Lecture 3/4: The Somatosensory System Flashcards
What is a sensation? What are the 5 senses?
Sensation
* Sensation entails the ability to transduce, encode, and ultimately perceive information generated by stimuli arising from both external and internal environments.
* Transduction: currency of the nervous system is Action Potential. Ttransfer the energy from light, sound-wave, mechanical touch to electrical energy (AP). You have ion channels that make the AP happen (Na and K). Protein channels that work with tthe energy and transduce it to mechanical energy.
Five basic senses:
* Somatic – pressure, temperature, vibration and pain
* Vision - light waves (opsins convert the light energy to electrical action potential)
* Audition - sound waves
* Taste- chemical
* Smell or Chemical senses
* Sixth sense - proprioception (sense of where your body is).
General info about somatic sensory system
All senses provide very different information. But they follow similar basic rules for sensation:
* Specialized cells (receptors) convert energy (mechanical forces – light) into afferent sensory signals – conveys information to the brain.
* Signals convey information about:
- Modality (touch versus pain: type of touch - sharp versus dull)
- Where it is (location)
- Intensity
- Time Course (sustained, temporary, gradual)
* Understanding deficits in sensory processing is very important in diagnosing various neurological problems.
Learning guide for sensory system - what do you need to know
- Mechanism of signal transduction?
(how does the receptor transform information into neural electrical signals?)
*Anatomical/synaptic pathway to the cortex?
(How does it get from the receptor to the cortex?) - Mapping rules represented?
(What is represented and how is it organized?)
Somatic sensory system
Mediates a range of sensations e.g. touch, pressure, limb position, temperature, pain
Three sub-systems
1) Fine touch (discriminative touch) , vibration, pressure
* Cutaneous mechanoreceptors
2) Proprioception: sense of relative position of our body parts in space
* Specialized receptors associated with muscles, tendons & joints (they sense force and movement)
3) Temperature, Pain and nondiscriminative (sensual) touch (covered in other courses)
Transmission of somatic sensory information
- Sensory information goes from nerve ending which can be in muscle, tendons or joints.
- They reach the axon of you afferent nerve fibers where the cell body are located in dorsal ganglia.(S1)
- If it is for the limb it will be for dorsal root ganglia, if it is for head or neck it will be trigeminal ganglia (body of sensory info for head and neck.
- From there reach to the CNS.
Red= mechanoreceptors and proprioceptors. (the ones we will talk about).
Internal structure of spinal cord
- The peripheral nerves that innervate much of the body arise from the spinal nerves (sensory afferent AND motor – efferent)
- Sensory information carried by afferent axons of the spinal nerves enters the cord via the dorsal roots
Transmission of somatic sensory info
Cell bodies of afferent nerve fibers are located in ganglia adjacent to spinal cord & brain stem
* Dorsal root ganglia : body
* Trigeminal ganglia: head
* Neurons of dorsal root ganglia are** Pseudounipolar – no synapse before entering the spinal cord!!!**
* The first synaptic terminals within the grey matter of the spinal cord.
* These PNS neurons supply the CNS with information about sensory events in the periphery
Bipolar neurons vs pseudounipolar neurons
Bipolar neuron
* Axon
* Dendrites
* Passes through the cell body
Pseudounipolar neurons
* One axon with two branches, no true dendrites
* Central : cell body to spinal cord
* Peripheral: cell body to periphery
* Action Potential does not need to go through cell body goes from one axon to the next.
*this allows them to send information much faster than bipolar neurons
Pseudounipolar sensory neurons
Found in dorsal root ganglia
* Cell body in DRG
* Axon exits DRG , splits into 2 branches
* Central branch to dorsal horn of spinal cord
* Peripheral branch travels through the spinal nerve then to skin, joint, muscle (proprioreceptors)
* Also found in sensory ganglia of cranial nerves
Sensory transduction
Sensory transduction converts energy from a stimulus into an electrical signal.
* Sensory stimulus produces a depolarizing current in the afferent nerve endings called a receptor potential
* Upon reaching a threshold, action potentials are generated in the afferent fiber
* APs then travel along the peripheral axon past the cell body in the dorsal root ganglion & along the central axon to reach the synaptic terminals in spinal cord
Afferent fiber terminals
Afferent fiber terminals can be:
1) the ending of the nerve has cells: Encapsulated by specialized receptor cells ‘mechanoreceptors’, which usually:
* special encapsulation of nerve ending help it tune in to a particular features (make it response better to a special case). Detect if the touching is transient or sustained (dynamic or static).
