Interfacing with the Sense of Touch: The Somatosensory ... haptic collaboration.pdf · Interfacing...

http://www.lsr.ei.tum.de

A. Peer

Institute of Automatic Control EngineeringTechnische Universität München

[email protected]

Interfacing with the Sense of Touch:The Human Somatosensory System (Network)

and Dynamic Tactile Displays

Human‐Centered Robotic SystemsHaptic Human-Robot Interaction

Analysis of human-human interaction

Synthesis of interactive robotic systems

Intention recognition

Advanced teleoperation controllers

Single and multi-user scenarios

Joint decision makingAdaptation

Telepresence and Teleaction Systems

Brain and BodyComputer Interfaces Human Motor Control

fMRI and TMS studieswith Max-Planck Institute

Intention recognition

fromphysiological

signals

Embodiment of intentionsand personalization of actions

IntentionrecognitionIntention

recognitionEmbodiment

ofIntentions

Embodimentof

Intentions

Bilateralcontrol

Sharedcontrol

Supervisorycontrol

Autonomouscontrol

degree of autonomy

Automatic shifting ofrobot autonomy

Design and control of human-system interfaces

Modellingof human

motor behavior

Haptics= from the greek haphe, the sense of touchhaptesthai, to touch something

3

Haptics – The Sense of Touch

The 5 human senses:

sight hearing smell taste touch

4

The Sense of Touch

Somatosensation:• Tactility

Perception of vibration, shear forces, pressure

• Thermal perception

• Pain perception

Kinesthesia:• Perception of motion and forces

Proprioception:

• Perception of posture and position

5

What is a Tactile Display?

Tactile properties:

• Texture

• Roughness

• Vibration

• Geometry of small objects (distribution of pressure)

• Sliding shear forces

• Temperature

A tactile display is a physical device that can display tactile propertiesto a user.

6

Application Areas of Tactile Displays• Teleoperation (medical application: palpation in minimal invasive surgery)

• Automobile industry

• Military application (nagivation, orientation‐assistance, increase of situation awareness)

• Assistance of the blind (Braille systems, orientation and navigation assistance)

• Commercial applications (vibration alarms, games,

enhancement of graphic displays)

• Psychophysical studies

[www.immersion.com]

[www.top‐braille.com]

[iDrive, BMW AG] [Erp et al. 2004]

A Special Type: Dynamic Tactile Displays

7

Dynamic tactile display: generates tactile patterns on a stationary skin surface

[MIT tactile vest]

[http://www.ultracane.com]

[www.freedomscientific.com]

Historical Displays: The Optacon

Optacon (optical‐to‐tactile‐conversion) (Livill, Telesensory, 1962‐2000)

• 24‐by‐6 matrix of tiny metal rods, each independently vibrated by a piezoelectric reed (bimorph)

• rods are vibrated according to black parts of the image, thus forming a tactile image of the letter being viewed by a camera module passed over the text

• Typical reading rates: 30‐50 wpm

• Also reading of images possible

1990s: page scanners with optical letter

recognition appear on the market, which are

less expensive, have a better learning curve,

and allow faster reading

8

Historical Displays: TVSS

Tactile Vision Substitution System (TVSS) [White et al., 1970]:

• Aimed at converting visual information to patterns for the skin

• Consists of TV camera and an array of vibrators built into the back of a chair

9

[White et al.,1970]

Pictorial vs Coded Approach

Tactons are structured, abstract, tactile messages which can be used to communicate information non‐visually.

10

Pictorial versus coded approach [Geldard, 1974]Analogic versus synthetic system approach [Sherrick, 1975]

What is the analogy in the tactile domain?

Natural parameters: frequency, amplitude, duration, waveform, rhythm

[Brown et al., 2005]

Known coded approaches in other modalities:• Vision: icons, which are defined by a meaningful image• Audition: earcons, which are defined by melody, pitch and timbre

Historical Displays: Vibratese Language

• Three intensities, three durations, five loci

• Most frequently occurring letters were assigned to shortest signals

• Numbers get longest duration

• Vowels are represented by each single vibrator

• Vibratory characters for most commonly recurring words: the, of, and, in

11

[Geldard, 1957]

Achieved speed:• 38 words per minute

12

Agenda

The somatosensory system

Types of tactile displays

Psychophysics of the sense of touch

Spatio‐temporal patterns on the skin

Tactile illusions – effects of motion

Application areas and selected examples

13

Anatomy of the Skin

Dermal layers:

• Epidermis

• Dermis/ Korium

• Subkutis

Hairy/glabrous skin:

• Different receptors

[www.kidsnet.at]

Epidermis

Subkutis

Dermis/Korium

Form: discoid receptorsLocation: in Epidermis close to Dermis

Form: Egg‐shaped stack of flattened cells with intertwined nerve fiber

Location: in Dermis under EpidermisSize: 30 x 80 um

Form: branched nerve fibers encloused by a cylinder‐shaped capsule

Location: DermisSize: 0.1 x 0.5‐2mm

Form: capsule build of multiple layersLocation: are located deep in the skin, also in inner

organs and jointsSize: 1x2mm, largest skin structure

14

Tactile Receptors

[E. B. Goldstein, Wahrnehmungspsychologie, 1996]

Merkel‘s disks

Meissner corpuscle

Ruffini corpuscle

Pacini corpuscle

15

Skin Receptors in Glabrous and Hairy Skin

Glabrous skin:• Merkel‘s disks• Meissner‘s corpuscle• Ruffini endings• Pacini corpuscle• Free nerve endings

Hairy skin:• Merkel‘s disks• Ruffini endings• Pacini corpuscle• (Hair follikel)• (Free nerve endings)

[Kandel, Schwartz, Jessell, Principles of neural science, 2000]

Microneurography

Insertion of electrode under the skin and recording of a single nerve fiber

[www.rrk‐berlin.de, 2008]

16

Nerve Fiber Types

4 types:

• Fast/slow adapting fibers (FA, „fast adapting“ / SA, „slowly adapting“)

• Small and large receptive fields

[Deetjen, Speckmann, Physiologie, 1994]

17

Receptive fields

Ada

ptat

ion

Small,sharp borders Large, washy borders

fast, onlydynamicresponse

slow,static

response

time time

time time

Nerve Fiber Types and Tactile Receptors

Type of fiberSize of

receptive field

ReceptorOptimal stimulus

Function

Small Pressure Intensity detectors

Small Tap on skin Velocity detectors

Large

Stretching of the skin, movement of joints

Intensity detectors

Large Fast vibrationAcceleration detectors

18

Merkel‘s disks

Meissner corpuscle

Ruffini corpuscle

Pacini corpuscle

time

time

time

time

19

Haptic Information Processing

↓ Skin receptors detect stimulus

↓ Nerve fibers enter spinal cord

↓ Two lines of afferent nerve fibers:– Medial Lemniscus: thick fibers, sensation of touch

and perception of position of limbs (proprioception)

– Spinothalamic tract: thin fibers, temperature and pain perception

↓ Entry into Thalamus and interconnection of fibers by means of synapses

↓ Forwarding from Thalamus to primary somatosensory cortex


signal travels to brainin the spinal cord

signal to spinal cord

stimulus

Somatosensory Cortex

Humunculus

• High resolution requires

⇒ small receptive fields

⇒ high receptor density

• Body regions with high receptor density are represented by larger areas in the Cortex

20

21

Agenda





Tactile illusions – effect of motion


Types of Tactile Displays

22

Categorization according to stimulation type:

1. Mechanical stimulation

a) Skin indentation

b) Vibration

c) Lateral displacement

2. Electro‐tactile stimulation

3. Functional neuromuscular stimulation

23

Pneumatic Displays

Principle:• Energy in form of compressed air is

transformed into motion

Technological realisation:• Micro air jets• Air pockets• Inflatable air‐rings

Advantages:• Simple, clean, cheap• Non invasiv

Disadvantages:• Can lead to discomfort• Low bandwidth (< 10 Hz)

air pocketsInflatable pneumatic rings

data glove forposition sensing

air jets

[Shimoga 1993]

Pin‐Array Displays

Principle:

• Stimulation of FA I und SA

receptors by vertical motion of

pins

Technological realisation:

• shape‐memory‐alloy

• Piezo linear motors

• Elektroactive polymeres

Elektroactive polymere[Nanoarchitecture.net, 2008]Piezo linear actuator

Shape memory alloy[Burdea 1996]

[Forschungszentrum Karlsruhe]

piezoelectric actuator moved object

friction

actuatedrod

slow elongationsticking

fast shrinkingsliding

Advantages:

• Display of localized forces and contact geometries

Disadvantages:

• Can lead to pain

• High energy consumption

• Actuator density limited by technology

24

25

Vibro‐tactile Displays

Principle:• Stimulation of FA II receptors by means of vibration

Technological realisation:• Voice coils• Miniature loudspeaker• Piezoelectric actuators• Motor with excentric load

Advantages:• Can lead to discomfort or pain • Compact construction

Disadvantages:• Miniature loudspeaker: high weight• Voice coils, Piezos: noise Piezoelectric actuators

Lautsprecher

[Wikipedia: Tauchspule, 2008]

26

Shear Force Displays

Principle:

• Shear forces simulate sliding over object

Technological realisation:

• Bimorph piezo actuators

• Elektroactive polymeres

• Rotation motors

Elektroactive polymere[Nanoarchitecture.net, 2008]

Bimorph piezoactuator[Nanoarchitecture.net, 2008]

[Fritschi et al., 2008][Chen & Marcus, 1994]

[Hayward et al.]

27

Electro‐tactile Displays

Principle:• Stimulation of receptors by electric pulses

Technological realisation:• Mini‐electrodes mounted on the skin

Advantages:• Low energy consumption and weight• Small moving parts• Uniform contact with skin

Disadvantages:• Danger of discomfort• Danger of pain in case of two small electrodes and

too high currents• Variable skin conductance

electrodes

[Shimoga 1993]

[Kaczmarek et al. 1995]

Functional Neuromuscular Stimulation (FNS) ‐ Displays

Principle:• Stimulation of neuromuscular systems (not the skin)

Technological realisation:• Mounting of electrodes on muscles and nervous system

Advantages:• No limitation by means of technological realisation

Disadvantages:• Invasive, risky• High complexity• Can cause pain• High responsibility

PerceptionNeuronalResponse

Stimulus

Psychophysics

Neuro‐physiology

Neuronalcoding

[Steve Hsiao]

28

29

Agenda







30

What is Psychophysics?

psychophysics

Psychophysics is the subdiscipline of psychology which focuses on the relationship between stimulus and its perception by a human being

stimulus perception

Why do we need knowledge about human perception?

• To design tactile displays appropriately

• To design an adequate and useful haptic rendering

31

Origin of Phsychophysics

1860 Fechner‘s Elements of Psychophysics→ introduced first methods to measure

mental processes→ established first relationship between physical

and mental states

1879 Wilhelm Wundt founded first laboratory of experimental psychology in Leipzig

Gustav Theodor Fechner

32

Absolute und Difference Threshold

Absolute threshold:

Smallest energy of a stimulus , that can lead to a sensation

Difference threshold/limen (DL) or just noticeable difference (JND):

minimum amount by which a stimulus intensity must be changed in order to produce a just noticeable variation in sensory experience (JND=DL)

physical psychologicalDL1=JND1 =ΔΦ1

DL2=JND2 =ΔΦ2

DL3=JND3 =ΔΦ3

DL4=JND4=ΔΦ4

Φ0

Φ1

Φ2

Φ3

Φ4

Ψ0=0

Ψ1=1

Ψ2=2

Ψ3=3

Ψ4=4

33

Linear relationship between the differential threshold and the

stimulus intensity :

Weber‘s Law

Example: Discrimination of weights

found Weber fraction: c = 1/30

Heavy weights need to differ more than leight, so that they are perceived differently

[Gescheider, Psychophysics, 1985]

Adaptation Problem

• Absolute threshold increases

• Changes in subjective magnitude of supra‐threshold stimuli

sensation magnitude declines during the exposure

• Recovery time ranges from a few seconds to several minutes

• Adaptation within one tactile channel does not affect the sensitivity of others

Adaptation: decrement in sensory magnitude and in the detection threshold following more prolonged stimulation

34

35

Perception of Vibrations with Different Frequencies

perception thresholds

• Receptors react differently to different frequencies

• Isolation of receptors by cooling and masking

• Perception of vibration by most sensitive receptor


smallest perceivablevalue at ~200‐300 Hz

reference value: 1μm amplitude of vibrator*

*

Frequency [Hz]

SA I

SA II

RA I

PC

Perception of Vibrations with Different Frequencies

• Dependence on contactor size:

36

[Verrillo, 1963]

Discrimination of Amplitudes and Frequencies

37

• Range: 20 – 1000 Hz

• Weber fraction:

≈ 0.03

[Franzen & Nordmark, 1975]

DL of vibrations with different frequencies:

[Geldard, 1957]

DL of vibrations with different intensities:

• Range: 0‐55dB SL

• Weber fraction:

0.05 (0.4 dB) [Knudsen et al., 1928]

0.11 (0.9 dB) [Schiller, 1953]

0.3 (2.3 dB) [Sherrick et al., 1950]

Discrimination deteriorates as frequency increasesDiscrimination deteriorates as intensity increases

• Two stimuli with same amplitude, but different frequencies are felt differently

• Subjective magnitude grows more rapidly for body loci with lower sensitivity

Perceptual Interaction of Amplitude and Frequency

38

[Verillo et al., 1972][Verillo et al., 1969]

max. intensity (above vibrations become

unpleasant)

Perceptual Interaction of Amplitude and Frequency

3‐5 values of vibration rates

39

Varying vibration ratesConstant sensory magnitude: 36 dB

Varying vibration ratesVarying sensory magnitude: 20, 28, 36 dB

5‐8 values of vibration rates

[Sherrick 1985]

Detection of Vibrations with Different Durations

40

[Verrillo, 1965]

Short stimulus is more difficult to detect than a long one

Discrimination of Vibrations with Different Durations

• Stimulus < 0.1 s can be mistaken for a „poke“ or a „nudge“

• Stimulus > 2 s is too time consuming

• 25 discriminable steps from 0.1 s to 2 s

41

[Geldard, 1957]

Number of Distinguishable Stimuli

Psychophysical dimension

Range Number of JNDs under laboratory conditions

Number of distinguishable stimuli under practical conditions

Intensity 0 – 55 dB SL 15 [Geldard, 1957] 3 [Geldard, 1957]

Frequency 20 – 1000 Hz 3 – 8 [Sherrick 1985]

Duration > 0 25 (from 0.1 – 2s)[Geldard, 1957]

4 – 5 (from 0.1 – 2s)[Geldard, 1957]

42

Waveform

43

• Subtle differences in waveform cannot be perceived on the skin

• Few tactile waveforms can be used without training unlike in audio

• Limited bandwidth of devices unifies waveforms

Square wave played with TACTAIDSine wave played with TACTAID


Amplitude Modulation

• Sinusoid amplitude modulation leads to perceptually different waveforms

• Known relationship between amplitude modulation and roughness:

– modulated sinusoid feels rougher than non‐modulated

– perceived roughness increases with decreasing modulation frequency

– unmodulated sine wave feels less rough than any other stimulus

44

250 Hz sinusoid modulated with 30 Hz sinusoid

Rhythm: Compound, Hierarchical, andTransformational Tactons

Compound tactons:

• Tactons can be combined to create compound messages

45

Hierarchical tactons:

• Tactons can be combined in a hierarchical way: each tacton is a node in a tree and inherits properties from the levels above it

Transformational tactons:

• Transformational tactons have several properties, each represented by a different tactile parameter

e.g. tactons for files in computer:

− file type rhythm

− size frequency

− creation date body location[Brewster & Brown, 2004]

Localization of Vibratory Stimuli

46

Law of mobility [Vierordt, 1870]: the more „mobile“ the body site , te greater is the sensitivity either to the location of a touch or to the separation between touched locations.

[Cholewiak and Collins, 2003]

Anhaltspunkte (anchor points) [Boring, 1942]: local signs that can provide reference points against which the observer can measure the position of less well localized tactile stimuli

Localization of Vibratory Stimuli

47

[Cholewiak et al., 2004]

Neurological redundancy of abdominal anchor points

Ecological significance

Artificial Anchor Points

• Artificial anchor points can be generated by including an odd site (stimulus that has a different quality compared to the rest of stimuli)

48


Effect of Age on Location Discrimination Capability

• With increasing age the density of receptors decreases drastically (e.g. Meissner corpuscles fall from 24 per mm2 in young people to 8 per mm2 in 70‐year old seniors)

• Age affects absolute discrimination threshold drastically

• Localization capability of vibrations is only little affected by age

49


Number of Stimuli: Channel Capacity

Confusion matrix:

50

Encoded information:

Transmitted information:

Overall recognition rate: 73,19 %

Channel of capacity of the observer for absolute judgements of nonvisual unidimensional stimuli has asymptote near level of 2.5 bits (about 6 alternatives) [Miller, 1956]

[Cholewiak et al., 2004]

51

Agenda







52

Spatial Resolution of Stimuli

• Spatial resolution:

– Determined by two‐point limen

– Increases with warmth and decreases with age

[Kandel, Schwartz, Jessell, Principles of neural science, 2000]

