Mechanisms of Perception Prepared by: Ace Alvin Lopez Sensation V.S. Perception While our sensory receptors are constantly collecting information from the environment, it is ultimately how we interpret that information that affects how we interact with the world. Sensation and perception are two separate processes that are very closely related •Sensation-input about the physical world obtained by our sensory receptors. It is a physical process •Perception- the process by which the brain selects, organizes, and interprets these sensations. It is a psychological process Sensation • Sensory receptors are specialized neurons that respond to specific types of stimuli. • When sensory information is detected by a sensory receptor, sensation has occurred •Transduction- the conversion from sensory stimulus energy to action potential Perception •Perception- the way sensory information is organized, interpreted, and consciously experienced •It involves two types of processing: •Top-down processing •Bottom-up processing •Bottom-up processing-refers to the fact that 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 Factors that affect Perception •Sensory adaptation •Attention •Motivation • Signal detection theory •Beliefs, Values, Prejudices, Expectations and Life Experiences • Müller-Lyer illusion •Personality Principles of Sensory System Organization • The sensory areas of the cortex are considered to be of three fundamentally different types: primary, secondary, and association. •Primary sensory cortex- an area that receives most of its input directly from the thalamic relay nuclei •The interactions among these three are characterized by three major principles: (1) hierarchical organization, (2) functional segregation, and (3) parallel processing. Hierarchical Organization • Sensory systems are characterized by hierarchical organization • Sensory structures are organized in a hierarchy on the basis of the specificity and complexity of their function. • Implication of this organization to the effects of damage to various levels. Functional Segregation • It was once assumed that the sensory areas of the cortex were each functionally homogenous. • Functional segregation- each levels of the cerebral cortex contains functionally distinct areas that specialize in different kinds of analysis. Parallel Processing • It was once believed that different levels of sensory hierarchy were connected in a serial fashion. • Parallel systems- systems in which information flows through the components over multiple pathways. • It features parallel processingthe simultaneous analysis of a signal in different ways by multiple parallel pathways of a neural network. • There appear to be two fundamentally different kinds of parallel streams of analysis: 1.one is capable of influencing our behavior without conscious awareness 2. one that influences our behavior by engaging our conscious awareness. Summary Model of Sensory System Organization • Sensory systems are characterized by a division of labor. Multiple specialized areas, at multiple levels, are interconnected by multiple parallel pathways. • Binding Problem Visual System The Visual System • The visual system constructs a mental representation of the world around us. • This contributes to our ability to successfully navigate through physical space and interact with important individuals and objects in our environments How does the Visual System works? • In the human visual system, the eye receives physical stimuli in the form of light and sends those stimuli as electrical signals to the brain, which interprets the signals as images. Anatomy of the Human Eye Parts of the Eye • The sclera- the white part of • The pupil- an opening where • The cornea and lens- are like • The choroid- • The iris-the colored part of • The ciliary muscle- the eye; functions as a structure where the eye muscles for movement are attached a tag-team as they bend light (refraction) together, focusing it on the retina the eye light enters; able to dilate (open wide) in a dim light environment or constrict (narrow) in bright light provide nourishment to the outer layers of the retina through blood vessels used to change the shape of the lens Parts of the Eye • The retina- the part where electromagnetic energy is turned into neural energy; transduction is possible through photoreceptors • Cones- light detecting cells; acuity, spatial resolution, & color perception • Rods- involved in our vision in dimly lit environment • The fovea- tiny pit; provides the clearest vision of all. Visual Pathway •Light enters the Eye and reach the Retina • Light reflected in your eyes are basis for vision •Light- waves of electromagnetic energy between 380- 760 nanometers in length. •Properties of light: