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Kornbrot, Blended Learning Unit, University of Hertfordshire 1October 05

Visual Illusions

Two Week Laboratory Investigations: Week 1

B.Sc. Psychology

B.Sc Cognitive Science

B.Sc. Combined Studies

October 05 Kornbrot, Blended Learning Unit, University of Hertfordshire 2

Learning Outcomes

� Psychology Knowledge

� What is an ‘illusion’?

� Visual illusion classification

� Illusions of length: Horizontal-Vertical, Muller-Lyer

� Psychological Mechanism(s)

� Empirical investigation

� Measuring size of an effect

� Method of adjustment, method of limits

� Reporting Psychological Investigations

� Structure of Report

� Content of Report


What is an ‘illusion’?

� Definition

� A situation where the immediate perception or judgment conflicts with more detailed inspection

� Situations where context alters perception or judgment

� Types of illusions

� Any sense modality: visual, auditory, taste, etc. Combination

� Audience give examples

� Higher level: cognitive

� Audience give examples

� Why do illusions exist?

� Draw on fast and effective rules that are usually correct


Why study illusions? Importance

� Identify underlying psychological mechanism� Several different mechanisms, even within modality

� Common feature 1: reduce information overload

� Common feature 2: constant judgment in changing world

� Evolutionary view: Purpose of Perception & Judgement� Identify objects and intentions

� E.g. tables - not components e.g. line length

� E.g. friend or foe - not all personality characteristics

� Guard against false judgements or perceptions� Cognitive: e.g. stereotyping. Pause for audience

� Enhance experience� Media ‘effects’ - films, internet games. Pause for audience


Visual Illusions

� Visual Illusion Demonstrations� http://www.michaelbach.de/ot/index.html

� http://www.at-bristol.org.uk/Optical/default.htm

� Context

� Constancy

� Fatigue


Illusions of magnitude, i.e. size

� T- illusion

� Demo http://www.michaelbach.de/ot/sze_t-illusion/index.html

� Note: Using + and - keys for adjustment may be easier than mouse

� Muller-Lyer illusion

� Demo http://www.michaelbach.de/ot/sze_muelue/index.htmlFor interest of students later


Theories of T- Illusion

� Theory 1: Gravity Asymmetry

� Vertical line appears longer because our bodies are longer in the vertical direction due to evolving in gravity. We have greater range of vision in the vertical frontal plane than the horizontal plane

� Hence will

� underestimate (<100%) if comparison is in vertical orientation

� overestimate (>100%) if comparison is in horizontal orientation

� Theory 2: Bisection

� Vertical line appears longer because it is a single line not two sections. There is tendency to match to a single section

� Hence will

� underestimate (<100%) if comparison is in either orientation


Psychophysical methods

� Psychological effect of physical stimulus features

� Judgment of single stimuli √

� Psychological size of single stimulus

� Method of adjustment √ Method of limits √

� Method of constant stimuli

� Judgment of many stimuli

� Magnitude estimation

� Category judgment (Likert items, scales)

� Use for ‘objective’ but non-physical stimulus features

� Objective if all agree definition of stimuli

� E.g. items in questionnaire, picture in film, Tsunami


Method of Adjustment

� Judge Equality

� Random start points

� Method of Limits

� Ascending

� Descending

� Fixed or varying start points?

� Types of error

� Perseverance

� Change


Student investigation

� Design

� Work in pairs

� Pair member1 participant 1st P1, experimenter 2nd E2

� Pair member2 participant 2nd P2, experimenter 1st E1

� Counterbalancing

� ascending & descending trials: P1 ascend-descend P2 descend-ascend

� Presentation mode of adjusted lineP1 vertical 1st, horizontal 2nd P2 horizontal 1st, vertical 2nd

� Participants

� Person characteristics. Age, gender, ?? Pause to discuss

� Apparatus and Materials

� WWW illusions

� http://www.michaelbach.de/ot/sze_t-illusion/index.html


Prepare Data Sheet

� Bold boxed items to surveymonkey

� Each student enters just TWO numbersparticipant type trial mode type % trial mode type %

abc P1 1 ascend vertical 9 ascend horizontal

abc P1 2 descend vertical 10 descend horizontal







abc P1 mean(% )

xyz P2 1 descend horizontal 9 descend vertical

xyz P2 2 ascend horizontal 10 ascend vertical







xyz P2 mean(% )

participant type trial mode type % trial mode type %









abc P1 mean(% )









xyz P2 mean(% )