* Lower threshold –more sensitive.
2) Free Terminals (pain):
* Higher threshold - do not have any special organ to detect them
* Same stimuli in higher intensities will produce ‘pain’.
Sensory transduction - converting physical stimuli to electrical signals
- The same basic mechanisms mediate sensory transduction in all somatic sensory afferents
- Cells that are sensitive to a stretch (mechanical deformation) - this is touch basically
- A stimulus changes the permeability of cation channels in the afferent nerve endings
- This generates a ‘receptor potential’ i.e. a depolarizing current
- These receptors are Piezo1 and Piezo 2. They were found recently (2010). They are mechanical recpetors that respond to mechanical deformation and produce AP. Only the nerve ending needs this characetirstic, the rest of the neuron can be the same
- If the stimulus is sufficient, the receptor potential reaches the threshold to generate an action potential in the afferent fiber.
- The rate of action potential firing is proportional to the magnitude of depolarisation.The rate of the AP tells you about the intensity of the touch.
- Because somatic sensory neurons are pseudounipolar, the electrical activity does not need to be conducted through the cell body membrane, but rather travels along the continuous peripheral and central axon
Specialization of somatic sensory afferents
- Distinct functional properties of somatic sensory afferents define distinct classes of afferents with specialized mechanoreceptors which convey unique sensory information
- Sensory afferents often encapsulated by specialized receptor cells that tune the fiber to specific stimulation
- Free nerve endings are important in pain sensation
- Receptor fields: closer to the skin = smaller receptor fields. Deeper in skin = bigger receptive field.
What are the key properties that characterize sensory afferents
- Axon diameter: bigger diameter = faster the sensory info can travel. Proprioception = want sensory info to travel fast. Pain does not have to reach the fastest because it is very intense so we dont want to be very fast.
- Receptive field: area that nerve respond. finger will have smallest receptor fields to allow us for finer touch, we can discriminate better = higher resolution.
- Temporal dynamics
- Quality of somatic sensory stimulation
Axon diameter
Axon diameter determines speed of conduction of action potential (larger, faster)
* we need some information to reach faster to the CNS because we need them for ongoing movements.
* bigger axon diameter = faster conduction
What is the receptor field
- The receptive field of a sensory afferent is the area of skin surface over which stimulation results in a significant change in the rate of action potential
- Receptive field size varies in different parts of the body
- Receptor field is defined by:
1) how much this nerve ending has branches. Do the dendrites have branches.
2) Smaller arbirizations = smaller receptive field.
The most fine tuned movements we want to do as humans are?
- Eye movement
- Using our hands
- Speech (tongue, lips.. all need to get information fast)
What determines the size of the receptive field
- The size of a receptive field i largely determined by:
1) The branching of the sensory afferents in the skin
Smaller arborization → smaller receptive field
2) Density of afferent innervation
More afferents→ smaller receptive field - You want a smaller receptive field for fine touch of fingers. That means we want less arborization of those dendrites. Now that you have a smaller receptive field, to cover all the area you need more density of afferent inervations. That means you need to dedicate more neurons to that specific part of skin because you want to cover the skin.
- more density of afferent inervations. More inervations = smaller receptive field.
How do you determine the size of the receptive field?
- Receptive field size determines spatial accuracy with which tactile stimulation can be sensed
- Two-point discrimination measures the minimum distance between two simultaneously applied stimuli that is perceived as two distinct stimuli.
- take two different stimuli (example: needles) and start by putting them right next to each other. Move them away from each other until the person can detect the two needles as two different stimuli. When they are close together, the person cannot detect that there are two needles. - Discrimination varies dramatically → it will be different for different body parts
*fingertips : 2mm - Forearm: 40mm (distance between two needles has to be 40mm for the person to detect as two different stimuli.
Two point discrimination threshold
- Two-point discrimination varies throughout the body
- Somatic acuity is much higher in fingers, toes and face than
in arms, legs, torso - This is the result of differences in receptive field size
- Species specific!
Fingertips (lowest), lips, head
Temporal Dynamics
- Sensory afferents respond to the same stimulus with different temporal dynamics
- You might have some sensory neurons that respond to only the dynamic part of that touch: which means when it is introduced and when it is removed (rapidly adapting neurons).
