Chapter 7 - Perception and Attention

Auditory System

Functions to perceive sound, which is vibration of air molecules that stimulate the auditory system

-Can only hear between frequencies of 20-20,000 hertz

-Amplitude, Frequency and Complexity of molecular vibrations are linked to Loudness, Pitch and Timbre

Pure Tones/Simple soundwave - Exist only in laboratories and sound recording studios

Rich Tones/Complex soundwave - Found in nature, consists of many pure tones

Fourier Analysis - Breaks down complex waves into their pure tone components, likely performed by the auditory system

The Ear: Tympanic Membrane and Ossicles

Sound waves (vibrating air molecules) travel from the outer ear into the auditory canal;

Tympanic Membrane - Ear drum, vibrates in response to soundwaves at their frequency, transfers vibration to the Ossicles

Ossicles - Small bones of the middle ear, the Malleus (hammer), Incus (anvil) and Stapes (stirrup), vibrations then trigger the Oval Window

Oval Window - Membrane that transfers vibrations from the Ossicles to the fluid of the Cochlea which also vibrates

The Ear: Cochlea

Long, coiled (snail-shaped) tube with internal structure (Organ of Corti)

-Filled with Cochlear Fluid

The Ear: Organ of Corti

Auditory receptor organ, pressure change at the oval window travels along the organ of Corti as a wave, organ composed of:

Hair Cells - Receptors of the auditory system

Basilar Membrane - Membrane of the organ of Corti where hair cell receptors are embedded

Reisner’s Membrane - Helps divide the inside of the cochlea into 3 compartments (alongside the basiliar membrane)

Tectorial Membrane - Cochlear membrane that rests on the hair cells

Hair Cell Activation

Deflection of the organ of Corti at any point produces a shearing force on hair cells at the same point

-Hair cells have hair-like projections (Stereocilia)

-Vibration in the fluid causes the basiliar membrane to move upward, lifting the hair cells

-When the hair cells push up, the stereocilia bend in one direction the other component of the wave then pulls the cells down, stereocilia then bind in the opposite direction

-Bending of the stereocilia, stimulates hair cells, increasing firing in axons of the auditory nerve

Round Window

Vibrations of the cochlear fluid are dissipated by the round window, elastic membrane in the cochlear wave

Place Coding

Place Coding:

-Higher frequencies produce greater activation of hair cells (better bending) closer to the windows, near the base of the basiliar membrane, neurons that respond best to high frequencies soundwaves are located here

-Lower frequencies produce greater activation (better bending) at the tip of the basilar membrane, neurons that respond best to low frequencies soundwaves are located here

Ear to Primary Auditory Cortex

Auditory nerve → Cochlear Nucleus - > Superior Olives → Inferior Colliculi → Medial Geniculate Nucleus → Primary Auditory Cortex

Sound Localization Mechanisms

Medial Superior Olive - Involved in responding to the differences in the time of arrival of auditory signals in both ears

• Ex; Speaker facing towards your right ear, the time difference between auditory signal arriving in left and right ear (faster time arrival = sound is closer to certain ear)

Lateral Superior Olive - Responds to differences in the amplitude of the sound waves at both ears to determine where the sound is situated

• Ex; Speaker facing 3 metres away from your left ear, as soundwaves travel their energy (amplitude) decreases (but amplitude is reduced by the same amount), but they decrease less when traveling to the left ear than the right ear

Superior Colliculus - Deepest layer (directly above inferior colliculus) has organization according to the map of 3D auditory space around us, purpose is to locate the source of auditory input in the 3D space around us

Cochlear Nuclei and Superior Olives

Axons of each auditory nerve synapse in the ipsilateral cochlear nuclei

-Many projections lead to the superior olives on both sides of the brain stem at the same level

Superior Olives - Medullary nuclei that play a role in sound localization

Inferior Colliculi and Medial Geniculate Nuclei

Axons of the olivary neurons project via the Lateral Lemniscus to the Inferior Colliculi - Structure of the Tectum that receives auditory input (from the superior olives)

Medial Geniculate Nuclei - auditory thalamic nuclei, receives input from inferior colliculi and project to primary auditory cortex

Primary Auditory Cortex

Receives majority of input from Medial Geniculate Nucleus, is located in the temporal lobe inside the lateral fissure, comprised of three adjacent areas (core region)

-Primary auditory cortex is organized in functional columns, neurons are organized into groups that respond optimally to sounds in the same frequency range

