تعدیل کنترل توجه ادراک شنوایی با دوپامین: اثرات ژنوتیپ DARPP-32 (PPP1R1B) بر رفتار و قشر پتانسیلهای برانگیخته

Abstract Using a specific variant of the dichotic listening paradigm, we studied the influence of dopamine on attentional modulation of auditory perception by assessing effects of allelic variation of a single-nucleotide polymorphism (SNP) rs907094 in the DARPP-32 gene (dopamine and adenosine 3′, 5′-monophosphate-regulated phosphoprotein 32 kilodations; also known as PPP1R1B) on behavior and cortical evoked potentials. A frequent DARPP-32 haplotype that includes the A allele of this SNP is associated with higher mRNA expression of DARPP-32 protein isoforms, striatal dopamine receptor function, and frontal–striatal connectivity. As we hypothesized, behaviorally the A homozygotes were more flexible in selectively attending to auditory inputs than any G carriers. Moreover, this genotype also affected auditory evoked cortical potentials that reflect early sensory and late attentional processes. Specifically, analyses of event-related potentials (ERPs) revealed that amplitudes of an early component of sensory selection (N1) and a late component (N450) reflecting attentional deployment for conflict resolution were larger in A homozygotes than in any G carriers. Taken together, our data lend support for dopamine's role in modulating auditory attention both during the early sensory selection and late conflict resolution stages.

Introduction Research on neuromodulation of cortical functions indicates that dopaminergic systems are critically involved in working memory and attentional control (for reviews, see Arnsten and Pilszka, 2011 and Seamans and Yang, 2004). Most studies on dopamine modulation of working memory maintenance have focused on processes related to prefrontal D1 and D2 receptors (Durstewitz et al., 2000, Phillips et al., 2004, Williams and Goldman-Rakic, 1998 and Vijyayraghavan et al., 2007). Given that multiple circuits connect striatal regions with regions in the frontal cortex (Alexander et al., 1986 and Pennartz et al., 2009), recent human research has begun to investigate the role of striatal dopamine in working memory and attention (e.g., Cools et al., 2004, Frank et al., 2001, Landau et al., 2005, Lewis et al., 2003 and McNab and Klingberg, 2008). 1.1. Dopamine and attention: evidence from animal and human studies Lesion studies in rats have shown that unilateral striatal dopamine depletion increases reaction times of responses contralateral to the lesion side in tasks that require visual attentional orienting (Brown and Robbins, 1989, Carli et al., 1985 and Ward and Brown, 1996). Deficits in selective attention processes (i.e., the inability to ignore irrelevant stimuli in blocking paradigms) have also been observed in rats with pharmacologically induced hyperdopaminergic activity (Crider, Blockel, & Solomon, 1986). A more recent study by Brown et al. (2010) investigating neurofibromatosis-1 mutant mice with reduced striatal dopamine function found impairments in non-selective and selective attention mechanisms as assessed by a variety of locomotor activities. Moreover, the mutants' attention dysfunctions could be reversed by treatment with methylphenidate, a dopamine agonist commonly used for treating attentional-deficit hyperactivity disorder (ADHD). Of particularly interest for the present study, Bao, Chan, and Merzenich (2001) found that pairing a tone with a transient dopamine signal through stimulation of the ventral tegmental area (VTA) increases the corresponding representation area in the auditory cortex, the selectivity of neural responses, and firing synchrony in response to the specific tone. In human research, a recent receptor imaging studies used 6-[18F]fluoro-L-DOPA (FDOPA) as a radioligand for assessing dopamine synthesis in the striatum. Vernaleken et al. (2007) found that changes in prefrontal blood-oxygen-level-dependent (BOLD) signal during attentional control were positively correlated with dopamine synthesis capacity in the ventral and dorsal striatum. Similarly, it has been observed that changes in BOLD signal in the anterior cingulate cortex and the dorsal lateral