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Chapter 7

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PSYC 271
Richard Beninger

Page 1 of 16 Chapter 7: Mechanisms of Perception: Hearing, Touch, Smell, Taste, and Attention • Exteroceptive sensory systems: sensory systems that interpret stimuli from outside the body – vision, hearing (audition), touch (somatosensory), smell (olfactory) and taste (gustatory). 7.1 PRINCIPLES OF SENSORY SYSTEM ORGANIZATION • Primary sensory cortex: Receives input directly from thalamic relay nuclei of that system. • Secondary sensory cortex: Receives input from primary or from other secondary areas. • Association cortex: Receives input from more than one sensory system, via various areas of secondary. • Interactions between these three types of sensory cortex and other sensory structures are explained here. Hierarchical Organization • Hierarchical organization: sensory structures are organized in hierarchy based on specificity and complexity of their function. From receptors up to association cortex, neurons respond optimally to stimuli of greater specificity and complexity. Each level receives most of its input from lower levels, and adds another layer of analysis. • The higher the level of brain damage, the more specific and complex the deficit. o E.g. destruction of a sensory system’s receptors produces complete loss of ability to perceive that sensory modality (e.g. total blindness), while destruction of association cortex produces complex and specific sensory deficits that leave fundamental sensory abilities intact. o The man who mistook his wife for a hat: Dr. P had excellent visual acuity, but could not seem to identify objects (could not recognize a glove, tried to grasp his wife’s head and put it on his own...) • Sensation: process of detecting presence of stimuli • Perception: higher-order process of integrating, recognizing, and interpreting complete patterns of sensations (Dr. P had problems with perception, not sensation). Functional Segregation • Not true that each area of sensory system is functionally homogeneous (all areas act together to perform same function); instead, there is functional segregation – in each level of sensory hierarchy, there are functionally distinct areas that specialize in different kinds of analysis. Parallel Processing • Levels of sensory hierarchy are not serial systems – system where information flows among the components over just one pathway, like string through strand of beads. • They are actually parallel systems – information flows through the components over multiple pathways. They have parallel processing, the simultaneous analysis of a signal in different ways by the multiple parallel pathways of a network. • Two different kinds of parallel streams – one to influence behaviour without our conscious awareness, and one that influences behaviour by engaging our conscious awareness. Page 2 of16 Summary Model of Sensory System Organization • Used to be: Hierarchical, functionally homogeneous, serial. • Now: Hierarchical, but functionally segregated and parallel. • Multiple specialized areas, at multiple levels, are interconnected by multiple parallel pathways – “division of labour”. • E.g. Complex visual stimuli are perceived as integrated wholes due to work of different systems perceiving shape, colour, movement, etc. • How does brain combine individual attributes to produce integrated perceptions? Binding problem. o Perception is a product of combined activity of different interconnected cortical areas. • Not just down to up: many neurons descend through the hierarchy and carry top-down signals. 7.2 THE AUDITORY SYSTEM • Auditory system perceives sound – perception of objects and events through the sounds they make. • Sounds are vibrations of air molecules that stimulate the auditory system. We hear between 20-20,000 Hz. • Amplitude = loudness; frequency = pitch; complexity = timbre • Timbre, the complexity of a sound, is what allows us to distinguish between a guitar and violin playing at the same loudness and same pitch. We visualize this with a spectral profile, which graphs the intensity of component frequencies. • In reality, pure tones do not exist outside a lab; sound is always associated with complex patterns of vibrations, a combination of sine waves of various frequencies and amplitudes. • Fourier analysis: mathematical procedure for breaking down complex waves into component sound waves. This may be how the auditory system analyzes complex sounds. • Relationship between tone frequency and perceived pitch for complex sounds: we interpret its pitch based on the fundamental frequency (highest frequency of which various component frequencies are multiples – numerically the lowest