Subjects detected on average 61.7% of targets, with a mean false alarm rate (# false alarms/# targets) of 4.7%. The detection rate was well below ceiling, indicating that the task was quite difficult, but also well above the false alarm rate, indicating above chance performance. Mean correct reaction time (RT) was 596 msec.
An ANOVA on percent detection rates with Cue Probability (i.e. probability of the cue immediately preceding the target: 0.14, 0.50, 0.86) and Cue Type (i.e. type of cue immediately preceding the target: stay, shift) as factors indicated no significant effect of Cue Probability (F(2,40) = 1.77), Target Type (F(1,20) = .37, ns), or Cue Probability by Cue Type interaction (F(2,40) = 1.71, ns). A similar ANOVA on RTs also yielded no effects: Cue Probability (F(2,40) = 1.06), Cue Type (F(1,20) = .70, ns), Cue Probability by Cue Type interaction (F(2,40) = 1.50, ns). These results indicate that attention to the cued stream was similar across conditions and that subjects did not prematurely shift attention to an uncued stream when shift cues were likely.
For each set of regions thought to be involved in stimulus-driven reorienting, i.e. TPJ, frontal cortex and basal ganglia, and dorsal fronto-parietal cortex, we examine the BOLD activations due to stimulus-driven reorienting, defined by significant differences between stay and shift cues, and then describe how those activations were modulated by expectation.
Results for R TPJ
Modulations due to stimulus-driven reorienting
R TPJ was activated by stimulus-driven reorienting. The left coronal slice in displays voxels in R TPJ that showed significantly different timecourses following shift and stay cues (i.e. voxels that showed a significant interaction of Cue Type (shift, stay) by Time (10 time points). All voxel-wise ANOVAs were corrected for multiple comparisons over the whole brain (see methods)). A strong activation was observed in R TPJ (52, −49, 17; z=6.45), which extended dorsally into the supramarginal gyrus (51, −45, 32; z=5.62). No significant activations were observed in L TPJ, consistent with the right hemisphere dominance that has previously been observed for this region during stimulus-driven reorienting (Corbetta and Shulman, 2002
Figure 2 A) The left coronal slice shows voxels that showed significant differences between shift and stay cues (from the group Cue Type by Time ANOVA map, corrected for multiple comparisons). The color bar indicates the equivalent z-score for the p-value from (more ...)
R TPJ activations during reorienting did not depend on the location of the cue, confirming previous results (Corbetta et al., 2000
). A regional ANOVA conducted on the R TPJ ROI formed from the shift vs. stay activation (i.e. the Cue Type by Time map) indicated no significant interaction of Cue Type by Cue Location (left, right) by Time (F(9, 180) = .67). The voxel-wise map for the Cue Type by Cue Location by Time interaction also showed no effects in R TPJ. The timecourse of R TPJ activation, shown in the leftmost graph of , indicated a transient response to a shift cue, relative to a stay cue, at both cue locations.
A difference between shift and stay cues was observed even when they were both highly likely. A voxel-wise ANOVA, conducted on just the stay and shift cue data from the p=0.86 conditions yielded a significant Cue Type by Time activation in R TPJ (50, Ȓ49, 14, z=4.88), as shown in .
Modulations due to expectation
R TPJ activity was affected by cue probability in addition to stimulus-driven reorienting. A regional ANOVA confirmed that the R TPJ ROI formed from the shift vs. stay activation showed a significant interaction of Cue Probability (0.14, 0.50, 0.86) by Time (F(18, 360) = 3.91, p<.0001). Moreover, the voxel-wise map for the interaction of Cue Probability by Time included a significant activation in R TPJ (47, −48, 17; z=5.18), shown in the coronal slice in , that was contained within the TPJ component of the more extensive shift vs. stay activation. However, the cue probability and shift vs stay effects did not interact, i.e. were additive. A regional ANOVA on the shift vs. stay ROI in R TPJ indicated no higher-order interaction of Cue Probability by Cue Type by Time (F(18, 360) = .19), and no higher-order interaction was observed for R TPJ in the voxel-wise map of the interaction.