prefrontal cortex while processing affective stimuli correlate positively with striatal dopamine synthesis in the caudate and putamen, which indicates that striatal dopamine contributes to attentional processing of affective stimuli (Siessmeier et al., 2006). Furthermore, striatal dopamine synthesis capacity is also related to working memory performance, with dopamine synthesis capacity being higher in individuals with better working memory performance (Cools, Gibbs, Miyakawa, Jagust, & D'Esposito, 2008). More specifically, as regarding dopamine's effect on mechanisms of selective attention, an early positron emission tomography (PET) study, which used 11C-labeled raclopride as the radioligand, found evidence for transient striatal dopamine release while young adults played a video game that required sustained and selective visual attention (Koepp et al., 1998). Also of relevance to the current study, earlier pharmacological studies that used target detection dichotic listening paradigms found that catecholamine antagonists (e.g., haloperidol or droperidol) attenuated the processing negativity, which reflected selective attention, only in later time windows, i.e. at least 200 ms after stimulus onset (Kähkönen et al., 2001 and Shelley et al., 1997). On the other side of the coin, a recent study showed that dopamine agonist (rotigotine) improved hemispatial neglect of patients' performance in visual search tasks that required selective attention (Gorgoraptis et al., 2012). 1.2. Dopamine and attention: clinical and molecular genetic evidence Evidence from clinical research also converges on the view that dysfunctional dopaminergic signaling in the cortical–striatal–thalamic–cortical pathways is one of the causes underlying symptoms of ADHD, such as impaired attentional regulation and poor impulse control (see Arnsten and Pilszka, 2011 and Swanson et al., 2007 for reviews). Abnormality of dopamine signaling in the prefrontal cortex contributes to hypoactivation of the ventral prefrontal and inferior parietal regions (see Casey & Durston, 2006). Furthermore, in ADHD patients alterations in striatal dopamine transporter (DAT) density (see Fusar-Poli, Rubia, Rossi, Sartori, & Balottin, 2012 for a meta-analysis of nine receptor imaging studies) as well as reduced volumes of striatal regions, such as the caudate nucleus and the globus pallidus that are rich in dopamine, were observed (Castellanos et al., 2002). Depending on the history of psychostimulant exposures, relative to healthy controls drug naïve ADHD patients tend to show lower DAT density in the striatum (e.g., Hesse et al., 2009 and Volkow et al., 2007), whereas patients with prior medication treatments tend to show higher DAT density (Fusar-Poli et al., 2012). Altered dopamine transporter density in ADHD patients could change mechanisms of recycling dopamine back into the presynaptic terminal, and consequently would result in suboptimal extracellular dopamine levels (Jones et al., 1998 and Shumay et al., 2010). Recent molecular genetic studies also showed that the dopamine transporter gene (DAT1) 10R/10R genotype, associated with lower levels of striatal synaptic dopamine and smaller caudate volume, is a risk factor for ADHD (Durston et al., 2005). Investigations of the effects of DAT1 gene genotype on spatial attention in healthy children and adolescents showed that DAT 10R homozygotes tend to perform below the levels of DAT 9R carriers (Bellgrove et al., 2007). Relatedly, a recent study of attentional regulation in healthy younger adults reported that DAT 9R carriers showed a larger effect of inhibition of return, likely reflecting greater attentional flexibility (Colzato, Pratt, & Hommel, 2010). Furthermore, another genotype also relevant for striatal dopamine function (i.e., the D2 receptor gene, DRD2 C957T) has been found to be associated with individual differences in attentional blink, in line with PET imaging studies suggesting a role for striatal dopamine in the regulation of attentional resources (Colzato, Slagter, de Rover, & Hommel, 2011). 