T-Illusion Procedure

� Adjust chair so participant eyes level with screen and about 40 cm distance. Try to keep distance fixed

� Get stimulus from: http://www.michaelbach.de/ot/sze_t-illusion/index.html

� Pair member 1 as participant� Ascending Trial Vertical: leave rotation at 0°°°°

� Move line to top fast Note: Using + and - keys may be easier than mouse

� Move down until vertical & horizontal appear equal: Note %

� Descending Trial Vertical: leave rotation at 0°°°°

� Move line to bottom fast

� Move up until vertical & horizontal appear equal: Note %

� Repeat 4 times� Ascending Trial Horizontal: change rotation to 90°°°°

� Move line to left fast

� Move right until vertical & horizontal appear equal: Note %

� Descending Trial Horizontal: with rotation to 90°°°°

� Move line to right fast

� Move left until vertical & horizontal appear equal: Note %

� Repeat 4 times

� Pair member 2 as participant� As above but starting with descending horizontal trial first


Enter Individual Data

� Illusion Experiment Questionnaire: COMPULSORY

� Record Results: http://www.surveymonkey.com/s.asp?u=295871446256

� Each student enters:

� Their own personal data

� NB Student number - will NOT show on group data sheet

� Mean results for T- illusion

� Summary of findings

� Extra activities (optional)

� Choose ‘favourite’ illusion, reasons & explanation

� Explore spatial frequencies

� How ‘fine’ is resolution needed to recognize face

� Record Results: http://www.surveymonkey.com/s.asp?u=676171446439


Preparation for Report

� You will be provided with template

� Structure and marking scheme will be provided

� Keep record of all activities this week to enable good write up next week


Illusions to explore 1

� Face and spatial frequency� http://www.michaelbach.de/ot/fcs_mosaic/index.html

� Guess what the picture represents & note down

� Click finer: Guess who the picture represents & note down

� Keep repeating

� Note how many clicks to recognize picture as face

� Note how many MORE clicks to recognize sex of picture

� Note how many MORE clicks to recognize identity of picture

� Results to be recorded on Questionnaire (optional)

� What is spatial frequency?

� Number of dark light changes per unit length

� High frequency = fine definition

� Low frequency = coarse definition or blurring

� Different mechanism for high and low frequency


Illusions to Explore 2

� Chose your ‘favourite’ illusion

� Note down exact title at top, e.g. Face in blocks

� Will need to be entered in Questionnaire

� You will also need to enter:

� A. single sentence on WHY it is your favourite

� B. single sentence explaining why illusion occurs


Activities and References

� Enjoy:

� Required illusions

� Play with any other illusions

� ReferencesBach, M. (2005, 5 September 05). 57 Optical

Illusions & Visual Phenomena. Retrieved 28

September, 2005, from

http://www.michaelbach.de/ot/index.html

Gregory, R. (1997). Eye and Brain: The Psychology

of Seeing (5th ed.). Oxford: Oxford University

Press.

Kornbrot, Blended Learning Unit, University of Hertfordshire 18October 05

Visual Illusions

Group Results

Writing Lab Reports

Two Week Laboratory Investigations: Week 2

B.Sc. Psychology

B.Sc Cognitive Science

B.Sc. Combined Studies


Learning Outcomes

� Analysing Group Results

� Use SPSS to produce histograms [EXPLORE}

� Use SPSS to summarize central tendency, variabilty and shape of a distribution [EXPLORE]

� Use SPSS to produce bar chart for 2 variables [GRAPHS]

� Reporting Psychological Investigations

� Produce a Report of a Psychological Investigation in APA structure format

� Produce Tables in Standard Format

� Produce Figures of Graphs in Standard Format


Instructions for SPSS

� Aim

� Students should learn to explore SPSS to find relevant features

� No screen shots, students should learn by doing, c.f. Computer Games

� Drop Down Menus

� The choices will be illustrated in order of choice

� E.g. Analyze > Descriptives > Explore should be intrepretted as:

� Analyze from top menu

� Descriptives from Analyze menu

� Explore from Descriptives menu

� Dialogue boxes

� Either directly what to put in a box

� E.g. Dependent List: Vertical, Horizontal

� tells one to put the Vertical and the Horizontal variables in dpendent box

� Or leads to further dialogue box

� E.g. Statistics: Percentiles

� Tells one that percentiles should be ticked in the statistics dialogue box


SPSS Data Analysis 1

� File: illusions.sav

� Description of Frequencies

� Analyze > Descriptives > Explore

� Dependent List: Vertical, Horizontal

� Statistics: Percentiles

� Plots: Stem and leaf, histograms

� Box Plot Dependents together

[to be explained in future lectures]

� Options Default [do not change]

� What to report?

� Central tendency, Variability, Shape

� Number greater than some target number, e.g. 100 or …..


SPSS Changing Graphs

� What is needed?

� Same axis for both frequncy histograms

� Normal distribution

� How to in change Graphs SPSS

� Double click on graph

� Axis > Interval > Custom

� Custom > Define

Minimum = 60, Maximum = 150, Width = 5

� Labels

Display = every 2 labels, Type = Range

� Chart > Options > Normal Curve


Instructions for using WORD Template

� Download File illusion_TEMPLATE.rtf from STUDYNET

� It is under assignment AND under teaching materials

� Double click on file to open

� It will warn you file is in rtf format [works on any machine]

� Save as: illusion_surname.rtf

� Make sure you ALSO choose rtf format in WORD Save as MENU

� Make sure filename does NOT have .rtf.doc in it - only .rtf

� Recommend you keep file on floppy or flash disc


Copying Graphics to WORD

� Click to select graphic [figure] in SPSS

� There will be a red arrow indicating graph is selected

� Edit > Copy Object

� This puts graph on clipboard

� Move to open WORD file

� Edit > Paste Special > rtf

� Resize picture to look good by dragging edges

There should then be no compatibility problems


Write Up Advice

� Advice in illusion_GUIDE.rtf

� Will go through this GUIDE

� Will use file illusion_TEMPLATE as example


Submission of Assignment

� Hard Copy To Office

� Electronic to STUDYNET via Assignment

� Dates and Times

� in Teaching Materials, by Lab Group

� Late submission

� Only gets mark if Extenuatin Cricumstances form acceptable

� Marks are provisional until confirmed by Board of Examiners

� MUST submit hard copy AND electronic copy


Submission by STUDYNET

� Go to Module Page

� MDA1 or EDA1 or Psychological Methods

� Click on Assignment in Left Panel Menu

� Choose the assignment labelled ‘illusions’

� Note for PSY students there will be distinct assignments for each lab group, e.g. ‘illusions Gp 1 Tues 12-2

� Choose the assignment for your lab group

� Follow instructions to upload the file

� There is also a link to a video clip if you need help


References

Arieh, Y., & Marks, L. E. (2002). Context effects in visual length perception: role of ocular, retinal,

and spatial location. Perception & Psychophysics, 64(3), 478-492. Availalble at:http://www.ingentaconnect.com/search/article?title=horizontal

+illusion&title_type=tka&year_from=1998&year_to=2005&database

=1&pageSize=20&index=13

Bach, M. (2005, 5 September 05). 57 Optical Illusions & Visual Phenomena. Retrieved 28

September, 2005, from http://www.michaelbach.de/ot/index.html

Gregory, R. (1997). Eye and Brain: The Psychology of Seeing (5th ed.). Oxford: Oxford University

Press.

Gregory, R. L. (1997). Knowledge in perception and illusion. Philosophical Transactions of the

Royal Society of London B, 352, 1121 - 1128. avialble at

http://www.richardgregory.org/papers/knowl_illusion/knowledge-in-perception.htm

Field, A., & Hole, G. (2003). How to design and report experiments. London: Sage Publications.

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