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PY2PN: Perception, Lecture #4

The body senses 2: Touch & temperature

Friday 4th Nov, 2011, 15:00-15:50


After this lecture, you should be able to:

  1. Describe the behavioural & psychophysical dimensions of somatosensory perception
  2. Explain how somatosensory perception depends on the properties of somatosensory receptors, their afferent nerves, & the part of the body stimulated
  3. Describe, compare, & evaluate different methods of assessing somatosensory spatial acuity
  4. Describe one or more somatosensory illusions & explain how they may depend on the properties of somatosensory receptors
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From sensory 'receptors' to perceptual 'channels'

Bolanowski et al. (1988)

(start audio recording)

Four separate 'channels' mediate touch perception

[last week's lecture]

By systematically changing the frequency & intensity of vibrations, 4 frequency-threshold functions were uncovered.

These channels may correspond to the four main cutaneous mechanoreceptor types

ChannelNameCutaneous receptorAfferent fibresFrequency rangeTemperature sensitive?Perception
MinMax
PPacinianPacinian corpusclePacinian corpuscle (PC)40800YESVibration
NP INon-Pacinian IMeissner's corpuscleRapidly adapting (RA)10100YESFlutter
NP IINon-Pacinian II?Slowly adapting (SAII)15400NO?
NP IIINon-Pacinian IIISlowly adapting (SAI)0.4>100YESPressure

"...at suprathreshold levels, the code for perceptual quality may be considerably more sophisticated than had been previously realized, requiring that several channels contribute information for this purpose. In other words, fundamental qualities like "pressure," "flutter," and "vibration" may combine to form the many sensory attributes ascribed to the somatosensory system [...] "roughness," "softness," or "intensity" (among myriad other attributes)" Bolanowski et al. 1988, p 1692

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Dimensions of somatosensory perception

  • Pressure
  • Vibration
  • Flutter
  • (& Warming/Cooling)

Can these basic dimensions identified by Bolanowski et al. (1988) be built into the many, more complex, dimensions of somatosensory (or even just tactile) perception?

  • Roughness-smoothness
  • Hardness-softness
  • Viscosity
  • Elasticity
  • Stickiness
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Pressure perception across the body

Threshold & spatial localization

[last week's lecture]

Pressure thresholds are lowest on the face (cheek, nose, lip, & forehead); highest on the feet (sole, big toe)

Localization of a single pressure stimulus is best for the nose, lip, digits, & big toe; worst for the back, legs, & arms

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Vibration perception

Body part dependence

Our fingers and hands are the most, while the buttocks are the least vibration-sensitive

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Vibration perception

Frequency dependence

Our fingertips are most sensitive to vibrations of ~100-500Hz

[last week's lecture]


This is my vibration sensitivity tested in my lab this year (it took 4 hours!)

This sensitivity relates closely to the combined sensitivities of different receptor types, most prominently the Pacinian corpuscle

The 'loudness' of perceived vibrations depends on their frequency, just like for hearing

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Spatial acuity of tactile perception

Two-point discrimination

The ability to discriminate between a single probe & two separate probes applied simultaneously, is a classic (clinical) test of somatosensation (originally from Weber in the 1850s

It can be done with a pair of blunt compasses



Or with specialised equipment

However, the two-point test is confounded by differences in intensity & stimulus size (area, length), so is not a pure measure of tactile spatial acuity (Johnson & Phillips, 1981): People can perform well when there is no gap!

Nor is it a reliable one in clinical settings (e.g., Lundborg & Rosén, 2004)

Despite such criticisms of the two-point discrimination method, it is still widely used...

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Spatial acuity of tactile perception

Gap detection


Gap detection may be a better method than two-point discrimination, since the overall size and shape of the stimulus can be held constant





However, some somatosensory receptors are very good at detecting edges, so performance could be based on edge detection (gaps have edges) rather than spatial acuity

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Spatial acuity of tactile perception

Grating orientation discrimination

Grating orientation discrimination is an even better method than two-point discrimination or gap-detection, since the size, shape, & number of gaps & edges are held constant


The only problem is that discrimination can be better when the gratings are aligned with the ridges of the skin, rather than across the ridges

But this is not problematic for the circular ridges on the fingertips

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Spatial acuity of tactile perception

Letter recognition


Unlike many tasks used to assess tactile spatial acuity, letter recognition requires no training & shows no learning effects. It also has clear relevance to the real world (e.g., Braille, object identification), & to other sensory modalities (i.e., vision)


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Spatial acuity of tactile perception

Comparing methods

The two-point discrimination technique (Method I) is the most widely-used, but it is unreliable & confounded


