U7-Mechanisms of Perception.docx

U7: MECHANISMS OF PERCEPTION Mar/8/2012 1. Sensation and Perception - Sensation is the simple process of detecting the presence of a stimulus - Perception is the complex process of integrating, recognizing and interpreting complex patterns of sensations. o Ex. The man who mistook his wife for a hat: Dr. P knew there was an object in his visual field but had a problem in perceptual processing, which 3 Sensory areas of the Cortex resulted in an inability to recognize that object. 1. Primary sensory cortex - Receives most of its input directly from the thalamic relay nuclei of that system 2. Secondary Sensory Cortex - Comprises the areas of the sensory cortex that receive most of their input from the primary sensory cortex of that system or from other areas of the secondary sensory cortex of the same system 3. Association cortex - Receives input from more than one sensory system - Sensory systems are characterized by a parallel, functionally segregated and hierarchical organization. - Parallel: information flows between different structures simultaneously along multiple pathways - There seems to be 2 kinds of parallel analysis, one that is capable of influencing our behaviour without our conscious awareness and one that influences our behaviour by engaging our conscious awareness. - Functionally segregated: different parts of various structures specialize in different kinds of analysis - It was thought that primary, secondary and association areas were functionally homogeneous whereby they acted together to perform the same function. - The three levels contains functionally distinct areas that specialize in different kinds of analysis. - Hierarchical: information flows through brain structures in order of the increasing neuroanatomical and functional complexity. o Receptors  Thalamic Relay Nuclei  Primary Binding problem Sensory Cortex  Secondary Sensory Cortex  Association Cortex o The higher the level of damage, the more specific Colour’s effect on Taste (Top-down processing) and complex the deficit Dubose et al. - If different types of information are processed in different specialized zones, how are complex stimuli perceived as an integrated whole? - One possible solution is that there is a single area of the cortex at the top of the sensory hierarchy that receives signals from all other areas of the sensory system and puts them together to form perceptions; however there are no areas of cortex to which all areas of a single sensory system report. - Perceptions are a product of the combined activity of different interconnected cortical areas. - People’s ability to identify taste and smell depends to some degree on colour. o Ex. People’s taste of cherry, orange, and lime beverages were judged correctly an average of 67% of the time if colours matched the beverages’ taste o Dropped to 37% if solutions were colourless o 28% if colours didn’t match the taste. 2. Auditory System - The function of this system is to perceive objects and events **The reason we hear 100 out of 200, 300, and through the sounds they make. 400 is because 100 is the greatest common - Amplitude, Frequency, and Complexity of the molecular denominator. vibrations are most closely linked to perceptions of loudness, pitch, and timbre. - Fournier analysis is the mathematical procedure for breaking down complex waves into their component sine waves. - The pitch of a complex sound may not be directly related to the frequency of any of the sound’s components o Ex. A mixture of pure tones with frequency of 200, 300, and 400 Hz would be perceived as having the same pitch as a pure tone of 100 Hz because it is the fundamental frequency of 200, Auditory Pathway 300, 400. o This is called the missing fundamental. - Vibrations in the air are transmitted through tympanic membrane (ear drum), ossicles (three small bones), oval window, into fluid of the cochlea. - Organization of organ of Corti is tonotopic because higher frequencies excite receptors closer to oval window. This is a tonotopic organization - Each pressure change at the oval window travels along this organ as a wave. - The organ of Corti is composed of 2 membranes: the basilar membrane that contains the auditory receptors, or hair cells and the tectorial membrane rests on the hair cells. - A deflection of the organ of Corti at any point along its length produces a shearing force on the hair cells at the same point, which stimulates them and increase firing in axons of the auditory nerve - The vibrations of the cochlear fluid are ultimately dissipated by the round window 1. Hair cells synapse on neurons whose axons enter the metencephalon and synapse in the ipsilateral cochlear nucleus. 