Psychophysics علی یونسی

 What is psychophysics  Methods  Applications

Cones in Retina

Thresholds General definitions (not assuming linearity):  Absolute threshold: intensity that the observer can just barely detect  Intensities below absolute threshold: undetectable  Intensities above absolute threshold: detectable  Difference threshold ( aka. just noticeable difference /JND/ and difference limen) : minimum intensity difference that is noticeable to the observer  A change in intensity that is smaller than the difference threshold: undetectable  A change in intensity that is larger than the difference threshold: detectable

Difference thresholds  Linear function  difference threshold (slope) is constant  An observer able to detect the difference between intensities 100 and 110 should also be able to detect the difference between 1000 and This is not the case: the observer is able to detect the difference only between 1000 and 1100  500 & 550 Hz tones  5000 & 5050 Hz tones  5000 & 5500 Hz tones  Difference threshold is not constant! Stimulus intensity Sensation magnitude Linear psychophysical equation constant slope

Difference thresholds  Difference threshold is not constant (changes with intensity)  function is nonlinear  Weber’s law: difference threshold is a constant proportion of the initial stimulus value ΔI / I = c  Previous examples: c=10%  Weber’s law holds only approximately! Stimulus intensity Sensation magnitude Nonlinear psychophysical equation slope changes with intensity

Absolute thresholds  Even in the absence of stimulation, there is some random firing on sensory nerves  This inner noise can even vary from moment to moment  Observers cannot distinguish inner noise from the effect of a weak stimulus  Even when there is no light (perfect darkness), observers may experience a dim light (dark light, intrinsic light)  Observers in an anechoic chamber often report hearing a whistling sound  Measuring truly „absolute” thresholds is problematic: observers may confuse inner noise with the real thing

Psychophysical methods  Threshold measurements: detection of small intensities (absolute thr.) and discrimination of small intensity differences (difference thr.) Is it intense enough to see? How small a difference can you see?  Fechner’s 3 methods  Method of constant stimuli  Method of limits  Method of adjustment  Modifications of Fechner’s methods  Staircase method  Modifications of the method of constant stimuli (adaptive, no standard)  Forced choice, objective methods  Sensory decision theory (SDT)  Psychophysical functions from psychometric data  Direct scaling: growth of sensation with intensity How bright do you see a light?  Magnitude estimation and the power law  Multidimensional scaling: degree to which stimuli are comparable along some dimensions Along which dimensions do you judge the similarity of two stimuli?

Three methods  Presenting one stimulus at a time  The stimulus is very weak  Possible responses: “Yes, I see it.” / “No, I don’t see it.” Absolute thresholdDifference threshold Method of constant stimuli  Method of limits  Method of adjustment  not used Presenting two stimuli at a time: –Standard: fixed, easily detectable –Comparison: either more or less intense than the standard Possible responses: “Comparison is stronger.” / “Comparison is weaker.”

Method of constant stimuli for measuring absolute thresholds 1.Select a range of light intensities from certainly invisible to certainly visible 2.Pick a few (4-7) points uniformly in this intensity range; this will be the constant stimulus set WeakStrong Light intensity

Method of constant stimuli for measuring absolute thresholds 3.Test each stimulus many times (20-25) in random order …

Method of constant stimuli for measuring absolute thresholds 4.Present the stimuli one at a time and ask the observer if it was visible or not Visible? YESNO

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Could you see the spot of light?

Method of constant stimuli for measuring absolute thresholds 5.Calculate the proportion of “yes” and “no” responses at each light level %5%20%50%80%95%100%

Method of constant stimuli for measuring absolute thresholds 6.Plot the percentages against stimulus intensity  psychometric function Stimulus intensity Percentage “seen” 0% 100% 50% 75% 25%

Psychometric function for absolute thresholds Ideal FIG (Sekuler)  Fixed absolute threshold  Step function Actual FIG (Sekuler)  Absolute threshold varies somewhat from trial to trial (due to constant fluctuations in sensitivity)  Conventionally, the intensity corresponding to 50% is considered to be the threshold sigmoid function

Method of constant stimuli for measuring difference thresholds 1.Standard stimulus has a fixed intensity 2.The intensities of comparison stimuli bracket the standard Light intensity Standard stimulus: Comparison stimuli:

Method of constant stimuli for measuring difference thresholds 3.All pairs of standard and comparison stimuli are tested many times

Method of constant stimuli for measuring difference thresholds 4.For each pair, the observer judges if the comparison stimulus was stronger or weaker than the standard STRONGERWEAKER

Method of constant stimuli for measuring difference thresholds 5.For each comparison level, the percentage of “stronger” responses is calculated and results are plotted as a psychometric function Light intensity of comparison stimuli Percentage “stronger” 0% 100% 50% 75% 25%

Psychometric function for difference thresholds  When the observer cannot see a difference, he/she chooses randomly between “stronger” and “weaker”; this corresponds to 50% on the psychometric function  point of subjective equivalence (PSE) Light intensity of comparison stimuli Percentage “stronger” 0% 100% 50% 75% 25% PSE

Psychometric function for difference thresholds  By convention, the intensity at 75% is considered to be just noticeably stronger than the standard  DS  A comparison intensity at 25% is just noticeably weaker than the standard  DW  Difference threshold = the average of DS and DW Light intensity of comparison stimuli Percentage “stronger” 0% 100% 50% 75% 25% DW DS

