Content Category 6A Sensing the Environment

AAMC Content Outline – Psychology and Sociology Foundational Concept 6: Biological, psychological, and sociocultural factors influence the ways that individuals perceive, think about, and react to the world Content Category 6A: Sensing the environment: Sensory Processing (PSY, BIO) Sensation Encoding of sensory stimuli • Stimulus Modality – type of stimulus, based on which type of receptors is firing (vision, balance, etc) • Sensory interaction • Stimulus location – communicated by the receptive field of the sensory receptor sending the signal o localization can be improved by overlapping receptive fields of neighboring receptors o discrimination can be improved by lateral inhibition of neighboring receptors • Stimulus intensity – coded by the frequency of action potentials • Stimulus duration – may or may not be coded explicitly o Tonic receptors – fire action potentials as long as the stimulus continues ▪ subject to adaptation—frequency of APs decreases as the stimulus continues at the same level o Phasic receptors – only fire action potentials when the stimulus begins, do not explicitly communicate the duration of the stimulus (It’s only a phase) ▪ important for communicating changes in stimuli ▪ adapt immediately if a stimulus continues at the same level Proprioception – awareness of self (body position); kinesthetic sense • muscle spindle – mechanoreceptor that is an example of an important proprioceptor; detects muscle stretch • Golgi tendon organs – monitor tension in tendons • joint capsule receptors – detect pressure, tension, and movement in the joints • Threshold • • Absolute Thresholds – minimum stimulus intensity required to activate a sensory receptor 50% of the time and thus detect the sensation) Difference Thresholds – minimum noticeable difference between any two sensory stimuli, 50% of the time • Weber’s Law (PSY) o Weber’s Law – two stimuli must differ by a constant proportion in order for their difference to be perceptible. For example, two objects must differ in weight by 2%, two lights must differ in intensity by 8%, etc • Signal detection theory (PSY) • Signal Detection Theory – attempts to predict how and when someone will detect the presence of a given sensory stimulus amidst all of the other sensory stimuli in the background (the signal and the noise). Four possibilities: a hit (signal present and detected), a miss (signal present and not detected), a false alarm (signal not present but detected), and a correct rejection (signal not present, not detected) • Sensory adaptation o Decrease in firing frequency when the intensity of a stimulus remains constant ▪ nociceptors, however, do not adapt under any circumstance Psychophysics • • • • the branch of psychology that deals with the relationships between physical stimuli and mental phenomena Psychophysics has been described as "the scientific study of the relation between stimulus and sensation" [1] or, more completely, as "the analysis of perceptual processes by studying the effect on a subject's experience or behavior of systematically varying the properties of a stimulus along one or more physical dimensions" Distal Stimulus – objects and event out in the world about you. Proximal stimulus – patterns of stimuli from these objects and events that actually are registered by the sensory receptors. Most of the time, perception reflects the properties of distal objects and events very accurately, much more accurately than you might expect from the apparently limited, varying, unstable pattern of proximal stimulus the brain gets • Sensory receptors • Sensory pathways • • • Axons cross over in the spine to other side o Processed in the cerebral cortex on the other side from where the stimulus comes in For cranial sense (with receptors in the face), crossing over still happens in the brain stem If one side of the brain is injured, there is a loss of somatosensation on the other side • • • • • Types of sensory receptor Exteroceptors – receptors that detect stimuli from outside world Interoreceptors – detect internal stimuli Mechanoreceptors – respond to mechanical disturbances o Pacinian Corpuscle – shaped like an onion, depolarized from firm pressure o Auditory hair cell – vibrations from sound waves o Vestibular hair cells – located within special organs called semicircular canals, also found in the inner ear—detect acceleration and position relative to gravity o stretch receptors Chemoreceptors – respond to particular chemicals o olfactory receptors – detect airborne chemicals and allow us to smell things o gustatory receptors – taste buds o Autonomic chemoreceptors in the walls of the carotid and aortic arteries – respond to changes in arterial pH, CO2 and O2 pressure levels • Nociceptors – pain receptors, detects chemical signs of tissue damage o Autonomic or somatic pain receptors ▪ Autonomic – frequently give a sensation of dull, aching pain • Referred pain – illusion of pain on the skin, when their nerves cross paths with somatic afferents from the skin • Thermoreceptors – stimulated by change in temperature, autonomic and somatic examples o cold-sensitive, warm-sensitive, and thermal nociceptors – detect painfully hot stimuli Electromagnetic receptors – stimulated by electromagnetic waves o rod and cone cells of the retina • • Vision (PSY, BIO) Structure and function of the eye Visual processing • Visual pathways in the brain • Light → cornea → anterior chamber → pupil → lens → vitreous chamber → retina (rods and cones) → bipolar cells → • • • • • • • • ganglion cells (optic nerve) → thalamus → occipital lobe Macula contains fovea centralis—contains only cones and is responsible