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Taxonomy of DO Ensembles

The dimensional overlap model provides a useful way of talking about compatibility tasks (more info: Why study compatibility effects?). 

When all the individual stimuli used in a task vary on a common stimulus dimension (e.g. red and blue varying on the color dimension), and all the individual responses that are used in that task also vary on a common response dimension (e.g. left and right varying on spatial dimension), they form a stimulus set and a response set, respectively. When these two sets are put together in a larger set, that larger set is called an ensemble.

In addition to a relevant stimulus dimension and a response dimension, a task may also have one or more dimensions: irrelevant stimulus dimensions that subjects are instructed to ignore .

Different task may be characterized by the properties of the ensemble that they use.  If the individual sets in the task are similar, be they a stimulus set and response set, or two stimulus sets, then we speak of that ensemble as having dimensional overlap. (more info: What is Dimensional Overlap?)

This allows us to define a broad framework for classifying different types of ensembles, or tasks,  based on the dimensional overlap, or set-level similarity, of the ensembles in the task. Dimensional Overlap (i.e similarity) is of course a continuous property; however, we can talk about tasks that either do, or not  have overlap between their different components.  A framework with three possible dimensions (relevant stimulus, irrelevant stimulus, response), all of which can either overlap or not overlap, produces a taxonomy of eight possible dimensional overlap ensembles or tasks types.

These eight types constitute the dimensional overlap ensemble taxonomy.

Type Relevant S-R
Overlap?
Irelevant S-R
Overlap?
S-S
Overlap?
Example Tasks
1 no no no CRT task
2 yes no no Fitts task
3 no yes no Simon task
4 no no yes Flanker Task
Stroop-like Tasks
Cross-modal tasks
5 yes yes no Hedge and Marsh task
6 yes no yes (not possible?)
7 no yes yes SS x SR task
8 yes yes yes Stroop task

The first ensemble in the taxonomy is a Type 1 task, which is a standard Choice Reaction Time (CRT) task with no dimensional overlap. This is technically not a compatibility task at all: it contains no dimensional overlap between any of its components. However, it is often used as a “neutral” condition in compatibility experiments in order to establish a comparison point for measuring the benefit of consistency versus the cost of inconsistency (more info: Choice Reaction Time (CRT) tasks).

After the Type 1 task, the taxonomy enumerates all of the possible combinations of dimensional overlap among the three dimensions, all the way up to the Type 8 task, which has overlap among all the dimensions: relevant stimulus, irrelevant stimulus, and response. The well-known Stroop task is an example of a Type 8 task (more info: The Stroop Task).

Click on the links in the above table for a more detailed description of the experiments and results of tasks belonging to each ensemble.

As an easy reference, the table below shows some sample stimulus and response sets that could be used to produce tasks of each type in the taxonomy.

Type Relevant Stimulus
Dimension
Irrelevant Stimulus
Dimension
Response
Dimension
Instructions
1 color vertical
position
left/right
key-press
press the left key for a green stimulus
press the right key for a blue stimulus
2 color digit color  names say “green” for a green stimulus
say “blue” for a blue stimulus
or
say “blue” for a green stimulus
say “green” for a blue stimulus
3 color horizontal
position
left/right
key-press
press the left key for a green stimulus
press the right key for a blue stimulus
4 color color word digit names say “two” for a green stimulus
say “four” for a blue stimulus
5 color horizontal
position
colored left/right
key-press
press the blue key for a blue stimulus
press a green key for a green stimulus
or
press the green key for a blue stimulus
press a blue key for a green stimulus
6 (not possible?)
7 color horizontal position
and color word
left/right
key-press
press a left key for a green stimulus
press a right key for a blue stimulus
8 color color word color names say “blue” for a blue stimulus
say “green” for a green stimulus
or
say “green” for a blue stimulus
say “blue” for a green stimulus

 


 

NOTE: Although the above taxonomy represents the official taxonomy of the ensembles explored by the dimensional overlap model, it is possible to explore extensions of this taxonomy by examining new task dimensions.

For example, Stevens and Kornblum (2000, July) presented an extension of the taxonomy that distinguishes between the response dimension and the goal dimension in a task. The goal dimension is defined as the intended outcome of a response action, which may be different from the response action itself.

Hommel (1993a) performed a variation of the Simon task in which subjects were told to light up a light by pressing a key when a stimulus appeared. In these tasks, subjects either needed to press a right key to light up a light on the right side, or press a left key to light up a key on the right side. Hommel (1993a) showed that this consistency between the response (key position) and goal (light position) had a measurable effect on reaction time. Stevens and Kornblum (2000, July) were able to extend the computational dimensional overlap model to include overlap with a “goal dimension” to account for this effect (more info: The Computational Model).

 

 
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