Psychometric function for difference thresholds

Method of limits for measuring absolute thresholds  Ascending and descending series may yield different results  use both  Even in the same direction, there is variability in the threshold (inner noise, etc)  average many measurements  Measured threshold corresponds to 50% point in a psychometric function (method of constant stimuli) Light intensity Trials pure-tone audiometry

threshold estimate Method of limits for measuring difference thresholds  Intensity of the comparison stimulus is decreased (descending) or increased (ascending) until the response changes  Threshold estimate: intensity difference between the standard and comparison stimuli where the response changes  Average results from multiple series in both directions Light intensity of comparison stimulus Trials +comparison brighter -comparison weaker

Types of Visual Stimuli  Dots  Motion dots  Lines  Sinusoidal stimuli  Grating  Wavelet

شباهتها و تفاوتها

Cognitive Neuroscience  visual and auditory processing  attention  change blindness  multimodal integration  consciousness  decision-making  learning  memory  language  mirror neurons  mismatch negativity

Bio-signals of the Human Brain  Electricity  Brain  Heart  Muscle  Chemical composition  Hgb  Neurotransmitters  Movement  Eye movement

Attention  Positive:  How to attract the attention of audience to the main point of the meme?  Negative:  How to distract the attention of the audience from some points of the meme?

Consciousness  Cigarette and Women  Subliminal advertising

Emotions

Language  تمایز بین حب و بغض  بفرمایید  بنشینید   تمایز بین رویکرد مثبت و منفی  Government – regime  دانشمند – محقق - پژوهشگر

کوکایین الکل ارزیابی ERP در اعتیاد

تصویر صورت تصویر خنثی مطالعه ادراکی مطالعه شناختی

انواع تصاویر در مطالعه ادراکی

Conditions Causing Low Vision

 Low vision can result from  Anomaly (usually congenital)  Disease (inherited or acquired)  Injury  Conditions can be progressive or stable  Affect different visual functions  Can be acquired or congenital

Conditions Causing Reduction in Central Visual Function  Decrease ability to see visual details including color  Q : what functional problems would be expected in a person who is unable to see visual detail?

Common Refractive Disorders  Cause reduced acuity ONLY if left uncorrected  Common types  Myopia  Hyperopia  Astigmatism  Corrected with lenses or surgery

Role of Psychophysical Tests in Monitoring Glaucoma

Merits and limitations of manual perimetry, SAP, SWAP, and FDT Perimetry TypeAdvantagesDisadvantages Manual (i.e., Goldmann) Long track record Easier to do than SAP Nonstandardized Well-trained technician required SAP Quantitative and standardized algorithms Diagnostic and progression statistics available Expensive and nonportable equipment SWAP May detect defects and change earlier than SAP More influenced by cataract FDT Sensitive to early changes Good patient acceptance Portable — screening tool No progression software available Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1  S93.

Dyslexia  Stein & Walsh (1997) M deficit theory  Dyslexia as a dysfunction of higher-order processing of visual information, as manifested by deficits (i) in contrast thresholds for low- spatial-frequency, achromatic stimuli and (ii) in impairments of the transient system, such as impaired visual motion sensitivity, performance on which some claim remains deficient even for stimuli of high contrast and illumination.

 Journal of the Neurological Sciences Volume 300, Issue 1, Pages , 15 January 2011 Volume 300, Issue 1  A differential color flicker test for detecting acquired color vision impairment in multiple sclerosis and diabetic retinopathy  Received 13 May 2010; received in revised form 10 August 2010; accepted 1 September published online 29 September  Abstract  Background  Optic neuritis related to multiple sclerosis and diabetic retinopathy are relatively selective post-retinal and retinal vision disorders. Vision impairment in both conditions is reliably measured by testing critical fusion frequency (CFF).  Methods  To examine color vision, we measured the CFF in response to red and blue stimuli, and tested CFF values in patients without evident vision impairment. To ensure that differences in CFF values in a given subject depended only on color perception we displayed red and blue flickering stimuli at equal luminance. CFF to red or blue stimuli were compared in patients with medical history of optic neuritis related to multiple sclerosis (post-retinal vision impairment), patients with diabetic retinopathy (retinal vision impairment) and healthy subjects.  Results  The test procedure disclosed altered CFF values for red and blue stimuli in both groups of patients studied. The comparison between the two groups disclosed a prevalent CFF impairment for red stimuli in patients with optic neuritis related to multiple sclerosis and for blue stimuli in patients with diabetic retinopathy.  Conclusions  The differential color flicker test appears highly accurate in detecting color vision impairment. Comparison of the two color CFFs differentiates retinal from post-retinal visual disorders.

 spatial vision in chronic alcoholism measured by psychophysical methods. Psychol. Neurosci. (Online) [online]. 2009, vol.2, n.2, pp ISSN  We used psychophysical tests to evaluate spatial vision in 15 subjects with a clinical history of chronic alcoholism by measuring luminance contrast sensitivity and color discrimination. The subjects were initially subjected to clinical inquiry and ophthalmological exam. Subjects then performed psychophysical tests to measure spatial contrast thresholds using sine wave gratings of different spatial frequencies and contrasts and chromatic discrimination thresholds using the Mollon-Reffin test. For the analysis, subjects were divided into three groups according to age and compared with age-matched controls. Ten subjects had some degree of color vision loss, which was quite severe in seven cases. All subjects had normal luminance contrast sensitivity. The results suggest that color vision changes related to chronic alcoholism can occur in the absence of impairment of spatial luminance contrast sensitivity and thus is an important aspect to be considered in the clinical evaluation of this condition.

چگونه یک آزمایش سایکوفیزیکی درست کنیم؟  Matlab and PsychToolbox  PsychoPy  VisionEgg  E-prime  Presentation

Summary  Psychophysical procedures can be used to diagnose and monitor several ophthalmic and neurologic disorders.  It is easy to implement and can be readily combined with other methods such as imaging (fMRI) and electrophysiological (EEG, VEP, AEP) prcedures.