SPATIAL VISION ICS 280 Visual Perception Spatial Frequency Theory So far we have considered feature detection theory Recent development Spatial Frequency Theory The fundamental elements are spatial frequency elements Does not preclude having feature detectors Spatial vision No good convergence in physiology and psychophysics yet Slide 2 Unlike color vision ICS 280 Visual Perception 1 Gratings Images representing sine waves Frequency Orientation Amplitude Phase Slide 3 ICS 280 Visual Perception Fourier Transform Any image can be expressed as a linear combination of a bunch of sine gratings of different frequency and orientation Slide 4 Amplitude Phase ICS 280 Visual Perception 2 Fourier Synthesis These component gratings can then be added together to create the original image back Slide 5 ICS 280 Visual Perception Spatial Frequency Content Lower frequencies Global pattern of light Higher frequencies Slide 6 Feature details like edges ICS 280 Visual Perception 3 Spatial Frequency Theory Each channel sensitive to particular range of frequencies and orientations Can overlap with each other Similar to the color primaries Slide 7 ICS 280 Visual Perception Contrast Sensitivity Function CSF Present a sine wave of particular frequency Start from 0 contrast and keep increasing contrast Note the contrast at which it becomes barely visible from an uniform gray field Defines the contrast threshold for that frequency Performed for a range of frequencies Slide 8 ICS 280 Visual Perception 4 Contrast Sensitivity Function CSF Minimum contrast required to detect a particular frequency Maximum sensitive at 4 5 cycles per degree Slide 9 ICS 280 Visual Perception Testing Contrast Sensitivity Slide 10 ICS 280 Visual Perception 5 Calculating Cycles per Degree Distance of the subject from the screen in inch d Resolution of the screen in pixels inch r No of pixels per degree 180 d r No of sine cycles in 180 d r pixels tells the number of cycles per degree Slide 11 ICS 280 Visual Perception Changes with Illuminantion Sensitivity decreases with dark Especially in high frequency regions Lower visual acuity in dark The peak sensitivity occurs at lower frequencies Slide 12 5 to 2 cycles degree ICS 280 Visual Perception 6 Development with Age Not great for babies Infants cannot recognize people Monkeys and macaque have similar CSF as humans Slide 13 ICS 280 Visual Perception Development with Evolution Slide 14 ICS 280 Visual Perception 7 Temporal Contrast Sensitivity Present image of flat fields temporally varying in intensity like a sine wave If the flicker is detectable Cycles per second Slide 15 ICS 280 Visual Perception Filters Low pass filters Band pass filters Blocks high frequencies Image blurring Blocks both high and low frequencies allowing only medium ones High Pass filter Slide 16 Blocks low frequencies Edge detection ICS 280 Visual Perception 8 CSF and filters Both spatial and temporal CSF act as band pass filters How do they interact At higher temporal frequency acts as low pass filter Slide 17 ICS 280 Visual Perception How does this help us Detecting objects versus illumination Illumination changes are low frequency Both in space and time Morning to day to night Changes over regions slowly Can phase out illumination and be more sensitive to reflectance Insensitive to afterimages Slide 18 Usually blurred low frequency ones ICS 280 Visual Perception 9 Selective Adaptation of Channels Adaptation to certain ranges of frequencies Selective adaptation aftereffects Slide 19 ICS 280 Visual Perception Selective Adaptation of Channels CSF changes before and after adaptation Subtraction from the original CSF gives the response of the cells that are adapting Slide 20 ICS 280 Visual Perception 10 Selective Adaptation of Channels Multiple channels that adapt to different ranges of frequencies Slide 21 ICS 280 Visual Perception Selective Adaptation to Orientations Similarly for orientation Orientation adaptation aftereffects Slide 22 ICS 280 Visual Perception 11 Further Support Checking the threshold for square and sine grating of same frequency above 4 5 cycles per degree Should be same Square wave made of many sine waves Will be visible as soon as one of the sine waves are visible The threshold for the higher sine waves are lower Slide 23 ICS 280 Visual Perception Physiological Support Infinite sine waves Eye has finite receptive fields Local piecewise frequency analysis Small patches of sine waves that fade out Garbor Functions Slide 24 Multiplying sine waves with a gaussian ICS 280 Visual Perception 12 Physiological Support Garbor Functions Slide 25 Cells with such response found in the simple cells of visual cortex ICS 280 Visual Perception 13