touch

Touch

The Sense of Touch

• Touch is actually a combination of three sensations:

  • Pressure

  • Temperature

  • Pain

• Experiences such as wet/dry, tickling, itching, tingling, and others result from combinations of these three primary sensations.

Our Skin

• Skin is an organ, defined as:

  • A collection of tissues joined together in a structural unit that performs a common function.

• Skin is the heaviest organ in our bodies.

• Functions of skin include:

  • Keeping things out and keeping things in.

• The outer layer of skin is called epidermis, which is made up of tough dead cells.

Underneath Our Skin

• Beneath the epidermis lies the dermis, which contains:

  • Mechanoreceptors

  • These are our touch sensory receptors.

  • They respond to:

    • Pressure

    • Stretching

    • Vibration

  • Tickling is a result of high-frequency activation of these receptors.

  • It is interesting to note that many mammals enjoy tickling, as referenced in a video link.

Touch Receptor Receptive Fields

• A receptive field is defined as:

  • The part of the skin where the mechanoreceptor neuron responds.

• When a mechanoreceptor's receptive field is stimulated, it will start firing action potentials.

Touch Receptors Close to the Surface

• Mechanoreceptors located closer to the surface of the skin detect:

  • Light touch

  • Pressure

  • Details, shapes, texture

  • Motion across the skin

  • Hand grip

• They have smaller receptive fields, allowing for higher spatial resolution.

Touch Receptors Deeper Under the Skin

• Mechanoreceptors located deeper under the surface detect:

  • Vibration

  • Stretching

• These receptors have larger receptive fields, which lead to lower spatial resolution.

Sensitivity of Different Body Parts

• Why are our hands more sensitive than our backs?

  • Our hands contain many mechanoreceptors with relatively small receptive fields.

  • Our backs have comparably fewer mechanoreceptors with larger receptive fields.

  • This difference affects our ability to detect the location of stimuli: we are less precise on our backs.

Two-Point Discrimination

• Two-point discrimination refers to:

  • The ability to differentiate two separate touch stimuli as distinct points.

• Our hands exhibit much lower thresholds for detection compared to other parts of the body like backs, legs, or arms, making them more sensitive.

Touch Receptors Overview

• There are four different types of touch receptors:

  • Two types are located closer to the surface of the skin and have smaller receptive fields.

  • Two types are found deeper under the skin and possess larger receptive fields.

Adaptation to Stimuli

• Why can’t I always feel the sweater on my skin?

  • Some mechanoreceptors are rapidly adapting.

  • They stop firing action potentials soon after a stimulus begins.

  • They will resume firing when the stimulus changes, which helps us adapt quickly to stimuli that don't require attention (like clothing).

The Long Journey of Touch Sensation to the Brain

• Touch information from all over the body travels up the spinal cord to reach the brain.

Touch in the Brain

• Most of the neuronal fibers carrying touch information are sent to the thalamus.

• The thalamus functions to filter and relay this information to several parts of the brain, notably the:

  • Primary Somatosensory Cortex located in the parietal lobe.

Body Maps in Somatosensory Cortex

• The primary somatosensory cortex contains a representation of the body, often referred to as a “homunculus” (meaning tiny man).

• Some regions of the skin are represented by much larger areas on the cortex due to the density of receptors.

Neuroplasticity Following Sensory Loss

• Our brains exhibit remarkable flexibility and are described as ‘plastic’.

• Upon losing a sensory modality, the brain can adjust by re-mapping.

• For example:

  • Sign Language activates the auditory cortex in individuals who are deaf.

  • Braille activates the primary visual cortex in individuals who are blind.

Kinesthesis and Equilibrium

Body Senses: Kinesthesis (Proprioception)

• We possess two body senses that inform us of our positioning in space:

• Kinesthesis (Proprioception) involves specialized nerves located in our:

  • Muscles

  • Tendons

  • Joints

• This sense allows our brains to understand the relative positioning of different body parts in relation to one another.

• It integrates with the visual system for enhanced spatial awareness.

Body Senses: Equilibrium

• Equilibrium refers to our sense of balance, localized to the inner ear.

• The inner ear contains the vestibular system, which consists of:

  • A maze of fluid-filled canals.

  • Hair cells that bend in accordance with the fluid's movement.

• When the head tilts, the movement of fluid activates these hair cells, sending electrical signals to the brain.

Motion Sickness

• Motion sickness occurs due to mismatched signals between the vestibular system and the proprioceptive and visual systems.

• Common scenarios include:

  • Visual input indicates movement not felt by the body.

  • Physical sensations of movement are not visually acknowledged.

Perceptual Concepts & Phenomena

Synesthesia: Blending of the Senses

• Synesthesia is a perceptual phenomenon affecting approximately 3-5% of the population.

• It involves the blending of senses, such that:

  • Stimulation of one sensory pathway leads to experiences in another sensory modality.

  • Examples include:

    • Seeing colors when hearing music.

    • Seeing shapes when smelling certain odors.

    • Experiencing taste while reading words.

• Corresponds to activation of multimodal sensory areas of the brain.

Perceptual Organization

• Human sensory perception is imperfect, requiring cognitive processing to create meaning from sensory input.

• Perceptual organization is the process whereby sensory information is grouped to form meaningful perceptions.

• Grouping involves how the brain organizes components of a visual scene into units or objects.

Visual Illusions: The Müller-Lyer Illusion

• Perception can sometimes be deceived, exemplified by visual illusions.

• Misapplied size constancy occurs when mechanisms for gauging relative object size fail in two-dimensional representations.

  • For instance, two lines can appear different in length even though they are of identical length (a common source of confusion).

Visual Illusions: The Ponzo Illusion

• In the Ponzo illusion, two identical objects appear to differ in size due to contextual cues created by converging lines, tricking our perception of distance.

Visual Illusions: The Ames Room

• The Ames Room illusion illustrates how spatial arrangement can create misleading perceptions.

• Two individuals of the same height seem to differ in size due to the room’s peculiar geometry, which affects visual angles.

Attention and Perception

• Attention is a cognitive process that selectively prioritizes certain sensory information over others.

• Inattentional blindness occurs when we fail to perceive something even while looking directly at it, indicating that focusing on one aspect often leads to neglecting others.

• Attention acts as a filter to prevent information overload.

What Captures Our Attention?

• Visual salience is defined as the characteristics of stimuli that make them stand out and capture attention, which includes:

  • High contrast

  • Bright colors

  • Highly visible orientations

  • Meaningful or recognizable stimuli

• Attentional capture refers to the qualities of a stimulus that can seize our focus unexpectedly, such as notifications from our phones, designed to attract attention effectively.

Can We Improve Our Attention?

• Attention is trainable:

  • With practice, our attentional skills can improve.

  • Learning to drive enhances our ability to focus on relevant stimuli while ignoring distractions.

  • Interestingly, as few as five days of daily meditation practice can increase attention spans and improve stress regulation.