-Auditory cortex has tonotopic organization; each area of the auditory cortex has gradient of frequencies from low to high along its length

-Adjacent auditory neurons (hair cells) activate adjacent neurons in the primary auditory cortex, and are organized based on frequencies they respond to (high frequency to low frequency)

-Some neurons in auditory cortex respond best to natural sounds (rich tones) and respond weakly to simple sounds (pure tones)

Secondary Auditory Cortex

Surrounds the core region of the Primary Auditory Cortex with a band (called the belt area), areas of the secondary auditory cortex outside the belt are called Parabelt Areas

-Neurons in the belt/parabelt regions (in monkeys) respond best to complex tones (rich tones like monkey calls)

Anterior and Posterior Auditory Pathways

Anterior auditory pathway (towards prefrontal cortex) - involved in identifying sounds, “what” pathway

Posterior auditory pathway (towards posterior parietal cortex) - involved in locating sounds, “where” pathway

Somatosensory Receptor Function

Each is specialized for a different function, tend to function in the same way:

-Stimuli applied to the skin deform or change the chemistry of the receptor, results in changes to permeability of receptor cell membrane to various ions (resulting in a neural signal)

-Each sensation is produced by interaction of multiple receptor mechanisms, and each receptor mechanism contributes to multiple sensations

Exteroceptive Systems

Mediates three types of external stimulation:

1. Mechanical - Sensation of touch

2. Thermal - Sensation of temperature

3. Nociceptive - Sensation of pain

-Each type of stimulation are conveyed by 4 types of sensory receptors in the skin: free nerve endings, pacinian corpuscles, merkel’s disks, ruffini endings

Stimulation → Deformation of Receptors (mechanical or chemical change) → Permeability (cell membrane more/less permeable to an ions) → AP

Cutaneous Receptors

Skin receptors, simple cutaneous receptors contain free nerve endings (neuron endings with no specialized structures), sensitive to temperature change and pain

-Largest and deepest cutaneous receptors are Pacinian Corpuscles, adapt rapidly and respond to sudden displacement of the skin but not constant pressure

Merkel’s Disks adapt slowly, respond to gradual skin indentation

Ruffini Endings adapt slowly, respond to gradual skin stretch

Dermatomes - Peripheral Pathways

-Sensory nerves in the anterior portion will enter spinal cord at the top level (cervical)

-Sensory nerves in the middle regions will enter spinal cord in its middle portion (thoracic)

-Sensory nerves in the anterior portion of the bottom will enter spinal cord at the Lumber region

-Sensory nerves in the posterior portion of the bottom will enter spinal cord at the saccral

These pathways are PERIPHERAL pathways

Dermatome - Specific area of skin supplied by sensory nerves that head to a single spinal nerve root, overlap with each other (T2, T3, T4 dermatomes overlapping)

Dr. Wilder Penfield

Studied the Central Pathways by electrically stimulated the different regions of the postcentral gyrus, patients reported feeling skin sensations in their body

-He was able to map out the Primary Somatosensory Cortex (S1), discovered it was organized according to the bodily map

-Cortical area dedicated to analyzing sensory signals of parts used for tactile discrimination (hands, feet) is far greater

Dorsal-Column Medial Lemniscus System (DCMLS)

Carries sensations of touch and proprioception

-Sensory neurons enter the spinal cord at the dorsal root, ascent ipsilaterally into the Dorsal Columns, which are somatosensory tracts that ascent in the dorsal portion of Spinal Cord white matter

-Axons of dorsal column nuclei neurons decussate (cross over), then ascend in the medial lemniscus (pathway between dorsal column nuclei and ventral posterior nucleus of the thalamus)

-It then moves to the contralateral ventral posterior nucleus of the thalamus, which also receives input through three branches of the trigeminal nerve that carry somatosensory information from contralateral areas of the face

-Neurons then project to the Primary Somatosensory Cortex (SI) or Secondary Somatosensory Cortex (SII) or the posterior parietal cortex

Anterolateral System

Conducts sensations of pain and temperature

Most dorsal root neurons of the system synapse once they enter the spinal cord

-axons of second-order neurons decussate, then ascend to the brain in the contralateral anterolateral portion of the spinal cord, but some continue to ascend ipsilaterally

Three Tracts:

-Spinothalamc tract

-Spinoreticular tract

-Spinotectal tract

-The three branches of the trigeminal nerve carry pain/temperature information from the face to the same-thalamic sites, it then reaches the thalamus and is distributed to somatosensory cortex/other associated parts