- Rapidly adapting afferents
- fire upon the initiation of stimulation
- quickly become quiescent if stimulation is maintained
- may fire again on termination - Slowly adapting afferents continue to fire with sustained stimulation. They increase their activity from background and start to firing.
- Some neurons are always firing = tonic activity. For these, when you have a stimuli you may inhibit them or reduce their activity.
- Some neurons may not have tonic activity = they are silent.
- Rapidly adapting afferents may be important for conveying information about changes in ongoing stimulation e.g. movement
- Slowly adapting afferents may convey information about spatial attributes of a stimulus e.g. size, shape
- Adaptation characteristics are determined in part by properties of mechanoreceptors
- The receptors that are closer to the skin will have a smaller receptor field. The deeper ones will have a larger receptive field.
- We can have a dynamic (fires when there is introduction and removal of stimuli) or static response (respond to the sustained touch).
- Meissner Corpuscle: dynamic
- Merkel Cells: static
- Pacinian corpuscle: dynamic
- Ruffini endings: static
Mechanoreceptors afferent for touch
- Cutaneous sensation is best understood for glabrous skin (i.e. palm, fingertips)
- Experiments recording from individual sensory afferents in nerves have identified the specific contributions of 4 distinct mechanoreceptors to somatic sensation
- Merkel cell-neurite complex (only merkel cells are not encapsulated, they are in epidermis)
- Meissner corpuscle (in epidermis)
- Pacinian corpuscle
- Ruffini corpuscle (deeper layer)
What is a mechanoreceptor?
- A mechanoreceptor is any receptor that provides an organism with information about mechanical changes in its environment
- Virtually all mechanoreceptors have specialized end organs surrounding the nerve terminal
- The sensitivity to mechanical displacement is a property of both the nerve terminal membrane and the specialized capsule
- Controversy as to whether these structures are neuronal or not
Different kinds of mechanoreceptors have distinct
properties and convey distinct sensory information
What kind of information do they transfer and what kind of information can we get from them.
* Receptor field: merkel, meissner = very small, others are much bigger
* what type of stimuli: sustained = merkel (the ones that help you detect the size of objects)
* Meissner = dynamic. motion detection and grip control
* Pacinian = detect vibration. use for toolss
* Ruffini = direction of the motion that you can detect.
* you need both fine tuned information and very rough idea aswell to have a better general info of the sensory.
Merkel Cell afferents
- Slow adapting- static aspect of touch
- 25% of mechanosensory afferents in hand
- Highest spatial resolution of all sensory afferents
- Enriched in fingertips
- Form Merkel cell-neurite complexes
- Both Merkel cells and the sensory afferents express Piezo2
- Highly sensitive to points, edges & curvature and suited to processing information about form & texture
- Only afferents sampling information from the epidermis
Meissner Corpuscles
- Express Piezo2
- Rapidly adapting
- High spatial resolution (small receptive field)
- 40% of mechanosensors in hand
- Skin indentation deforms the corpuscle to
trigger receptor potentials - Removal of stimulus relaxes the corpuscle to resting position also generating receptor potentials
- Meissner corpuscles are formed by connective tissue capsule of flattened cells derived from Schwann cells with the center of each capsule containing 2-6 afferent nerve fibers
In comparison to Merkel:
* Closer to skin surface
* More sensitive to skin deformation
* Larger receptive field → reduced spatial resolution
* Sensitive to vibration of objects moving across skin
* Detect slippage between the skin and an object held in the hand, essential feedback information for the efficient
control of grip.
Pacinian afferents
Pacinian afferents
* ** Rapidly adapting**
* 10-15% of mechanosensors in hand
* Pacinian corpuscles located deep in dermis or subcutaneous tissue.Concentric layers of membranes
around a single fiber (like an onion)
* Laminar structure of Pacinian corpuscle filters out all but high frequencies. Only high frequencies are transferred (this is why they are good for tools - high frequency vibration received)
* Lower response threshold than Meissner corpuscles
* Can respond to skin displacements as small as 10 nm
* Large receptive fields
* Detect vibrations transmitted through objects in touch with the hand. Capsule help the nerve ending to tune it to specific features. Tune it to high frequencies. Also help detect smallest changes. Much more sensitive than any other one we talked about - more sensitive to movement.
