Parallel versus serial processes in multidimensional stimulus discrimination

1. Parallel versus serial processes in multidimensional stimulus discrimination Egeth, H. W. (1966). Perception & Psychophysics, 1, 245-252.

2. Introduction • Considerable effort has been devoted to the description of processes underlying discriminations along single dimensions. • But few attempts to determine how the processes may be combined when discrimination requires the consideration of more than one stimulus dimension.

3. Introduction • whether humans discriminate between multidimensional objects by … • Serial: comparing them one dimension after the other. • Parallel: comparing them on several dimensions simultaneously. • Template: comparing unitary representations of them without regard to their component dimensions.

4. Introduction • The prediction was made by regarding two important relations: • “Same” reaction time • “Different” reaction time and number of relevant dimensions

5. Assumptions • Subjects are uninfluenced by the presence of irrelevant information, which inferred their capability of completely filtering it.

6. Before the prediction … • A set of three hypothetical two valued dimensions will be referred: X, Y, and Z. • all dimensions are independent from each other. • How quick the two levels within each dimension can be discriminated: X > Y > Z.

7. Prediction – in Serial models • “Same” RT should increase with the number of relevant dimensions and in a linear fashion. • Two attributes: stopping rules and processing order

8. Prediction – in Parallel models • Two attributes: stopping rules and processing order

9. Prediction – in Templatemodels • Replicateseachofthetwostimuliinapair,andthenmakesanoverallcomparisonbetweenthesetworeplicas. • “Different”RT:equallyfastregardlessofthenumberofdifferentdimensions. • “Same”RT:multidimensionalstimuliwouldneverbeslowerthantheconditionwithsingleworstdimension. • ParallelExhaustivewithconstantorder

10. Method • Threedimensions:Color,Form,andTilt • 7conditions:C,F,T,CF,CT,FT,CFT • theprobabilitiesofanswering“Same”and“Different”were50%each • 64possiblestimuli

11. Design&Procedure • The orders between and within sets were counterbalanced. • 30 trials for one- and two- dimensional condition, 90 for three-dimensional condition. • 5 min rest between sets, 2 min rest within sets

12. Design & Procedure

13. Result • Difference between 7 categories of “different stimuli” were significant. • Serial self –terminating random order • Parallel self-terminating random order • The relation between two axis was nonmonotonic. • Support non of the none models.

14. Discussion • 3 possibilities: • Subjects adopted a lower “decision criterion” in condition CFT • Positive correlation across conditions between speed and accuracy • CFT had lower error rate than any of the bidimensional conditions • May be related to the Gestalt principles of perceptual organization • Compare stimuli as a whole • Leads to prediction that CFT’s RT is lower than C, F, T • Subjects did not completely ignore the irrelevant dimensions

15. Discussion • Irrelevant dimensions of variation are not necessarily ignored at the command of the experimenter. • Until more is known about the processing of irrelevant information, it will be difficult to achieve a good understanding of the processing of relevant information.

16. The End