for extreme visual acuity Rods and cones contain special pigment proteins called opsin Rods respond equally to all visible wavelengths, but cones only respond to specific wavelengths o each opsin is bound to one molecule and contains one molecule of retinal, which is derived from vitamin A o In the dark, retinal has several trans double bonds, and one cis double bond —this confirmation keeps sodium channels open, depolarized o In light, retinal is converted to the all-trans form, which closes sodium channels, hyperpolarization o Prevented from releasing glutamate, which originally inhibits bipolar cells o bipolar cell can depolarize, and depolarize ganglion cells Night vision is accomplished by the rods, which are more sensitive to dim light and motion, concentrated in the periphery of the retina Normal vision – emmetropia myopia – nearsightedness – corrected by diverging lens hyperopia – farsightedness, corrected by converging lens Presbyopia – inability to accommodate (focus) – results from loss of flexibility of lens, which occurs with aging o Once light travels through the eye and is detected by the photoreceptors, info must be conveyed to higher levels of the nervous system ▪ after light strikes cones and rods, series of events leads to hyperpolarization of the membrane of the photoreceptor ▪ has an inhibitory effect by reducing the rate of neurotransmitter release • glutamate – usually excitatory, but not for the cells that receive it from photoreceptors o Bipolar cells – may be inhibited or excited by changes in amount of glutamate released by photoreceptors ▪ different bipolar cells respond to different receptive fields o o o • • bipolar cells also affected by signals from horizontal cells, which provide “horizontal” information from photoreceptors at the edge of the bipolar cell’s receptive field ▪ If bipolar cells experience an overall excitatory effect from both the vertical and horizontal inputs, they release neurotransmitter at an increased rate, producing an excitatory effect on the ganglion cells—these are the sensory neurons that finally produce action potentials The axons of ganglion cells gather to form the optic nerve, which leaves the eye to convey visual information to the brain ▪ Some of the axons from each eye cross to the opposite side of the brain ▪ The optic nerve travels to the lateral geniculate nucleus (LGN) of the thalamus, which preserves the visual map created by the ganglion cells and projects this information to the primary visual cortex, in the occipital lobe Neurons project from the visual cortex along two visual pathways, which detect different features of visual stimuli ▪ ventral (“what”) pathway travels to the temporal lobe towards the base of the brain and is involved in object recognition ▪ dorsal (“where”) pathway projects to the parietal cortex and is more involved in perceiving the location of objects • these two pathways communicate with each other, and information from both is ultimately integrated in other areas of the brain Parallel processing (PSY) • Parallel Processing – where many aspects of a visual stimulus (such as form, motion, color, and depth) are processed simultaneously instead of in a step-by-step or serial fashion. The occipital lobe constructs a holistic image by integrating all of the separate elements of an object, in addition to accessing stored information. • Feature detection (PSY) • Feature-detection theory – neurons in the visual cortex fire in response to very specific information; feature detecting neurons are specific neurons in the brain that fire in response to particular visual features. Feature-detection theory explains why a certain area of the brain is activated when looking at a face, and a different area is activated when looking at letters. Hearing (PSY, BIO) Structure and function of the ear • • • • Auricle/pinna and external auditory canal – Outer Ear Middle Ear – tympanic membrane (eardrum), ossicles (malleus, incus, stapes), oval window Inner ear – oval window, round window – releases excess pressure, cochlea, semicircular canals (balance) Eustachian tube (auditory tube) – passageway from the back of the throat to the middle ear; functions to equalize pressure on both sides of eardrums, is the cause of the “ear popping” one experiences at high altitudes or underwater Auditory processing (e.g., auditory pathways in the brain) • Malleus receives the vibrations, function with the incus and stapes to amplify sound vibrations, vibrates oval window • Pressure waves in the perilymph and endolymph, the fluids in the cochlea, cause vibration in the basilar membrane (covers lining of the cochlea) o Basilar membrane is covered with hair cells, which have cilia, bend when the basilar membrane vibrates and moves across the tectorial membrane (roof), open ion channels in the hair cells, results in neurotransmitter release o Stimulates dendrites from bipolar auditory afferent neurons o Basilar membrane, hair cells, and tectorial membrane = Organ of Corti ▪ organ of Corti is thus within the cochlea • Pitch – frequency, distinguished by regions of basilar membrane that vibrate, stimulation of different auditory neurons o Thick and sturdy near oval window, become thin and floppy near apex of the cochlea o Low frequency detected farthest away from oval window, high-pitched sounds stimulate hair cells near oval window • Loudness – large vibrations cause more frequent action potential in the auditory neuron • Sound is processed in the auditory cortex, located in the temporal lobe of the brain Sensory reception by hair cells Other Senses (PSY, BIO) Somatosensation (e.g., pain perception) Taste (e.g., taste buds/chemoreceptors that