Both hypotheses would clearly have to assume that the ipsilateral P1 should always be smaller than the contralateral P1. It could be objected, however, that the large ipsilateral P1 simply is an artifact which is due to volume conduction. Depending on the location and spatial orientation of a dipole, ERP components on the scalp will vary in amplitude size and/or polarity. Because volume conduction is extremely fast (operating at the speed of light), the peak latencies of the components TSA HDAC concentration must be identical for all recording sites. Inspection of Fig. 1B,

however, clearly indicates that all ipsilateral P1 components are shifted in latency by about 5 ms. The extent of the latency shift is even more pronounced in the examples shown in Fig. 2 and Fig. 4, Bleomycin manufacturer where the ipsilateral P1 components are delayed by about 20 ms or more. These findings are in

good agreement with other studies showing that the delayed ipsilateral P1 must be modeled by a separate dipole that is clearly distinct from that which is used to model the contralateral P1 (cf. Di Russo et al., 2002). This remarkable finding of a large ipsilateral P1 appears to be even more pronounced in type 2 paradigms. The reason for this may be seen in the fact that in type 2 paradigms a cue directs attention to different locations on a trial per trial basis. Thus, the attentional top–down control may be more effortful (and require more inhibitory control) than in type 1 paradigms where over an entire run of trials attention remains focused on the same location. As an example for a type 2 paradigm, Freunberger et al. (2008a) found that P1 amplitudes are actually larger (and delayed) over

ipsi- as compared to contralateral recording sites (cf. Fig. 2). In this experiment, targets were white bars on black background presented either right or left from the center of the computer monitor. Subjects had to indicate 4-Aminobutyrate aminotransferase by a button press, whether the bar was small or large. Frequencies for small and large targets were 50% and were equally distributed to the different experimental conditions. In half of them attention was cued to the right and in the other half attention was cued to the left hemifield. In 75% of the trials, cue and target locations were congruent (valid condition) and the remaining 25% were incongruent (invalid condition). Cue predictability is closely related to top–down control. High predictability enables focused, top–down controlled attention, whereas low predictability is associated with unfocused attention. If a cue is non-predictive, the P1 for cued and uncued locations is of equal magnitude (e.g., Hopf and Mangun, 2000) which means that top–down controlled attention is unfocused and equally distributed to cued and uncued locations.