wavelengths (color) and intensity (brightness) • Light enters the eye through the opening of the iris which is the pupil • Light is focused by the lens (adjustable) and cornea (not adjustable) and projected onto the retina. • Light from the left side of the world strikes the right side of the retina and vice versa • Light from above strikes the bottom half of the retina and vice versa •Adjustment of pupil size: • In response to changes in illumination •Sensitivity- ability to detect the presence of dimly lit objects •Acuity- ability to see the details of objects. •Adjustment of lens: • In response to changes in distance of an object •Near objects- lens assumes natural cylindrical shape; increases its ability to refract light •Far objects- lens is flattened •Accommodation- process of adjusting the configuration o lenses to bring the images into focus in the retina •The Retina and Translation of light into Neural Signals • After light passes and reaches the retina, it converts light to neural signals and conduct them towards the CNS •Five layers of the retina: • Receptors ( rods and cones) • Horizontal cells (lateral communication) • Bipolar cells • Amacrine cells (lateral communication) • Retinal ganglion cells •Retina has an inside-out arrangement • Two visual problems: • Distortion of incoming light •Blind spot • Solutions for visual problems: •Fovea- minimizes the first visual problem •Completion - filling in for the blind spot. Visual Receptors: Rods and Cones • Vertebrae retina contains two types of receptors (photoreceptors): •Rods- abundant in the periphery of the human retina; respond to faint light; not useful in daylight because it bleaches them. •Cones- abundant in and near the fovea, are less active in dim light; essential for color vision • Because of the distribution of rods and cones, you have good color vision in the fovea but not in the periphery. • Cones provide 90% of the brains input even if rods outnumber cones. • Reason: • Each cone has its own line to the brain ( fovea) • Rods shares a line with tens or hundreds ( periphery) Human Foveal and Peripheral Vision Characteristic Foveal Vision Peripheral Vision Receptors Cones Proportion of Rods increases toward periphery Convergence of input Each Ganglion cell excited by a single cone Each Ganglion cell excited by many receptors Distinguishes among Responds well to dim Brightness sensitivity bright lights; responds light poorly to dim light Sensitivity to detail Good detail vision Poor detail vision Color Vision Good (many cones) Poor (few cones) Color Vision • Visible light consists of electromagnetic radiation within the range of less than 400 nm and more than 700 nm • We perceive the shortest visible wavelengths as violet • Progressively longer wavelengths are perceived as blue, green ,yellow, orange, and red. Color Vision •Black is experienced when there is an absence of light. •White is produced by an intense mixture of a wide range of wavelengths in roughly equal proportions •Gray is prodded by the same mixtures at lower intensities. •The correct term for colors is hues. QUESTION: What is there about a visual stimulus that determines the color we perceive? ANSWER: To a large degree, the perception of an object’s color depends on the wavelengths of light that it reflects into the eye. However, outside the laboratory, one never encounters objects that reflect single wavelengths. The Trichromatic (YoungHelmholtz) Theory • First proposed by Thomas Young • He recognized that color required a biological explanation • He proposed that we perceive color by comparing the responses across few types of receptors • Modified by Hermann von Helmholtz The Trichromatic (YoungHelmholtz) Theory • We perceive color through the relative rates of response by three kinds of cones, each one maximally sensitive to a different set of wavelengths. •Trichromatic- three colors •Question: How did Helmholtz decide on number 3? •Answer: He found out that people could match any color by mixing appropriate amounts of just three wavelengths The Trichromatic (YoungHelmholtz) Theory • Three kinds of receptors (cones) are sufficient to account for human color vision. • We discriminate among wavelengths by the ratio of activity across the three types of cones. • The perception depends on the frequency of response in one cell relative to the frequency of another cell. • Long and medium-wavelength cones are far more abundant than shortwavelength cones Let’s have an activity! I need six volunteers The Opponent- Process Theory • The trichromatic theory is incomplete as a theory of color vision. • Activity: Do the afterimage activity • Proposed by Ewald Hering • We perceive color in terms of opposites. • The brain has a mechanism that perceives color on a