Psychophysics in Clinical Settings

Psychophysics What is psychophysicsMethodsApplications

Cones in Retina

ThresholdsGeneral definitions (not assuming linearity):Absolute threshold:intensity that the observer can just barely detectIntensities below absolute threshold: undetectableIntensities above absolute threshold: detectableDifference threshold (aka. just noticeable difference /JND/ and difference limen):minimum intensity difference that is noticeable to the observerA change in intensity that is smaller than the difference threshold: undetectableA change in intensity that is larger than the difference threshold: detectable8Limen [laimen]Difference thresholdsLinear function difference threshold (slope) is constantAn observer able to detect the difference between intensities 100 and 110 should also be able to detect the difference between 1000 and 1010. This is not the case: the observer is able to detect the difference only between 1000 and 1100500 & 550 Hz tones5000 & 5050 Hz tones5000 & 5500 Hz tonesDifference threshold is not constant!Stimulus intensitySensation magnitudeLinear psychophysical equationconstant slope

9Example: you initially show a spot of light with intensity of 100 (easily visible) then start increasing the intensity. It is at 110 when the observer reports seeing a change. Difference threshold=110-100=10. Repeat this starting with 1000. Observer needs a much larger change in intensity to be noticeable, e.g. changing to 1100 units.Difference thresholdsDifference threshold is not constant (changes with intensity) function is nonlinearWebers law: difference threshold is a constant proportion of the initial stimulus valueI / I = cPrevious examples:c=10%Webers lawholds only approximately!Stimulus intensitySensation magnitudeNonlinear psychophysical equationslope changes with intensity10Slope helyett behzott vonalakkal (~Wolfe Fig 1.11)?rint = tangentDelta I: difference thresholdI: initial stimulus intensityc: constantAbsolute thresholdsEven in the absence of stimulation, there is some random firing on sensory nervesThis inner noise can even vary from moment to momentObservers cannot distinguish inner noise from the effect of a weak stimulusEven 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 thing11Trained subjectsPsychophysical methodsThreshold 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?Fechners 3 methodsMethod of constant stimuliMethod of limitsMethod of adjustmentModifications of Fechners methodsStaircase methodModifications of the method of constant stimuli (adaptive, no standard)Forced choice, objective methodsSensory decision theory (SDT)Psychophysical functions from psychometric dataDirect scaling: growth of sensation with intensityHow bright do you see a light?Magnitude estimation and the power lawMultidimensional scaling: degree to which stimuli are comparable along some dimensionsAlong which dimensions do you judge the similarity of two stimuli?Three methodsPresenting one stimulus at a timeThe stimulus is very weakPossible responses:Yes, I see it. /No, I dont see it.

Absolute thresholdDifference thresholdMethod of constant stimuliMethod of limitsMethod of adjustmentnot usedPresenting two stimuli at a time:Standard: fixed, easily detectableComparison: either more or less intense than the standardPossible responses:Comparison is stronger. /Comparison is weaker.

Method of constant stimuli for measuring absolute thresholdsSelect a range of light intensities from certainly invisible to certainly visiblePick a few (4-7) points uniformly in this intensity range; this will be the constant stimulus setWeakStrongLight intensity14I suspect the threshold to be around here (e.g. by introspection)Method of constant stimuli for measuring absolute thresholdsTest each stimulus many times (20-25) in random order15Illustration: each level tested only 4 timessorrend: Matlab mod(randperm(16),4)+1Method of constant stimuli for measuring absolute thresholdsPresent the stimuli one at a time and ask the observer if it was visible or notVisible?YESNO16Click to start18randperm(7) = 1 2 4 7 3 6 5Could you see the spot of light?Could you see the spot of light?Could you see the spot of light?Could you see the spot of light?Could you see the spot of light?Could you see the spot of light?Could you see the spot of light?32randperm(7) = 1 2 4 7 3 6 5Could you see the spot of light?Could you see the spot of light?Could you see the spot of light?Could you see the spot of light?Could you see the spot of light?Could you see the spot of light?Could you see the spot of light?Method of constant stimuli for measuring absolute thresholdsCalculate the proportion of yes and no responses at each light level+-+++---+++----+0%5%20%50%80%95%100%46Method of constant stimuli for measuring absolute thresholdsPlot the percentages against stimulus intensity psychometric functionStimulus intensityPercentage seen0%100%50%75%25%47Psychometric function for absolute thresholdsIdeal

FIG (Sekuler)

