l4 human factorsruth/year4ves/slides09/l4.pdf · 2012-01-08 · vestibulo-occular reflex to bealto...
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Human factors
Ruth Aylett
Topics
Human senses and their limitations– Sight– Hearing– Touch– Vestibular system
Health and safety issues
Why are human factorsimportant?
Human capabilities define desired technologyattributes
Different senses have different data rates Guidelines for best display and organisation
of information for understanding Health and safety issues Predict overall impact of a VE
Human senses
Visual Auditory Tactile Olefactory Vestibular (balance) Others:
– Proprioception: sense of position andmovement of body
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Visual perception
Field of view (FOV) Acuity
– Resolution– Dark v light
Stereopsis– Depth cues– Binocular rivalry/eye dominance
Vision
Visual process
Light enters through cornea/pupil– Focused by lens on retina– Lens has to adjust focus:accommodation
• Uses ciliary muscles
Responds to a narrow band ofelectromagnetic radiation– 400nm to 700nm, overall peak at 559nm– Well-matched to spectral emission of sunlight
Response from single photon to fluxes onetrilliontimes stronger
Visual process cont. Colour
– Via RGB cones (rods do night vision)• Three kinds of cones, sensitive to different
wavelengths– “long”: 575nm, about 64%– “middle”: 535nm, about 32%– “short”: 445nm,
• Peripheral cells very sensitive to movement
Blind spot– Where optic nerve emerges
Eye in permanent motion– Saccades at about 4Hz– Stabilized images disappear– Saccadic suppression leaves these motions largely
unnoticed
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FOV Horizontally
– -59˚ to +110˚– 118º overlap where stereopsis occurs
Vertically– -70˚ to +56˚
Visual acuity– Can separate 2 bright light points 1.5mm apart at 10m– 40’’of arc; 2µm on retina– To match this, requires “typical” desktop display of
4800x3840 (18.4 million pixels) Light to dark adaptation: 25000:1 Visual persistence
– Critical fusion frequency: c 20 Hz
Visualprocessing
StereopsisThe geometry
Can detect depthdistances of 0.05mat .5m or 4mm at5m
Represented as:θD=IPD Δr — —— r r +Δr
IPD= Inter-PupilDistance
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Stereo cues
Motion parallex cues– Image speed across the retina to judge depth– Only needs one eye
Convergence– How much eyes turn in
Perspective depth cues– Where we ‘know’ the size of an object
Textural cues– Light and shadow
Very important for greater distances
Oculomotor cues Accommodation
– Physical stretching and relaxing of the lens– Parallel rays entering the relaxed eye will focus on the retina– Relaxed eye has a depth of field of 6m to infinity– To focus objects within 6m it is necessary to alter the optical
system of the eye Vergence
– Rotation of the eyes (convergence: inward rotationcorresponding to viewing closer object)
– Muscular feedback in converging and focusing the eyes iscue to the depth of viewed object
– Relatively weak, but coupled depth cues!
Hearing Air vibrations (rapid changes in air pressure)
converted to mechanical vibrations in middle ear Sound has:
– Amplitude: Magnitude of the pressure variation– Frequency: Pressure variation rate– Phase
Acoustic reflex: Adaptation to high-intensity sounds;temporarily reduced auditory sensitivity
Acoustic stimuli necessarily have temporalcomponent
Constant sounds drop out of conscious awareness Sounds are perceived from sources in all directions
Localisation
Different factors influence ability to perceivelocation of sound sources– Interaural level difference: Difference in volume of
sound reaching each ear– Interaural time difference:Time difference of sound
reaching each ear (sound is slow)– Motion cues
• Doppler effect: Frequency shift resulting from relativemotion between sound source and observer
• Changing volume:Sound perceived as approachingwhen volume gradually increases (and vice versa)
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External ear - pinnae
Why do stereo speakers reproduce theoriginal sound stage?– Effect of head + pinnae– Hence in-ear speakers produce stage
‘inside head’
Head-related transfer functions (HRTF)– Modify signal– Simulate effect of head and pinnae
Haptic perception
Touch is a mechanical or thermal contact with theskin.
Mechanical stimuli produce sensation of touch:– Displacement of skin over an extended period– Transitory (milliseconds) displacement of skin– Transitory displacement repeated at constant or variable
frequency– Different below-skin sensors for skin with/without hair
Also thermal sensing Difficult to characterize in quantitative way Sensations of skin adapt with exposure to stimuli
Skin Force sensing
Receptors in muscles and joints– Inside the body
Perception of movement, position, andtorque of limbs and other body parts
Varying joint angles and muscularlength
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Vestibular system
Movement andorientation in space– See
http://en.wikipedia.org/wiki/Vestibular_system
Inner ear: vestibulum– Orthogonal canals for
rotation– Saccule/utricle for
linear acceleration
Vestibulo-occular reflex
To be able tosee whilemoving– Push-pull
between RHand LH canals
– 3 neurons linkto eye
– <10 ms lag ineye for headmovement
Health and safety issues
Flicker– Can trigger epilepsy
Eyestrain– Dissociation of accommodation and
convergence– Lack of visual calibration, especially HMDs
Use of laser light Movement injuries
Cybersickness
Reverse form of motion sickness– Conflicting signals from visual and vestibular
systems
Caused by:– Vection: illusion one is moving in a VE– Lag: delay in visual feedback
• Especially bad in HMDs
– FOV: wide and narrow. Impacts vection– Lack of interactive control
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Symptoms
Vestibular disturbances– Head spinning
Reduced hand-eye coordination Nausea Effects take time to reduce after
exposure Nervous system does adapt
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