Human performance is influenced by various factors including task-relevant and task-irrelevant information. We previously reported that task-irrelevant visual motion stimuli more greatly disrupted accuracy for a rapid serial visual presentation (RSVP) task when the motion coherence was low than when the motion coherence was larger or zero. Based on this, we defined the Performance Dip as a lowered accuracy of a main task with presentation of a parathreshold task-irrelevant stimulus. We also found that the dorsolateral prefrontal cortex (DLPFC) accounts for the Performance Dip, in that the DLPFC failed to detect and therefore to suppress very weak coherent motion signals. A similar Performance Dip has also been reported with low coherence task-irrelevant motion when the central task required lexical decision on moving words (Meteyard et al. 2008
). These studies identified important aspects of how the brain suppresses task-irrelevant visual stimuli that could distract an observer from performing on a task-relevant visual feature and/or cause a conflict with the task-relevant signals, particularly when the available attentional resource is limited (Miller and Cohen 2001
However, the underlying neural mechanisms for the Performance Dip, and its generality to other tasks, is largely unknown. The previous studies investigated the Performance Dip in tasks where the interfering task-irrelevant visual motion stimuli (Tsushima et al. 2006
; Meteyard et al. 2008
) were largely distracting to discriminating the identity of linguistic stimuli such as letters or words. This may suggest the possibility that the Performance Dip is a particular phenomenon confined to letter or words. To test whether the Performance Dip is also generated in the directional ‘motor’ response presented with task-irrelevant visual motion stimulus, here we used a modified version of the Simon effect paradigm (Simon and Small 1969
), which utilizes random dots (Bosbach et al. 2004
; Wittfoth et al. 2006
). We asked the subjects to make a manual directional tilt in either the left or right direction by the lever depending on the color of the moving dots. At the same time, we manipulated the coherence of the random dots so that the perceived global direction of the dots was either congruent or incongruent to the required tilt direction, but task irrelevant.
A question arises as to whether the DLPFC is the unitary system that involves a Performance Dip. In Experiment 1, we tested whether a Performance Dip occurs in a directional motor task and confirmed that it did when the task-irrelevant coherent motion was incongruent to the main task. In Experiment 2, we conducted an functional magnetic resonance imaging (fMRI) analysis to test which of the following two models, the DLPFC model or the pre-SMA/SMA model best accounts for the Performance Dip. The DLPFC model is predicted by the previous Performance Dip study (Tsushima et al. 2006
), where the DLPFC plays a key role in the inhibitory modulation of inputs to the visual cortex (Knight et al. 1999
; Tsushima et al. 2006
; Rossi et al. 2009
). In contrast, several other frontal regions including bilateral anterior insula (Wager et al. 2005
), the right ventral lateral prefrontal cortex (VLPFC) (Forstmann et al. 2008
), and anterior cingulated cortex (ACC) (Lau et al. 2006
; Fan et al. 2008
) have been regarded as candidates to process response conflicts stemming from stationary
visual signals and motor response. However, in the present study, the conflicting information is between the directions of coherent motion as a task-irrelevant feature and the direction of the lever tilt response appropriate to the color cue. It has been reported that pre-SMA, but not the DLPFC, is involved in a conflict between the directions of moving dots
and motor response (Wittfoth et al., 2006
). Thus, as opposed to the DLPFC model, we built the pre-SMA/SMA model, where the SMA and pre-SMA, which usually suppress motor plans (Brass and von Cramon 2002
; Crone et al. 2006
; Isoda and Hikosaka 2007
; Sumner et al. 2007
; Imamizu and Kawato 2008
), fail to suppress weak task-irrelevant signals that lead to conflicting motor plans. In this model, the failure of pre-SMA and/or SMA to suppress task-irrelevant information causes a Performance Dip.
We describe the predictions of the two models below after the brief summary of the previous study. Suppose there were three levels of coherence for the task-irrelevant visual motion: zero (totally random), very weak, and very strong, and the Performance Dip occurred when the coherence of the task-irrelevant visual motion was very weak. The results of the previous study (Tsushima et al. 2006
) indicated two things. First, activation of middle temporal areas (MT+) did not increase monotonically as a function of the coherence of the task-irrelevant motion signal. MT+ was most highly activated when task-irrelevant motion coherence was very weak, when the Performance Dip occurred, and activation of MT+ was lower for weaker and stronger coherent motion signals, for which performance was higher. Second, no significant difference was found between blood oxygen level–dependent (BOLD) signals in the DLPFC between zero coherence and low coherent motion stimuli, but DLPFC was significantly activated for strong motion signals. The previous study (Tsushima et al. 2006
) suggested that the weak coherent motion signal was below the threshold of perception, resulting in a failure of the DLPFC to detect and inhibit the MT+ response to this distracting visual signal.
If the Performance Dip in the present study is subserved by the same mechanism as in the previous study (Tsushima et al. 2006
), that is, if the DLPFC hypothesis is true, then it is predicted that activation in MT+ will be suppressed for strong motion signals relative to weak coherent motion signals and that DLPFC will be significantly activated for strong motion signals compared with weak or zero motion signals.
In contrast, the pre-SMA/SMA hypothesis provides a different prediction. This model predicts that the dip will be associated with the low activation of the pre-SMA/SMA. In addition, since there are no known direct connections between MT and pre-SMA/SMA (Desimone and Ungerleider 1986
; Ungerleider and Desimone 1986
; Luppino et al. 1993
), and the pre-SMA and SMA have not been reported to send inhibitory modulations to the visual cortex, the activation of MT+ may not be suppressed in our task, in contrast to the previous study (Tsushima et al. 2006
). Rather, MT+ activation in our task will increase monotonically as a function of the coherence of the task-irrelevant motion signal.
Thus, in Experiment 2, we conducted an fMRI experiment to measure the brain activation in the four specified regions: MT+, DLPFC, pre-SMA, and SMA, to test which of the aforementioned hypotheses is more likely to account for the Performance Dip.