Bolstering the evidence for separate reorienting and probability signals, R TPJ showed significant effects of cue probability even when the cue did not evoke a shift of attention. An analysis of the effect of cue probability in just the stay cue condition identified a significant voxel-wise activation in R TPJ (47, −52, 14; z=3.75), and a significant effect in a regional ANOVA on the R TPJ ROI defined from the shift vs stay activation (F(18, 360) = 2.06, p = .01).
The timecourse of R TPJ activation as a function of cue type and probability is shown in the right graph in . Low probability cues (red symbols) produced larger activations than middle or high probability cues (green and black symbols) irrespective of whether the cues signaled stay (open symbols) or shift (filled symbols). Notably, TPJ activity did not change linearly as a function of cue probability, with very similar responses for the medium and high probability cues (i.e. green and black symbols). This pattern indicates that R TPJ was activated when an expectation concerning the cue was breached, as in the low probability condition. A strong expectation was not present in the 0.50 condition, since stay and shift cues were equally likely, while the expectation in the 0.86 condition was confirmed rather than violated, leading to equivalent and lower activity in the latter two conditions. Voxel-wise ANOVAs in which the cue probability factor was confined to the 0.14 and 0.50 conditions or to the 0.50 and 0.86 conditions confirmed that R TPJ was significantly activated by the former comparison (46, −51, 16, z=3.93) but not by the latter. Finally, the timecourses supported the earlier conclusion that the reorienting and breach of expectation effects were additive. The BOLD signal in R TPJ was reasonably described by increments due to reorienting (shift 50, shift 86), breached expectation (stay 14), or their sum (shift 14).
To summarize, R TPJ showed significant effects of stimulus-driven reorienting to either visual field even when shifts of attention were likely and frequently performed, and these activations were not increased when reorienting was unlikely. R TPJ was independently (additively) activated by low probability cues, indicating separate signals for reorienting and breaches of expectation. R TPJ activations to unexpected targets in previous studies reflected a sum of reorienting and breach of expectation signals (Arrington et al., 2000
; Corbetta et al., 2000
; Marois et al., 2000
; Macaluso et al., 2002
Results for frontal/insula cortex and basal ganglia
Interaction of expectation and stimulus-driven reorienting
In contrast to R TPJ, several regions in frontal and insula cortex and basal ganglia () were primarily activated by unexpected shift cues, resulting in a strong interaction of stimulus-driven reorienting and cue probability (see for z-scores and coordinates of significant foci). Significant interactions in the voxel-wise map for Cue Probability by Cue Type by Time were observed in left anterior insula, anterior cingulate, and left dorsolateral prefrontal cortex (DLPFC) (), and in basal ganglia (), including bilateral dorsomedial globus pallidus and nearby regions in caudate, and right ventral striatum.
Figure 3 Voxels in insula and frontal cortex (A) and basal ganglia (B) that showed a significant interaction of the shift vs. stay effect with Cue Probability (transverse and coronal slices from the group Cue Type by Cue Probability by Time ANOVA map, corrected (more ...)