1.3. DARPP-32 gene, dopamine modulation, and cognition Another well-studied molecular candidate for striatal dopamine signaling is the DARPP-32 protein (now also known as PPP1R1B, protein phosphatase 1, regulatory inhibitor subunit 1B), which is richly expressed in the striatum. The DARPP-32 protein is phosphorylated by dopamine D1 receptor stimulation, and dephosphorylated by D2 receptor stimulation (Nishi, Snyder, & Greengard, 1997). The protein modulates striatal dopamine cellular excitability and synaptic plasticity related to the dopamine receptors (Calabresi et al., 2000, Fienberg et al., 1998 and Gould and Manji, 2005). It should be noted, however, given that the striatum integrates excitatory glutamatergic inputs, and there are other neuromodulators, such as adenosine and nitric oxide, which also regulate striatal phosphorylation, it is likely that DARPP-32 also interacts with other neurotransmitters besides dopamine (Svenningsson et al., 2004). Although as reviewed above the effects of a few other dopamine genes (e.g., the DRD2 or the DAT genotypes) on attention or working memory functions have been studied, much less is known about the potential contributions of the DARPP-32 gene on attentional mechanisms. Extant findings, however, suggest that DARPP-32 may also regulate executive control and attention functions in the frontal cortex via the frontal–hippocampal–striatal pathway. For instance, other than expressions in the striatum, the DARPP-32 protein is also expressed in other regions innervated by dopaminergic projections, such as in the anterior cingulate cortex (Narita et al., 2010) and other regions of the prefrontal cortex (Albert et al., 2002 and Kunii et al., 2011). Moreover, the DARPP-32 protein has been shown to modulate the functional interaction between the striatum and the prefrontal cortex (Meyer-Lindenberg et al., 2007 and Frank and Fossella, 2011) that is critically involved in attention-demanding tasks (e.g., Casey, 2005, Cools et al., 2004 and Nagano-Saito et al., 2008). There is also evidence indicating that variations in the DARPP-32 gene affect the functional connectivity between the inferior frontal gygus and the parahippocampus during an associative emotional memory task (Curcic-Blake et al., 2012). Thus, individual differences in mRNA expression of the DARPP-32 protein may also account for individual differences in attentional control of auditory processing. Of particular interest in this context is the allelic variation of a SNP (rs907094) in the DARPP-32 (PPP1R1B) gene. While the precise functional genetic changes invoked by this SNP still need to be elucidated, it is noteworthy that it is located near the splice donor site of the intron between exons 5 and 6 (+31 bp, using transcript ENST00000254079 as a reference). As such it could affect mRNA processing, e.g. splicing and/or expression. There is already some molecular evidence for the latter, in terms of higher mRNA expression and better striatal receptor function (Calabresi et al., 2000, Fienberg et al., 1998 and Meyer-Lindenberg et al., 2007). In human, a haplotype of the DARPP-32 gene that includes the A allele of SNP rs907094 was found to be associated with higher mRNA expressions of the DARPP-32 protein isoforms (Meyer-Lindenberg et al., 2007). Furthermore, variations in this polymorphism have been found to be associated with fMRI BOLD responses or ERPs in frontal brain networks implicating attention. Individuals carrying a haplotype of the DARPP-32 gene including the A allele of the rs907094 SNP showed greater changes in BOLD response in the striatum as well as greater frontal–striatal connectivity during cognitive performance, among others during attention (Meyer-Lindenberg et al., 2007). In an emotional associative memory task that implicates the frontal–hippocampal network, variations in SNPs of the DARPP-32 gene, including rs907094, were also found to be associated with higher functional connectivity between the inferior frontal gyrus and the parahippocampal gyrus (Curcic-Blake et al., 2012). In the context of reinforcement learning, the A homozygotes of the rs907094 SNP showed a greater advantage than G carriers in learning from positive than from negative outcomes (e.g., Doll et al., 2011 and Frank et al., 2009; for review, see Frank & Fossella, 2011). Recently, in a larger sample covering a wider age range from childhood to old age, our own results also showed that feedback-related ERPs assessed at frontal electrodes were larger in A homozygotes of this SNP than any G carriers, particularly in children and older adults (Hämmerer et al., 2013). 