Longer or Shorter?

An examination of line length discrimination using manipulated feedback

Amanda Hostiuc, Alicia Kim, Melanie Laking, and Matt Pachai

Outline

1) First steps and hypothesis generation– The Peter Jansen paradigm– Failures and revisions– The Vertical-Horizontal Illusion

2) Experimental Design3) Results4) Conclusions

The Peter Jansen Paradigm

• The original task flashed two lines on screen simultaneously and asked if the second line was longer or shorter

• This experiment served as the starting point in our research program

The Original Motivation

• What is the effect of attention on our line length acuity in the Peter Jansen paradigm?

• Could attentional cueing lead to increased performance on a length discrimination task?

• Would an invalid cue decrease performance?

A Snag in the Literature

• A disturbing picture began to emerge

• Many articles described the “well known fact” that vertical lines are perceived as longer than horizontal lines

The Vertical-Horizontal Illusion

• When a horizontal line and a vertical line of the same length are presented together, the vertical line is perceived as on average 10% longer

Craven (1993)

• Systematically examining the perception of

The New Design

• We decided it was unlikely that cuing would cause a significant change in like acuity if our perceptions are already biased

• But what if we could change the perception of the illusion?

• How might you go about designing an experiment to manipulate

Programming a Pilot Run

• We decided to program our experiment from scratch in Matlab due to prior experience

• After extensive tweaking, Amanda and Melanie ran as control subjects

• The result NO vertical-horizontal illusion was present!

Back to the Drawing Board

• After our initial failure, we began to scour journal articles to find an ex

Vertical-Horizontal Illusion

• Vertical-Horizontal Illusion (V-H)– Perception of vertical lines as longer than

horizontal

Purpose

• Increase or eliminate vertical-horizontal illusion

Idea Generation (edit)

• Consecutive lines• Cues– Same/different– Shorter/longer

• Feedback– Correct– biased

Herzog and Fahle, 1999 --Vernier Task

Hypothesis

• Valid feedback– Decrease V-H

• Biased feedback– Increase V-H

Method

• Conditions– Reduction: valid feedback– Control: no feedback– Strengthened: biased feedback

Method

• Blocks (Phases)– Block 1: establish baseline threshold– Block 2: learning trials– Block 3: examine the effect of learning trials

compared to baseline threshold

DesignBlock 1Block 1 Block 2Block 2 Block 3Block 3

ReductionReduction No No feedbackfeedback

Valid Valid feedbackfeedback

No No feedbackfeedback

ControlControl No No feedbackfeedback

No No feedbackfeedback

No No feedbackfeedback

StrengthenStrengtheneded

No No feedbackfeedback

Biased Biased feedbackfeedback

No No feedbackfeedback

Method

Method

Method

• Subjects– 8 subjects

• Stimuli– Vertical/horizontal lines between -9 to +3 pixels

Method

• Measure– Accuracy of the responses

• Trials– 10 practice trials– 240 trials for each block– 20 trials per length per block– Total of 720 trials

Method

• Procedure– Fixation cross– Mask– Stimulus– Response: left or right

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ResultPhase 1Phase 1 Phase 3Phase 3 DifferenceDifference

Condition 1Condition 1

Subject 1Subject 1 -3.2651-3.2651 -2.66190-2.66190 1.60321.6032

Subject 2Subject 2 -3.1660-3.1660 -2.5070-2.5070 0.65900.6590

Condition 2Condition 2

Subject 3Subject 3 -3.2703-3.2703 -4.0302-4.0302 -0.7593-0.7593

Subject 4Subject 4 -3.8623-3.8623 -4.4380-4.4380 -0.5757-0.5757

Subject 5Subject 5 -3.5175-3.5175 -3.5103-3.5103 0.00720.0072

Condition 3Condition 3

Subject 6Subject 6 -3.3370-3.3370 -4.0723-4.0723 -0.7353-0.7353

Subject 7Subject 7 -2.1695-2.1695 -1.8156-1.8156 0.35390.3539

Subject 8Subject 8 -5.6957-5.6957 -3.8027-3.8027 1.89301.8930

Results (Condition 1)

Results (Condition2)

Results (Condition 3)

Results

• Replicated the v-h illusion• Inconclusive results in reducing/

strengthening the illusion with valid/biased feedback

Discussion

• Finding 1: Vertical-horizontal illusion was replicated– Craven (1993)

Discussion

Finding 2:• Unable to decrease illusion using valid

feedback

Finding 3:• Unable to increase illusion using biased

feedback

Discussion

Source of error– Number of trials– Number of N

Discussion

Suggestions for future research– More time to train/learn– More trials for Block 2– Better incentive