Haptic Perception

Sensitivity to Touch

Sensitivity to Touch

• Tactile acuity: Ability to detect fine details through touch.

• Measured using: Physical gratings (raised ridges).

• Low spatial frequency: Fewer, widely spaced ridges → easier to distinguish.

• High spatial frequency: More, closely spaced ridges → harder to distinguish.

• Key Finding: Touch and vision both depend on spatial frequency to detect details.

Exploratory Movements

Exploratory Movements

Different hand movements detect different object properties:

• Lateral Motion (rubbing): Texture

• Pressure (pressing): Hardness

• Static Contact (touching): Temperature

• Unsupported Holding (lifting): Weight

• Enclosure (grasping): Overall shape and volume

• Contour Following (tracing): Exact shape

Key Takeaway: Different hand movements help us perceive different object properties through touch.

Weinberg et al. (1970)

Weinberg et al. (1970)

• Method: Recognized 20 geometric objects using vision, hands, or mouth.

• Procedure: Studied objects, then completed a recognition test.

• Results:

  • Vision: 99%

  • Hands: 90%

  • Mouth: 74%

• Key Finding: Touch is highly effective for recognizing 3D objects, nearly as accurate as vision.

Klatzky et al. (1985)

Klatzky (1985)

• Study: Identify 100 objects by touch.

• Free hand movement: 96% accuracy.

• Restricted movement: 20% accuracy.

• Conclusion: More freedom to explore → more haptic (touch) information → better object recognition.

Skin & Touch

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Skin & Touch

• Skin: One of the body's largest organs.

• Functions: Protection, insulation (warmth), and sensing touch.

• Two skin layers:

  • Epidermis: Outer layer, mostly dead cells.

  • Dermis: Inner layer, living cells and receptors.

4 Mechanoreceptors (Touch Receptors)

4 Mechanoreceptors (Touch Receptors)

• Merkel Discs: Pressure, fine details, texture.

• Meissner Corpuscles: Light touch, low-frequency vibration.

• Ruffini Cylinders: Skin stretching.

• Pacinian Corpuscles: Deep pressure, high-frequency vibration.

Key Takeaway: Different skin receptors detect different types of touch.

Temporal Properties of Mechanoreceptors

Temporal Properties of Mechanoreceptors

- Slowly Adapting (SA): Fire continuously while stimulus is present (sustained response).

• Receptors: Merkel discs & Ruffini cylinders.

- Rapidly Adapting (RA): Fire when stimulus starts and stops (transient response).

• Receptors: Meissner & Pacinian corpuscles.

Key Takeaway: SA = continuous pressure; RA = changes in touch.

Mechanoreceptors — Response Patterns

Mechanoreceptors — Response Patterns

- Slowly Adapting (SA): Sustained response → fire throughout stimulation.

• SA1: Merkel discs

• SA2: Ruffini cylinders

- Rapidly Adapting (RA): Transient response → fire when stimulus starts and stops.

• RA1: Meissner corpuscles

• RA2: Pacinian corpuscles

Key Takeaway: SA = continuous firing; RA = firing at changes (on/off).

Spatial Properties of Mechanoreceptors

Spatial Properties of Mechanoreceptors

• Receptive Field (RF): Area of skin where a receptor detects touch.

• Punctate RF: Small, precise boundaries.

• Diffuse RF: Large, fuzzy boundaries.

Receptor	Adaptation	RF Size
Merkel	Slow (SA1)	Small
Ruffini	Slow (SA2)	Large
Meissner	Fast (RA1)	Small
Pacinian	Fast (RA2)	Large

Key Takeaway: Small RF = precise touch; Large RF = less precise touch.

Mechanoreceptors — Stimulus Properties

Mechanoreceptors — Stimulus Properties

Receptor	Adaptation	RF Size	Detects	Frequency
Merkel	Slow	Small	Steady pressure	0.3–3 Hz
Ruffini	Slow	Large	Skin stretching	15–400 Hz
Meissner	Fast	Small	Flutter/movement	3–40 Hz
Pacinian	Fast	Large	Vibration	10–500 Hz

Key Takeaways:

• Slow adapting: Merkel & Ruffini (sustained touch).

• Fast adapting: Meissner & Pacinian (changing touch).

• Small RF: Merkel & Meissner (precise).

• Large RF: Ruffini & Pacinian (less precise).

Two Pathways for Haptic Perception

Two Pathways for Haptic Perception

1. Reflex Pathway: Mechanoreceptors → Spinal Cord → Motor Neurons (muscles)

• Automatic response; does not reach brain.

2. Ascending Touch Pathway: Mechanoreceptors → Spinal Cord → Brain Stem → Thalamus → Somatosensory Cortex

• Reaches brain for conscious touch perception and analysis.

Key Takeaway: Reflex = automatic response; Ascending = conscious touch perception.

Thalamus — Center-Surround Receptive Fields

Thalamus — Center-Surround Receptive Fields

• Vision (LGN): Center-surround organization.

• Touch: Similar center-surround organization.

• Excitatory Center (+): Touch increases neuron firing.

• Inhibitory Surround (−): Touch decreases neuron firing.

Key Takeaway: Both vision and touch use center-surround receptive fields to improve spatial discrimination.

Somatosensory Cortex

Somatosensory Cortex

• Function: Processes complex touch (orientation, direction, shape).

• Somatotopic Map: Body parts mapped to specific areas of the cortex.

• Cortical Magnification: Sensitive body parts receive more brain space, regardless of physical size.

  • Fingers: More cortical space, small/densely packed RFs → precise touch.

  • Arms: Less cortical space, large/sparsely packed RFs → less precise touch.

  • Lips: Receive disproportionately large cortical representation.

• Sensory Homunculus: Body diagram showing cortical magnification (larger body parts = more brain space devoted to them).

Key Takeaway: Greater touch sensitivity = more cortical space + smaller, densely packed receptive fields.

These notes now cover all the important testable concepts from your slides without unnecessary details.