CONTRAST AND COMPLEXITY INDEPENDENTLY AFFECT CENTRAL CAPACITY-LIMITED MECHANISMS

Using the psychological refractory period (PRP) paradigm, we examined whether visual stimulus encoding is subject to central capacity-limited mechanisms. Therefore two experiments were conducted in which second task difficulty was manipulated by varying stimulus intensity and stimulus complexity. The first task was a speeded 2 AD task consisting of pure tones. The second task was a same-different matching task consisting of patterns of different stimulus complexity and intensity. Both tasks were performed temporally overlapping at varying stimulus onset asynchronies (SOAs). Main effects for SOA, intensity and complexity were found. Furthermore, no interaction between these factors occurred. From the intensity effect it is concluded that visual stimulus encoding does require central mechanisms. Furthermore, mechanisms susceptible to stimulus complexity are interpreted as using limited capacity as well. However, it still remains unclear whether these mechanisms belong to the encoding stage. In the psychological refractory period (PRP) paradigm, two stimuli are presented in rapid succession at varying stimulus onset asynchrony (SOA), with each stimulus requiring a fast and discrete response. The typical finding is a slowdown of second task performance. This slowdown is bigger the smaller the SOA is. After a certain SOA, the slowdown-effect disappears completely. This effect is attributed to capacity-limited central mechanisms processing stages involved in task performance (Welford, 1952; Pashler, 1984, 1994; Pashler & Johnston, 1989). Due to the capacity-limitation of these mechanisms, only one central capacity requiring processing stage can be performed at a time. Consequently, if in a dual task situation in which processing stages of both tasks require central capacity at the same time, an information processing bottleneck occurs, and processing for one of the tasks needs to be postponed. At the stage of postponement, a waiting period or cognitive slack (Schweickert, 1978; cited in Schubert, 1996) arises reflecting the psychological refractory period (PRP). After the bottleneck has been cleared, central capacity becomes available for the other task. Whereas most consistent empirical support has been provided for the assumption that response selection does require central mechanisms (e.g. Welford, 1952; Pashler, 1984; McCann & Johnston, 1992), a controversial discussion has recently arisen over whether or not, and under what conditions encoding is also subjected to these mechanisms (see Jolicoeur, 1999 for a review). According to Jolicoeur and Dell ́Acqua’s (1998) short-term consolidation, a subprocess of encoding requires central capacity. 1 This work was supported by Grant La 1281/2 by DFG to Thomas Lachmann. 2 Thomas Lachmann is at RIKEN BSI as a Guest Scientist until May 2003. 3 Krisztin Pataki is now at the Technical University of Berlin. However, the PRP-paradigm proved to be a helpful tool to investigate whether or not certain processing stages, as well as mental operations, such as memory search (Heil et al1998) or mental rotation (Ruthruff, Miller, & Lachmann, 1995), require central mechanisms. Therefore, a manipulation of second task`s difficulty is required, and the duration of a specific processing stage is affected. If this processing stage refers to central mechanisms, the manipulation of the second task`s difficulty and SOA should turn out to operate additively on second task`s reaction time (RT2). If, however, the second task difficulty factor affects a stage prior central mechanisms, there is an underadditive interaction between difficulty factor and SOA expected. In Lachmann & Pataki (in preparation) as a second task, a visual same-differentmatching was required. Participants had to decide whether two simultaneously presented fivedot-patterns, first used by Garner & Clement (1963), are the same or different. These patterns were presented with varying stimulus intensity (figure-ground contrast). The intensity of presentation is assumed to influence the speed of encoding. If encoding refers to central capacity limitations, additive effects on RT2 should result from this manipulation. Therefore, RT2 should increase by decreasing SOAs and low stimulus intensity. Stimulus complexity was varied by using Garner`s information concept (1962). Therefore, patterns of five dots on an imaginary 3 × 3 grid were used, leaving no row or column empty (Garner & Clement, 1963). The total of 90 patterns falls into 17 disjunctive subsets consisting of pattern elements that are equivalent in the sense that they can be transformed into each other by reflection and/or multiples of 90° of rotation (R&R equivalence sets). These sets differ in size, depending on the symmetries of their pattern elements. Two patterns are invariant against the R&R transformations and therefore represent a set by themselves with set size 1. As members of R&R subsets, these patterns are assumed to be highly redundant, i.e. they represent a low complexity (Garner, 1962). Eight subsets consist of four elements that can be transformed into each other. Furthermore, there are seven subsets of eight equivalent pattern elements. Garner & Clement (1963) could show that the equivalence set size (ESS), as an objective measure of the degree of redundancy (or complexity) of the pattern elements, can predict the subjective Goodness ratings of their participants. Therefore, it was suggested that ESS is a measure of Goodness. Consequently, using the introduced pattern sets for the same-different-task entails a variation of complexity and thus of Goodness, measured by the ESS.