1.4. Aim of study and hypotheses The goal of this study was to investigate the effects of dopamine signaling on auditory attention in humans by studying the effects of the DARPP-32 gene on attentional control of auditory perception. To this end, we assessed behavioral performance and ERPs in younger adults while performing a specific variant of the dichotic listening task that was particularly amenable for investigating the effects of conflicts between attentional focus and the relative perceptual saliency of competing auditory inputs (cf. Passow et al., 2012, Passow et al., in press and Westerhausen et al., 2010). Early or late auditory evoked potentials have been shown to reflect neural correlates of sensory-driven or conflict-related processes, respectively. We tested whether the two genotype groups differ in attention modulation of sensory processing as reflected in amplitude differences in P1, N1 or P2 component (e.g., Clark and Hillyard, 1996, Hillyard et al., 1973, Lange et al., 2003 and Sanders and Astheimer, 2008). As for conflict-related processing, we focused on a late negativity occurring approximately in the time window of 450–550 ms after stimulus onset. This late negativity has been shown in previous studies to be sensitive to sensory conflicts between auditory inputs during dichotic listening in younger adults (e.g., Bayazit, Öniz, Hahn, Güntürkün, & Özgören, 2009). Furthermore, in other cognitive paradigms (e.g., the Stroop interference task) a modulation effect of the ERP in a similar time window was previously shown to reflect the demands of attentional control in conflict processing (e.g., Frühholz et al., 2009, Larson et al., 2009, Liotti et al., 2000 and West and Alain, 1999) as well as attention orienting (Kanske, Plitschka, & Kotz, 2011). Given their higher dopamine function, we expected that A homozygotes of the DARPP-32 gene would show more flexible attentional control of auditory perception than any G carriers, especially under conditions in which attentional focus conflicts with the perceptual saliency of sensory inputs. Accordingly, we also expected that the amplitude of the late negativity (hereafter referred as the N450) would be more strongly modulated by the extent of attentional–perceptual conflict in A homozygotes than in any G carriers.

Results 4.1. Behavioral performance A three-way repeated-measures ANOVA with the LIs derived from the behavioral session as dependent variable revealed significant main effects of attentional focus, F(1.19,26.12)=40.84, p<0.05, η2=.25, and perceptual saliency, F(2.15,47.26)=120.36, p<0.05, η2=0.54. The two-way attentional focus×perceptual saliency interaction, F(5.08,111.82)=3.62, p<0.05, η2=0.01, was also significant. Of particular interest are the interactions involving DARPP-32 genotype. Both the two-way attentional focus×DARPP-32 genotype interaction, F(1.19,26.12)=12.16, p<0.05, η2=0.07, and the three-way attentional focus×perceptual saliency×DARPP-32 genotype interaction, F(5.08,111.82)=2.93, p<0.05, η2=0.01, were significant. The two-way perceptual saliency×DARPP-32 genotype interaction was not significant (p>0.05). To follow up the significant three-way interaction, separate analyses for each genotype group showed a larger effect of attentional focus in DARPP-32 A homozygotes, F(1.05,11.60)=32.33, p<0.05, η2=0.55, and a much weaker effect in any G carriers, F(1.43,15.75)=9.38, p<0.05, η2=0.06. The main effect of perceptual saliency was also significant in both groups; however, in contrast to the effect of attentional focus, the effect was weaker in DARPP-32 A homozygotes, F(1.70,18.67)=40.71, p<0.05, η2=0.37, than in any G carriers, F(2.68,29.45)=85.61, p<0.05, η2=0.76. Together these patterns of results indicate that DARPP-32 genotype affects interactions between attentional focus and perceptual saliency. In A homozygotes, the