2. From the cochlear nucleus, some fibers project to the nearby superior olives  inferior colliculus via lateral lemniscus 3. From the inferior colliculus, fibers ascend to the medial geniculate nucleus of the thalamus 4. Fibers ascend to the primary auditory cortex in the lateral fissure. - The sensitivity of the cochlea (0.2% difference in pure tone that differ in frequency) brings up the problem of auditory processing whereby you can sort out individual frequency activations on your basilar membrane when in a crowded room. - Semicircular canals are the receptive organs of the vestibular system that carries information about the direction and intensity of head movements, to help maintain balance. - Localization of sounds in space is mediated by the lateral and medial superior olives - When a sound originates on the left side, it reaches the left ear first and is louder there, some neurons in the medial superior olives respond to slight differences in the time of arrival of signals from the two ears whereas some neurons in the lateral superior olives respond to slight differences in amplitude of sounds from the two ears. - These olives travel to the superior colliculus and inferior colliculus which take auditory input and lay it out according to a map of auditory space - The primary auditory cortex receives the majority of input from the medial geniculate nucleus is located in the temporal lobe, within the lateral fissure. Localization of Sound - This cortex, along with 2 other areas make up the core region, which is surrounded by the belt, or secondary cortex and areas outside this are parabelt areas - Located in the lateral fissure that separates the temporal and frontal lobes - Tonotopically organized into functional columns such that neurons in a given column all respond maximally to tones in the same frequency range. - Lesions in the auditory cortex don’t cause deafness, even if the lesions include secondary auditory cortex. - Can cause difficulty localizing brief stimuli or recognize Auditory Cortex rapid sequences of sounds. - The major permanent effects of bilateral lesions are loss of ability to localize sounds and impairment of the ability to discriminate frequencies. - Research has lagged behind in this field because we have no idea what the auditory cortex since neurons respond only weakly to simple stimuli such as pure tones. - The effects of unilateral auditory cortex lesions suggest that the system is partially contralateral - Unilateral lesions disrupts the ability to localize sounds in space, contralateral but not ipsilateral to the lesion Deafness in Humans - Severe hearing problems usually result from damage to the inner ear or middle ear or nerves leading from them, instead of central damage. - There are 2 classes of hearing impairments: 1. Conductive deafness – associated with damage to the ossicles 2. Nerve deafness – damage to the cochlea or auditory nerve, occurring as a loss of hair cell receptors. - If only part of the cochlea is damaged, individuals may have nerve deafness for some frequencies but not others, usually due to age. - Tinnitus – ringing of the ears. When nerves from the ringing ear are cut, this has no effects which suggests that changes to the central auditory system that were caused by the deafness are the cause of tinnitus. Two Streams of Auditory Cortex - There are 2 streams of auditory analysis – anterior auditory pathway is involved in identifying sounds (what) and the posterior auditory pathway is involved in locating sounds (where) Auditory Visual Interactions - Some posterior parietal neurons have both visual and auditory receptive fields - fMRI results showed that sensory system interaction is not tagged on after unimodal analysis are complete, it is an early and integral part of sensory processing. - Bypass damage to the auditory hair cells by converting sounds picked up by the microphone on the ear to electrical signals which are then carried to the cochlea by a bundle of electrodes. - Small, complex device that provides a sense of sound to a person who is profoundly deaf or severely hard of hearing - Electrodes are inserted in the cochlea to create hearing by electrically stimulating auditory nerve fibres - Implants are helpful for patients with damaged hair cells because they only amplify sounds but don’t cause that sound to be translated into electrical signals in hair cells. Parts of a Cochlear Implant: (External – behind ear and Internal portion – beneath skin) 1. Microphone – picks up sound from environment Cochlear Implants 2. Speech processor – which selects and arranges sounds picked up by microphone 3. Transmitter/ Receiver – receive signals from the speech processor and convert them into electric impulses 4. Electrode array – group of electrodes that collects the impulses from the stimulator and sends them to different regions of the auditory nerve. 