생리현상에 대한 공학적해석 10: The Eye as a Transducer ABSTRACT • Visible light; 380nm~825nm : lens, retina • Cone cell; at the foveola(center of the eye) : 3um in diameter, 0.7' visual angle • Amplitude range : 4500:1(cone) ; 7 min dark adaptation time : 22:1(rod); 30 min dark adaptation time • Photoreceptors : generate graded potential : proportional to logarithm of visual stimulus 1 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer ABSTRACT • Retina; five layers 1. photoreceptors; receptor potential 2. bipolar cells ; subtraction 3. horizontal cells; gather receptor potentials → local value 4. ganglion cells; translate to APs 5. amacrine cells; detection of illuminance change • Cones 1. red cones; 65%, respond maximally to yellow 2. gg;reen cones; 33%, resppygond maximally to green 3. blue cones; 2%, respond maximally to indigo 2 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer ABSTRACT • SbjtiSubjective sensati on; sat ura tion &h& hue : depend on the ratio red: green: blue(CR:CG:CB) :white: white =(CR :C: CG :C: CB) = 90:45:1 • Standard chromaticity diagram : 0.9 log CR + 0.1log CB -log CG ↔ log CG -log CB - topologically similar to standard diagram : 3-D map in visual cortex; 2-D color map + illuminance : chromaticity diagram for color blindness • Contrast enhancement : B&W enhancement, color enhancement : hypothetical retinal model 3 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer Charac ter is tics o f the Eye • Viibllihisible light; e lectromagnet ic wave : 825nm(3.6×1014Hz)~380nm(7.9 ×1014Hz) : atoms & molecules act like tiny antennas • Why sensitive to only this small range? : only this range reach to the earth from sun : for some radiowave, reach through, -but much to small to act as a antenna : phthotorecep tor cell ; organ ic compoun d(hdd(rhodops i)in) - visible light → receptor potential - by electrochemical cycles 4 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer Cross section of Human eye • Lens; squeeze using accommodation muscle • Normal; - relaxed lens; looking at distant object on retina - 10yrs old; change focal length by 2.3mm (focus an object 8cm away) - 70yrs-old; change focal length by 0.22mm (sufficient for an object 1m away) 5 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer 5 layers in Retina Fig.50-11 6 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer 5 layers in Retina • Reversed direction -wrong evolution direction & not able to turn back - photoreceptor; require high oxygen tension : closer to blood supply • Light → ganglion → amacrine → bipolar → horizontal → photoreceptor - essentially transparent • Optic disk - optic nerve through the hole in the retina -15° toward nasal side → blind spot - blend in with surroundings 7 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer Foveola • Central area for "looking at" an object : 34,000 photoreceptors : 0.6mm in diameter : each receptor - 3um= 3000nm = 3.6 × wavelength of red light → visual acuity - (3um/15000um) × (360° /2π) = 0.011° = 0.7’ ≅ 1' → limiting resolving power of the eye • 1' for a TV screen? - minimum viewing distance? - (screen height/495) ×(360' × 60/2π) = 7 × height 8 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer Photoreceptors • Rod - rod shape, sensitive to dim light, free of color - maximally sensitive to 512nm(green; dark gray) - no rod in fovea; increase away from the fovea - maximal visual acuity; 7% of cone - high sensitivity sacrificing resolution • Cone - cone shape - dominate & inhibit rods under "daylight" condition - color vision 9 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer Relative Time Response • Lux : SI unit f or ill um inance : power : 1 lux = 1 lumem/m2 • √lux : measure of amplitude : 33020 √lux ~ 0 .003 2 √lux → 100,000:1(107 for ear) 10 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer Relative Time Response • Cone photoreceptor : from 320 to 0. 5 : sudden decrease from maximal tolerable white to black : pupil start to open; - opening slowly increase to maximum in 7 min. - 0.07 √lux ; completely absence of light - some chemical & neural change occur • RdRod p hthotorecep tor : 0.07 √lux; cones cease to inhibit the rods : start to increase sensitivity : due to slow chemical & neural factor : 0.0032 √lux after 30 min from the 320 √lux level 11 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer Retinal Responses to Steady Light • Retina : Basic sense of light : 2 layer - photoreceptor - ganglion cell : 3 more layers Fig. 10-3 - enhance contrast - signal motion • Simple Circuit : for steady state input : excluding - motion detection 12 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer Signa l Process ing in Re tina • Photoreceptor(P) -coppypge