continuum from red- green; yellow-blue; white-black The Opponent- Process Theory • After you stare at one color in one location long enough, you fatigue that response and tend to swing to the opposite • Part of the explanation for this process pertains to the connections within the retina. The Retinex Theory QUESTION: “ If we change our light bulbs into red and blue light, will the color of our classroom and everything in it change also in color?" The Retinex Theory Why do you think colors will stay the same even if we change the color of the light source? • Color constancy- ability to recognize colors despite changes in lighting. • is an example of subjective constancy and a feature of the human colour perception system that ensures that the perceived colour of objects remains relatively constant under varying illumination conditions The Retinex Theory Colour is one of the most important aids for recognition of objects. But the level and colour of the illumination may vary widely. The human vision system is highly efficient at compensating such changes and, as a result of this adaptation, the perceived colour of the object remains approximately constant. The Retinex Theory • Your brain compares the color of one object with the color of another, in effect subtracting a certain amount of green from each. • Similarly, we perceive the brightness of an object by comparing it to other objects The Retinex Theory • Retinex theory of color vision- accounts for both color and brightness constancy • The cortex compares information from various parts of the retina to determine the brightness and color for each area • the color of an object is determined by its reflectance- the proportion of light of different wavelengths that a surface reflects The Retinex Theory • visual perception requires reasoning and inference, not just retinal stimulation. Color Vision Deficiency • Color deficiency results when people with certain genes fail to develop one type of cone, or develop an abnormal type of cone • In red-green color deficiency, the most common form of color deficiency, people have trouble distinguishing red from green because their long- and medium- wavelength cones have the same photopigment instead of different ones. •From Retina to Primary Visual Cortex • Many pathways carry visual information •Retina-geniculate-striate pathways- most thoroughly studies. •Conduct signals from each retina to the primary cortex via lateral geniculate nuclei •About 90% of axons of retinal ganglion cells become part of the retina-geniculate-striate pathways • All signals from the left visual field reach the right primary visual cortex either ipsilaterally (from the temporal hemiretina of the right eye) or contra laterally (via the optic chiasm from the nasal hemiretina of the left eye) • Each lateral geniculate nucleus has six layers ( each layer of each nucleus receives input from all parts of the contralateral via field of one eye) • Most of the lateral geniculate neurons that project to the primary visual cortex terminate in the lower part of the cortical layer IV producing stripe or striation when viewed in cross section, hence the name striate cortex. The M and P Channels • At least two parallel channels of communication flow through each lateral geniculate nucleus •Parvocellular layers (P layers)- composed of neurons with small cell bodies •Responsive to color, fine pattern details, stationary or slow moving objects •Magnocellular layers (M layers)composed of neurons with large bodies •Responsive to movement • The primary visual cortex is located in the posterior region of the occipital lobes • Secondary visual cortex: • Prestriate cortex • Inferotemporal cortex • Association cortex • Posterior parietal cortex Damage to Primary Visual Cortex: Scotomas and Completion ● Damage to an area of the primary visual cortex produces a scotoma ● Perimetry test ● Many patients with extensive scotomas are not consciously aware of their deficits. ● One of the factors that contributes to this lack of awareness is completion. Damage to Primary Visual Cortex: Scotomas and Completion ● Conscious awareness ● Blindsight- ability of patients with scotomas to respond to visual stimuli in their scotomas even though they have no conscious awareness of the stimuli. ● Of all visual abilities, perception of motion is most likely to survive damage to primary visual cortex. Prosopagnosia ● Is also considered as visual ● agnosia for faces ● Agnosia- is a failure of recognition that is not attributable to a sensory deficit or to verbal or intellectual impairment ● Visual agnosia is a specific agnosia for visual stimuli ● It is presumed that damage to an area of secondary visual cortex that mediates the recognition of a particular attribute