While a small number of fMRI studies 
utilizing continuous fMRI scanning on cued auditory attention shifting have been published, the current study was specifically designed to compare auditory fMRI activations between attention shifting to predictable cues and stimulus-driven orienting to unexpected novel sounds. Noting the essential role of sensory areas in automatic change detection processes triggering involuntary attention in the auditory domain, we used a “mixed” event-related/sparse sampling approach to mitigate both sensory and attentional confounds caused by acoustical scanner noise to achieve this goal. In addition to similarities between activations to putative endogenous and exogenous processes consistent with previous studies 
, our approach also revealed some notable activation differences between cued attention shifting, novelty-triggered attention shifting, and target discrimination.
Areas activated selectively by cued attention shifting were found in the bilateral precentral/FEF regions, and in posterior parietal areas. In these areas, the foci of activations were clearly different between cued attention shifting (more posteriorly/superiorly in IPS), novelty-triggered attention shifting (inferior/posterior to IPS), and target discrimination (more anteriorly in IPS/SMG). In line with the theory 
of distinct dorsal vs. ventral attention systems, novelty-triggered attention shifting activated selectively posterior aspects of the STS/medial lateral temporal cortex, inferiorly to the activations of the two other more goal-directed attention conditions. Interestingly, the anterior insula, which has often been associated with more fundamental processes of stimulus-driven change 
and salience detection 
, was activated during cued attention shifting and target discrimination, but not during attention shifting to sound novelty. In the prefrontal cortices, activations associated with target discrimination were widespread and found in regions more anterior and superior to the frontal areas activated by other conditions.
In the precentral areas including the FEF and lateral PMC areas (BA 6 and 44), most widespread activations were observed for the contrast associated with cued attention shifting. Within these areas, the region probably most closely corresponding to FEF was significantly activated also during novelty-triggered attention shifting, but only very weakly during target discrimination. Our results, thus, would suggest that these areas are related to the orienting of auditory attention, and most strongly, during cued attention shifting. This interpretation is consistent with the long-held view that FEF constitutes a critical locus for the control of spatial attention 
, as it is presumably interconnected with other frontoparietal regions, such as IPS, and because it may also be involved in multisensory attention 
and orienting 
. The increased FEF activations associated with auditory attention shifting, during a condition in which subjects were instructed to fixate on a cross in the center of the screen for all tasks, is also in line with the notion 
that this area is involved in more than just the control of eye movements and overt gaze orienting. However, similar to previous observations 
, our results suggest that the right FEF is activated by distracting events that catch attention in a bottom-up manner as well. Indeed, it was recently suggested that regions of human FEF and IPS may reflect the representation or integration of attentional priority 
, instead of constituting strictly a “voluntary” attentional system. In other words, the function of FEF may, instead of voluntary control only, be more essentially associated with orienting of spatial attention. Intriguingly, the lateralization of present FEF effects for novel and distracting auditory events is also in line with the traditional view that the triggering of involuntary auditory attention is specifically lateralized to the right frontal cortex 
. Meanwhile, the voluntary shifting condition seemed to activate precentral areas including the FEF more bilaterally.
Our results on precentral areas (beyond FEF) may be interesting in light of the recent debate on the attentional role of lateral PMC. Some studies 
suggest that lateral PMC is involved in the detection of salient and behaviorally relevant stimuli, especially in unattended and task-irrelevant locations (stimulus-driven attention). This finding has led to a proposition that these regions constitute a part of the same ventral fronto-parietal network that also includes the anterior insula and TPJ 
. However, the present evidence of more widespread activations in the lateral PMC during cued rather than novelty-triggered attention shifting suggests that these regions are involved in top-down/voluntary attentional control.