LIs clearly varied as a function of instructed task goals (i.e., attending to the right or left ear, or attending to both), whereas in any G carriers, auditory perception was mainly driven by the perceptual saliency of the stimulus inputs, regardless of attentional focus (see Fig. 1A). Mean Laterality Indices across all inter-aural intensity difference conditions ... Fig. 1. Mean Laterality Indices across all inter-aural intensity difference conditions and for each attentional focus condition for (A) the extended behavioral version (with nine perceptual saliency conditions) of the dichotic listening task and (B) for the EEG version (with three perceptual saliency conditions) of the dichotic listening task for DARPP-32 A homozygotes (left panels) and DARPP-32 any G carriers (right panels). Error bars indicate 1 SE of the mean. Figure options The three-way repeated-measures ANOVA analyzing the LIs assessed in the EEG version of the task with three levels of perceptual saliency also revealed a similar pattern of results (compare Fig. 1B with 1A). The analyses revealed significant main effects of attentional focus, F(1.23,27.09)=30.99, p<0.05, η2=0.39, and perceptual saliency, F(1.32,29.03)=120.36, p<0.05, η2=0.32. The two-way attentional focus×perceptual saliency interaction, F(1.73,30.05)=4.79, p<0.05, η2=0.01, and of particular interest, the two-way attentional focus×DARPP-32 genotype interaction, F(1.23,27.09)=4.05, p<0.05, η2=0.05, were also significant. The three-way attentional focus×perceptual saliency×genotype interaction was only marginal in this case (p=0.16), presumably reflecting the more restricted range of perceptual saliency in the EEG version of the task. Follow-up analyses of the significant attentional focus×DARPP-32 genotype interaction for each genotype group separately revealed a larger effect of attentional focus in DARPP-32 A homozygotes, F(1.14,12.51)=28.45, p<0.05, η2=0.55, and a weaker effect in any G carriers, F(1.24,13.62)=6.38, p<0.05, η2=0.23. The main effect of perceptual saliency was significant in DARPP-32 A homozygotes, F(1.26,13.80)=65.22, p<0.05, η2=0.32 as well as in any G carriers, F(1.30,14.28)=39.96, p<0.05, η2=0.34. In line with the results reported above, independent t-tests also revealed significantly higher mean ATTIndices in DARPP-32 A homozygotes compared to any G carriers in the behavioral session, t(16.76)=3.55, p<0.05, d=1.45, and in the EEG session, t(22)=2.11, p<0.05, d=0.89 (see Fig. 2A). Full-size image (37 K) Fig. 2. (A) Mean Selective Attention Indices for DARPP-32 AA (black bars) and any G carriers (patterned bars) in the behavioral (left) and EEG (right) session. Error bars indicate 1 SE of the mean. Scatterplots showing the relations between the Selective Attention Index (ATTIndex) derived from the extended behavioral version of the task (black circles, solid line) as well as the EEG version of the task (open rhombs, dashed line) and the N1 amplitude (B) and the N450 modulation effect (C) (⁎p<0.05). Figure options Other than the above findings specific to the aims of our study, we also found the commonly observed right-ear advantage (REA) effect of auditory verbal processing (i.e., more report of verbal stimuli presented to the right relative to the left ear; see Hugdahl et al., 2003 for reviews) in the neutral focus condition of our experiment when both ears were presented with the same input intensity. Specifically, we observed an effect of REA as reflected in the main effect of ear in the behavioral session, F(1,22)=12.90, p≤0.05, as well as in the EEG session, F(1,22)=15.12, p≤0.05). The REA effect, however, was not related to attentional flexibility as reflected in the selective attention indices (ps>0.05) nor to genotype (ps>0.05 for ear×genotype interaction). 