Gap detection (Method II) is better, but confounded by edge-detection

Orientation discrimination (Method III) is more reliable, not confounded, & provides a complete psychophysical curve, from chance to 100% correct

Embossed letter recognition (Method IV) also provides a meaningful task & requires no training

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"The body senses 2: Touch & temperature"

Interim Summary

  1. There are four channels of cutaneous mechanoreceptors, but natural touch involves information from several channels simultaneously
  2. Somatosensory sensitivity varies with body part location, just as visual acuity depends on location, & auditory sensitivity depends on frequency
  3. The classic two-point technique for measuring tactile spatial acuity is confounded by intensity & area; Gap detection is better, but reflects edge-detection for small gaps; Grating orientation & letter recognition are best
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Thermoception

Relative versus absolute temperature

John Locke's (1690) 'three buckets' experiment* shows that temperature perception is relative: We rapidly adapt to mild deviations in temperature

Yet, we are also extremely sensitive to temperature change: Changes of just 0.1C over a few seconds are felt as warming (W) and cooling (C)


*Place one hand in a bucket of cool water (~20C), and the other in warm water (~40C). Leave to adapt for several minutes. Then, place both hands into a third bucket of tepid water (~30C). The cooled hand will feel the tepid water as warm, and the warmed hand as cool.
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Thermoception

The role of the skin

Skin temperature

We are more sensitive to temperature changes when our skin is warmer or cooler than the normal range (32-34C)

Skin area

We are more sensitive to temperature changes when a larger area of skin is warmed or cooled

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Somatosensory illusions

and similar phenomena


Along with the paucity of somatosensory textbook knowledge, somatosensory illusions are not widely understood, yet there are many!

Object perception (spatial)

  • Tactile funnelling
  • Cutaneous rabbit
  • Size-constancy failure
  • Crossed-hands deficits
  • Pattern & movement illusions
    (comb, lens, fishbone, barber pole)
  • Tactile versions of visual illusions (Muller-Lyer, Ponzo, Poggendorf)
  • Orientation illusions
  • Movement-based illusions (active vs. passive touch)
  • Virtual-object perception

Object perception (other)

  • Aristotle's illusion
  • Body-part temporal order failures
  • Size/weight/temperature illusions
  • Tactile numerosity failure
  • Tactile change numbness
  • Heat grill illusion
  • Hot/cold illusion & after-effects (buckets)
  • Weight/shape after-effects

Body perception

  • Finger-toe correspondence failure
  • Fingers-in-between failure
  • Pinocchio illusion(s)
  • Tendon vibration illusion(s)
  • Muscular/postural after-effects (door-frame)
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Tactile 'funneling'

Separate closely-spaced stimuli on the skin are perceived as a single, more intense stimulus at the central location

This even works for just two stimuli, inducing a 'phantom' touch between the two real touches




Gardner & Spencer (1972) [above] showed the illusion in both human judgements & cat nerve fibre responses - three stimuli were perceived as one, stronger, central stimulus

Chen et al. (2004) [below] showed with optical imaging that area 3b in the primary somatosensory cortex represents the illusory location of the stimulus, not the real location

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The cutaneous rabbit

First described by Geldard & Sherrick (1972), the cutaneous "rabbit" is a spatio-temporal tactile illusion

Two separate locations stimulated multiple times in quick succession feels like a "rabbit", hopping across the skin between the two locations



The illusion was recently investigated with fMRI (Blankenburg et al., 2006), who showed that activity in the primary somatosensory cortex representing the middle location was just as high for the cutaneous rabbit as compared to actual stimulation at that point

These data, just like the 'funnelling illusion' data, show that primary somatosensory cortex represents the perceived location of stimuli on the skin, not the actual location

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'Aristotle's illusion'

(also called 'tactile diplopia' or 'tactile diplesthesia')


two fingers are crossed over, and an object is placed, tapped, or rubbed between them. Many people feel two distinct sensations (objects) in separate locations, rather than one


Notes: 1) I find it works best with the middle and ring fingers (i.e., don't use the index); 2) 'Aristotle's illusion' also works for other body parts (Ponzo, 1910)

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Tactile size-constancy

(and the failure thereof)


Originally described by Weber, distances on the face feel bigger than distances on the back (~80% of times); distances on the index finger feel bigger than on the forearm (77%), back (70%), & middle finger (63%)



But, these illusions are much smaller than predictions based on receptor density, homunculus proportions, or spatial acuity, suggesting a tactile size-constancy mechanism (or some other effect)