Fixed absolute thresholdStep functionActual

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 functionMethod of constant stimuli for measuring difference thresholdsStandard stimulus has a fixed intensityThe intensities of comparison stimuli bracket the standardLight intensityStandard stimulus:Comparison stimuli:49Method of constant stimuli for measuring difference thresholdsAll pairs of standard and comparison stimuli are tested many times50Itt minden pr csak 3x Matlab mod(randperm(21),7)+1Method of constant stimuli for measuring difference thresholdsFor each pair, the observer judges if the comparison stimulus was stronger or weaker than the standard

http://www.yorku.ca/psychoSTRONGERWEAKER51Method of constant stimuli for measuring difference thresholdsFor each comparison level, the percentage of stronger responses is calculated and results are plotted as a psychometric functionLight intensity of comparison stimuliPercentage stronger0%100%50%75%25%52Psychometric function for difference thresholdsWhen 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 stimuliPercentage stronger0%100%50%75%25%PSE53Jobb lenne, ha 25% s 75% adatpontok kz esne (interpolci)Psychometric function for difference thresholdsBy convention, the intensity at 75% is considered to be just noticeably stronger than the standard DSA comparison intensity at 25% is just noticeably weaker than the standard DWDifference threshold = the average of DS and DWLight intensity of comparison stimuliPercentage stronger0%100%50%75%25%DWDS54Psychometric function for difference thresholds

55http://www.yorku.ca/psycho experiment results (range = 30, otherwise default settings)X axis different on top left chart!!!Individual differences in PSE, threshold, slope (DT: closely related to slope)Other phenomena: attentional problems, no detectionMethod of limits for measuring absolute thresholdsAscending and descending series may yield different results use bothEven in the same direction, there is variability in the threshold (inner noise, etc) average many measurementsMeasured threshold corresponds to 50% point in a psychometric function (method of constant stimuli)+-+++++-----+++++----Light intensityTrialspure-tone audiometry56threshold estimateMethod of limits for measuring difference thresholdsIntensity of the comparison stimulus is decreased (descending) or increased (ascending) until the response changesThreshold estimate: intensity difference between the standard and comparison stimuli where the response changesAverage results from multiple series in both directions+-++++Light intensity of comparison stimulusTrials+comparison brighter-comparison weaker+----+-++++-----+57Nem igazn j: PSE-nl tallgats miatt hamis vlaszvltozs lehet (staircase-nl ezrt hasznlnak 3-up-1-down mdszert) ezt a flit kihagyni?Types of Visual StimuliDotsMotion dotsLinesSinusoidal stimuliGratingWavelet

Cognitive Neurosciencevisual and auditory processingattentionchange blindnessmultimodal integrationconsciousnessdecision-makinglearningmemorylanguagemirror neuronsmismatch negativity

Bio-signals of the Human BrainElectricityBrainHeartMuscleChemical compositionHgbNeurotransmittersMovementEye movementAttentionPositive: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?

ConsciousnessCigarette and Women

Subliminal advertising

Emotions

Language ...... Government regime -

ERP

Behavioral Assessment & Intervention Commons3/1/2012OHSU 6-month review70

Conditions Causing Low VisionConditions Causing Low VisionLow vision can result fromAnomaly (usually congenital)Disease (inherited or acquired)InjuryConditions can be progressive or stableAffect different visual functionsCan be acquired or congenitalConditions Causing Reduction in Central Visual FunctionDecrease ability to see visual details including colorQ: what functional problems would be expected in a person who is unable to see visual detail?Common Refractive DisordersCause reduced acuity ONLY if left uncorrectedCommon typesMyopiaHyperopiaAstigmatismCorrected with lenses or surgeryRole of Psychophysical Tests in Monitoring GlaucomaGlaucoma slides - July 297676Merits and limitations of manual perimetry, SAP, SWAP, and FDTPerimetry TypeAdvantagesDisadvantagesManual (i.e., Goldmann)Long track recordEasier to do than SAPNonstandardizedWell-trained technician requiredSAPQuantitative and standardized algorithmsDiagnostic and progression statistics availableExpensive and nonportable equipmentSWAPMay detect defects and change earlier than SAPMore influenced by cataractFDTSensitive to early changesGood patient acceptancePortable screening toolNo progression software availableCanadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93.DyslexiaStein & 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 SciencesVolume 300, Issue 1 , Pages 130-134, 15 January 2011A 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 2010. published online 29 September 2010.

AbstractBackgroundOptic 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).MethodsTo 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.ResultsThe 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.ConclusionsThe 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. 179-187. ISSN 1983-3288.

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.

Matlab and PsychToolboxPsychoPyVisionEgg

E-primePresentationSummaryPsychophysical 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.