Experiment 1

How efficient is simultaneous search for either of two targets when they are specified by different values from the same feature dimension?

Possible outcomes:

1. Simultaneous search: exposure time for multiple targets equals exposure time for a single target.
   
2. Consecutive searches: exposure time for n multiple targets greater than or equal to the sum of exposure times for n single targets.

Method

• Stimuli: 16 colour patches distributed in CIE colour space.
  
• Each pair of targets selected from opposite sides of colour space.
  
• The remaining 14 colours used as distractors

Figure 1: CIE (x,y) co-ordinates of colour stimuli

• Display size = 8
  
• Participants: N=26
  
• On each trial, a single target could appear
  
• Task: respond "yes" if specified target(s) present; "no" if target(s) absent.
  
• Three Search conditions
  1. target A
  2. target B
  3. target A or B

Figure 2: Sequence of events in each trial.

Two-up-one-down staircase procedure used to calculate minimum exposure time for target detection at 71% accuracy.

Analysis

Figure 3: Illustration of method of analysis.

Exposure times were compared for (i) a simultaneous search for target A or B (ii) the sum of exposures for single target A and target B searches.

Results

Summed exposure times for separate A and B searches significantly shorter than the exposure time required for the simultaneous A or B search.

Indicates the relative inefficiency of simultaneous compared to single target searches.

Figure 4: Mean Exposure Thresholds for simultaneous target searches and summed single target searches.

Conclusions

Search for multiple colours is inefficient when the colours are widely separated in feature space.

Search for multiple targets produces performance costs over and above those associated with two consecutive single-target searches.

Pattern of data may reflect: (i) the top-down activation of a subset of items in the single target search conditions, or (ii) a cost of target switching in the simultaneous search.

Figures 5a to 5d illustrate what may be happening during searches for single and multiple targets.  

On to Experiment 2.

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