5.1 Sensation versus Perception

Sensation

• Sensory receptors: specialized neurons that respond to specific types of stimuli.

• Sensation: when sensory information is detected by a sensory receptor

• Transduction: the conversion from sensory stimulus energy to action potential

• We have sensory systems that provide information about balance (the vestibular sense), body position and movement (proprioception and kinesthesia), pain (nociception), and temperature (thermoception).

• Absolute threshold: the minimum amount of stimulus energy that must be present for the stimulus to be detected 50% of the time.

  • generally measured under incredibly controlled conditions in situations that are optimal for sensitivity

• Subliminal messages: when we receive messages that are presented below the threshold for conscious awareness.

• Just noticeable difference (jnd) or difference threshold: how much difference in stimuli is required to detect a difference between them.

  • changes depending on the stimulus intensity.

  • Weber’s law: the difference threshold is a constant fraction of the original stimulus

Perception

• Perception: the way sensory information is organized, interpreted, and consciously experienced.

  • Perception involves both bottom-up and top-down processing.

    • Bottom-up processing: perceptions are built from sensory input.

    • Top-down processing: how we interpret those sensations is influenced by our available knowledge, our experiences, and our thoughts.

• Although our perceptions are built from sensations, not all sensations result in perception.

• Sensory adaptation: we often don’t perceive stimuli that remain relatively constant over prolonged periods of time.

• Attention plays a significant role in determining what is sensed versus what is perceived.

  • Inattentional business: failure to notice something that is completely visible because of a lack of attention

• Motivation to detect a meaningful stimulus can shift our ability to discriminate between a true sensory stimulus and background noise.

• Signal detection theory: The ability to identify a stimulus when it is embedded in a distracting background.

• Our perceptions can also be affected by our beliefs, values, prejudices, expectations, and life experiences.

5.2 Waves and Wavelengths

Amplitude and Wavelength

• Two physical characteristics of a wave are amplitude and wavelength.

• Amplitude: the distance from the center line to the top point of the crest or the bottom point of the trough.

• Wavelength: the length of a wave from one peak to the next.

  • Wavelength is directly related to frequency.

    • Frequency: the number of waves that pass a given point in a given time period

      • Often expressed in terms of hertz (Hz), or cycles per second.

    • Longer wavelengths will have lower frequencies, and shorter wavelengths will have higher frequencies.

Light Waves

• Visible spectrum: the portion of the larger electromagnetic spectrum that we can see.

  • associated with wavelengths that range from 380 to 740 nm

• Electromagnetic spectrum: encompasses all of the electromagnetic radiation that occurs in our environment

• In humans, light wavelength is associated with perception of color.

• Within the visible spectrum, our experience of red is associated with longer wavelengths, greens are intermediate, and blues and violets are shorter in wavelength.

• The amplitude of light waves is associated with our experience of brightness or intensity of color, with larger amplitudes appearing brighter.

Sound Waves

• The physical properties of sound waves are associated with various aspects of our perception of sound.

• The frequency of a sound wave is associated with our perception of that sound’s pitch.

  • High-frequency sound waves are perceived as high-pitched sounds, while low-frequency sound waves are perceived as low-pitched sounds.

• The loudness of a given sound is closely associated with the amplitude of the sound wave.

  • Higher amplitudes are associated with louder sounds.

  • Loudness is measured in terms of decibels (dB), a logarithmic unit of sound intensity.

• Different musical instruments can play the same musical note at the same level of loudness, yet they still sound quite different.

  • Timbre: a sound’s purity

    • It’s affected by the complex interplay of frequency, amplitude, and timing of sound waves.

5.3 Vision

Anatomy of the Visual System

• The eye is the major sensory organ involved in vision.

  • Light waves are transmitted across the cornea and enter the eye through the pupil.

    • Cornea: the transparent covering over the eye that serves as a barrier between the inner eye and the outside world

      • It’s involved in focusing light waves that enter the eye.

    • Pupil: the 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.

      • When light levels are low, the pupil will expand to allow more light to enter the eye. When light levels are high, the pupil will become smaller to reduce the amount of light that enters the eye.

      • The pupil’s size is controlled by muscles that are connected to the iris, which is the colored portion of the eye.

  • After passing through the pupil, light crosses the lens

    • Lens: a curved, transparent structure that serves to provide additional focus.

      • It’s attached to muscles that can change its shape to aid in focusing light that is reflected from near or far objects.

      • In a normal-sighted individual, the lens will focus images perfectly on a small indentation in the back of the eye known as the fovea, which is part of the retina, the light-sensitive lining of the eye.

