VISUAL SENSORY SYSTEMS

Chapter 4

The Stimulus: Light Visual stimulation is a wave of electromagnetic energy Visual spectrum has a point along a wavelength

Wavelength determines hue (color) – 400 -700 nm Amplitude determines brightness

Pure colored wavelengths are said to be saturated May be diluted with achromatic light (gray) e.g., pure red diluted with achromatic light yields pink

Measurement of brightness or intensity of stimulus reaching the eyeball is more complex Luminous intensity/flux 1 candela or 12.57 lumens Iluminance Foot candle or 10.76 LUX Luminance Candela/M2 or foot lambert Reflectance luminance/iluminance Brightness Actual experience of intensity

Illustration of Relationships

Receptor System: Eyeball & Optic Nerve

Visual Receptor System1. Location – Retina center Fovea (cones) & Periphery

(mostly rods)2. Acuity (ability to resolve fine detail) – much greater

when image on cones3. Sensitivity (ability to detect light) – rods much more

sensitive Scotopic vision – night vision where only rods active Photopic vision – enough light for both rods & cones

4. Color sensitivity – only cones can discriminate all wavelengths

5. Adaptation – Light stimulation causes rods to rapidly lose sensitivity (slow response); cones insensitive to changes (sometimes hypersensitive with little stimulation causes night glare)

6. Differential wavelength sensitivity – cones sensitive to all wavelengths; rods are particularly insensitive to long wavelengths (red)

Sensory Processing Limitations Contrast sensitivity – ability to detect contrast

is essential to detect and recognize shapes C = (L-D)/(L+D) CS = 1/CM

Contrast itself

Level of illumination

Sensory Processing Limitations (cont.) Reading Print – optimum print size >= 3

cycles/degree (stroke width 1/6 th degree); use familiar fonts; don’t use all uppercase or blocked letters

Color Sensation – best in well illuminated environment; 7% of males colorblind (protanopia). Most prevalent red-green Simultaneous contrast – when two colors next to each

other look the same Negative After Image – occurs when focusing on one

color too long. Night Vision –

loss of contrast sensitivity due to age and low illumination is big problem

at night rods are actively used – lack of perception leads to speeding or overdriving head lights

Bottom-up VS Top-down Prcessing

Bottom-up – stimulus that is there; Top – down – what we expect to be there.

Depth PerceptionPictorial Cues Linear perspective – converging parallel lines Relative size – objects known to be similar size

appear be different size Interposition – one object obscures contour of

another Light & shading – shadows provide evidence as to

location Textured gradients – distant objects have finer

gradient Relative motion or motion parallax – distant object

appear to move slower across the visual field

Visual Search & Detection Eye movements

Pursuit (following a moving object – plane in the sky)

Saccadic Characteristics – initiation latency, destination,

movement time, dwell duration, & useful field of view

Visual search Serial search model – discriminating target

from non-target (distracters) images, T=(NI)/2, top-down

Conspicuity – how well target stands out, bottom-up

Expectancies – experience/knowledge lead to target

Implications of Visual Search Knowledge

Knowledge of conspicuity should lead designer to enhance visibility of target

Knowledge of serial aspects should forewarn the designer about the cost of cluttered displays

Knowledge of the role of top-down processing should lead the designer to structure the search field to take advantage of past experience & intuition

Knowledge of all these influences should lead to visual search models that can predict how long it will take to find particular targets

Signal Detection Theory SDT assumes world can be modeled with

the signal being present or absent (e.g., to luggage inspector weapon is signal and hair blowers, calculators, etc. are noise)

Combination of 2 states of the world present 4 joint events – hits, false alarms, misses, and correct rejections

Sensitivity & Response Bias

Interventions & Vigilance

Difficulties in Mid-Air Detection Other aircraft inconspicuous – occupies

very small visual angle in field of vision Two aircraft flying toward each other

cover 1 mile in as little as 5 seconds Two aircraft flying toward each other do

not appear to be moving in the field of vision

Target may be camouflaged by clouds and ground noise

Not expecting other aircraft affects sensitivity because of top-down processing

Discrimination Ability to discriminate one signal from another Recognize that long wave length colors are

not easily recognized at night Recognize that some letters and numbers are

similar to others – 0 & O, E & F, 1 & l, etc. Recognize that placement and design of signs

& symbols can be confusing Recognize that too close proximity can cause

confusion Design accordingly

Absolute Judgement