Outline. Basic principles of psychophysical techniques. Αντίληψη της όρασης. Οπτική συµπεριφορά - αξιολόγηση. Ψυχοφυσικές µέθοδοι (Psychophysics)

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1 ΟΠΤΙΚΗ Κ ΟΡΑΣΗ Outline Basic principles of psychophysical techniques Psychophysical methodology Absolute threshold of vision (laws of visual detection) Temporal aspects of vision Σωτήρης Πλαΐνης, PhD ΒΕΜΜΟ Visual Science Lab Μάιος 2005 ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 1 ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 2 Αντίληψη της όρασης Οπτική συµπεριφορά - αξιολόγηση Η επεξεργασία του σήµατος στην οπτική και στην ανώτερη νευρωνική οδό είναι µη-γραµµική (non-linear) και πολύπλοκη. Η Η όρασή µας συλλέγει σηµαντικσ µαντικό αριθµό πληροφοριών (ως προς την λεπτοµέρεια, το χρώµα, την κίνηση κ κτλπ) που απαιτείται ι να αποσαφηνίσουµε για να αποκριθούµε κατάλληλα Απαραίτητη η διερεύνηση µεθοδολογιών για την αποκωδικοποίηση των φυσιολογικών και ψυχολογικών χαρακτηριστικών της όρασης. Τρεις προσεγγίσεις χρησιµοποιούνται ιούνται για την αξιολόγηση της οπτικής συµπεριφοράς: Ανατοµική (µελέτη ανατοµικών χαρακτηριστικών νευρώνων/δοµών) Νευροφυσιολογική (µελέτη επεξεργασίας των οπτικών πληροφοριών σε διάφορα στάδια της οπτικής οδού) Ψυχοφυσική (διερεύνηση λειτουργικών χαρακτηριστικών του οπτικού ύ συστήµατος, π.χ. ο τρόπος κατά τον οποίο χιλιάδες νευρώνες συνεργάζονται για την «καταγραφή» του χρώµατος, σχήµατος, λεπτοµερειών των εικόνων) ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 3 ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 4 Modern psychophysics Ψυχοφυσικές µέθοδοι (Psychophysics) Any examination of the eye and the visual system consists of a series of psychophysical tests (eg visual acuity, refraction, visual fields, phoria testing, colour vision). Visual psychophysicists seek to contribute to neurology, ophthalmology or optometry by devising non-invasive procedures for: strengthening differential diagnoses of diseases monitoring therapy (effectiveness of treatment) developing screening tests for high-skill tasks (eg driving a car, flying an aircraft) Ψυχοφυσικές µέθοδοι: συσχέτιση των φυσικών παραµέτρων των ερεθισµάτων (input) µε τη φυσιολογική δραστηριότητα (αποκρίσεις) των νευρώνων και τις αντιλαµβανόµενες αποκρίσεις (output), δηλ. αυτό που ο εξεταζόµενος αντιλαµβάνεται και αναφέρει. Fechner (1860) proposed that a motor response could be used as a measure of the output of a sensory system. Sensory system ~ black box Input A Output Input A B C Output ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 5 Γραµµικό σύστηµα περιγραφής οπτικής επεξεργασίας (µε ή χωρίς υποσυστήµατα) ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 6

2 Ψυχοφυσικές µέθοδοι Ερέθισµα ραστηριότητα Νευρώνων Αντίληψη Example: Human vs. Neuronal Contrast P cells have much poorer sensitivity to luminance contrast compared to M cells Human CSF Inner Νευροφυσιολογία Psychophysics Έρευνα συσχέτισης Outer Psychophysics ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 7 Contrast Neuronal CSF P cell M cell Spatial frequency (c/deg) Kaplan and Shapley (1982) Log Contrast Spatial frequency (c/deg) The concept of threshold Ψυχοφυσικές µέθοδοι: Πως υπολογίζεται η ουδός (threshold)? An index of howh sensitive a sensory system is. ( = 1 / threshold) Determined by measuring how much of a particular stimulus is required to reliably detect that stimulus Threshold is defined as the minimum quantity of a stimulus required to elicit a response (for a light stimulus is the intensity which allows it to be "just seen ). Stimuli which exceed threshold are referred to as suprathreshold. ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 9 ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 10 Threshold determination Methods of threshold determination: Method of adjustment (ρύθµισης) method of limits (ορίων) method of constant stimuli (προ-καθορισµένων τιµών) Forced-choice choice procedure (υποχρεωτικής επιλογής) adaptive methods (προσαρµογής) Μethod Οf Αdjustment (ΜΟΑ) Subject controls the intensity (chromaticity) of stimulus Subject makes small adjustments to the stimulus values UNTIL: it is judged to be (in the case of a visual stimulus) "just visible" (in a detection ask) it matches a standard (comparison/matching) stimulus (in a matching experiment) Matching task ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 11 ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 12