TPJ, SMG, Prefrontal, anterior insular, and basal ganglia regions that showed significant differences between stay and shift cues in voxel-wise statistical maps, corrected for multiple comparisons
The timecourses of the frontal, insula, and basal ganglia responses () were quite different than those for R TPJ. Low probability shift cues produced a time-locked response that was sometimes extended, but low probability stay cues and high probability shift cues evoked no activity. Therefore, these regions did not respond to unexpected cues that did not reorient attention, or to expected cues that evoked stimulus-driven shifts of attention. Rather, they mainly responded to unexpected cues that evoked a shift of attention. This interaction pattern was consistent with that expected for control regions that activate new attentional sets or inhibit competing processes (Fuster, 1989
; Miller and Cohen, 2001
). Anterior insula and anterior cingulate have been identified as the regions most consistently involved in cognitive control across a variety of tasks in human neuroimaging studies (Dosenbach et al., 2006
A prior study demonstrated that greater activity in striatum was produced by infrequent than frequent randomly located peripheral stimuli that evoked shifts of attention (Zink et al., 2003
), but did not test for an interaction of frequency and reorienting. Interestingly, this study manipulated expectation about temporal onset (i.e. infrequent stimuli were less expected), holding constant expectations about the spatial location of the peripheral stimulus. Here, the temporal predictability of cue onset was held constant across conditions (i.e. in all scans a cue appeared roughly every 2 to 6 seconds) and expectations about spatial location were varied.
In addition to the basal ganglia/frontal/insula circuitry defined by the interaction of stimulus-driven reorienting and expectation signals, we were interested in evaluating R VFC, the frontal core of the putative ventral attention network. VFC includes a collection of regions in R anterior insula, R IFG, and R MFG, which tend to be variably recruited in different experiments (Corbetta and Shulman, 2002
). Of these regions, only R IFG showed an interaction of cue probability and stimulus-driven reorienting (), similar to the basal ganglia/frontal/insula regions discussed above, although the timecourse of BOLD activity was noisier. While activations were again mainly observed in the low probability shift cue condition, the transient and time-locked nature of the activation was less clear. However, the response in this region was quite different than that observed in R TPJ. The resting-state connectivity results presented below provide strong evidence that the present R IFG focus falls within the ventral network.
To summarize, basal ganglia and prefrontal/insula regions were only activated by stimulus-driven shifts of attention that were unexpected. Neither a breach of spatial expectation (i.e. spatial prediction error) nor reorienting alone was sufficient to activate these regions.
Results for dorsal fronto-parietal cortex
Transient modulations due to reorienting in precuneus and parts of FEF/precentral
Large regions within dorsal fronto-parietal cortex showed transient responses that differed for shift and stay cues, averaged over cue location, as indicated in , which shows the voxel-wise Cue Type (shift, stay) by Time ANOVA map. Parietal activations were observed most robustly in bilateral precuneus, extending into superior parietal lobule (SPL), and to a lesser degree in bilateral intraparietal sulcus (IPS) extending into postcentral sulcus (see ). Significant dorsal frontal activations were observed most robustly in bilateral human frontal-eye field (FEF), extending laterally and ventrally in bilateral precentral sulcus, and in SMA. The graphs in , which show timecourses for the four regions in dorsal fronto-parietal cortex with the highest z-scores for the shift vs stay effect (), indicate the transient character of the bilateral precuneus/SPL and L FEF responses, which resembled a ‘shift’ signal (Serences and Yantis, 2006a
), as previously reported for these regions during symbolically-directed shifts of attention (Yantis et al., 2002
; Kelley et al., 2007
Figure 4 A) Dorsal and medial views of voxels that showed significantly different activations following shift and stay cues (Cue Type by Time ANOVA map, corrected for multiple comparisons). The graphs show the timecourse of the BOLD signal as a function of Cue (more ...)
Dorsal frontal-parietal regions that showed significant differences between stay and shift cues in voxel-wise statistical maps, corrected for multiple comparisons
Three of the four ROIs shown in did not show significant spatial selectivity in regional ANOVAs, matching a previous report (Yantis et al., 2002
). However, this result should be treated cautiously. Spatially selective signals have been reported in precuneus (Hagler et al., 2007
; Jack et al., 2007
; Sylvester et al., 2007
; Saygin and Sereno, 2008
) and both L FEF and L precuneus/SPL ROIs showed the expected trend for greater contralateral activity, a trend that was significant in a regional ANOVA on the R FEF ROI (Cue Type by Cue Location (left, right) by Time (F(9,180)=2.70, p=.0095)). Spatially selective signals may be easier to image when the underlying neural processes are sustained (e.g. maintenance of attention) rather than transient (e.g. shifts of attention), as shown in the next section.
