The Effects of Proportion Congruent on the Magnitude of Stroop Interference:
Controlling for the Display Frequency Confound
Jaeyong Lee, Toby Mordkoff, & Eliot Hazeltine
Department of Psychology, University of Iowa
Proportion Congruency Effect
25/75% Congruent Block
GREEN
WHITE
1
3
blue●
2
2
3
1
green●
2
2
Low
Low
BLUE
3
1
yellow●
2
2
1
3
white●
2
2
High
High
Proportion
Congruent
Original Analysis
Schmidt & Besner (2008)
750
Incongruent
Congruent
700
Active - Neutral (ms)
Response Latancy (ms)
Incongruent
Congruent
650
600
550
Low
Medium
Response Latency (ms)
700
600
550
Low
Medium
High
Medium
600
Low
550
Display Frequency
High
650
Low
Medium
High
“Proportion Congruent”
Congruency Trials
120
Neutral Trials
120
100
80
60
40
20
0
-20
100
80
60
40
20
0
-20
Low
Medium
High
Proportion Congruent
Low
Medium
High
“Proportion Congruent”
650
120
600
550
High
Response Latency (ms)
600
700
Medium Medium Medium Medium
Reanalysis
Jacoby, Lindsay & Hessels (2003)
"Congruent"
Proportion Congruent
• The manipulation of congruency was confounded with the display frequency of the
individual item pairs (Schmidt & Besner, 2008)
• The congruent trials in the low proportion congruent condition and the incongruent trials in
the high proportion congruent condition were more frequent, suggesting an alternative
display frequency account
750
650
550
YELLOW
"Incongruent"
650
50% Congruent Block
WHITE
Incongruent
Congruent
Frequency Effect (ms)
GREEN
750
700
700
High
Proportion Congruent
Medium
Congruency Trials – Neutral Trials
100
Low
Display Frequency
• Note that the proportion congruent (in a Stroop task) cannot be manipulated without
changing the frequency of the color/word pairings
Measuring the Effect of Display Frequency
Congruency Trials
Neutral Trials
Stroop Effect (ms)
YELLOW
Neutral Trials
750
Incongruent
Congruent
Stroop Effect with
Frequency effect (ms)
BLUE
750
Response Latency (ms)
• The Stroop effect is known to be dependent on the proportion of congruent trials: larger
when there are more congruent trials in a block (e.g., Tzelgov, Henik, & Berger, 1992)
• This had been shown to be true for a within block manipulation: words which are mostly
congruent (e.g., GREEN and BLUE) show larger Stroop effect than words which are mostly
incongruent (e.g., WHITE and YELLOW; Jacoby, Lindsay & Hessels, 2003)
Congruency Trials
80
60
40
20
0
RED
BLUE
YELLOW
GREEN
CAR
LINE
FOLDER
SHIRT
red●
1
2
4
1
1
2
4
1
blue●
4
1
1
2
4
1
1
2
yellow●
1
4
2
1
1
4
2
1
green●
2
1
1
4
2
1
1
4
Proportion
Congruent
Low
Low
Medium
High
Low
Low
Medium
High
• Neutral trials were used to measure the display frequency effect
• All features had equal marginal frequency
• The true proportion congruency effect was estimated by subtracting neutral trials from the
congruency trials
-20
Low
Medium
High
Proportion Congruent
Reference
Tzelgov, J., Henik, A., & Berger, J. (1992). Controlling Stroop effects by manipulating expectations for color words. Memory & Cognition, 20,
727-735.
Jacoby, L. L., Lindsay, D.S., & Hessels, S. (2003). Item-specific control of automatic processes: Stroop process dissociations. Psychonomic
Bulletin & Review, 10, 634-644
Schmidt, J. R. & Besner, D. (2008). The Stroop effect: Why proportion congruent has nothing to do with congruency and everything to do
with contingency. Journal of Experimental Psychology: Learning, Memory, and Cognition, 34, 514-523