**1. Introduction**

Individuals with higher compared to those with lower mental ability (MA) have been reported to have shorter reaction times (RTs) in a wide range of elementary cognitive tasks (ECTs) [1–3]. ECTs are so easy that individuals with higher and lower MA do not differ in the number of errors or the use of cognitive strategies but only in speed of task completion. The most common explanation of the faster information processing in individuals with higher compared to those with lower MA refers to a more efficient information transmission in the central nervous system [4,5]. It should be noted that MA-related differences in mental speed can be observed in simple RT tasks but usually increase with increasing task demands [6–8]. Only after exceeding a certain level of task demands the relation between MA and RT decreases in favor of an increasing relation between MA and error rates in the experimental task [9,10].

To further elucidate the mechanisms underlying the relation between MA and speed of information processing, psychophysiological studies have probed whether MA-related speed differences can also be identified in the latencies of the event-related potential (ERP) [5,11]. ERP is an electrophysiological response to specific events or stimuli [12], which can be observed in an electroencephalogram (EEG). Different aspects of stimulus processing have been demonstrated to be

related to the positive and negative components of ERP [13]. The P3 component, also referred to as P300 and first described by Sutton [14], is a very pronounced positive wave with a maximum peak at about 300 ms after presentation of a stimulus. If the stimulus is presented but not attended to, the P3 component does not (or only rudimentarily) emerge, indicating that the P3 component reflects the allocation of attentional resources [13,15–17]. More specifically, the P3 component is assumed to represent attention-related inhibition of ongoing brain activity to facilitate the consolidation of the target's mental representation in working memory [15,16]. P3 latency, defined as the time interval between stimulus onset and the peak of the P3 wave, has been assumed to be a reliable index of the time needed to evaluate and categorize a presented stimulus [18–22]. As suggested by Verleger's [23] thorough review, however, the view of P3 latency as a pure speed measure of cognitive processes unrelated to response processes might be premature, since P3 latency is also sensitive to delays in response selection when responses are given fast.

As an electrophysiological and reliable measure of speed of information processing [24], P3 latency also received much attention as a possible correlate of MA. In contrast to RT, however, P3 latency was found to be less consistently related to MA, with the majority of studies investigating young adults (but see Reference [25]). In simple and choice RT tasks, for example, a relation between P3 latency and MA could not be obtained [22,26,27]. Houlihan et al. [28] reported a positive relationship between MA and P3 latency in a short-term memory scanning task, whereas McGarry-Roberts et al. [22] reported MA to be negatively related to the P3 latencies derived from a short-term and a long-term memory task. The only task showing consistently the expected negative functional relation between P3 latency and MA was the oddball task with shorter P3 latencies in higher- compared to lower-MA individuals [25,29–33].

From the inconsistent results on the relation between MA and P3 latency, it can be concluded that P3 latency is not in general related to MA. Rather, this relation seems to depend on the respective task used to elicit the P3 component or, in other words, on the specific cognitive processes required by the given task. For example, P3 latency associated with simple or choice reaction time was consistently unrelated to MA [22,27,28], while studies on MA and P3 latency associated with short-term memory scanning produced inconsistent results [22,28]. To date, little empirical support is available for a functional relation between MA and P3 latency associated with long-term memory retrieval [22]. Only the information processing required by the oddball task led to a consistent relation between the associated P3 latency and MA. Thus, the oddball task represents a good starting point for a systematic investigation of the task conditions and, thus, the required cognitive processes necessary to yield faster P3 latencies in individuals with higher than those with lower MA. In the following, we outline why we expect that selective attention is the crucial cognitive process underlying the negative relation between P3 latency and MA.

The oddball task consists of a series of standard stimuli (e.g., the letter "O") infrequently interrupted by the "oddball" (e.g., the letter "X"), to which participants respond. In other words, the task requires to direct attention selectively to an infrequently presented target and to respond with a key press. Given these task characteristics, the oddball task is reminiscent of the continuous performance test (CPT) [34]. With the CPT, the participants' task is to monitor a stream of letters successively presented on a monitor screen and to press a designated key in response to a prespecified target letter (e.g. "X"). According to Riccio et al. [35], the CPT is one of the most popular clinical tasks to assess sustained attention and vigilance by means of RT and error scores. A most obvious difference between the oddball task and the CPT is that the distractors are always the same (frequent) stimulus in the oddball task, whereas different distractor stimuli are used in the CPT. Nevertheless, the attentional demands of both tasks (i.e., identifying a target among distractors for a given period of time) are highly similar.

Given these similarities between the CPT and the oddball task, the first assumption to be investigated in the present study was that the target-related P3 latency in the CPT is negatively associated with MA as suggested by the findings with the oddball task. In addition, we assumed that the selective-attention demands on the identification of a target among distractors are decisive for the relation between P3 latency and MA. To investigate this hypothesis, two further CPT conditions were applied in the present study. In a control condition, the process of selective identification will be eliminated by omitting distractor stimuli from the task and presenting only target stimuli. The absence of distractors should reduce the demands on selective attention. If these demands, in fact, account for the observed relation between MA and P3 latency, the negative association between MA and P3 latency would be expected to vanish in the control condition.

In the case that the process of selectively identifying a target among distractors is the decisive process underlying the relation between P3 latency and MA, this relation should become stronger with increasing task demands on selective attention. To test this hypothesis, in an attention-enhanced CPT condition, the demands on selective attention were experimentally increased. For this purpose, the stream of letters contained a regular as well as an italic 'X' as 'invalid' and 'valid' target letter, respectively. The italic '*X*', but not the regular 'X', was defined as the valid target stimulus. Participants were instructed to identify and to respond to the valid target letter (*X*) but to ignore the invalid target letter (X) as a distractor. Thus, during the process of correctly identifying the valid target stimulus, the letter as well as the font type needed to be attended to. If selective-attention demands for identifying targets among distractors represent the crucial source underlying the functional relationship between P3 latency and MA, the association between MA and the target-related P3 latency should increase in this latter condition compared to the standard CPT condition. With this approach, the present study aims to elucidate the necessary preconditions for a negative relationship between MA and P3 latency to occur. Learning more about these preconditions will contribute to a better understanding and conceptual expansion of the mental-speed approach to MA.

#### **2. Materials and Methods**
