Visual Sensory Systems

1. Visual Sensory Systems Chapter 4

2. 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

4. Illustration of Relationships

5. Receptor System: Eyeball & Optic Nerve

6. Visual Receptor System • Location – Retina center Fovea (cones) & Periphery (mostly rods) • Acuity (ability to resolve fine detail) – much greater when image on cones • 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 • Color sensitivity – only cones can discriminate all wavelengths • Adaptation – Light stimulation causes rods to rapidly lose sensitivity (slow response); cones insensitive to changes (sometimes hypersensitive with little stimulation causes night glare) • Differential wavelength sensitivity – cones sensitive to all wavelengths; rods are particularly insensitive to long wavelengths (red)

7. 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

8. 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

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

10. Depth Perception Pictorial 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

11. 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

12. 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

13. 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

14. Sensitivity & Response Bias

15. Interventions & Vigilance

16. 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

17. 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

18. Absolute Judgement