        • The fovea contains densely packed specialized photoreceptor cells called cones, which are light-detecting cells that are directly involved in our ability to perceive color.

• Rods: a type of specialized photoreceptor that’s located throughout the remainder of the retina, and are involved in our vision in dimly lit environments and our perception of movement on the periphery of our visual field.

• Rods and cones are connected to retinal ganglion cells. Axons from the retinal ganglion cells converge and exit through the back of the eye to form the optic nerve.

  • Optic nerve: carries visual information from the retina to the brain.

    • Optic chiasm: the point just below the brain where the optic nerve from each eye merges

      • Information from the right visual field is sent to the left side of the brain, and information from the left visual field is sent to the right side of the brain.

        • Once inside the brain, visual information is sent to the occipital lobe at the back of the brain for processing.

          • It might be processed in parallel pathways which can generally be described as the “what pathway” and the “where/how” pathway. The “what pathway” is involved in object recognition and identification, while the “where/how pathway” is involved with location in space and how one might interact with a particular visual stimulus

• Blind spot: a point in the visual field we can’t see

  • We’re not consciously aware of our blind spots because the blind spots don’t overlap and our visual system fills in the blind spot

Color Vision

• Normal-sighted individuals have three different types of cones that mediate color vision.

  • Each of these cone types is maximally sensitive to a slightly different wavelength of light.

  • Trichromatic theory of color vision: all colors in the spectrum can be produced by combining red, green, and blue.

    • The three types of cones are each receptive to one of the colors.

• Opponent-process theory: color is coded in opponent pairs: black-white, yellow-blue, and green-red; some cells of the visual system are excited by one of the opponent colors and inhibited by the other.

• Afterimage: the continuation of a visual sensation after removal of the stimulus.

Depth Perception

• Depth perception: our ability to perceive spatial relationships in 3-D space.

• A variety of cues are used in a visual scene to establish our sense of depth.

  • Binocular cues: cues that rely on the use of both eyes.

    • Binocular disparity: the slightly different view of the world that each of our eyes receives.

  • Monocular cues: cues that require only one eye

    • Linear perspective: we perceive depth when we see two parallel lines that seem to converge in an image.

5.4 Hearing

Anatomy of the Auditory System

• Outer ear

  • Pinna: the visible part of the ear that protrudes from our heads,

  • Auditory canal

  • Tympanic membrane: the eardrum.

• Middle ear

  • Ossicles: three tiny bones known called the malleus (or hammer), incus (or anvil), and the stapes (or stirrup).

• Inner ear

  • Semi-circular canals: involved in balance and movement (the vestibular sense)

  • Cochlea: a fluid-filled, snail-shaped structure that contains the sensory receptor cells (hair cells) of the auditory system.

• Sound waves travel along the auditory canal and strike the tympanic membrane, causing it to vibrate.

  • This vibration results in movement of the three ossicles.

  • As the ossicles move, the stapes presses into a thin membrane of the cochlea known as the oval window.

  • As the stapes presses into the oval window, the fluid inside the cochlea begins to move, which in turn stimulates hair cells, which are auditory receptor cells of the inner ear embedded in the basilar membrane.

    • Basilar membrane: a thin strip of tissue within the cochlea.

• The activation of hair cells is a mechanical process:

  • The stimulation of the hair cell leads to activation of the cell.

  • As hair cells become activated, they generate neural impulses that travel along the auditory nerve to the brain.

  • Auditory information is shuttled to the inferior colliculus, the medial geniculate nucleus of the thalamus, and finally to the auditory cortex in the temporal lobe of the brain for processing.

Pitch Perception

• Temporal theory of pitch perception: frequency is coded by the activity level of a sensory neuron

• Place theory of pitch perception: different portions of the basilar membrane are sensitive to sounds of different frequencies. More specifically, the base of the basilar membrane responds best to high frequencies and the tip of the basilar membrane responds best to low frequencies.

  • Hair cells in the base portion would be labeled as high-pitch receptors, while those in the tip of basilar membrane would be labeled as low-pitch receptors

• Both theories explain different aspects of pitch perception.

Sound Localization

• The auditory system uses both monaural (one-eared) and binaural (two-eared) cues to localize sound.

• Each pinna interacts with incoming sound waves differently, depending on the sound’s source relative to our bodies.

  • Monaural cue: sounds that occur above or below and in front or behind us.

  • Binaural cue: provides information on the location of a sound along a horizontal axis by relying on differences in patterns of vibration of the eardrum between our two ears.

