## 4 Discussion
### Adjacent ERP Overlap, Jittered SOI, and SSVEP  
Our research hypotheses were formed under the assumption that systematic distortion of averaged ERPs--which we observed as increased signal variance in the non-jitter condition--would translate to changes in performance of a BCI RSVP system. We also suspected that this rhythmic distortion, which we conceptualized as unwanted SSVEP artifact, contributed to changes in an alpha attenuation effect which were observed previously between slow and fast RSVP paradigms [19]. However, because our measures were generally unchanged by jittered SOIs, one should consider possible problems with the logic underpinning our assumptions. Specifically: (1) jittered SOI is not a one-size-fits-all solution to adjacent ERP overlap; (2) it is unclear whether SSVEP artifact has deleterious effects on event-related responses during RSVP; and (3) there are notable differences between the current design and previous investigations which may help to explain our results.  
With regard to the first of these points, high-pass filtering can attenuate unwanted long-latency potentials, though in the case of P300 measurement, this solution perhaps is not practical [38]. Additionally, in cases of random stimulus presentation, target versus non-target differences are understood to be somewhat resilient to distortions caused by adjacent ERP overlap, though not necessarily all types of order effects [17]. Critically, this scenario accurately describes the current RSVP design, where the classifier is focused on class differences. Some have even concluded that detection of a relevant target response can occur without precise knowledge of the timing of stimulus onset [24].  
As for SSVEP artifact, we acknowledge that many ERP-based RSVP systems continue to function with acceptable accuracy [13], despite a general lack of concern over overlapping adjacent ERPs. ERP- and SSVEP-based BCIs are generally characterized as separate mechanisms, to be sure, and have even been fused in hybrid designs [47]. Of importance to the question of jittered SOI, code-modulated visual-evoked potentials (c-VEP) also use irregular pseudo-randomized stimulus presentation durations, but there is no consensus that c-VEP is functionally beneficial relative to routine SSVEP [48]. Dramatic changes in alpha attenuation between slow and fast RSVP presentations [19], on the other hand, may very well be the result of qualitative endogenous changes in the task (e.g., cognitive load), rather than exogenous factors, such as interference from the SSVEP or an associated harmonic.  
Lastly, we would like to consider our results as they relate to previous relevant investigations. To our knowledge, no previous work has explored jittered SOI in the context of theRSVP BCI paradigm, so we have no ideal candidates for comparison. The visual changes observed in our ERP waveforms with increased jitter latencies were ostensibly similar to those illustrated previously [17]. As noted above, however, stimulus randomization can be an effective tool for dealing with overlap when looking at target versus non-target differences. Additionally, compared to relevant discussion in [17] which assumed equal target and non-target likelihoods, the present RSVP design possessed unequal classes. As well, while previous reports described changes to ERP attention effects with increasing SOI durations, the present study used dramatically shorter latencies overall than [18], and another comparison of different SOA values on ERPs and VEPs did so using a block design [26]. Increasing N200 and P300 amplitudes observed in that study and a lack of such effects in the present study might be explained by our use of uniform random distributions, which result in an unchanging mean SOI value across conditions.