Sustained, spatially-selective modulations in IPS and parts of FEF/Precentral
Regions in IPS and parts of FEF/Precentral sulcus showed sustained BOLD responses in which the difference between stay and shift cues significantly depended on whether the cue was contralateral or ipsilateral, as shown in by the voxel-wise ANOVA map for Cue Location (left, right) by Cue Type (shift, stay) by Time (). Highly significant, spatially-selective responses were also observed throughout occipital cortex (e.g. lateral view in and Supplementary Figure 1
). Because the ANOVA compared stay and shift cues that were presented at the same location, controlling for purely sensory activations, the activations reflected a spatially-selective attentional modulation. Previous studies have reported spatial selectivity in these regions (Sereno et al., 2001
; Schluppeck et al., 2005
; Silver et al., 2005
; Hagler and Sereno, 2006
; Serences and Yantis, 2006b
; Jack et al., 2007
; Molenberghs et al., 2007
; Swisher et al., 2007
The timecourses of the BOLD signal in dorsal IPS and FEF are shown in . Shift cues to a location produced a sustained increase in the contralateral hemisphere and a sustained decrease in the ipsilateral hemisphere that eventually returned to the level of activation produced by a stay cue, which showed smaller differences between ipsilateral and contralateral locations. The sustained decrease was not a true deactivation, however, since it did not occur relative to a resting baseline but only relative to a baseline that included both sensory-evoked and task-evoked activity; i.e. the important result was the difference between the ipsilateral and contralateral cue timecourses. The sustained nature of the timecourses likely reflected the maintenance of attention and/or the modulation of stimulus-evoked activity from the RSVP stream at the cued location.
While in parietal cortex there was a clear functional-anatomical segregation between the IPS regions that most strongly showed sustained signals and the precuneus regions that most strongly showed transient ‘shift’ signals, the functional-anatomical segregation between the two types of signals in FEF/precentral cortex was less clear and suggested some mixing of the two signal types (e.g. R FEF foci in ).
Interaction of expectation and reorienting: regions showing transient signals
We next consider how expectation affected reorienting in dorsal fronto-parietal regions that showed transient ‘shift’ signals. , which shows timecourses for the four ROIs showing the largest shift vs. stay z-scores, indicates that activations were greater for low probability than middle or high probability cues. The latter two conditions showed similar activation magnitudes, again indicating that the effect of cue probability was best described as a breach of expectation. Importantly, however, the probability effect was largely confined to shift cues, with weak or absent effects for stay cues, unlike the additive effects of probability and reorienting observed in R TPJ.
Figure 5 A) The graphs show the timecourse of the BOLD signal as a function of Cue Type and Cue Probability in ROIs defined from the Cue Type by Time map, corrected for multiple comparisons. B) The graphs show the timecourse of the BOLD signal for shift cues only (more ...)
The interaction of reorienting and expectation was significant in both of the medial parietal regions showing transient spatially non-selective shift signals, i.e. R precuneus (6, −56, 54, F(18, 360) = 2.17, p=.01) and L precuneus/SPL (−12, −58, 55, F(18, 360) = 2.29, p=.006), and was marginally significant in R FEF (25, −9, 54, F(18, 360) = 1.81, p=.039). These regional analyses were corroborated by voxel-wise results shown in Supplementary Figure 2
and . Finally, the same two medial parietal ROIs that showed a significantly greater effect of shifting attention when cues were unexpected, nevertheless showed highly significant shift vs stay differences in regional analyses when only expected (high probability) cues were analyzed (L precuneus/SPL: −12, −58, 55: F(9,180) = 4.99, p<.0001; R precuneus: 06, −56, 54: F(9,180) = 4.15, p<.0001) (see Supplementary Figure 2
and Supplementary Figure 3
for corresponding effects in the voxel-wise maps).