3 Μethod οf Limits The method of limits: : stimulus valuev (eg g intensity) is presented in an ascending series es (non-seeing to seeing) and/or descending (seeing to non-seeing) sequence. The subject indicates on each trial (on each presentation) whether er the stimulus was "seen" or "not seen" Μethod οf constant stimuli increment Α variety of pre-determined stimulus values, above and below stimulus, l threshold, are presented in random order, so the subject cannot Background (L) anticipate the intensity of the stimulus on any given trial. The percent (Y) responses can be plotted as a function of stimulus strength, and a psychometric function can be described Alternating ascending and descending ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 13 A perfect observer would give the same response (threshold) each time the experiment is performed Threshold is usually defined as the intensity that results in detection of the stimulus on half of the presentations (or 75%) Percent of responses Frequency-of-seeing curve stimulus ideal oberver Intensity Threshold Forced-Choice Procedures (FC) Adaptive Μethods (staircase) Subjects are presented with two or more alternatives, and must select one on each trial even if the stimulus was not clearly seen. The choice can thus be coded as "correct" or "incorrect". Alternatives can be presented sequentially (temporal forced-choice), or can be presented simultaneously (spatial forced-choice). choose the odd A modification of the Method of limits An example of descending staircase for which stimulus intensity is decreased when the stimulus is perceived and increased when it is not perceived. An example of using two interleaved staircases. Stimuli from the respective descending and ascending staircases are presented on alternate trials. ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 15 ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 16 Absolute threshold Absolute threshold of vision Absolute threshold: the minimum amount of a stimulus required to simple detect it against a background (e.g., detecting light in an absolutely dark room). Increment (difference) threshold: : the threshold for detecting that two stimuli differ in some characteristics, such as intensity, when the stimuli are adjacent or superimposed (is also referred to as just noticeable difference, JND). ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 17 ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 18

4 Tasks vary in complexity Stimulus threshold depends mainly on its characteristics (eg( intensity,, wavelength, size, exposure duration, shape, location on the retina), but also on the complexity of the task Detection task: : measures whether a subject does or does not see something (eg( increment threshold) Recognition task: : the object is already visible and the subject is asked to name (or categorise) the object (eg( Visual Acuity) Discrimination task: : measures the threshold for detecting that a test stimulus is different from a reference stimulus (eg( contrast matching functions) Visual Detection ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 19 ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 20 Visual function light levels Background luminance is important Luminance (log cd m -2 ) Luminance of white paper in Starlight Moonlight Indoor lighting Sunlight Scotopic Visual Mesopic Photopic Function Rod Cone Rod saturation threshold threshold begins Damage possible Total darkness 10-3 cd m cd m cd m -2 No colour vision Poor acuity Good colour vision Good acuity Cone saturation ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 21 1asb = 0.1 cd/m 2 Angular distance from fixation Visual threshold Background luminance Photoreceptor distribution 180 N 60 Scotopic Mesopic Photopic T Eccentricity, deg.. Density, 1000/mm Rods Cones Blind spot Eccentricity ( ) ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 23 replotted from Oesterberg (1935) ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 24

5 Dark Adaptation (DA) Dark adaptation - Background levels Two distinct regions: -1 initial fast adapting segment - cones final protracted phase - rods Increase in sensitivity (~ 5-7 log units) Factors affecting DA: wavelength stimulus size retinal eccentricity Duration (+intensity) of pre-adapting light ambient illuminance Log Absolute Threshold (cd / m 2 ) cone curve rod curve Time (min) Log Threshold (log cd/m 2 ) Time (min) 5.0 lux 0.5 lux 0.1 lux Absolute Darkness ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 26 Dark Adaptation - Ageing Weber s law. Increment threshold Both initial (cone) and final thresholds increase with age. Decrease in scotopic visibility is partially due to: df the ageing lens, pupillary miosis changes in retinal metabolism degeneration of visual pathways Absolute threshold (cd m -2 ) L.G. (19 yrs L.L. (22 yrs P.K. (26 yrs P.H. (61 yrs T.D. (76 yrs in dark (min) Time (mins) ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 27 How much brighter the target has to be from its background? Increment threshold ( I) or Just Noticeable difference is not an absolute amount of stimulus, but is a constant proportion of the background "standard" stimulus, I. Weber fraction ( I / I) increment stimulus, l Background (I) distance l Intensity Weber s law. luminance matters When background luminance > 0.1 cd/m 2, the Weber fraction stays constant, ie the larger the background luminance the larger the stimulus luminance has to be to be seen as just noticeably different. Ricco s law size is important Reciprocal relationship between background luminance (L) and area of stimulus (A): L*A= constant, L = k L L / L = constant At low background levels L increases approximately with the L (De-Vries law) At very high luminances, neither relationship holds, because both rod and cone systems saturate Ricco s law is due to spatial summation and holds for stimulus areas less than a critical area. It depends on retinal location: at 4-7 eccentricity: critical area = 30 at 35 eccentricity: critical area = 2. For larger areas there is partial (spatial) summation: threshold declines with increasing spot size at a slower rate (Piper s law) L* A=Constant Ricco s law Piper s law Barlow, 1958

6 Bloch s law.duration matters Reciprocal relationship between background luminance (L) and stimulus duration (t) L*t = constant Bloch s law is due to temporal summation and holds for stimulus areas less than a critical duration (~100 ms) Temporal aspects of vision For flashed stimuli with durations longer than the critical duration, threshold depends solely on the luminance of the stimulus. Barlow (1958) Critical Flicker Frequency (CFF) Ferry-Porter law (CFF) CFF (in Hz) is the transition point from a flickering appearance to one of a continuous light. CFF increases with luminance of the stimulus, rising in direct linear proportion to the logarithm of flash luminance (L) A simple measure of temporal resolving power of the visual system Variables of stimuli that affect CFF luminance spectral composition size retinal position CFF = klogl+b, This linearity breaks down at high luminances k=slope, b=constant ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 33 Hecht, 1933 ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 34 CFF - Spectral composition Equal energy spectral lights have different luminances Spectrally different lights have different CFF values The photopic (cone) branch is the same for all wavelengths At low luminances CFF is higher for the shorter wavelengths (rods absorb better at these wavelengths) CFF - Effect of eccentricity CFF depends on the relative proportion of the rods/cones confined to a limited area. at 0 deg - Ferry Porter Law - only cones at 5 deg - rods operate below 0.1 trolands at 15 deg - more rods - slight increase in CFF at.001 photons blue note very low levels of CFF with rods (<10Hz photopic) blue red red Hecht, 1936 ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 35 ΠΛΑΙΝΗΣ, ΒΕΜΜΟ 36