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 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 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; 2 µ m 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: θ D =IPD ⎡ Δ r ⎤ — —— r ⎣ r + Δ r ⎦ 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 original 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 orientation 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