common denominator), e.g. complex sound of 100, 200 and 300Hz has fundamental frequency of 100Hz. o Missing fundamental: when perception of pitch not directly related to sound’s component frequencies. E.g. Mixture of 200, 300 and 400Hz perceived as having same pitch as 100Hz pure tone – it is the fundamental frequency, but is not a component. The Ear • Sound waves travel through the ear: 1) Auditory canal 2) Vibrates tympanic membrane (eardrum) 3) Vibrates middle ear bones, the ossicles (malleus, incus, stapes) Page 3 of 16 4) Stapes triggers vibrations on the oval window 5) Vibrational energy transferred to fluid of the cochlea, a long coiled tube with an internal membrane running through the middle. The internal membrane is the organ of Corti, the auditory receptor organ. • It has 3 chambers: the vestibular duct (scala vestibule), the cochlear duct (scala media) and the tympanic duct (scala tympani). • The cochlear duct is composed of the basilar membrane and the tectorial membrane with the organ of Corti in between. The basilar membrane is stiff while the tectorial membrane is soft. • The auditory receptors, the hair cells, are mounted on the basilar (base = bottom) and poke through the tectorial (on the top). Inner Ear Middle Ear OUTER EAR Outer Ear Pinna funnels sound into the auditory canal. Sound energy vibrates the tympanic membrane (eardrum). MIDDLE EAR Malleus (hammer) Incus (anvil) Stapes (stirrup) The middle ear bones Tympanic and vestibular ducts are perilymph (like ECF) are the smallest in the Cochlear duct has endolymph, human body. which has high K concentration and Stirrup presses on the oval window, which +80mV. connects to the inner Cochlear duct (scala ear. Vestibular duct media) INNER EAR (scala vestibuli) Semicircular canals – balance, equilibrium Cochlea – audition Tympanic duct At (scala area of hair Hair cells are the auditory sensory higtympani)entration. The shearing force opens the mechanically-gated ion channels (stereocilia) so K enters the hair cell and causeslls are connected to Endolymph afferent nerves to send signals to depolarization. the brain. At the base of the cell, the surrounding fluid hasted to + efferent nerves from the brain. low K concentration. To recover from +he depolarization (receptor potential), K channels at the base open to let K out of cell (down concentration gradient). Meanwhile, Ca channels also open here inenergy from oval window initiat+s fluid waves respond to the depolariin cochlear, organ of Corti deflects to help repolarization.on its pivot points, produces Perilymph shearing force on hair cells. Action potential causes vesicles of transmitter (maybe glutamate?) to be expelled and initiate is described action potential. Signal is transmitted down the auditory nerve (a branch of the auditory-vestibular Page 4 of16 • Sound energy is dissipated at the round window, an elastic membrane in the cochlea wall. • The cochlea is very sensitive: we can hear differences in pure tones that differ in frequency by only 0.2% • Different frequencies produce maximal stimulation of hair cells at different points along the basilar membrane with tonotopic organization. o High frequencies are perceived (hair cells here most sensitive to) at the base of the cochlear – high frequencies have a lot of energy and are registered quickly; low frequencies are perceived at the apex of the cochlear – low energy so won’t be registered till it reaches the tip. The characteristic frequency is the frequency a region of hair cell is most sensitive to. o Another explanation: basilar membrane is narrow and stiff at the base – so high frequency resonates best here; it is wide and floppy at the apex, so low frequencies resonate here (high frequencies can’t). o Characteristic frequencies on basilar membrane are logarithmically spaced – for same distance, wider range of high frequency (squished closer together) at apex than for low frequency at base (spaced widely apart). This is why the elderly lose high frequency hearing early – damage to small area causes loss of a wide, noticeable range of hearing. o For a complex sound, the component frequencies would together activate hair cells at different points along the membrane; the many signals created are carried by different auditory neurons to the brain. • Tonotopic organization of Organ of Corti demonstrated by hair cell tuning curves – testing each hair cell with pure tones of different frequencies and of different intensities to find characteristic frequency. Intensity can excite a cell at a lower-than-characteristic frequency, but for its characteristic frequency the cell will be excited even at a low intensity. • The vestibular system, with its semicircular canals, is involved in the maintenance of balance. From the Ear to the Primary Auditory Cortex • Network of auditory pathways: 1. Axons of each auditory nerve synapse in the ipsilateral cochlear nuclei 2. To the superior olive in the brain stem (here signals from each ear are combined) 3. Olivary neurons project to the lateral lemniscus. 