Interaction of expectation and reorienting: regions showing sustained, spatially-selective signals
shows the effect of expectation on the timecourse of activation following a shift cue in the four dorsal fronto-parietal regions from that showed sustained, spatially selective differences between stay and shift cues. Low probability shift cues increased the activation from both ipsilateral and contralateral cues, resulting in an effect of cue probability that was largely independent of spatial location and therefore did not enhance the spatial selectivity of the activation. While breaches of expectation produced overall increases in the BOLD signal, the interaction of expectation and reorienting was less robust. Regional ANOVAs on the four ROIs yielded a marginally significant Cue Type by Cue Probability by Time interaction in one parietal region, L IPS (−27, −58, 50, F(18,360) = 1.83, p=.036) and in R FEF (31, −12, 51, F(18, 360) = 1.74, p=.049), indicating that in some ROIs the probability effect modestly differed for stay and shift cues. The weaker nature of the effects of expectation on reorienting was also evident in the corresponding voxel-wise statistical map (Supplementary Figure 2
), which was not significant in IPS.
To summarize, in dorsal fronto-parietal regions, breaches of expectations enhanced signals due to reorienting. This effect was more pronounced in medial parietal (precuneus) regions showing transient signals to the cue than in more lateral parietal regions (IPS) showing sustained spatially-selective signals, possibly reflecting different functional roles of these regions in shifting and maintaining attention, respectively. As with the basal ganglia/frontal/insula regions discussed above, interacting effects of expectation and reorienting cannot be ascribed to a general increase in arousal when an expectation is breached, but indicate a specific effect of expectation on shifts of attention. However, unlike basal ganglia/frontal/insula regions, dorsal fronto-parietal regions showed highly significant differences between stay and shift cues even when those cues were expected, consistent with the primary role of the dorsal network in shifting attention.
Interaction of expectation and reorienting across networks
The preceding analyses have indicated that R TPJ, basal ganglia/frontal/insula regions and dorsal fronto-parietal ‘shift’ regions fall along different points on a continuum that reflects the effect of expectation on reorienting. TPJ falls on one end of the continuum with equivalent shift vs stay activations under high and low probability conditions, the most robust dorsal fronto-parietal ‘shift’ regions fall in the middle, with shift vs stay activations that are present under high probability conditions but are increased under low probability conditions, and basal-ganglia/frontal/insula regions and R IFG fall on the other end, with shift vs stay activations only under low probability conditions.
We illustrate this point using a metric that allows the different regions to be more readily compared. The graph in displays a shift minus stay magnitude for each probability level, normalized by the magnitude at the 0.14 probability level, where a single line in the graph displays the magnitudes from a single region. Magnitudes were computed from the observed timecourses using a standard hemodynamic response function (Boynton et al., 1996
). shows the normalized magnitudes for TPJ and SMG (from the Cue Type by Time map; , ), the 4 dorsal fronto-parietal regions that showed the largest z-scores for the difference between shift and stay cues (from the Cue Type by Time map; , ) and the 8 IFG, frontal/insula and basal ganglia regions from the voxel-wise map for the interaction of Cue Type by Cue Probability by Time (, ). The distribution of the three region sets corresponded to the description above, with TPJ/SMG and basal ganglia/frontal/insula regions on the extremes and dorsal fronto-parietal ‘shift’ regions in the middle.
Figure 6 The graphs display the shift cue minus stay cue magnitude at three probability levels, normalized by the magnitude at the 0.14 probability level. Each line in the graph corresponds to a different ROI. The R TPJ and R SMG ROIs are taken from the multiple-comparison (more ...)
