ELEMENT 3

The Human Body3.5 The Senses

3.6 The Eye and Vision3.7 The Ear and Hearing

3.8 The Inner Ear and Balance3.9 Sensory Inputs and Spatial Disorientation

AIM & END STATE

• Aim:– To discuss the senses and how they relate to

human performance limitations• End state:

– A greater understanding of the human body’s strengths and weaknesses

• References: In the CRM Reference Manual.

3.0

SENSES• How many senses do we have?

– Classic senses are five but perhaps up to 21– Nine senses are commonly agreed upon and

are tested in aircrew medicals• A sense is an ability to

interpret information about our environment that has been detected througha sensor, eg chemorecptors in the nose.

3.5

SENSES• The extra senses we shall investigate:

– Equilibrioception – balance– Thermoception – ability to detect heat/cold– Proprioception – ability to tell where the body is

in relation to the environment– Nocioception – ability to detect pain.

• Equilibrioception and Proprioception are tested by aviation physicians during aircrew medicals.

3.5.1.2

SENSES• Sensory Thresholds

– The limit of the ability to detect a change in a stimulus: Absolute, Recognition, Differential, Terminal.

• Sensory Sensitivity– Detection ability (1st meaning) or a disorder

where a person experiences a debilitating sensory overload from everyday experiences, eg touching fabric or a bad smell.

3.5.2

SENSORECEPTORS• Stimuli detected through SensoReceptors:

3.5.4

THE EYE• Most important

sensor for humans• More than 80% of

our sensory information about our environment comes through our eyes.

3.6

THE EYE• Light passes through the cornea and lens

and falls upon theretina, focussing inparticular on thefovea centralis

• Accommodation isthe way the lens isbent by ciliary musclesto focus light.

3.6.2.1

THE EYE• Rods and Cones are photoreceptors that

detect light– Cones detect colour and

detail – day/central vision– Rods detect low light

levels and movementnight/peripheral vision

• Rhodopsin and Iodopsinare two chemicals thathelp in light detection.

3.6.2.2

THE EYE• Visual Acuity (VA) is the ability to

discriminate the fine details of an object thus is a requirement of depth perception

• VA is measured as a fraction (20/20 imp 6/6 metric)– 20/40 or 6/12 is half as good as the average

• VA can be affected by disorders such as– Myopia (shortsightedness)– Hyperopia (long sightedness).

3.6.3

THE EYE• Presbyopia

– Loss of VA due to age– Becomes very noticeable in a person’s 40s– Reduction in suppleness of the lens and

weakness in the ciliary muscles causes a reduction in accommodation

– Hard to read fine print; things are a little blurry; hard to focus between distances; eye strain after reading for a long time.

3.6.3.2

THE EYE• Visual Field

– What a person can see. Depends on light falling on the retina (central and peripheral vision)

• Binocular Vision– FOV 200 deg both eyes;

160 deg one eye– Uses parallax error for

depth perception to approx 12m– Provide Visual Summation: two eyes are 41%

more effective at detecting stimuli than one eye.

3.6.4

THE EYE• Depth Perception: the ability to estimate

distances without measuring them– Relative Size– Interposition– Linear Perspective– Aerial Perspective– Light and Shade– Monocular

Movement Parallax

3.6.4.2

THE EYE• Blind Spots: areas on the retina that do not

receive light information– Day Blind Spot:

• Optic Disc• No cones

– Night Blind Spot• Fovea Centralis• No rods• Offset vision

by 10-20 deg.

3.6.5

THE EYE• Glaucoma: Pressure build up in the corneal

area (anterior and posterior chambers) causes damage to the fovea– Usually gradual– Can easily be

checked– Easily fixed if

caught earlybefore damage to retina is done.

3.6.6

THE EYE• Hypoxia and Colour Vision: Vision affected

by loss of oxygen, especially at night– 10% at 5,000’ and 28%

at 10,000’ PA especially during mesopic (dawn/dusk) periods

– Breathing oxygen increases visual acuity and colour discrimination.

3.6.7

THE EAR• The Ear: Primary organ of audition; second

most important organ for humans– Detects sounds– Inner ear detects balance– Pressure waves in the air

(sound waves) are caughtby the pinna, funnelled intothe auditory canal to theeardrum which vibrates.

3.7

THE EAR– The vibrations are transmitted through the

ossicles to the cochleawhere they are detected bymechanoreceptorsthat induce a signal to the brain

– Semi-Circular Canalsdetect movementby detecting radial and linear accelerations.

3.7.1

THE EAR• Hearing Loss: Two main categories:

– Conductive• Damage or blockage

due to injury or diseaseto the outer and/ormiddle ear

– Sensorineural• Usually caused by

damage to the inner ear’s stereocilia cellsdue to exposure to loud noise.

3.7.3

THE EAR• Noise Induced Hearing Loss:

– Part of the speech range in the audible range is first to go.

3.7.3

THE EAR• Balance: Equilibrioception is through the

organs of the inner ear– Semi-Circular Canals

arranged in x,y,z axes– Otoliths arranged in

x,y axes in utricle– Detects movement by

relative movement ofendolymph fluid in SCCor by small crystals in gelatinous mix in utricle.

3.7.3

DISORIENTATION• Spatial Disorientation (SD) is the inability of

the operator (eg pilot) to understand where the apparatus (eg aircraft) is in relation to the earth.

• Usually due to poor interpretation of, or weak sensory stimuli, especially:– Vision, Equilibrioception and Proprioception

• ATSB says that the career incidence of SD occurs to 90 to 100% of pilots… (it will happen to you)

3.9

DISORIENTATION• Types of SD:

– Type 1: SD is not recognised by individual• Acft usually remains under control but is flown into a

dangerous situation (eg terrain)– Type 2: SD is confusing but not incapacitating

• Operator knows something is wrong. Information does not correlate with what the body/mind is saying

– Type 3: SD is incapacitating, potentially fatal• Operator is physically/psychologically overwhelmed

and is usually unable to recover from the situation.

3.9.1

DISORIENTATION• Vertigo – usually associated with an

abnormality in the vestibular organ. Similar effect to SD– Flicker vertigo: caused by flashing light

1 Hz to 20 Hz– Often in helicopter ops:

• Front seats occupantsor patients lying in rearwhere the sun canflicker throughrotor blades.

3.9.2

DISORIENTATION• Pressure Vertigo – due to imbalance in

pressure between middle ears – often due to imbalanced valsalva attempt

• Medical Vertigo – usually due to temporary problem in middle ear

• All types of vertigo will cause dizziness, nausea and disorientation to some degree

• Can happen to anyone at anytime.

3.9.2

DISORIENTATION• Categories of SD:

– Somatogyral: Dark Night Take Offs– Somatogravic: Graveyard spiral– Visual:

IllusionscausingSD

3.9.3

DISORIENTATION• Black Hole Approach: Illusion of excessive

altitude when there are no lights below the aircraft during an approach– Often leads to incorrect control inputs

• Boeing study found that 52% of accidents/fatalities occur during the last 4% of flight (approach/landing phase)– Most of those were due to pilot error when no

precision approach aid (glidepath) was used.

3.9.3