3. Somatosensory System: Touch and Pain - Sensations from your body are called somatosensations - The somatosensory system is made up of three separate but interacting systems: 1. Exteroceptive system – senses external stimuli that are applied to the skin 2. Proprioceptive system – which monitors information about the position of the body that comes from receptors in the muscles, joints, and organs of balance 3. Interoceptive system – general information about conditions within the body Skin Receptors - This chapter will deal with the exteroceptive system. - The simplest cutaneous/ skin receptors are free nerve endings (neuron endings with no specialized structures on them), which are particularly sensitive to temperature change and pain. - Pacinian corpuscles are large and onionlike receptors that respond to sudden displacements of the skin but not to constant pressure. - Merkel’s discs and Ruffini endings both respond to gradual skin indentation and skin stretch respectively. - When you identify objects by touch (Stereognosis), you manipulate them in your hands so that the pattern of stimulation continually changes Dermatomes - Neural fibers that carry information from skin receptors and other somatosensory receptors gather together in nerves and enter the spinal cord via dorsal roots - Area of the body that is innervated by the left and right Two Major Somatosensory Pathways dorsal roots is called a dermatome and can be plotted on a map. - The dorsal-column medial lemniscus system carries information about touch and proprioception (knowing the presence of your own body parts) - The anterolateral system tends to carry information about pain and temperature Dorsal System 1. The sensory neurons of the dorsal column enter the spinal cord via a dorsal root, ascend ipsilaterally in the dorsal columns and synapse in the dorsal column nuclei 2. The axons decussate/ cross over and then ascend in the medial lemniscus to the contralateral ventral posterior nucleus of the thalamus 3. The ventral posterior nuclei also receives input from the three branches of the trigenimnal nerve which carries somatosensory information from contralateral areas of the face. Anterolateral system 1. Most dorsal root neurons of the anterolateral system synapse as soon as they enter the spinal cord 2. Second order neurons decussate/cross over but then ascend to the brain in the contralateral/opposite side. However, some do not decussate but ascend to the brain on the ipsilateral side. 3. There are 3 tracts 1. Spinothalamic tract – projects to the ventral posterior nucleus of the thalamus 2. Spinoreticular tract – projects to reticular formation 3. Spinotectal tract – projects to the tectum 4. Three branches of the trigeminal nerve carry the pain and temperature information from the face. - Lesions to ventral posterior nuclei, which receive input from spinalthalamic tract and dorsal-column system produce loss of skin sensitivity to touch, temperature change and sharp pain - Lesions of parafascicular and intralaminar tract receive input from spinoreticular tract reduced deep chronic pain without disrupting cutaneous sensitivity. Cortical Areas and Homunculus Penfield - Penfield applied electrical stimulation to various sites on the cortex and patients described what they felt. - Discovered that human primary somatosensory cortex(SI) was somatotopic – organized as a map of the body surface - The greatest proportion of SI is dedicated to receiving input from the parts of the body that we use to make tactile discriminations (hands, lips, tongue) whereas a small areas of SI receives input from large body areas like the back. - A second somatotopically organized area SII is ventral to SI and extends into lateral fissure, receiving most input from SI - SII also receives substantial input from both sides of the body and the output of SI and SII goes to the association cortex of the posterior parietal lobe. - Primary somatosensory cortex is composed of 4 functional strips, each with a similar but separate somatotopic organization. - If one were to record from neurons in a horizontal line across the four strips, one would find neurons that preferred 4 different kinds of tactile stimulation, all to the same part of the body. - As one moves from anterior to posterior, the preferences of the neurons would tend to become more complex and specific, suggesting an anterior-posterior hierarchical organization. Q: Clarify this left hand thing? - There have been 2 steams of perception that projects to posterior parietal cortex and participates in multisensory integration and direction of attention and a ventral stream that project to SII and participates in perception of objects’ shapes. - Somatosensory signals are ultimately conducted to the Effects of Damage to the Primary highest level for the sensory hierarchy, to areas of Somatosensory Cortex association cortex in prefrontal and posterior parietal cortex. - The posterior parietal cortex has bimodal neurons that respond to both somatosensory and visual stimuli. - The visual and somatosensory receptive fields are spatially related – if a neuron has a somatosensory receptive field Somatosensory Agnosia centered in the left hand, its visual field is adjacent to the left hand. - Damage to primary somatosensory cortex are mild because this system has numerous parallel pathways - In a unilateral excision that included SI, the patients displayed 2 minor contralateral deficits: a reduced ability to detect light touch and a reduced ability to identify objects by touch. - There are 2 major types: 1