* May be important in tool use
Ruffini afferents
- Slow adapting and they are found in the hairy part of skin
- 20% of mechanoreceptors in hand
- Ruffini corpuscles are elongated, spindle-shaped capsules in dermis and also found in ligaments and tendons
- Long axis of corpuscle lies parallel to stretch lines in skin making them sensitive to cutaneous stretching with digit or limb movement
- Contribute, along with muscle receptors, to sensation of finger position, and hand conformation
Experiment that showed the Specialization of touch mechanoreceptor
The different kinds of information that sensory afferents convey to central structures were first illustrated in experiments conducted by K. O.
Johnson and colleagues, who compared the responses of different afferents as fingertip was moved across a row of raised Braille letters
* Person ran their finger along brail and then they record all the sensory to see how the information was transferred.
* You can see that you can almost detect the dots with the merkel cells (sustained part).
* can see speed of fingers move.
Receptors in hairy cells
- the only difference is that here, you have merkel cells that are close to the hair dome. Merkel cells that respond to the sustained part of the stimuli.
- The merkel cells that are close to the hair dome respond differently.
- If you are stroking skin or there is a breeze, the merkel cells at the hair dome will respond.
- Otherwise it is mostly the merkel cells at glaborous skin that respond (touch).
Mechanoreceptors specialized for proprioception: Muscle spindles
Muscle spindles
* Found in striated (skeletal) muscle
* They are parallel to your extrafusal muscles (the ones that produce force).
* Consist of 4-8 specialized intrafusal muscle fibers surrounded by capsule of connective tissue, distributed in parallel with extrafusal fibers
Mechanoreceptors specialized for
proprioception: sensory afferents
look at notes on ipad!!
Sensory afferents
* The sensory afferent coil around the central part of the intrafusal spindle and are able to detect the rate of change of muscle length.
* When the muscle is stretched, tension of the intrafusal fibers activates mechanically gated ion channels, triggering action potentials.
* They only respond when their muscle muscle spindle stretches. When the opposing muscle (agonist) contracts.
* Muscle spindle is innervated by two types of fibers:
- Primary endings (Group Ia afferent axons)
- Secondary endings (Group II afferent axons)
Mechanoreceptors specialized for proprioception: Primary endings, secondary endings
Primary endings
* Group Ia afferents (the largest myelinated sensory axons)
* Rapidly adapting responses to changes in muscle length
* Transmit information about limb dynamics
- detect sudden changes of movements — onset and offset of movements
Secondary endings
* Group II afferents
* Sustained responses to constant muscle lengths
* Information about static limb position
* slow adapting
- Also innervated by efferent 𝛾 motor neurons in the ventral horn of the spinal cord, which change intrafusal fiber tension (how much stretch is allowed) and increase sensitivity of the afferents to changes in muscle length.
mechanoreceptors specialized for proprioception: density of spindles
Density of spindles in human muscle varies with function
* Muscles that generate coarse movement have fewer spindles than muscles that generate very fine movements (need more information)
* More muscle spindles = more rich information (more info). Ex: less muscle fiber for legs rather than hands
* More precise movement requires more refine sensory input (eyes, hand, neck).
- eyes have lots of muscle spindles
Illusionary movements
- Artificial stimulation of spindles by vibration produces sensory illusion of altered limb position in stationery limbs
- Illusion only produced if visual input is prevented (when you don’t have vision, eyes closed). It will feel like you are flexing your muscles but you actually are not. If the vibrations are strong enough it will feel like your hand is somewhere that is is not actually.
- Whenever there is a conflict between visual and somatic, you trust the vision. We are visual animals → vision is the primary sense we will use.
- In normal conditions, proprioception is achieved by integration of somatic & visual cues
- Joint angle perception arise from the integration of afferent signals from muscle spindles and efferent motor commands.
Mechanoreceptors specialized for
proprioception: golgi tendon organ
Golgi tendon organs
* Arranged in series with extrafusal muscle fibres
*Low-threshold mechanoreceptors in tendons
* Sense changes in muscle tension
* Distributed along collagen fibers that form tendons
* Innervated by branches of group Ib afferents
* Contribute less to conscious sensation of muscle activity.
* Important role in reflex circuits protecting the muscle from injury.