with the 600:1 dynamic input range - measure average intensity - i/o characteristics can be changed by " chemical & neural changes" - transfer function for average light intensity : lateral communication between cones : feedback from horizontal(H) cell : constant saccadic motion - wander within a field randomly - 50 cone diameter in 4s : eachllifftdbllh cell is affected by local average 13 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer Signa l Process ing in Re tina • Compressifion of output : 600:1 : w/o compression - 20Hz for minimum, 12,000Hz for maximum → ? : with compression - 20~12,000 → 20~500Hz AP frequency • Color dependent : only weakly to brightness : extract ratio(red/green , blue/green.. ) : logarithm + subtraction : red(270), green(135); log270 - log135 = 5. 60 - 491=0694.91 = 0.69 : red(27), green(13.5); log27 - log13.5 = 3.29 - 2.60 = 0.69 (same hue) 14 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer • Logarithmic compression : transfer fn . changes as average intensity change : VP = 16 + 5 log(LZ/LZ0) - valid 0.0408 < LZ/LZ0 < 24.5 → 0.0408:24.5=600:1 - LZ0 = average of local light intensity -VP = cone's output potential -LZ = input light intensity : maximum value = 16 + 5 log(24.5) = 32mV : minimum value = 16 + 5 log(0.0408) = 0mV : input: local average VP = 16 + 5 log(LZ/LZ0 ) =16mV 15 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer Idea lize d Neura l • P cells Model - respond to light - logarithmic compression • H cells - wired uppg to measure average • B cells - excited by P cell Fig. 10-3 - inhibited by H cell - contrast enhancement : VB=VP-0.5VH - spherical bipolar structure : up & down • A cells - concerned with motion • G cells - convert GP to AP 16 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer CtContras t • Visual scene presentation Enhancement : resulting bipolar cell output; VB : average input light intensity (LZ0) = 50 • First Experiment : dark background except for a bright center : VP; 16mV(background), 32mV(saturation) Fig. 10-5 : VH; 21mV(center) to 16mV(peripheral) - remote H cell; attenuation of long dendrite -VH is biased for nearest cone neighbors -r² : ΔVH = (ΔVP/π) e ; Gaussian model -r² -r² : ΔVH = (16/π) e → VH = 16 + 5.093 e :V: VB = VP - 05V0.5VH - at center; VB=32-0.5(21.093)= 21.45mV -far away; VB =16 - 0.5(16) = 8mV 17 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer CtContras tEht Enhancemen t • Step transition -black(2.04; 0mV) → white(97.96;19.36mV) → 19mV jump Fig. 10-7 - VP: step function -VH = (ΔVP/2)( 1+ erfc x ) -VB = VP -0.5VH : 0mV[left] → -4.84mV → 14.52mV(19mV jump) → 9V9.7mV[i[rig h]ht] 18 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer Contrast Enhancement Fig. 10-8 • 5 vertical gray strips - differences in subject and actual intensity variation - effect of large contrast change 19 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer CtContras tEht Enhancemen t • Incorrect assessment of small area brightness - medium gray(30:VP=13.45) : in dark gray (10: VP =7.95) : in bright gray (90: VP=18.94mV) Fig. 10-9 - strips are narrow → not seen by the H cell -VB = VP -0.5VH : VB= 9.47mV(left) : VB = 3.98mV((gright) : left strip looks brighter than right one 20 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer CtContras tEht Enhancemen t • Sinusoidal gratings - 0.2 cycles, 1 cycle/unit - compression of high intensity : spatial harmonics -H cell : smooth out the VP variation : DC + fundamental component -VH0 =14.88mV 2 - VH1 =VP1exp(-w /4) : VH1 = - 2.695cos(1.257x) mV : VH1 = - 0.0002cos(6.283x) mV (H cell doesn't see horizontal gratings) -VB =8.3mV(lower f.), VB =11mV(higher f.) : more sensitive to higher freq.(smaller object) 21 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer CEhContrast Enhancement • Ramp transition : VP = 16 + 5 log(LZ/50) : cusp in VB of enhanced contrast Fig. 10-11 : more slope discontinuity : enhancement is less in high brightness 22 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer MhBdMach Bands • EitExperiments : 60 revolution/sec → smooth transition w/o flicker : 135° black ↔ 45° black : displays bumps at edges Fig. 10-12 - MhBdMach Band - white & black bands at edges : calibration ramps → location of equal brightness → estimate size of bumps : Mach Bands in low contrast case 23 서울대학교 대학원 의용생체공학 협동과정 생리현상에 대한 공학적해석 10: The Eye as a Transducer MhBdMach Bands • Difference from the out put of bi pol ar cell VB (1) rounded edge instead of sharp edge (()2) chang ing slop e - small slope; no Mach Bands - increased slope; increased Mach Bands - vertical slope; decreased Mach Bands! (3) only in central region of eye(foveola) ↔ contrast enhancement is retina