Touch

Functions: Provides tactile information. Warns us of damaging stimuli. Contains body fluids and organs. Protects against bacteria. Regulates body temperature. The Skin

Touch Touch helps us identify objects and provides unique information (e.g., texture) Touch is important in development and social interaction (e.g., Harlow’s monkeys).

The skin on the hand contains thousands of pressure- sensitive mechanoreceptors. Touch acuity is best when fingers move over the object of interest.

Measuring sensitivity and acuity of touch Pressure sensitivity is greatest for fingertips and lips (the lightest touch). The back is least sensitive to pressure. Females more sensitive than males. Vibrotactile sensitivity is greatest for palms. Females are more sensitive than males.

Touch acuity (pattern acuity) is measured using the two- point threshold test. Measuring sensitivity and acuity of touch Tactile contrast sensitivity. Spatial frequency Sensitivity Linked to centre/surround receptive fields in the thalamus. Braille.

Neural Processing for Touch Receptive field sizes of the different mechanoreceptors determines our ability to discriminate fine details. Sensitive body parts have higher density of RA1 and SA1 fibers.

Receptive Fields thalamus Receptive fields in the thalamus have centre-surround organization. Cortical receptive fields (left) Cortical receptive fields (left) are smaller in the fingers and larger on the hand and forearm.

Neural Processing for Touch Two-point threshold: Two-point threshold: the smallest discriminable distance between two points

Localization of tactile stimulation Localization ability enables judgment of where stimulus has been applied to the skin. Tactile judgments of relative position are highly accurate.

Perception of surface texture Surfaces have unique “texture signatures” (e.g., coarse vs. fine). Gratings of different spatial frequencies measure tactile acuity.

Tactile sensitivity (temperature) Touch temperature is the perception of surface temperature. Objects differ in thermal conductivity. Touch temperature is based on temperature gradient between object and skin.

Thermoreceptors Located just below the skin. Continuous nerve impulses at a certain temperature. Small receptive fields (less than 1mm 2 ). There are spaces between receptive fields (“blind spots”). Two classes: (1) cold receptors (2) warm receptors Responds to CHANGES in temperature!!!! Perceived temperature depends on the state of the receptors.

Thermoreceptors Warm Fibers: - increased responding with increasing temperature - sustained firing - decreased firing when temperature decreases - do not respond to mechanical stimulation Cold Fibers: - increased responding with decreasing temperature - sustained firing - decreased firing when temperature increases - do not respond to mechanical stimulation

Mental set and tactile sensitivity Uncertainty makes tactile discrimination more difficult. Advance information improves the identification of a tactile stimulus. Practice also improves tactile discrimination.

Touch Fibers Each nerve fiber signals touch to a specific area of the skin (the fiber’s receptive field).

Temporal properties Slowly adapting (SA) fibers respond to initial stimulation and continue responding (perception of light, uniform pressure). Rapidly adapting (RA) fibers respond only to start and stop points of stimulation (perception of buzzing vibration).

Spatial properties Punctate fibers have small receptive fields with sharply defined boundaries. Diffuse fibers have large receptive fields with fuzzy boundaries.

The four-channel model: Mechanoreceptors Spatial Property Punctate Diffuse Temporal Property RA SA Meissner corpuscles (transient stimulation) Pacinian corpuscles (very sensitive with large RFs) Merkel disks (steady pressure of small object) Ruffini endings (steady pressure and stretching)

Receptors Frequency Range PerceptionFiberRF Size Merkel HzPressureSA1Small Meissner 3-40 HzFlutterRA1Small Ruffini HzStretchingSA2Large Pacinian HzVibrationRA2Large Properties of Mechanoreceptors

Ascending pathways for touch Fibers (receptors)spinal cord interneuronsmuscle Fibers (receptors) lemniscal neuronsbrainstem spinal cord Reflex (OUCH!!!!) Sensory Analysis

Somatosensory Cortex Damage to somatosensory cortex destroys ability to recognize objects by touch.

The body is mapped topographically onto somatosensory cortex, but body parts are not represented equally. Homoculus

Homunculi

Tactile Object Recognition Haptic perception: Haptic perception: exploration of 3D objects with the hand Exploratory procedures Exploratory procedures (Lederman & Klatzky) Passive Touch Active Touch Passive Touch vs. Active Touch

Exploratory Procedures

Visual and Haptic Object Recognition Amedi, Jacobson, Hendler, Malach and Zohary (2002). Cerebral Cortex, 12(11), Haptic perception: Haptic perception: involves coordination of motor, sensory, and cognitive systems

Somatosensory Cortex Certain cortical neurons respond selectively to orientation and direction.

A person with unilateral neglect denies ownership of limbs on one side of the body. A person with a phantom limb experiences sensation from a limb that no longer exists. Disorders related to somatosensory cortex

Phantom Limbs Flor, Elbert, Wienbruch, Pantev, Knecht, Birbaumer, Larbig & Taub (1995). Amputees often report that they can still feel their missing limb, and sometimes this is painful! Referred sensation: Referred sensation: stimulation of one part of the body results in a sensation on another part of the body (i.e. the phantom limb). The amount of functional cortical reorganization is positively correlated with the degree of phantom limb pain.

Pain perception Nociceptors respond to painful stimuli. Two categories: (1) mechanical receptor - severe pressure on skin - tearing (2) thermal receptor - responds to very high and very low temperatures

Pain Nociceptors: Nociceptors: receptors in the skin that respond to intense pressure, extreme temperature, or burning chemicals. The perception of pain can be modulated by cognitive factors: expectation, placebo, shifting attention, emotional distraction, individual differences

Endorphins, Opiates and Pain Relief The “reward pathway” contains opioid receptors for exogenous and endogenous substances. - neurotransmitters (dopamine) - opiate drugs (morphine, heroin, cocaine) - endorphins

Somatosensory cortex plasticity Cortical reorganization occurs in monkeys when fingers are surgically connected. PET studies reveal differences in the brains of musicians that suggest somatosensory cortical changes occur in humans.

Cortical Plasticity The amount of cortical magnification was correlated with the age at which the person began to play. Elbert, Pantev, Wienbruch, Rockstroh & Taub (1995) String instrument players have larger representation in primary sensory cortex for their left hands than normal controls.

Cross-Modal Plasticity Cohen, Celnik, Pascual-Leone, Corwell, Faiz, Dambrosia, Honda, Sadato, Gerloff, Catala & Hallett (1997). People who have been blind from a very young age show activity in visual cortex during Braille reading. A TMS pulse to the visual cortex impaired Braille reading. Evidence of functional reorganization of the brain!

Summary The homuculus describes the amount of cortex devoted to processing sensory and motor information from the different parts of the body. Haptic perception results from active touch and exploratory hand movements. Haptic and visual object recognition share an overlapping region in the ventral “what” visual stream. Nociceptors provide information about painful stimuli.

Central neural mechanisms, such as emotional state and drugs, modulate our perception of pain. The somatosensory system can undergo substantial reorganization after intensive practice and injury.