detect specific chemicals) Smell • Olfactory cells/chemoreceptors that detect specific chemicals • Pheromones (BIO) • Olfactory pathways in the brain (BIO) Kinesthetic sense (PSY) Kinesthesis, also referred to as kinesthesia, is the perception of body movements. It involves being able to detect changes in body position and movements without relying on information from the five senses. Vestibular sense The Vestibular Senses. Awareness of body balance and movement are monitored by the vestibular system. The vestibular senses (the sensations of body rotation and of gravitation and movement) arise in the inner ear; the sense organs are the hair cells that send out signals over the auditory nerve. Perception (PSY) Bottom-up/Top-down processing Bottom-up Processing – begins with the sensory receptors and works up to the complex integration information occurring in the brain. For example, information enters the eyes in one direction (“bottom”), and then is turned in to an image by the brain (“top”). We tend to use bottom-up processing when we have no or little prior experience with a stimulus Top-down processing – occurs when the brain applies experience and expectations to interpret sensory information Perceptual organization (e.g., depth, form, motion, constancy) of an object, in addition to accessing stored information. Depth Perception – describes the ability to see in 3 dimensions despite the fact that images are imposed on the retina in only 2 dimensions. Appears to be largely innate Binocular Cues – depth cues that depend on information received from both eyes and are most important for perceiving depth when objects are close to us in our visual field Retinal Disparity – a binocular cue whereby the brain compares the images projected onto the two retinas in order to perceive distance. The greater the difference or disparity between the two images on each retina, the shorter the distance to the observer. Convergence – a binocular cue that describes the extent to which the eyes turn inward when looking at an object. The greater the angle of convergence or inward strain, the closer the object. Monocular Cues – Depth cues that depend on information that is available to either eye alone. They are important for judging distances of objects that are far from us because the retinal disparity is only slight. • Relative Size – if objects are assumed to be the same size, the smaller one appears more distant • Interposition – If one object blocks the view of another, we perceive it as closer • Relative Clarity – we perceive hazy objects as being more distant than sharp, clear objects • Texture Gradient – change from a coarse, distinct texture to a fine, indistinct texture indicates increasing distance • Relative height – we perceive objects that are higher in the visual field as farther away • Relative motion – as we move, stable objects appear to move as well. Objects that are near to us appear to move faster than objects that are farther away. • Linear Perspective – Parallel lines appear to converge as distance increases. The greater the convergence, the greater the perceived distance. • Light and Shadow – Closer objects reflect more light than distant objects. The dimmer of two identical objects seem farther away. Gestalt principles Gestalt Psychology – idea that the organized whole exceeds the sum of its parts. When humans perceive an object, rather than seeing lines, angles, colors, and shadows, they perceive the whole—a face or a table or a dog. Gestalt does not explain how the brain does this, merely that it does • Emergence – when attempting to identify an object, we first identify its outline, which then allows us to figure out what the object is. Only after the whole emerges do we start to identify the parts that make up the whole, such as a dog’s face or legs • Figure/Ground – describes our perceptual tendency to separate the figure or object from everything else (background) based on a number of possible variables, like size, shadow, or contrast. According to this principle, everything that is not figure is ground. Consider the famous face-vase illusion. The Figure/Ground principle helps explain why it is impossible to perceive the image as both a vase and two faces simultaneously. • Multistability – tendency of ambiguous images to pop back and forth unstably between alternative interpretations in our brains. Examples include the images of impossible object (How many legs does this elephant have?). Gestalt Laws of Grouping – pg 77 • Law of Proximity – suggests that things that are near each other seem to be grouped together (for example, you would see a square composed of dots rather than 25 individual dots). • Law of Similarity – things that are similar tend to be appear grouped together (viewing a column of dots and a column of squares, if they are next to each other) • Law of continuity – the points that we perceive the smooth, continuous lines and forms, rather than disjoined one. (We see two overlapping circles rather than two black semicircular lines and a football shape in the middle) • Law of closure – predicts that we will perceive things as complete a logical entity, because our brains fill in gaps in the information (seeing a triangle when looking at an image of unfilled concentric circles at the angles) • Law of common fate – predicts that objects moving in the same direction or moving in synchrony are perceived as a group or unit (flock of birds moving together) • Law of connectedness – predicts that things that are joined or linked or grouped are perceived as connected (square around nine circles in a square of 25 circles leads us to perceive that the 9 are in some way different)