Primary Somatosensory Cortex (SI)

Primary somatosensory cortex is organized like a map of the body surface (somatotopic);

-Greatest proportion of SI is dedicated to receiving input from parts of the body used to make tactile discriminations (hands, lips, tongues)

-Only small areas of SI receive input from large areas of the body (the back, etc)

-Largely contralateral (receives information from one side)

-Output of SI goes to association cortex of the posterior parietal lobe

Secondary Somatosensory Cortex

Receives most of its input from the SI, lies just ventral to SI in the post-central gyrus and extends into the lateral fissure

-Receives substantial input from both sides of the body (unlike SI)

-Output of SII goes to association cortex of hte posterior parietal lobe

Columnar Organization in Primary Somatosensory Cortex (SI)

Similar to columns in visual/auditory cortices;

-Neurons had a receptive field on the same part of the body, and responded robustly to same type of tactile stimuli (light touch, heat)

-Primary somatosensory cortex has 4 functional strips, each with similar (but separate) somatotopic organization, each strip is sensitive to different type of inputs (light touch, pressure)

Streams Proceeding from the SI (Primary Somatosensory Cortex)

Dorsal Stream - Projects to posterior parietal cortex and association cortex, involved in multisensory integration (integrating auditory, visual, etc sensory info) and direction of our attention

Ventral Stream - Projects to SII, participates in perception of object’s shapes

Association Cortex in Somatosensory System

Somatosensory signals conducted from highest level of sensory hierarchy (SI, SII) to association cortex in prefrontal/parietal cortex

-Posterior Parietal Cortex has bimodal neurons that respond to somatosensory and visual stimuli, receptive fields of each neuron is spatially related (ex; neuron has somatosensory receptive field centered in the left hand, and its visual field adjacent to the left hand)

Pain

Unpleasant sensory and emotional experience associated with or resembling that associated with, actual or potential tissue damage

Paradoxes of Pain

1. Seems bad, but is essential in survival

2. No clear locus of pain (not one specific region associated with pain), painful stimuli activate many areas of the cortex (thalamus, SI, SII, anterior cingulate cortex), patients with one hemisphere removed can still perceive pain from both sides of the body

• Anterior Cingulate Cortex is most linked to pain, associated with expectation to pain, emotional reaction to pain, and adaptive responses to pain

3. Pain is modulated by Cognition and Emotion (influence of religious ceremonies, wounds suffered by soldiers aren’t painful until out of the threat)

Pain-Control Circuit

Blocks pain signals from reaching the brain

Periaqueductal gray has pain-blocking (analgesic) effects due to opioid receptors activated by the body’s endogenous opioids (endorphins) → Raphe Nuclei, has cell bodies of neurons → Activates spinal inhibitory interneurons → Inhibit incoming pain signals

-PAG and other areas of the brain have specialized receptors for opioid analgesic drugs (like morphine)

Neuropathic Pain

Severe chronic pain in absence of recognizable pain stimulus, typically develops after injury and persists even when injury heals

-Can be triggered by an innocuous stimulus

After an injury, microglia produce neuroplastic changes that produce pain after the injury has healed

-Another factory is the individual’s epigenetics (past experience influencing genetic mechanisms)

Olfaction and Gustation

Chemcial senses, monitor the chemical content of the environment

-Smell is due to airborne chemicals drawn through inhalation via receptors in nasal passages

-Taste is response of gustatory system to chemicals in solution in the oral cavity

Pheromones

Chemicals that influence the physiology and behaviour of conspecifics (members of the same species)

-Ex; sexual/aggressive behaviours in hamsters is due to pheromones

Olfactory System

Olfactory Mucosa - Layer of mucus-covered tissues where olfactory receptor cells are embedded in

-Dendrites of olfactory receptor cells are located in the nasal passages

-Axons pass through the porous portion of the skull (cribriform plate), enter the olfactory bulbs

Olfactory Bulb - Location where axons of olfactory receptor cells synapse on neurons that project via the olfactory tracts to the brain (amygdala and piriform cortex)

Damage to Primary Somatosensory Cortex

-Damage to SI results in mild affects, due to numerous parallel pathways in the SI (general)

Reduced ability to detect light touch and reduced ability to identify objects by touch due to unilateral lesions on the SII (not SI)