The posterior parietal cortex, especially IPS, is essential for spatial attention 
supported by converging evidence from monkey physiology and human neuroimaging studies. There is even some evidence that further suggests a topographic organization of spatial attention signals within IPS 
. Our findings that cued attention shifting activates superior IPS, novelty-triggered attention shifting activates inferior IPS, and target discrimination activates more anterior/superior IPS areas suggest possible functional differentiation within these posterior parietal areas. These findings are also principally in line with the proposed dorsal vs
. ventral distribution of networks dedicated to goal-driven (cued attention shifting, target discrimination) and stimulus-driven (novelty-triggered attention shifting) attentional processes 
Another intriguing finding in our study is that the anterior insula was, in both hemispheres, more significantly activated in cued attention shifting and target discrimination than novelty-triggered attention shifting. Previous studies on executive control of auditory spatial attention have reported activations in the bilateral anterior insula 
. However, the anterior insula has not, traditionally, been viewed as a task-control region, and its activations have typically been considered subsidiary to IFC 
or they have not been extensively discussed 
. Nevertheless, our data are in line with the accumulating human and non-human primate evidence 
suggesting that the anterior insula, as a part of a cingulo-opercular system, might play a more significant role in voluntary cognitive control than previously assumed. The notion has been further supported by animal 
and human 
evidence on anatomical connectivity between the anterior insula and mSFC areas, as well as by histological evidence 
. It has been, however, also debated whether the anterior insula has a more executive role in maintaining a sustained task mode and strategy 
, or whether it is merely a transient saliency detector that initiates attentional control signals in other higher-order areas 
. The present lack of anterior insula activations to the most salient sounds of the present design, the novel sounds, would seem to be clearly at odds with the latter idea. Our findings would, instead, seem to be more consistent with an alternative theory 
that the anterior insula activity does not express perceptual salience, per se
, but rather the recruitment of processing resources when faced with a given sensory event, whatever the source of that recruitment, bottom-up or top-down. Finally, it is also noteworthy that, in addition to the actual redirection of attention, attention shifting presumably involves endogenous processes that allow us to disengage from previous activity and to maintain heightened top-down control on the new task 
. Consequently, noting the recent evidence by Wu et al. 
and Alain et al. 
showing anterior insula activations during working memory processing and goal-directed actions, it is also possible that activations in the anterior insula during cued attention shifting and target discrimination are most essentially related to engagement of attention control.
It has been proposed that the anterior insula and the cingulate gyrus may belong to the same cingulo-insular system involved in top-down cognitive control. Our data is consistent with this proposal, as the mSFC regions (including bilateral pre-SMA and extending to aMCC, and rostral cingulate zones) and the anterior insula were both activated during cued attention shifting and target discrimination, but not in novelty-triggered attention shifting. The cingulate gyrus has also been identified as a major component in a distributed network subserving the dynamic relocation of spatial attention 
. Previous studies also show that aMCC is associated with conflict resolution 
and decision making 
. Here we provide data suggesting that aMCC is specifically involved in top-down spatial attention control.
In general, our data are consistent with previous studies on auditory attention shifting. However, we also observed certain discrepancies. Such discrepancies may, obviously, be in part explainable by differences in paradigms and methods between the studies. For example, our results differ slightly from a previous event-related fMRI study by Mayer et al. 
where subjects were instructed to localize targets following informative (75% valid) or uninformative (50% valid) cues. In contrast to present findings, as well as those reported in several recent auditory studies 
, the authors found that automatic orienting elicited by the uninformative cue condition increased activations in the precentral areas and the insula, both of which in the present study were associated with voluntary instead of stimulus driven processes. However, the study of Mayer et al. apparently did not aim at separating the processes of auditory cued attention shifting and target identification. Moreover, based on the behavioral data, it is not entirely clear that uninformative cues used in Mayer et al. were, in fact, followed by less intensive top-down processing than informative cues.
At the same time, the present results differ slightly from the event-related fMRI study of Salmi et al. 
, in which the authors used a dichotic target-discrimination design that was in many ways analogous to the present study. Specifically, Salmi and colleagues asked their subjects to detect occasional targets in the attended stream. Instead of novel sounds presented only to the uncued ear, involuntary attention shifts were triggered by unexpected loudness deviations presented to either ear. Finally, unlike in the present study, attention shifts were guided using central visual cues. Their results were quite similar to the present findings in regards to top-down controlled attention shifting. Salmi et al did not, however, observe top-down related activations in the anterior insula. At the same time, using the visually presented shifting cues, Salmi et al. observed visual-cortex activations that were absent during cued shifting in the present study. For the bottom-up driven attention shifting, the present study showed more extensive activations in bilateral auditory cortex (possibly due to auditory stimulation and scanning parameter differences explicated above), posterior insula, IPS, and posterior cingulate than the study of Salmi and colleagues. In addition to aforementioned differences in stimulation and scanning parameters (continuous vs. sparse sampling), some of these discrepancies may be explainable by anatomical interpretation approaches. For example, the present surface-based approach may produce different results in terms of the exact anatomical boundaries between the anterior insula and IFC than the fully volumetric atlas that was used by Salmi and colleagues.