Temporal Resolution and Order of StimuliAbsolute identification of temporal order of several sequentially presented stimuli to different

body sites requires very slow rates of presentation:

• Temporal resolution without order judgements: ~5ms

• Threshold for temporal order judgements: ~ 20ms for two stimuli

• Threshold increases significantly with more stimuli (about 500ms)

53

[Hirsch and Sherrick, 1961]

• Correctness of discrimination between pairs of successive vibratory patterns, requires to avoid as much as possible common elements in the patterns

• No single vibrator contributes to error production more than another

Discrimination of Patterns ‐ Communality

[Geldard and Sherrick, 1965]

Effects of Multiple Stimulation Sites

• Masking: reduced ability to detect a stimulus in the presence of a background or masking stimulus (simultaneous, forward, and backward masking)

• Enhancement: occurs when the presence of a brief stimulus causes a second stimulus to appear to be of greater intensity than when it is presented alone

• Summation: refers to the total or combined sensation magnitude of two stimuli occurring close together in time

• Suppression: occurs when the presense of one stimulus decreases the ability of the subject to detect a second stimulus when the two simuli are delivered to different places on the skin. Masking from a remote site.

55

[Verrillo and Gescheider, 1991]

Masking: Change Numbness

„Change numbness“ [Gallace et al., 2006]:

• Changes between two vibrotactile patterns is perceived significantly less when:

– a pause is made between patterns

– a masking patterns (e.g. all vibrators activated) is shown in between

56

[Gallace et al. 2006]

sensitvity of observer

Enhancement and Summation of Successive Stimuli

• Summation:

– 3 dB effect when all frequencies are within one mechanoreceptor system

– effect more pronounced across all time intervals when frequency of each stimulus activates different tactile systems

57

1st stimulus

2nd stimulus comparisonstimulus

Summation

Enhancement

• Enhancement:

− Enhancement effect is strong for short time intervals when the intensity of the first stimulus is raised 10 dB above the second

− Effect diminishes when time interval is lengthened

− Enhancement effects disappear when the first and second stimuli are presented in different tactile systems

[Verrillo and Gescheider, 1975]

Suppression: Threshold Elevation

Temporal factors have an effect on

threshold elevations:

• Threshold elevation is maximal if no difference in the arrival time exists between signals coming from test site and masking site (neural signalling speed 50‐70 m/s)

• If arrival time is compensated for, masking effect is independent of stimulus loci

Vibrotactile masking is controlled by differential time delays and not by marker placement

58

maskerlocated at I

[Gilson, 1969]

59

Agenda







Tactile Illusions – Effects of Motion

Apparent motion, „Cutaneous phi“:

• „powerful ‚gouging‘ that moves from one stimulus site to the other at a rate depending on the distance between sites and the duration of the time between onsets“ [Sherrick & Rogers, 1966]

• Distances < 30 cm can be covered

• Transient pressure pulse at each

stimulus site:

(visual movement)

(tactile movement)

(tactile movement)

[Sherrick and Rogers, 1966]

Setting for optimal effect of illusion


„The Cutaneous Rabbit“ – Sensory Saltation [Geldard and Sherrick, 1972]

• Train of taps is presented consecutively at different locations on the skin

• Perceived vibration „hops“ from one place to the other

– With constant timing, but reduced number of taps at each locus, hops get longer

– Increasing number of taps makes hops shorter

61

[Geldard, 1975]T ~ 25-200 ms

perceived Phantomvibrations vibration

actuators

2-35 cm


„Funneling“ [Gardner and Spencer, 1972]

• Simultaneous activation of multiple vibrators: instead of one vibration at the location of the single vibrations, only one position in the middle is perceived, which is stronger in intensity than the single vibrations

• Works also with electrostimulation

62

T ~ 5ms

perceivedPhantomvibration

vibrationactuators

~ 2cm

Two-dimensional:

[Tachi 1985]Possible reasons:• Mechanical properties of skin• Interaction of receptors• Central information processing

63

Agenda







Application Area I: Enhancement of Desktop Interfaces

• Reduction of overload by visual information

• Increase of focus of attention: users can concentrate visual attention on primary task, while tactile feedback provides information from secondary task

• Enhancement of interaction with buttons, scrollbars, menus

• Improvement of pointing, targeting, and steering type interactions

64

[iFeel mouse of Logitech]

Eight task properties of a progress bar[Conn et al., 1995]

• Acceptance• Scope• Initiation• Progress• Hearbeat• Exception• Remainder• Completion