results to agnosia Akinetopsia ● Akinetopsia is a deficiency in the ability to see movement progress in a normal smooth fashion. ● it can be triggered by high doses of certain antidepressants ● It is often associated with damage to middle temporal (MT) area of the cortex. The location of MT- near the junction of the temporal, parietal and occipital lobes. Auditory System Physics and Psychology of Sound •Sound waves- are periodic compressions of air, water, or other media. •Varies in amplitude and frequency •Amplitude- is the sound wave’s intensity (loudness) •Frequency- the number of compressions per second (pitch) •Sound is measured in Hertz. Humans hear a range from 20 Hz to 20, 000 Hz. • Children hear higher frequencies than adults because the ability to perceive high frequencies decreases with age and exposure to loud noises. •Timbre- means the tone quality or tone complexity • Makes a particular musical sound different from another • People communicate emotion by alterations in pitch, loudness, and timbre. •Prosody- conveying emotions information by tone of voice. “That was interesting” Structure of the Ear • Three parts of the ear: •Outer Ear •Middle Ear •Inner Ear Outer Ear •Pinna- structure of flesh and cartilage attached to each side of the head •Helps us locate the source of sound ( reflector and attenuator) •Auditory canal- passage way •Tympanic membrane- vibrates at the same frequency as the sound waves that strike it. Middle Ear • Composed of three small bones (ossicles) • Connects tympanic membrane to oval window •Hammer ( malleus) •Anvil (Incus) •Stirrup (Stapes) Inner Ear •Liquid filled. That is why a conversion of low pressure to higher pressure is needed. •Cochlea- a snail-shaped structure. •Where sound waves are transformed to electrical signals ( transduction) •Contains three long fluid-filled tunnels: (a) scala vestibuli; (b) scala media; (c) scala tympani •Organ of Corti- composed of hair cells ( transduction). It forms the auditory nerve. •Auditory nerve (hearing) & vestibular nerve (balance) forms the Vestibulocochlear nerve Pitch Perception • Your ability to understand speech or enjoy music depends on your ability to differentiate among sounds of different frequencies •“How do you do it?” •Place theory- the basilar membranes resembles the strings of a piano. Each area along the membrane is tuned to a specific frequency. •Each frequency activates the hair cells at only one place along the basilar membrane •Nervous system distinguishes among frequencies based on which neurons respond. •Frequency theory- the basilar membrane vibrates in synchrony with a sound, causing auditory nerve to produce action potentials at the same frequency •Example: a sound at 50 Hz would cause 50 action potentials per second in the auditory nerve •Downfalls of both theories: • place theory- various parts of the basilar membrane are bound together too tightly for any part to resonate like a piano • Frequency theory- the refractory period of a neuron is typically about 1/1000 second so the maximum firing rate of a neuron is 1000 Hz. • The current theory is a modification of both theories • In low-frequency sounds (up to 100 Hz) , the basilar membrane vibrate in synchrony with the sound waves ( frequency theory) • As sound exceed 100 Hz, it becomes harder for a neuron to continue firing in synchrony with the waves. • Each wave of a high-frequency tone excites at least a few auditory neurons. •Volley principle of pitch discrimination- groups of neurons respond to a sound by firing AP slightly out of phase with one another so when combined, a greater frequency can be encoded and analyzed •auditory nerve as a whole produces volleys if impulses for sounds up to about 4000 per second. •Most human hearing takes place below 4000 Hz ( e.g. highest C octave in the piano) • When we hear higher frequencies, we use a mechanism similar to the place theory • Basilar membrane varies from stiff at its base and apex at the end of the cochlea. • Hair cells along the basilar membrane have different properties based on their location •Amusia (tone deafness)- impaired detection of frequency changes •Although not entirely tone-deaf, they generally do not detect a change in sound ( e.g C vs. C sharp) •Have a hard time recognizing tunes, singing off-key, “wrong” note in a melody •Have trouble gauging people’s mood from tone of voice. •Have no trouble imitating one’s intonation. •Reason: Pitch information reaches some parts of the brain but not in others • Amusia has a genetic basis. • People with amuse have a thicker than average auditory cortex in the right hemisphere but fewer than average connection from it to the frontal cortex • When they here two tones that slightly differ, the brain’s initial response