The present sparse sampling design may help make the results more easily comparable to cognitive neuroscience studies conducted using other methods that are not confounded by factors such as acoustical scanner noise. That is, in comparison to the relatively small number of auditory studies on voluntary attention shifting, there has been a profusion of MEG and EEG research on involuntary attention shifting to unattended sound changes 
. In these studies, involuntary auditory attention shifting has been proposed to be triggered by an automatic change-detection process, reflected by the MMN response 
, followed by a sequence of brain events associated with attentional orienting and conscious detection of this change (however, see also 
). Indeed, in the present study, quite remarkable differences between conditions emerged in the superior temporal auditory areas. In these areas, the activations during novel sound processing extended all over the superior temporal plane, while activations to attention shifting cues were only significant in posterior aspects of auditory cortex (pSTG, PT). This distribution difference of effects could, in principle, be interpreted to be in line with the suggestion 
that automatic deviance detection (reflected by the MMN process) originates more anteriorly in the auditory cortex than responses to more predictable shifting cues. At the same time, previous studies also suggest that non-primary auditory cortex processes sound identity and location in parallel, through the anterior “what
” and posterior “where
” pathways 
. In the current study, the novels contained much richer identity features than the cues used to trigger voluntary attention shifting. Hence, the enhanced spreading of auditory cortex activations to the putative “what
” regions might reflect stimulus-driven activations in the sound-object identification system. A sound-identification process might also explain some of the IFC activations during novelty-triggered attention shifting, given the theory that the “what” streams extend to ventral frontal cortex areas 
. Meanwhile, the broader activations associated with auditory target discrimination, compared to cued attention shifting, could be partially explained by the more enhanced top-down influences needed for the more difficult process of discriminating the targets from the repetitive standards, as established by numerous imaging studies 
. Our previous work on auditory attention has also demonstrated correlations between attentional modulation of auditory cortex activation and behavioral discrimination of target tones (as measured from the difference in the hit rate between easier vs. more difficult targets delivered to the ear).
We observed extensive activations in the visual cortices associated with target discrimination, similar to Wu et al. 
(note however that Wu and colleagues asked their subjects to keep their eyes shut throughout the study). This may be the result of cross-modal influences between the auditory and visual cortices. That is, previous studies have shown that the visual cortex can be activated by auditory input 
and that there are direct anatomical connections between the superior temporal and occipital regions in primates 
and humans 
. Meanwhile, a recent fMRI study 
also showed that auditory occipital activations depend strictly on the sustained engagement of auditory attention and are enhanced in more difficult listening conditions.
The “standards only” condition, during which subjects were instructed to listen carefully and wait for the cue, revealed significantly increased activations in several frontal, temporal and parietal regions. Interestingly, the activated areas included the paracentral region, which according to recent studies is activated during maintenance of attention 
. These paracentral activations might have also overlapped with a supplementary eye field region, which has been previously proposed to be involved in visuospatial control processes and performance 
. The areas activated during the standards only condition also included the SFC, which has been previously associated with high-level cognitive control processes such as monitoring 
and anticipatory spatial attention 
. However, it has to be noted that, in contrast to other comparisons that were conducted across active task/stimulation conditions, the “standards only” comparison was contrasted with the fixation condition, in which no explicit task was included.
Benefits of Sparse vs. Continuous Scanning
Here, we utilized sparse sampling to control for biases caused by the acoustical scanner noise. Acoustical scanner noise is potentially a problematic variable in all fMRI experiments, but it is of particular concern in studies of audition and language processing. First, as discussed above, these effects might obviously modulate stimulus-driven orienting, which presumably receives a major contribution from auditory cortex 
. Although scanner noise does not necessarily entirely abolish change-detection activations that trigger involuntary orienting 
, the benefits of sparse sampling in studies on stimulus-driven auditory cortex activities have been well documented 
. Second, the ongoing acoustic and somatosensory stimulation associated with continuous scanning may also confound attentional top-down effects, both in auditory cortices and higher-order association areas. The longer-term effects of continuous environmental noise on our ability to concentrate have been very well documented 
. Not surprisingly, in fMRI studies, it has been shown that increasing the intensity of acoustical scanner noise modulates extra-auditory activations during working memory performance, resulting activation increases in certain areas (inferior, medial, and superior frontal gyri) and decreases in others (e.g.
, anterior cingulate) 
. Using PET, it has been further shown that recorded scanner noise may increase regional blood flow in anterior cingulate and Wernicke’s areas during visual imagery 
. Event-related potential (ERP) studies also suggest that continuous fMRI scanner noise may reduce and delay certain “endogenous” components that are related to auditory attention 
. Consistent with these results, active behavioral auditory performance may be improved during sparse vs. continuous fMRI scanning 
. Finally, recent sparse-sampling fMRI studies 
also suggest that top-down attention effects may produce detectable modulations in auditory cortex even in the absence of any acoustic stimuli. Such endogenous feedback activations could be easily masked or cofounded in by acoustical scanner noise. These kinds of confounds have been also discussed 
, for example, in the context of interpreting top-down modulations of auditory cortex activity by visual stimuli during continuous scanning.
