Strategies for Studying Perception (Cont) PDF

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BCS 151
Dr. Tadin

Notes go over how we study perception for research....

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BCS 151 Prof. Tadin 6 September 2018

Strategies for Studying Perception (Cont.) 

What are the Limits of Perception? (Psychophysics: thresholds, JND, PSEs) Psychophysical Methods: -Signal Detection Theory1 →if someone’s threshold is lower, does that mean they’re more sensitive? (dependent on each person’s criterion) →problem of criterion: ~we must never take a person’s testimony, however sincere, that he has felt nothing, as proof positive that no feeling has been there (there is a stimulus, but it hasn’t reached our own threshold) ~perceptual decisions are influenced my many factors: *signal strength relative to background level of noise *person’s sensitivity to the signal *relative frequency of occurrence of the signal *person’s level of motivation *costs associated with hits and false alarms *personal biases →solution: ~more sensitivity independently of criterion = SDT →explains perceptual performance (& decision making) under conditions of uncertainty →goal = distinguish performance changes/differences attributable to sensitivity to those attributable to criterion

1 https://isle.hanover.edu/Ch02Methods/Ch02SDT_ROC.html; https://www.youtube.com/watch?v=WnfQ2CPHtwk

(errors can be more costly than others; miss vs false alarm)

→four possible outcomes: 1) hit2 2) miss3 3) correct rejection4 4) false alarm5 →each observation is either a case of “just noise6” or a “signal plus noise”

Signal weaker on the left and stronger on the right Dotted curve is noise, Line is signal; the more separated it is, the easier your task is

2 signal was present and person says “yes” (ex: correct diagnosis of disease) 3 signal was present and person says “no” (ex: physician tells patient that no disease exists when one does) 4 signal was not present and person says “no” (ex: unnecessary operation/treatment) 5 signal was not present and person says “yes” (ex: patient properly diagnosed with no disease) 6 “noise” is anything that complicates detection of “signal” by introducing uncertainty about whether or not the signal is present

in the middle, we are uncertain and try to make a decision

everything to the left is “no I don’t hear it”, everything to the right is “yes I hear it” Curve is probability of distribution saying “no” even though there is a signal, therefore, miss

Criterion can shift can change the types of errors you make The shift is your mental bar

liberal not a lot of misses, but a lot of false alarms conservatives miss a lot, but not a lot of false alarms

→the ROC (Receiver Operating Characteristic) curve7 ~traced out by plotting Hits against False Alarms as the criterion moves ~ex: likeliness of false alarms relative to hit rate

7 https://isle.hanover.edu/Ch02Methods/Ch02SDT_ROC.html

0,0 100,100 diagnose everyone (pick up all hits, but also a lot of false alarms) d’0 hits and false alarms the same

D is doing the better job (a lot of hits, no false alarms) E an C doing the same (same curve) B almost equal

Liberal, Neutral, Conservative want to know if false alarms sensitivity is the same

→explains yes-no decisions →detecting signal in noise8 (S + N) →sensitivity (discriminability) vs criterion (bias) →sensitivity (d’) depends on: ~signal strength ~noise strength ~observer sensitivity →criterion depends on: ~personal bias ~cost/benefit factors (risk) ~signal frequency →ROC curve: ~used to visualize SDT concepts & results

When does perception produce mistakes? (Study of Illusions) 8 what makes detection hard (internal + external noise)

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How can we make visible “invisible”?

-Degrading visual stimulation →ex: lowering contrast to where you can’t see it

-Crowding →focusing on limitations in concentration visualizing periphery

-Masking →don’t perceive a target but your brain processed it, therefore affecting your performance

-Motion Induced Blindness →we don’t know why it works; subconscious?

-Binocular Rivalry →showing things in different eyes

-Bistable Figures →flipping perspectives back and forth

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Where in the Brain does Perception Occur? (fMRI, EEG, TMS) MRI images brain structure: -how brain is made up; structural differences fMRI images brain function (BOLD9 signal): -see brain in action -1increased neural activity -2increased metabolic demand -3increased blood oxygenation -4higher concentration of oxy-hemoglobin level in blood, changes magnetic susceptibility of hemoglobin -increased bold signals (go through 1-4) -fMRI measures blood flow NOT NEURAL ACTIVITY DIRECTLY -BOLD signal is sluggish (peaks about 5 sec after neural activity which is a lot in brain time) -blind to neural changes that do not alter local metabolic demand -fMRI is a correlational method

9 Blood Oxygenated Level Dependent...