Tactons for Progress Information

Tactile progress bar:

• Amount of remaining time proportional to the time between two pulses

• Time between two pulses is scaled to the amount being downloaded

Reaction time to task completion:• Visual: 13.54 seconds (SD 5.2) • Tactile: 8.7 seconds (SD 5.6)

[Brewster and King, 2005]

Application Area II: Mobile and Wearable Devices

• Mobile information systems:

– Tactile patterns provide information on the caller, replace or enhance items on the display, aid in the navigation of the devices menus so that the user does not need to look at the screen

– Tactons can describe information such as the type of building, type of shop, number of stairs, location of rooms, …

– Information on stock market data

• Mobile orientation systems

– Information about orientation in space for pilots, microgravity environments

• Mobile navigation systems

– Directional information, compass‐like display of direction

• Mobile communication systems

– Tactile icons, tactile melodies

66

[www.immersion.com]

[Erp et al., 2004]

Transformational Tactons to Code Messages and Calls

• Type of message coded by 3 roughness grades: smooth, rough, very rough

• Type of call coded by 3 rhythms:

– voice call

– text message

– multimedia message


• Spatial disorientation is a serious threat to pilots

in aircrafts

• Somatogyral illusion may lead to graveyard spin

• Tactile displays can help to overcome this threat

• Two rendering modes:

− Inside‐out: instrument presents Earth‐fixed

orientation

− Outside‐in: signal rotates at the same angular

velocity and in the same direction with respect

to the observer as the observer rotates with

respect to Earth

Orientation‐Aid

68

24x2tactors

[Erp et al., 2006]

Vibrotactile In‐Vehicle Navigation System

Typical threats of in‐vehicle support systems:

• Overload of traditional sensory channels (vision, audition)

use free sensory channel

• Overload of the cognitive capacities that a driver has

at disposal

use intuitive information presentation that

minimizes information processing

Vibrotactile display for navigation tasks

• Eight vibrating elements mounted in a

driver‘s seat

• Coding of distance to next waypoint by rhythms

and direction of course change by location of

activated tactors

69

[Erp et al., 2004]

Waypoint Navigation at Sea and in the Air

• Eight vibrating elements around torso

• Direction coded by location of vibration

• Passing of waypoint by activation of all

vibrators

70

Pilot following instruction of the navigator

Pilot following instruction of the tactile navigation system

[Erp et al., 2004]

Application Area III: Visually Impaired Users

• Display of symbolic, graphical, and multidimensional data

– Tactons advance Braille like Earcons advance synthetic speech

– Representation of graphical information

– Visualization of multidimensional data by mapping to different dimensions of a stimulus (amplitude, frequency, locus, ...)

• Tactile waypoint navigation systems

• Blind walking canes

• Tactile game consoles

71

[http://www.ultracane.com][VTPlayer mouse]

Reading of Charts by Visually Impaired

• Combination of graphics tablet as absolute positioning device and VTPlayer mouse on which information is displayed dynamically

• Axes and bar representation with tangible tactile relief

• Buttons on stylus to trigger audio information on the bars

[VTPlayer mouse]

[Wall and Brewster, 2006]

Summary

• Dynamic tactile displays are not well researched yet

• Many open research questions:

– Psychophysics of touch:• Information carrying capacity of different stimuli

• Spatiotemporal masking effects

• Modelling of dynamic effects

– Technology: • Adaptation to psychophysical findings (actuator density, bandwidth, etc.)

• Ergonomic aspects

– Haptic rendering:• Integration of knowledge from psychophysics

– Multimodality: How do multiple modalities interfere with each other?

73

Selected Literature

• S. Wall, S. Brewster, Sensory substitution using tactile pin arrays: human factors, technology and application, Signal Processing 86, 3674‐3695, 2006

• S. A. Brewster, S. A. Wall, L. M. Brown, E. E. Hoggan, Tactile Displays, The Engineering Handbook on Smart Technology for Aging, Disability and Independence (John Wiley & Sons), ISBN 978‐0‐471‐71155‐1, 2008

• R.T. Verillo and G.A. Gescheider, Tactile Aids for visually impaired: ch1: Perception via the sense of touch. Ed. Summers, Whurr Publishers Ltd. London, 1992

• J.C. Craig, C. E. Sherrick, Dynamic tactile displays, Tactual perception, a sourcebook edited by William Schiff and Emerson Foulke, Cambridge University Press, 1982

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