* at the junction of muscle and bone
Central patwhways conveying tactile information
- Tactile afferents, enter through dorsal horn of spinal cord
- The MAIN ascending branches (direct projections) extend ipsilaterally through the dorsal columns (also called the posterior funiculi) of the cord to the lower medulla, where they synapse on neurons in the dorsal column nuclei
- Information goes through dorsal root and will reach your spinal cord.
- Related to both sense of touch (mechanorecptors) and proprioception.
How does the information reach your cerebellum?
- Similar to tactile afferents, proprioceptive afferents enter through dorsal horn of spinal cord
- However, many fibres then bifurcate to form both ascending and descending branches , collaterals synapse on neurons of the dorsal and ventral horn
-
Proprioceptive information also reaches cerebellum where it is required in control of voluntary movement (touch information does not reach the cerebellum→touch not needed for movement). It reaches the cerebellum through the:
- Spinocerebellar tract: First order neuron (from dorsal root ganglion) collaterals from lower body synapse in Clarke’s nucleus. Neurons in Clarke’s nucleus send their axons via the dorsal spinocerebellar tract to the cerebellum, with a collateral to the dorsal column nuclei.
- Proprioceptive afferents for the upper body ascend via the dorsal column to the dorsal column nuclei; the cuneate nucleus, in turn, relays signals to the cerebellum.- dorsal root ganglion will reach to the cuneate nucleus. From there there will be a branch that will reach to cerebellum.
- lower body —> follows dorsal spinocerebellar tract via Clarke’s nucleus
- upper body —> similar to tactile pathway (external cuneate nucleus)
How does information reach the somatosensory area (S1) of the brain?
-
First order neuron - Ipsilateral (outside of the spinal cord)
Dorsal Root ganglion (has body of the first order neurons)(for lower body and upper body) / Trigeminal ganglion (for head and neck)
Dorsal column (ascend ipsilaterally)
- Cuneatus tract → for upper body
- Gracile tracts → for lower body
- Trigeminal tract → for head and neck
-
Second order neuron - Ipsilateral
Gracile, Cuneiform nuclei / Trigeminal nucleus
Second order neurons go through Medial Lemniscus // Trigeminal Lemniscus Axons cross the midline - the second order neurons cross the midline, travel up the medial lemniscus to the thalamus.
-
Third order neuron – Contralateral
Lateral nuclei in thalamus//medial nuclei in thalamus
Ventral posterior complex / Medial Thalamic and Parabrachial (nuclei in the thalamus)
Internal capsule - third order neurons get information from the thalamus and bring it to S1.
- Cerebral cortex – Primary somatosensory cortex –postcentral gyrus – parietal lobe // Anterior
cingulate and insula. Also, secondary somatosensory cortex.
Somatic Sensory in thalamus.
what happens when the sensory info reaches the thalamus?
Ascending somatic sensory pathways from the spinal cord and brainstem converge in the ventral posterior complex of the thalamus in a highly organized manner
* ventral posterior latteral nucleus = info from limb and body
* ventral posterior medial nucleus = info from neck and head
Afferents terminate in a somatotopic representation of the body & head
* VP lateral: relay from body (via medial lemniscus)
* VP medial: relays from face (via trigeminal lemniscus)
* Parallel pathways (body ad head) → all the information (proprioception, touch…) will be parallel, will not be mixed up, will be segregated.
- Inputs carrying different types of somato-sensory information terminate on separate populations of relay cells
- Information from distinct somatosensory receptor types remains segregated in passage to cortex
Primary somatic sensory cortex (S1)
- Most neurons from VP thalamus project to layer 4 of primary somatic sensory cortex (SI)
- SI is located in postcentral gyrus of the parietal lobe and
has 4 regions
- Brodmann’s area 3a, 3b, 1, & 2 - Each region contains a complete somatotopic map of the body in a medial to lateral arrangement → maps the areas of your body it represents.
Note on somatotopic maps in SI
- Foot, leg, trunk, forelimbs and face are represented in a medial to lateral arrangement.
- Do not represent the body in its actual proportions
- Bigger area (and number of neurons) for more richly innervated region
- Homunculus- Grossly enlarged representation of face & hands - The proportionality of representation reflects the neural circuitry required to govern the associated functions
- e.g. facial expressions, speech, manual manipulation of objects - Same representation on other side of the brain but for the motor cortex.