4. To o the inferior colliculi in the tectum (mesencephalon) where they synapse onto the next neurons 5. Project to the medial geniculate of the nuclei in the thalamus 6. To the primary auditory cortex Page 5 of16 • There are at least 5 synapses from cochlea to cortex • Signals are transmitted both ipsilaterally and contralaterally to the primary auditory cortex. Subcortical Mechanisms of Sound Localization • Medial superior olives: respond to differences in theLateral timing of arrival of Olive Medial sound signals between the two ears (phase Olive difference). For frequencies < 800Hz. • Lateral superior olives: respond to differential loudness (amplitude) of sounds received by the two ears. For frequencies > 1600Hz (we have poor ability to localize sounds between 800 and 1600Hz). • The olives project to both the inferior and superior colliculi; the layers of the superior colliculi are organized not tonotopically, but according to a map of auditory space. • The general function of the superior colliculi is locating (localizing) sources of sensory input in space; it also receives visual input, and is organized retinotopically for that modality. • There is not only an ascending pathway of auditory information, but a descending pathway which exercises inhibitory control over lower-level organs to determine what will be picked up (e.g. attention to one person at a loud party). • The auditory neurons of a barn owl`s superior colliculus are very finely tuned with small receptive fields. Auditory Cortex WHAT WHERE • Primary auditory cortex is located in the superior temporal lobe, in Heschl’s gyrus (HG), and hidden from view in the lateral fissure. • Adjacent to the primary auditory area two other areas, together called the core; surrounding it is a belt region of secondary cortex (over 6 distinct areas). Surrounding that is the parabelt. • Organization of Primary Auditory Cortex: Organized in functional columns with neurons in Lateral the same vertical area (all neurons encountered during vertical microelectrode penetration) fissure responding optimally to sounds in same frequency range. It is also arrange tonotopically, based on frequency. • What sounds should be used to study the auditory cortex? Progress in research of auditory system is limited by complexity and variety of system even at low levels. • Two Streams of the Auditory Cortex: o WHAT – ANTERIOR: Recognition (“ventral”) = identifying sounds, using the anterior auditory pathway to the prefrontal cortex. o WHERE – POSTERIOR: Spatial (“dorsal”) = localizing sound, using the posterior auditory pathway to the posterior parietal cortex. o However, newest evidence suggests that the WHERE pathway may not really project to the posterior parietal cortex – instead projects to the posterior part of the prefrontal cortex. o Evidence of imaging of fibre (white matter) tracts to look at connections. Page 6 of 16 • Auditory-Visual Interactions: Interactions take place in the association cortex (especially the posterior parietal cortex) - combination of visual and auditory receptive fields, of the same location in the subject’s environment. • fMRI studies allow recording of activity throughout the brain: show us that interactions occur not only in the association cortex, but even at the lowest level of the primary sensory cortex. These interactions are an early and integral part of sensory processing, not after unimodal analyses, but alongside it. • Where Does the Perception of Pitch Occur? Observation that when stimuli have different frequency and pitch (using missing fundamentals), neurons respond to changes in frequency, not pitch. An area anterior to the primary auditory cortex responds to pitch most, while also containing neurons that respond to frequency – this may be where frequencies are converted to the perception of pitch (Bendor & Wang, 2005). Effects of Damage to the Auditory System • Auditory Cortex Damage: Auditory cortex rarely entirely destroyed due to its being hidden in the lateral fissure, and when it is, it’s also accompanied by extensive damage to surrounding tissues; we rely on lesion-based research on animals. • Large lesions surprisingly do not produce severe permanent deficits – subcortical circuits must serve complex and important processing functions before signal reaches cortex (at tectum, brainstem levels). • In humans with bilateral lesions, there is often complete loss of hearing, but it recovers in ensuing weeks – deafness must have due to shock of the lesion. Permanent effects include loss of ability to localize sounds, and impairment of ability to discriminate frequencies. • A unilateral lesion disrupts ability to localize sounds in space contralateral, but no ipsilateral, to the lesion; this effect is observable but smaller for other auditory deficits. This suggests the system is partially contralateral. • Deafness in Humans: Total deafness is rare, affecting 1% of the hearing-impaired; diffuse parallel network of auditory pathways ensures that if one is destroyed, alternative pathways can still transmit the information. • Severe hearing problems typically result from damage to inner ear, middle ear, or the nerves leading from them – rarely central damage. • Conductive hearing impairment is due to damage to the outer or middle ear; this blocks the signal from getting to the cochlea. Can be caused by earwax buildup, tear in tympanic membrane, problems with ossicles or oval window (such as due to birth defects). • Sensorineural hearing impairment is due to damage to cochlea and loss of receptive hair cells, or to the auditory nerve. Causes include noise exposure, aging process, or ototoxic drugs. • If only part of the cochlea is damaged, may have nerve deafness for some frequencies but not others. In age-related hearing loss, high-frequency perception is often lost first. • Tinnitus: ringing of the ears without outside stimulation, common in hearing loss. Observed that cutting the nerve from the ringing ear does not stop the ringing – suggests changes to the central auditory system caused by deafness is the cause of tinnitus. Can be one or both ears, temporary or permanent. • Cochlear implants bypass damage to the auditory hair cells, replacing function of middle and inner ears. • They convert sounds picked up by microphone in patient’s ear to electrical signals; a speech processor selects and arranges sounds picked up by the microphone; a transmitter and receiver/stimulator converts this into an electrical signal; a bundle of electrodes collect electrical impulses and excite the auditory nerve. • There are many varieties – behind the ear, full or half shell, in the canal, or “in the crest”. Page 7 of 16 • Cochlear implants do not restore normal hearing, but implantation soon after deafness can be very beneficial (disuse leads to degeneration of auditory neural pathways). Vision vs. Audition (Lecture) • Vision has better spatial resolution than audition. However, audition has better temporal resolution: when we see a quick flash and hear a quick beep simultaneously, and sensory modalities conflict (see 1 flash, hear 2 beeps) we trust audition because it has better temporal resolution (and we think that we saw two flashes). • Vision has limited spatial extent – we can only see what’s in front of us – while audition is omni- directional. 7.3 SOMATOSENSORY SYSTEM: TOUCH & PAIN • The somatosensory system is made up of 3 separate but interacting systems: an exteroceptive system that senses external stimuli applied to the skin; 2) a proprioceptive system that monitors information about the position of the body, from receptors in the muscles, joints, and organs; 3) an interoceptive system that provides general information about conditions within the body, like temperature and blood pressure. • The exteroceptive system comprises of 3 divisions: one for perceiving mechanical stimuli (touch), one for thermal stimuli (temperature), and one for nociceptive stimuli (pain). Cutaneous Receptors • Free nerve endings: The simplest receptors, nerve endings with no specialized structures on them. Most sensitive to temperature change and pain. • Pacinian Corpuscles: Largest and deepest receptors, enclosed in an onion-like structure of tissue. They adapt rapidly and respond to sudden displacements of the skin (fast-adapting), though not to constant pressure. They are mechanically-gated Na channels. • Ruffini endings and Merkel’s disks adapt slowly and respond to gradual indentation and stretch, respectively. • Stereognosis is the identification of objects by touch. When we do this, we manipulate the object in our hands so the pattern of stimulation continually changes – otherwise only the slow-adapting receptors would be active and the sensation would be markedly different. • Having information from both quick and slow-adapting receptors provides information about both the dynamic and static qualities of tactual stimuli. • Somatosensory receptors are specialized, being most sensitive to a particular type of tactual stimulation. However, the general mechanism is similar: stimuli changes the chemistry of the receptor, changing permeability of various io
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