At the level of individual subjects, the normalized magnitude measure was very unstable and sensitive to noise since some subjects had small magnitudes in the denominator. Therefore, we used the non-parametric Mann-Whitney U-test to compare the group-averaged distributions between region sets at the 0.86 probability level normalized values, which by hypothesis should most strongly differentiate the three region sets. Significant differences were found between dorsal fronto-parietal cortex and basal ganglia/frontal/insula cortex (U=0, U`=32; p=.007) and basal ganglia/frontal/insula vs TPJ/SMG (U=0, U`=16, p=.037). The difference between dorsal fronto-parietal cortex vs TPJ/SMG was marginal (U=0, U`=8; p=.064), entirely because of the small number of regions involved (n=6). However, each of the 24 dorsal fronto-parietal ROIs listed in from the Cue Type by Time and Cue Type by Cue Probability by Time voxel-wise maps had a value for the normalized 0.86 shift vs. stay magnitude that was less than that for either R SMG or R TPJ.
Resting-state functional connectivity
The task activation analyses indicated that three sets of brain regions were differentially modulated by reorienting and expectation. To determine if these regions formed coherent networks, we measured resting-state functional connectivity (rs-fcMRI), using the task-evoked foci as seeds, in an independent set of subjects (n=11).
shows the rs-fcMRI maps that used as seeds the dorsal basal ganglia ROIs from the Cue Type by Cue Probability by Time interaction. A striking pattern of selective rs-fcMRI was observed with the task-evoked frontal/insula ROIs in anterior cingulate, L anterior insula, and L DLPFC that also showed the Cue Type by Cue Probability by Time interaction (indicated in by the white circles). Supplementary Figure 4
shows that strong inter-regional rs-fcMRI was obtained between the cortical regions of this network, namely L DLPFC (the more inferior focus), anterior cingulate, and L anterior insula. These observations from the voxel-wise maps were confirmed with pair-wise regional statistical analyses (see Supplementary Text
for detailed statistics). As summarized in , rs-fcMRI for all of the ROI-ROI pairs within the basal ganglia/frontal/insula network exceeded the p<.01 significance threshold, except for a single pair with p<.05.
Figure 7 A) Flat-maps of voxel-wise rs-fcMRI statistical maps in a separate group of subjects (n=11) using as seeds the left and right dorsal basal ganglia ROIs that showed a significant interaction of Cue Type by Cue Probability by Time, corrected for multiple (more ...)
p-values of resting-state fcMRI between indicated regions.
In summary, a robust basal-ganglia/frontal/insula resting-state network was observed in regions that were only activated by unexpected cues to shift attention. Interactions between human basal ganglia and frontal and insula cortex (Postuma and Dagher, 2006
; Di Martino et al., 2008
) are thought to be consistent with the principle of parallel cortico-striatal loops (Alexander et al., 1986
; Alexander and Crutcher, 1990
), although the disparate frontal/insula regions of the current network may not be subsumed within a single loop.
Previous reports have indicated that R IFG and R TPJ show strong functional connectivity (Fox et al., 2006
; He et al., 2007
), supporting the hypothesis of a ventral attention network. While in the present study R IFG and R TPJ showed different activation patterns, they nonetheless showed strong and selective rs-fcMRI in voxel-wise maps that used each focus as a seed (). A regional analysis confirmed highly significant rs-fcMRI between the two regions (t(10)=6.44, p<.0001).
Basal-ganglia/frontal/insula vs R TPJ/IFG
When all possible ROI pairings between the basal-ganglia/frontal/insula and R TPJ/IFG networks were averaged, the overall rs-fcMRI between networks was not significant (t(10)=1.67, p>.1) (; see and Supplementary Text
for regional statistics on pairwise comparisons). Therefore, although both networks were recruited when subjects shifted attention to an unexpected stimulus, they showed at best modest interactions in the resting state.
