Transcript Basics of Stimuli
Basics of Stimuli
John H. Krantz Hanover College
Outline The nature of the beast Basic Perceptual Issues Spatial Temporal Color Luminance
The General Beast
Screen Mosaic Triad Arrangement
Screen Mosaic Color Strip Arrangement
Interlaced Projection
Progressive Projection
The Electron Beam 0.5
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Seeing the Flicker If can’t get movie by clicking on the picture, click here .
Spatial Issues
Spatial Inhomogeneity Aliasing Edge Effects and Contrast
Spatial Inhomogeneity The same output value leads to different luminance outputs at different screen locations.
Thus, you get different outputs at different screen locations.
In general, draw to center if critical
CRT % Change from Minimum 45 40 35 30 25 20 15 10 5 0
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How to Handle Spatial Inhomogeneity Keep stimuli in central region
Aliasing Technical definition: When an image contains frequencies beyond the range of the sampling matrix, the wrap-around and occur as lower frequencies, distorting the image
Aliasing Description of aliasing Generally try to draw and image that has too fine of detail or sharp edges Causes “jaggies” And pixel artifacts That is you can notice the pixels
Aliasing and Pixel Effects
Craik-Cornsweet
Craik-Cornsweet: Described
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The figure above is an exaggerated map indicating the light levels across the image on the previous slide. Note how the center and edges have identical luminance. That can be seen by sitting far enough away from the screen
Minimal Contours
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Minimal Contours Described There are two circles below. Both circles have the same luminance (intensity level) at the center.
Click on your mouse and This one changes abruptly watch as the edges are blurred to the level at the center.
and the circle disappears.
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Anti-aliasing Solution to aliasing Bandlimiting or anti-aliasing Essentially blur the edges or image so fine detail is lost In a lot of Software Ideally use a gaussian filter This is the shape of CRT electron beam which is why dots on screen never show up Infante (1985); Silverstein et al. (1990) Many graphics software and even some languages anti-alias (Java2D)
Temporal Issues
Motion reproduction Flicker Stimulus Timing
Motion Reproduction Frame rates of monitors far exceed what is needed for movement update NTSC video: 30 updates per second Movies: 24 updates per second Most web video 10 to 15 frames/sec
Computer Video Most Update rates are 10-15 frames/sec 5 fps 10 fps 15 fps Click on pictures to start each of the movies. If that does not work: click here for the left hand movie, click here for the middle , and here for the right .
Flicker Critical threshold of flicker rates is about 60 Hz in the fovea But gets higher for larger stimuli Recommended flicker rates between 66 Hz and 120 Hz (Bridgeman, 1998) Most monitors are adequate in this value Notice difference between flicker and frame update rate.
Stimulus Duration Pixels are not on continuously during a frame In part this is necessary for clean motion Typical CRT phosphors last about 4 msec. (Bridgeman, 1998) On LCD and other technologies, persistence is longer Makes motion less clean but flicker less noticeable
Differences in Persistence Click on pictures to start movies. If that does not work, click here for the crt and click here for the lcd .
Determining Stimulus Duration Possible errors: Not knowing which frame the stimulus is first presented on Assuming the stimulus is visible from the beginning of the display Stimuli are not visible at beginning of frame but some time later Delay depends upon where on frame stimulus is and frame rate This is a constant error
Effects on Timing If no more than one frame delay: Additional error of up to 17msec added to all conditions No longer msec timing but really 60 Hz timing If frame delay is larger effects are worse
Example Results from Myors (1999)
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Luminance Issues
Gamma
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Gamma
Gamma Correction Typically well described by: L=L 0 +
a
(V-V 0 ) g g is typically near 2.3; L, L 0 ,
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, g are affected by contrast and brightness settings Can’t be done on web Make sure stimuli are not sensitive to distortions in gray scale High contrast Photo’s may be distorted slightly and impact results (Ruppertsberg et al., 2001) Don’t require fine distinctions
Chromatic Issues
The Trichromatic Theory Color Picker The Color Gamut Glare Color Blindness
The CIE Color System A set of Equations that allow predictions of matching.
Used in photo printing, TV and film.
Wyszecki & Stiles (1967)
The Color Gamut The range of colors reproducible by any system 0.9
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The Effect of Illuminance on Gamut 0.9
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In Dark With Lights on With Lights on and 30% Luminance Level
Illuminance Agostini & Bruno (1996) Accuracy of the perceived stimulus is affected by the amount of illuminance.
D. F. Neri (1990) Combination of light and gamut changed chromaticity Request close windows and turn off unnecessary light Do not use subtle color differences
Color Blindness Description Click on picture to start movie. If that does not work, click here .
Color Blindness Click on picture to start movie. If that does not work, click here .
Color Blindness Click on picture to start movie. If that does not work, click here .
Color Blindness Click on picture to start movie. If that does not work, click here .
How to handle Screening questions?
Large sample and random assignment Pre-test on color blind
Summary Spatial Aliasing Edges Temporal Frame per second in video Luminance Gamma Color Glare Color deficiencies