is the same with other people but they fail to process the information further •Absolute pitch (perfect pitch)- ability to hear a note and identify it. •Factors that contributes to absolute pitch: •Genetic predisposition •Early musical training •Absolute pitch is also more common among people who speak tonal languages ( e.g. Vietnamese and Mandarin Chinese) The Auditory Cortex • Information from the auditory system passes through subcortical areas, axons cross over in the midbrain to enable each hemisphere of the forebrain to get most of its input from the opposite ear •Superior temporal cortexauditory cortex (A1) primary • Auditory system has a “what” pathway (anterior temporal cortex) sensitive to patterns of sounds; “where” pathway (posterior temporal and parietal cortex) sensitive to sound location • Damage in parts of the superior temporal cortex become motion deaf • A1 responds to imagined sounds as well as real ones. • Development of the auditory system depends on experience. • Damage to area A1 does not produce deafness • People with damage to A1 have trouble with speech and music but they identify and localize single sounds reasonably well. • The cortex is not necessary for hearing, just for processing the information. Watch Videos Hearing Loss • Although few people are totally insensitive to all sounds, man people have enough impairment to reduce or prevent speech comprehension. •Conductive deafness (middle-ear deafness)- caused by diseases, infections, or tumorous bone growth by preventing the middle ear from transmitting sound waves to the cochlea. •Nerve deafness (inner-ear deafness)- results from damage to the cochlea, hair cells or the auditory nerve. •It can be inherited, result from diseases or exposure to loud noises •Tinnitusthe ears. frequent ringing in Somatosensory System Somatosensory System: Touch and Pain • Sensation from your body are referred to as somatosensations. • Mediates bodily sensations • Three separate interacting systems: •Exteroceptive system- senses external stimuli applied to skins •Proprioceptive system- monitors information about position of the body •Interoceptive system- provides general information about conditions within the body Somatosensory System: Touch and Pain • This discussion deals most exclusively with the exteroceptive system • Three divisions: •Mechanical stimuli (touch) •Thermal stimuli (temperature) •Nociceptive stimuli (pain) Cutaneous Receptors Receptor Location Responds to Free nerve ending Near base of hair and elsewhere in skin Pain, warmth, cold Hair-follicle receptors Hair-covered skin Movement of hairs Hairless areas Sudden displacement of skin; low-frequency vibration Pacinian corpuscles Both hairy and hairless skin Sudden displacement of skin; high-frequency vibration Merkel’s disks Both hairy and hairless skin Light touch Ruffini endings Both hairy and hairless skin Stretch of skin Krause end bulbs Mostly or entirely hairless areas (including genitals) Uncertain Meissnner’s corpuscles Dermatomes • Neural fibers that carry information from cutaneous receptors and other somatosensory receptors gather together in nerves and enter the spinal cord via the dorsal roots. • Dermatome- area of the body that is innervated by the left and right dorsal roots of a given segment of the spinal cord • Considerable overlap between adjacent dermatomesdamage produces little somatosensory loss. Two Major Somatosensory Pathways •Dorsal-column medial-leminiscus system- tends to carry information about touch and proprioception •Anterolateral system- tends to carry information about pain and temperature •The key words in the preceding sentence are “tends to” • Implications of damage on these areas • If both ascending somatosensory paths are completely transected by a spinal injury, the patient can feel no body sensation from below the level of cut. • When it comes to spinal injury, lower is better. • Research by Mark, Ervin, and Yakolev (1962) on “effects of lesions to the thalamus on chronic pain of patients in advanced stages of cancer” • Results:(1) lesions to the ventral posterior nuclei produced loss of cutaneous sensitivity ( touch, temperature and sharp pain) but not to deep chronic pain; (2) lesions to parafascicular and intralaminar nuclei reduced deep, chronic pain without disrupting cutaneous sensitivity. Cortical Areas of Somatosensation • Penfield and colleagues ( 1937) mapped the primary somatosensory cortex of patients during neurosurgery. (electrical stimulation to cortical surface) • When stimulation was applied to the post central gyrus, the patients reported somatosensory sensations in parts of their bodies •Somatosensory homunculus Cortical Areas of Somatosensation • Somatosensory homunculus is distorted • The greatest