    • If a sound comes from an off-center location, it creates two types of binaural cues: interaural level differences and interaural timing differences.

      • Interaural level difference: a sound coming from the right side of your body is more intense at your right ear than at your left ear because of the attenuation of the sound wave as it passes through your head.

      • Interaural timing difference: the small difference in the time at which a given sound wave arrives at each ear.

      • Certain brain areas monitor these differences to construct where along a horizontal axis a sound originates.

Hearing Loss

• Deafness: the partial or complete inability to hear.

• Congenital deafness: when people are born deaf

• Some degree of hearing loss is inevitable.

• Conductive hearing loss: when the hearing problem is associated with a failure in the vibration of the eardrum and/or movement of the ossicles.

• Sensorineural hearing loss: when the hearing problem is associated with a failure to transmit neural signals from the cochlea to the brain

5.5 The Other Senses

Taste

• Umami: our fifth taste; associated with a taste for monosodium glutamate

• Molecules from the food and beverages we consume dissolve in our saliva and interact with taste receptors on our tongue and in our mouth and throat.

• Taste buds: formed by groupings of taste receptor cells with hair-like extensions that protrude into the central pore of the taste bud

  • Have a life cycle of ten days to two weeks

  • Taste molecules bind to receptors on the extensions and cause chemical changes within the sensory cell that result in neural impulses being transmitted to the brain via different nerves, depending on where the receptor is located.

  • Taste information is transmitted to the medulla, thalamus, and limbic system, and to the gustatory cortex

Smell

• Olfactory receptor cells are located in a mucous 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.

  • Once an odor molecule has bound a given receptor, chemical changes within the cell result in signals being sent to the olfactory bulb

    • Olfactory bulb: a bulb-like structure at the tip of the frontal lobe where the olfactory nerves begin.

  • From the olfactory bulb, information is sent to regions of the limbic system and to the primary olfactory cortex

• Many species respond to chemical messages, known as pheromones, sent by another individual.

  • Pheromonal communication often involves providing information about the reproductive status of a potential mate.

Pain Perception

• Pain: an unpleasant experience that involves both physical and psychological components.

  • Makes us aware of an injury, and motivates us to remove ourselves from the cause of that injury.

• Inflammatory pain: pain that signals some type of tissue damage

• Neuropathic pain: when pain is caused by damage to neurons of either the peripheral or central nervous system, and the pain signals that are sent to the brain get exaggerated.

• Thermoception: temperature perception

• Nociception: a signal indicating potential harm and pain

The Vestibular Sense, Proprioception, and Kinesthesia

• Vestibular sense: contributes to our ability to maintain balance and body posture, and collects information critical for controlling movement and the reflexes that move various parts of our bodies to compensate for changes in body position.

  • The major sensory organs (utricle, saccule, and the three semicircular canals) of this system are located next to the cochlea in the inner ear.

  • The vestibular organs are fluid-filled and have hair cells that respond to movement of the head and gravitational forces.

    • When these hair cells are stimulated, they send signals to the brain via the vestibular nerve.

  • Proprioception (perception of body position) and kinesthesia (perception of the body’s movement through space) interact with the information provided by the vestibular system.

    • They gather information from receptors that respond to stretch and tension in muscles, joints, skin, and tendons

    • The information travels to the brain via the spinal column.

    • Several cortical regions in addition to the cerebellum receive information from and send information to the sensory organs of the proprioceptive and kinesthetic systems.

5.6 Gestalt Principles of Perception

• Gestalt psychology: the whole is different from the sum of its parts; the brain creates a perception that is more than the sum of available sensory inputs, and it does so in predictable ways.

• Gestalt Principles

  • Figure-ground relationship: we tend to segment our visual world into figure and ground.

    • Figure: the object or person that is the focus of the visual field

    • Ground: the background.

    • Our perception varies depending on what is perceived as figure and what is perceived as ground.

  • Proximity: things that are close to one another tend to be grouped together,

  • Similarity: things that are alike tend to be grouped together

  • Continuity: we are more likely to perceive continuous, smooth flowing lines rather than jagged, broken lines

  • Closure: we organize our perceptions into complete objects rather than as a series of parts

• According to Gestalt theorists, pattern perception, or our ability to discriminate among different figures and shapes, occurs by following these principles.

• Our perceptions are based on perceptual hypotheses: educated guesses that we make while interpreting sensory information.

  • These hypotheses are informed by a number of factors, including our personalities, experiences, and expectations.

  • We use these hypotheses to generate our perceptual set.