Human factors Ruth Aylett Topics Human senses and their - - PowerPoint PPT Presentation

Human factors

Ruth Aylett

Topics

 Human senses and their limitations

– Sight – Hearing – Touch – Vestibular system

 Health and safety issues

Why are human factors important?

 Human capabilities define desired technology

attributes

 Different senses have different data rates  Guidelines for best display and organisation

• f 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 and movement of body

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 of

electromagnetic radiation

– 400nm to 700nm, overall peak at 559nm – Well-matched to spectral emission of sunlight

 Response from single photon to fluxes one

trilliontimes 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

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

Visual processing

Stereopsis

The geometry

 Can detect depth

distances of 0.05m at .5m or 4mm at 5m

 Represented as:

θD=IPD⎡ Δr ⎤ — —— r ⎣r +Δr⎦ IPD= Inter-Pupil Distance

Stereo cues

 Motion parallex cues

– Image speed across the retina to judge depth – Only needs one eye

 (Con)vergence

– 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 rotation corresponding to viewing closer object) – Muscular feedback in converging and focusing the eyes is cue 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 temporal

component

 Constant sounds drop out of conscious awareness  Sounds are perceived from sources in all directions

Localisation

 Different factors influence ability to perceive

location 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 relative

motion between sound source and observer

• Changing volume:Sound perceived as approaching

when volume gradually increases (and vice versa)

External ear - pinnae

 Why do stereo speakers reproduce the

• riginal 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 the

skin.

 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, and

torque of limbs and other body parts

 Varying joint angles and muscular

length

Vestibular system

 Movement and

• rientation 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 to

see while moving

– Push-pull between RH and LH canals – 3 neurons link to eye – <10 ms lag in eye for head movement

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

Symptoms

 Vestibular disturbances

– Head spinning

 Reduced hand-eye coordination  Nausea  Effects take time to reduce after

exposure

 Nervous system does adapt