proportion of SI is dedicated to receiving input from the body that we use to make tactile discriminations (e.g., hands, lips, and tongue) • Only small areas of SI receive input from large areas of the body (e.g., back) which are not used to make somatosensory discriminations. Effects of Damage to the Primary Somatosensory Cortex • Effects of damage to the primary somatosensory cortex are often remarkably mild- because of numerous parallel pathways. • Research: “ the somatosensory abilities of epileptic patients before and after a unilateral excision that included SI” • Results: minor deficits; reduced ability to detect light touch and identify objects by touch. Somatosensory Agnosias • Two major types of somatosensory agnosia: •Astereognosia- inability to recognize the objects by touch •Asomatognosia- failure to recognize parts of one’ own body. Pain •Pain, the experience evoked by a harmful stimulus, directs your attention toward a danger. •The prefrontal cortex (attention) typically responds only brief to any new light, sound or touch •With pain, it continues responding as long as the pain lasts. Stimuli and Spinal Cord Paths • Pain sensation begins with the least specialized receptors (bare nerve ending) • Axons carrying pain information have little or no myelin ( slow conduction; 2 to 20 m/s) •Thicker and faster axons convey sharp pain •Thinner ones convey dull pain (post-surgical pain) • Motor responses to pain are faster than motor responses to touch stimuli. •Mild pain releases the neurotransmitter glutamate •Stronger pain releases several neuropeptides including substance P and CGRP ( calcitonin generelated peptide Emotional Pain • Pain stimuli also activate a path that goes through the reticular formation of the medulla then to several of the central nuclei of the thalamus, amygdala, hippocampus, prefrontal cortex and cingulate cortex • These areas react not to sensation but to its emotional associations • If you watch someone in pain, you experience sympathetic pain • A hypnotic suggestion to feel no pain decreases the responses in the cingulate cortex without much effect on the somatosensory cortex • Hurt feelings do resemble physical pain • Research : “Virtual-ball tossing game” • Results: People’s brain activity increased in the cingulate cortex (emotional aspects of pain) • More intense hurt feelings increases activity in both emotional areas ( especially cingulate cortex) and sensory areas ( responsive to physical pain) • Hurt feelings are like real pain in another way: you can receive it with pain-relieving drugs Ways of Relieving Pain • Insensitivity to pain is dangerous • People with a gene that inactivates pain axons suffer repeated injuries and generally fail to learn to avoid dangers •Opioids and endorphins • After pain alerts you to a danger, continuing pain messages are unnecessary •Opioid mechanismssimilar chemicals systems that respond to opiate drugs and •The discovery was important: •Opiates act on the NS rather than the injured tissue •It implies the that the NS has its own opiate-type chemicals •Endorphins- relieve different types of pain (pain from cut v.s. burn); released during sex or when listening to thrilling music •Gate theory- spinal cord neurons that receive messages from pain receptors also receive input from touch receptors and from axons descending from the brain; these other inputs can close the “gates” for pain messages •Placebos • A drug or other procedure with no pharmacological effects • People who receive placebos do not jut say the pain decreased, scans of the brain and spinal cords also show a decreased response. • Conversely, if someone is told to expect pain to increase , the spinal cord response to a painful stimulus does increase •Cannabinoids and capsaicin •Cannabinoids- chemicals related to marijuana also blocks certain kinds of pain; act mainly on the periphery of the body. •Capsaicin- a chemical in jalapeños and similar peppers that stimulates receptors for heat. Itch • Have you ever wondered “ What is itch anyway?” • It is a separate sensation (special receptors and special spinal cord paths) • Itch pathways are slow to respond • Itch is useful because it directs you to strict the itchy area ad remove whatever is irritating your skin Active Touch • Your sense of touch operates over time. Your brain is sensitive to overall patterns of cutaneous input that may be quite complex. When you rub your fingertips (which are particularly rich in cutaneous receptors) over a surface like cloth, plastic, or wood, the distinct texture of the surface depends on activity from thousands of touch receptors firing in sequence and in combination. This is called active touch and is used, for example, in reading Braille. Let’s have an activity!