- This is species-specific, in a rat, facial whiskers are overrepresented, while raccoons overrepresent their paws and the platypus its bill in a naked mole rat, the teeth dominate!!!
What are the inputs in the thalamus?
Despite similar topography across SI areas, functional properties of neurons in each region are distinct, reflecting segregated, parallel inputs from VP thalamus
* Neurons in 3b & 1 respond primarily to cutaneous stimulation (touch information)
* Neurons in 3a respond primarily to proprioceptive stimulation
* Neurons in 2 respond to both tactile & proprioceptive stimuli (receives both)
* Area 3a and 3b are basically down the central sulcus. Area 1 and 2 on top of the gyri.
Segregation of neurons in SI
- Neurons in SI form functionally distinct columns
- For example, neurons with responses to rapidly and slowly adapting mechanoreceptors cluster into separate zones
within the representation of single finger - This modular organization is a fundamental feature of cortical organization but the functional significance is still being determined.
- They stay segregated even when they reach the SI, they will have seperate zones.
- in the representation of one single finger, the area for the fast adapting and slow adapting mechanoreceptors are completely segregated.
Schematic showing how information reaches the SI
- Functional hierarchy with 3b as an obligatory first step in cortical processing
- 3b receives largest input from VP thalamus & sends dense projections to 1 & 2 → majority of information reaches area 3b.
- Lesions of area 3b in non-human primates - deficits in all forms of tactile sensations mediated by cutaneous mechanoreceptors (severe case, not getting all the sensory ingormation)
- 3b also sends output to 1 and 2, so all this information reaches area 1 and 2 through 3b → 3b is so important for cutaneous sensation(lesion in 3b is most devastating).
- lesions limited to areas 1 or 2 - partial deficits to discriminate either the texture of objects (area 1 deficit) or the size and shape of objects (area 2 deficit)
Once the information is in the SI, what happens?
- Substantial connections between SI areas → Corticocortical connections. All the cortical-cortical connections help to integrate all this information that comes segregated.
- All regions of SI project to secondary somatosensory cortex. It is a higher order area and combines this information.
- Also projections to parietal areas for motor integration & limbic areas for learning & memory.
- We can act uppon information that we receive from out somatosensory area through the parietal areas 5 and 7 to motor and premotor cortical areas (react to the sensory and send it to premotor and then motor.
Secondary somatosensory cortex
- SII sends projections in turn to limbic structures such as the amygdala and hippocampus – tactile learning and memory
- SI also project to parietal areas 5a and 7b.
- These areas, in turn, supply inputs to neurons in motor and premotor areas of the frontal lobe.
- That’s how proprioceptive afferents signaling the current state of muscle contraction gains access to circuits that initiate voluntary movements – sensorimotor integration
Desscending projections
- There is an influx of afferent information from a limb to the primary somatosensory cortex, but there are also descending output neurons from somatosensory area to the thalamus brainstem and the spinal cord.
- Descending projections outnumber ascending projections
- Descending projections to thalamus, brainstem & spinal cord
- Their function is not well understood but assumed they modulate sensory information flow in thalamus & brainstem
Plasticity in the adult cerebral cortex
- Primary somatosensory cortex responses adapt to differences in stimulation
*experiment to discover this plasticity - amputated finger number 3 of the monkey. After a few months, there is no finger three anymore so the area that responds to area numbr 3 is taken over by other fingers. - Lesioning an input:
- Initial lack of response in corresponding cortical area
- Gradual increase in responding to stimulation of neighboring regions
Another case of plasticity in the adult cerebral cortex
- Changes in cortical representation also induced by less drastic changes in sensory or motor experience
- e.g. Training a monkey to use specific fingers to perform a task expand associated cortical representation
- e.g. local anesthetic induce temporary remapping of
receptive fields
- e.g. local anesthetic induce temporary remapping of
- e.g. Training a monkey to use specific fingers to perform a task expand associated cortical representation
- Rapid plasticity suggests likely reflect changes in synaptic strength of existing synapses.
Another study made a monkey get their food from a very small well from rotating a disk. They needed to use finger 2 and 3 in a very precise movement to get the food off the well. Area 2 ans 3 is represented much bigger from what it was before!
- This kind of ”functional remapping” appears to be a general property of neocortex, also observed in visual, auditory & motor cortices.