Based on the above notions, it might seem quite obvious that sparse sampling is the best approach for any study involving auditory functions. However, it has to be also noted that with sparse sampling designs, a much smaller number of volumes can be acquired in a given experiment, which may reduce the signal-to-noise ratio in comparison to continuous scanning. Indeed, a recent auditory cortex mapping study 
(which however also utilized 70 dB noise masking on the background in both fMRI scanning conditions) showed relatively small differences between sparse and continuous scan experiments. In terms of more complex designs, a disadvantage of sparse sampling is the reduced temporal resolution that will make it difficult to extract stimulus specific BOLD time courses. Finally, a trade-off in sparse sampling is the fact that the clustered scan noise may, itself, become a “rare sound” that triggers strong activations of the alerting and orienting networks. Although the BOLD responses of such activations will not be necessarily caught by fMRI when the TR is long enough, the cognitive significance and relative saliency of the subsequent stimuli of interest may still be modulated. However, a novel feature in the present orienting design was that these biases were controlled by using the noise stimulus produced by each fMRI volume acquisition as a part of the task design.
It is noteworthy that in experimental conditions, it is difficult to produce and document activations that are purely stimulus driven vs. endogenous. For example, novelty-triggered attention shifting may involve a number of top-down processes that are associated with, for example, the suppression of involuntary attention shifting, reorienting to the relevant task (if this is part of the instruction), and conflict resolution processes to “evaluate the situation” after the automatic orienting response (see, e.g., Escera et al. 
, Schröger and Wolff 
). Cued attention shifting is, in turn, contaminated by stimulus-driven processes triggered by the cue itself. At the same time, voluntary attention shifting may involve active disengagement from the previous strategy, as well as engagement to the new attentional task (termed “cued attention,” in Petkov et al. 
, also see 
). In other words, although the process is collectively referred to as “attention shifting”, the parts that are actually the most “voluntary” or “goal directed/endogenous” might not be related to the orienting, per se
It is also possible that differences in auditory cortex activities during the triggering of involuntary and voluntary attention are related to the context and predictability of the stimulation. Strong unpredictable stimuli, such as novel sounds, tend to result in widespread sensory responses from the bottom up, which then triggers an involuntary orienting process that, according to previous ERP studies, is related to the strength of the auditory cortex response. A more predicable and repeated stimulus, such as the cue, may trigger less prominent responses – but even in this case, the cue plays a role in orienting. However, proportionally speaking, the bottom-up influence is smaller than in the case of novelty-triggered attention (consistent with our predictions and conclusions). At the same time, these processes are essentially modulated by top-down attention, especially when the discrimination task is difficult (such as in the case of targets that result in a stronger auditory cortex response than the cues).
Also note that in most visual studies, the cue that triggers voluntary shifting is usually an arrow, which is physically different from the target. A related consideration is whether the present cues were more prone to induce stimulus-driven activations than the symbolic arrows, which have been utilized in many visual and also auditory attention shifting studies 
. It has been thought that because arrow symbols do not occur in the physical location of the target, they might trigger purely goal-driven processes. However, it is worth noting that the processing of any simple cue probably gets rapidly automatized during the course of an experiment, and as the simple symbol is repeated, the account of stimulus-driven processes subsequently increases 
. Most importantly, the present study showed notable differences between activations during cued and novelty-triggered auditory attention shifting, clearly beyond areas associated with sensory cortices processing physical properties of the sounds. Further, activations in these sensory areas, where one might expect particularly strong stimulus-driven activations, were clearly weaker during attention-shifting cues than during novel-sound or target-sound detection.
In conclusion, our study revealed distinct activations during cued auditory attention shifting, involuntary orienting to novel sound, and auditory target discrimination. Areas most selectively involved with cued voluntary shifting included the superior/posterior IPS and precentral areas (including FEF and PMC), which provides important evidence supporting these regions’ involvement in top-down/voluntary attentional control. Activations specific to involuntary attention shifting to novel sound were found in posterior STS, inferior IPS and TPJ, which is principally consistent with models suggesting more ventral distribution of stimulus-driven attention 
, but also from the right IFC. Interestingly, our results also revealed marked differences in the anterior insula and IFC activations associated with goal-driven attentional processing (cued attention shifting and target discrimination) and novelty-triggered involuntary attention shifting, suggesting that the anterior insula may play a more executive role in auditory attention than previously thought.