Figure 8 The graph shows the group-averaged Fisher z-transformed value for all ROI pairs within a network for three different networks, or for all ROI pairs between networks. Error bars are computed over the 11 subjects. dAtt = the dorsal attention network, comprising (more ...) Dorsal fronto-parietal regions vs basal ganglia/frontal/insula and R TPJ/IFG networks
Because of the primary role of the dorsal network in controlling attention and because dorsal activations for shifts of attention were enhanced when those shifts were unexpected, we measured the resting correlation of the dorsal network with the basal ganglia/frontal/insula and TPJ/IFG networks recruited by unexpected shifts of attention. Average connectivity scores were computed within and between networks (). Modest but significant overall positive rs-fcMRI was observed between the basal ganglia/frontal/insula network and dorsal fronto-parietal regions (t(10)=3.39, p=.007). Pairwise regional analyses indicated that dorsal fronto-parietal rs-fcMRI with the basal ganglia/frontal/insula network was strong through L DLPFC (t(10) = 6.86, p<.0001; see Supplementary Figure 4
) and to a lesser extent L anterior insula (t(10) = 4.06, p=.0023). Significant connectivity of DLPFC with posterior parietal cortex is consistent with previous anatomical studies in monkeys (Goldman-Rakic, 1988
) and functional connectivity studies in humans (Seeley et al., 2007
In contrast, no significant overall rs-fcMRI was observed between R TPJ-IFG and dorsal frontal-parietal regions (t(10)=−1.65, p > .1). While this null result could indicate that these regions did not interact during task states, it could also indicate the highly contingent nature of those interactions. We have previously argued that the resting-state independence of R TPJ from both dorsal fronto-parietal and default networks allows TPJ to flexibly switch between those networks during task states (Corbetta et al., 2008
Eye movement recording
Two summary measures were applied to the eye movement data for each subject. First, the mean eye position during periods in which the left RSVP stream was attended was compared to the mean eye position during periods in which the right RSVP stream was attended, indicating whether there was any overall bias to fixate nearer the attended stream. Second, the mean changes in eye position evoked by stay and shift cues were measured, indicating whether eye movements affected event-related BOLD responses to the cues. In order to compute the second measure, the mean eye position during the 0 msec to 100 msec period following cue onset was used as a baseline while the mean eye position in the 1750 msec to 2000 msec period following cue onset was used to assess movements. Measures in other time periods (e.g. from 550 msec to 775 msec or from 775 to 1000 msec) yielded similar results. Both measures identified two subjects who made excessive movements. For one subject, the mean difference in eye position when the left and right streams were attended was −6.2 deg (where negative numbers refer to a leftward shift), while the mean change in eye position following left and right shift cues was, respectively, −5.3 and 6.5 deg (where negative numbers refer to leftward shifts). For the other subject, the analogous measures were −2.0, −2.4, and 2.0 deg. The data from these two subjects were not included in the results reported above.
For the remaining subjects, the mean difference in eye position when attending to the left and right streams was −0.11 deg. The mean change in eye position following left and right field shift cues was −0.17 deg. and 0.10 deg. respectively. These small deviations in eye position from fixation indicate that subjects largely followed instructions. In addition to the quantitative data given above, each subjects’ eyes were carefully monitored via a camera on all scans irrespective of whether eye movement records were also collected.
Finally, our results cannot be explained by eye movements. The regions in TPJ, frontal cortex, insula, and the associative division of the basal ganglia, which provided many of the primary results of the present study, are not classically involved in the control of eye movements. Dorsal fronto-parietal regions do show eye movement responses, but the effects of expectation on reorienting in these regions were actually intermediate between those for TPJ on the one hand and basal ganglia/frontal/insula cortex on the other. Moreover, dorsal parietal regions, as well as occipital regions, showed strong spatially selective attentional activations, which would not have occurred if subjects moved their eyes and fixated the attended stream, and the spatial selectivity of the activations was independent of expectation. Finally, it is unlikely that eye movements occurred during the resting-state scans, in which no peripheral stimuli were presented and the subject’s only task was to remain fixated. Yet there was a striking correspondence between the TPJ-IFG and basal ganglia/frontal/insula networks identified from the resting-state scans and the regions identified from the scans involving the RSVP task.