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Tactile numerosity

In vision, we can immediately 'count' (subitise) up to three or four separate items. More than this, we have to count them explicitly


Gallace et al. (2006) showed that the same phenomenon exists for vibrations distributed across the body, but we can only subitise two vibrations

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Visceral sensation

(is even harder to study than touch)

The Cannon-Washburn Stomach Balloon

Contractions of the stomach wall coincide with self-reported feelings of hunger

But, many subsequent studies clarified and qualified the roles of stomach contractions
(e.g., people who have had their stomachs removed still feel hunger)

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"The body senses 2: Touch & temperature"

Summary

  1. Temperature perception is relative & complex: We can perceive cooling & warming of fractions of a degree, yet can rapidly adapt to large (>5deg) changes in temperature. The combination of cool & warm can be felt as hot
  2. There are many, yet little known or understood, illusions & strange phenomena in somatosensory perception

References (APA style)

Ponzo, M. (1910). Intorno ad alcune illusioni nel campo delle sensazioni tattili, sull'illusione di Aristotle e fenomeni analoghi. Archives für die Gesammte Psychologie, 16, 307–345. [NB]

Geldard, F. A., & Sherrick, C. E. J. Jr (1972). The cutaneous "rabbit": A perceptual illusion. Science, 178(57), 178–179. [NB]  

Johnson, K. O., & Phillips, J. R. (1981). Tactile spatial resolution. I. Two-point discrimination, gap detection, grating recognition, and letter recognition. Journal of Neurophysiology, 46(6), 1177–1191. [NB]  

Benedetti, F. (1985). Processing of tactile spatial information with crossed fingers. Journal of Experimental Psychology: Human Perception and Performance, 11(4), 517–525. [NB]  

Chen, L. M., Friedman, R. M., & Roe, A. W. (2003). Optical imaging of a tactile illusion in area 3b of the primary somatosensory cortex. Science, 302(5646), 881–885. [NB]    

Gardner, E. P., & Spencer, W. A. (1972). Sensory funneling. II. Cortical neuronal representation of patterned cutaneous stimuli. Journal of Neurophysiology, 35(6), 954–977. [NB]  

Taylor-Clarke, M., Jacobsen, P., & Haggard, P. (2004a). Keeping the world a constant size: Object constancy in human touch. Nature Neuroscience, 7(3), 219–220. [NB]    

Benedetti, F. (1986). Spatial organization of the diplesthetic and nondiplesthetic areas of the fingers. Perception, 15(3), 285–301. [NB]  

Benedetti, F. (1986). Tactile diplopia (diplesthesia) on the human fingers. Perception, 15(1), 83–91. [NB]  

Benedetti, F. (1988b). Exploration of a rod with crossed fingers. Perception & Psychophysics, 44(3), 281–284. [NB]    

Flach, R., & Haggard, P. (2006). The cutaneous rabbit revisited. Journal of Experimental Psychology: Human Perception and Performance, 32(3), 717–732. [NB]  

Hayward, V. (2008). A brief taxonomy of tactile illusions and demonstrations that can be done in a hardware store. Brain Research Bulletin, 75(6), 742–752. [NB]    

Blankenburg, F., Ruff, C. C., Deichmann, R., Rees, G., & Driver, J. (2006). The cutaneous rabbit illusion affects human primary sensory cortex somatotopically. Public Library of Science Biology, 4(3), 69 [NB]    

Gardner, E. P., & Spencer, W. A. (1972). Sensory funneling. I. Psychophysical observations of human subjects and responses cutaneous mechanoreceptive afferents in the cat to patterned skin stimuli. Journal of Neurophysiology, 35(6), 925–953. [NB]  

Rivers, W. H. R. (1894). A modification of Aristotle's experiment. Mind, 3(12), 583–584. [NB]

Bolanowski, S. J. J., Gescheider, G. A., Verrillo, R. T., & Checkosky, C. M. (1988). Four channels mediate the mechanical aspects of touch. Journal of the Acoustical Society of America, 84(5), 1680–1694. [NB]    

Lundborg, G. N., & Rosén, B. N. (2004). The two-point discrimination test: Time for a re-appraisal? Journal of Hand Therapy, 29(5), 418–422. [NB]    

Gallace, A., Tan, H. Z., & Spence, C. (2006c). Numerosity judgments for tactile stimuli distributed over the body surface. Perception, 35(2), 247–266. [NB]    

Miyazaki, M., Hirashima, M., & Nozaki, D. (2010). The "cutaneous rabbit" hopping out of the body. Journal of Neuroscience, 30(5), 1856–1860. [NB]  


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