” colors.

SENSORY ADAPTATION

• Adaptation is an unconscious, temporary change in response to environmental stimuli

• Habituation is the process by which we become accustomed to a stimulus, and notice it less and less over time.

• Dishabituation occurs when a change in the stimulus, even a small change, causes us to notice it again.

ATTENTION

• The term attention refers to the processing through cognition of a select portion of the massive amount of information incoming from the senses and contained in memory.

• A good example of attention in action is selective attention, by which we try to attend to one thing while ignoring another.

• An example of selective attention is called the “cocktail party phenomenon,” which refers to our ability to carry on and follow a single conversation in a room full of conversations.

• Shadowing - The participant is instructed to repeat only one of the conversations.

• Filter theories propose that stimuli must pass through some form of screen or filter to enter into attention.

• Attentional resource theories, in contrast, posit that we have only a fixed amount of attention, and this resource can be divided up as is required in a given situation.

• Divided attention, trying to focus on more than one task at a time, is most difficult when attending to two or more stimuli that activate the same sense, as in watching TV and reading.

• Inattentional blindness, also known as change blindness, demonstrates a potential weakness of selective attention.

PERCEPTUAL PROCESSES

• Perceptual processes—how our mind interprets these stimuli.

• Bottom-up processing achieves recognition of an object by breaking it down into its component parts.

• Top-down processing, by contrast, occurs when the brain labels a particular stimulus or experience.

• Visual perception is quite complex.

• Monocular depth cues are those that we need only one eye to see.

• Relative size refers to the fact that images that are farther from us project a smaller image on the retina than do those that are closer to us.

• Interposition, also known as occlusion, which occurs when a near object partially blocks the view of an object behind it.

• Linear perspective is a monocular cue based on the perception that parallel lines seem to draw closer together as the lines recede into the distance.

• Aerial perspective, another perceptual cue, is based on the observation that atmospheric moisture and dust tend to obscure objects in the distance more than they do nearby objects.

• Relative clarity is a perceptual clue that explains why less distinct, fuzzy images appear to be more distant.

• Motion parallax is the difference in the apparent movement of objects at different distances, when the observer is in motion.

• Binocular depth cues rely on both eyes viewing an image.

• Stereopsis refers to the three-dimensional image of the world resulting from binocular vision.

• Retinal convergence is a depth cue that results from the fact that your eyes must turn inward slightly to focus on near objects.

• The complement to stereopsis is binocular disparity, which results from the fact that the closer an object is, the less similar the information arriving at each eye will be.

• The Gestalt approach to form perception is based on a top-down theory.

  • This view holds that most perceptual stimuli can be broken down into figure-ground relationships.

• Some basic Gestalt principles of figure detection include the following:

  • Proximity— the tendency to see objects near each other as forming groups

  • Similarity— the tendency to prefer grouping like objects together

  • Symmetry— the tendency to perceive forms that make up mirror images

  • Continuity— the tendency to perceive fluid or continuous forms, rather than jagged or irregular ones

  • Closure—the tendency to see closed objects rather than those that are incomplete

• The Law of Prägnanz is a Gestalt psychology principle which states that the mind will attempt to simplify and organize complex stimuli into the simplest and most organized form possible.

  • This principle is used to explain why people tend to perceive objects in their simplest form, rather than as a collection of individual parts.

• Constancy is another important perceptual process.

• One of the most complex abilities we have is motion detection.

• (phi phenomenon); a motion picture, where still pictures move at a fast enough pace to imply movement (stroboscopic effect); and still light that appears to twinkle in darkness (autokinetic effect).

Next Chapter: Chapter 10: Learning