Types of Olfactory Receptors

Humans have 300 different kinds of olfactory receptor proteins, each receptor cell contains only one type of protein molecule

-Olfactory receptor proteins are in the membrane of the dendrites, and are stimulated by circulating airborne chemicals in the nasal passage

-All types of olfactory receptors are scattered throughout the mucosa, no clear organization

-Odors are encoded through pattern of activity across different receptor types

Vestibular System

Carries information about direction and intensity of head movements, maintaining our balance

Semicircular Canals - Receptive organs of the vestibular system

Astereognosia and Asmatognosia

Major types of somatosensory agnosia;

Astereognosia - inability to recognize objects by touch

Asomatognosia - failure to recognize parts of one’s own body, usually unilateral (affecting only left side of body) due to damage to right temporal/posterior parietal lobes

-Often accompanied by anosognosia, failure to recognize your own symptoms in neuropsychological patients

-A component of contralateral neglect (tendency to not respond to stimuli that are contralateral to a right-hemisphere injury)

Rubber Hand Illusion

Feeling that an extraneous object (rubber hand) is part of one’s own body

-Generated by a participant’s hand being hidden by a screen, and a rubber hand placed next to the hidden hand but in clear sight

-Experimenter repeatedly strokes the hidden hand and the rubber hand at the same time, participant eventually feels like the rubber hand is part of their own body (temperature in hidden hand also drops)

May be due to frontal and parietal bimodal neurons with their two visual and somatosensory fields

Olfactory Golmeruli

Discrete clusters of neurons that lie near the surface of the olfactory bulb, and is where axons of olfactory receptors terminate

-Each glomerulus receives input from thousands of olfactory receptor cells with the same receptor protein

Systematic Layout of Glomeruli (Chemotopic)

-Mirror symmetry between left and right olfactory bulbs (glomeruli sensitive to an odor are located at the same sites of the two bulbs)

-Glomeruli sensitive to particular odors are arrayed on the olfactory bulbs in the same way in different members of the same species

-Layout of glomeruli is similar in related species

Piriform Cortex

Located in the medial temporal cortex, location where olfactory bulb projects axons

-Axons also project towards the amygdala (in the medial temporal lobe)

-often considered the “primary olfactory cortex”, but the olfactory system is the only system that reaches the cerebral cortex without passing through the thalamus

Major Olfactory Pathways

-Limbic Pathway projects to the limbic system, mediates emotional response to odors (ie, amygdala, etc)

-Thalamic-Orbitofrontal Pathway projects via the medial dorsal nuclei of the thalamus to the orbitofrontal cortex (area of cortex on the inferior surface of the frontal lobes, next to the eye sockets)

-thalamic-orbitofrontal pathway mediates conscious perception of odors

Gustatory System

Taste receptor cells occur in clusters of 50-100 called tastes buds

-Located around papillae, small protuberances on the surface of the tongue

-Receptors cells composed of three types

1. Cells that detect bitter, sweet and umami

2. Cells that detect sour

3. Cells that detect salty

-In each tastebud, only one receptor cell (presynaptic cell) synapses onto the neuron carrying signals away from the bud (communication between the other cells is done via gap junctions)

Taste Transduction

Taste transduction for sweet, umami and bitter is mediated by metabotropic receptors; two receptors types for sweet, one for umami, and 25 for bitter

Taste transduction for salty and sour is mediated by ionotropic receptors; two receptor types for salty and 3 for sour

Pathway in the Gustatory System

Gustatory afferences leave mouth via the Facial (VII) nerve (brings information from the front of the tongue) → Glossopharyngeal (IX) (brings information from the back of the tongue) and Vagus (X) (brings information from the back of the oral cavity) cranial nerves

-Fibers all terminate in the Solitary Nucleus of the medulla, synapse on neurons that project to the ventral posterior nucleus of the thalamus

-Gustatory axons of the VPN project to the primary gustatory cortex, cortex is located in the lateral fissure (insula)

-Different parts of the primary gustatory cortex represents taste, secondary gustatory cortex is located in the orbitofrontal cortex

-Projections of gustatory system are ipsilateral (unlike the other sensory systems)

Anosmia and Ageusia

Anosmia - Inability to smell

-Often due to displacement of the brain within the skull that shears the olfactory nerves where the pass through the cribriform plate

-Less complete deficits in smell due to Alzheimer’s disease, down syndrome, epilepsy, MS, korsakoff’s syndrome, and parkinson’s disease

Ageusia - Inability to taste