4.2. Relations between cortical evoked potentials and task performance At the sample level without subdividing the two genotype groups, the mean averaged amplitude of the N1 component across all conditions correlated positively with the ATTIndex of the behavioral, r=0.49, p<0.05, and EEG session, r=0.48, p<.05 (see Fig. 2B). Furthermore, we were interested in the late evoked potential (N450), which reflects attentional regulation of conflict resolution when attentional focus conflicts with perceptual saliency (cf. Passow et al., in press). At the sample level, the magnitude of N450 modulation effect (i.e., the enhancement of N450 magnitude in the conflict relative to the no-conflict condition) correlated positively with the ATTIndex derived from the behavioral session, r=0.54, p<0.05, and the EEG session, r=0.61, p<0.05 (see Fig. 2C). Individuals who showed a stronger N450 modulation in response to conflicts between attentional focus and perceptual saliency also yielded higher ATTIndices. Thus, N450 modulation reflected individual differences in the flexible allocation of attentional control. 4.3. Genotype Effect on N1 at frontal–central electrodes In light of results from earlier studies (Ceponiene et al., 2008) and the scalp topography of our data (see Fig. 3B), we focused on N1 component derived from the frontal–central electrodes. The repeated-measure ANOVA for the entire sample revealed a significant DARPP-32 genotype effect on the N1 component, F(1,22)=10.41, p<0.05, η2=0.32. This effect, however, did not interact with conflict (p>0.05), reflecting that the N1 amplitude was larger in DARPP-32 A homozygotes than in any G carriers in all conditions independent of whether attentional–perceptual conflict was involved (see Fig. 5). As comparisons, we also analyzed two other early auditory evoked potentials: In contrast to N1, no genotype effects were found for the P1 and P2 components, ps>0.05 (see Fig. 3A and C). Topographical voltage maps of the grand average ERP waveforms averaged across ... Fig. 3. Topographical voltage maps of the grand average ERP waveforms averaged across all conditions in the time window of (A) the P1, i.e., 80–120 ms, (B) the N1, i.e., 120–200 ms, and (C) P2, i.e., 200–300 ms. Maps display top views of the scalp distribution for A homozygotes (left) and any G carriers (right). Based on the observed topographical map, the fronto-central (FCz, C1, C2) region of interest was used to define the three early EPR components (see main text for details). Figure options 4.4. Genotype Effect on N450 at parietal electrodes Guided by previous studies showing a more posterior distribution of the N450 when not using the classical Stroop but other conflict paradigms (e.g., Frühholz et al., 2009 and Schirmer and Kotz, 2003) and the scalp topography from our data (see Fig. 4), we focused on N450 component derived from parietal electrodes. Results of the repeated measures ANOVA revealed a main effect of conflict, F(1,22)=13.66, p<0.05, η2=0.03, and a trend for a conflict×DARPP-32 genotype interaction, F(1,22)=2.39, p=0.09, η2=0.01. As most methods of multiple comparisons, including Tukey's test, can be applied regardless of whether the F test is significant ( Ryan, 1959a, Ryan, 1959b and Wilcox, 1987), we followed up the marginally significant interaction separately for the two genotype groups. The results indicated a main effect of conflict in DARPP-32 A homozygotes, F(1,11)=29.71, p<0.05, η2=0.11, but not in any G carriers, p>0.05. Follow-up t-test in A homozygotes revealed a significantly larger N450 in conditions involving conflict between attentional focus and perceptual saliency than in conditions without such conflicts, t(11)=−2.37, p<0.05, dz=0.68 for the attending right condition and t(11)=−3.36, p<0.05, dz=0.97 for the attending left condition (see Fig. 6). Topographical voltage maps of the difference waveforms between high minus low ... Fig. 4. Topographical voltage maps of the difference waveforms between high minus low attentional control demand conditions in the time window of the N450 modulation effect, i.e., 450–550 ms. Maps display back views of the scalp distribution for A homozygotes (left) and any G carriers (right). Based on the topographical map, the parietal (Pz, P3, P4) region of interest was used to derive the N450 component (see main text for details). Figure options Grand averages of the stimulus-locked event-related potential (ERP) waveforms at ... Fig. 5. Grand averages of the stimulus-locked event-related potential (ERP) waveforms at frontal–central electrodes highlighting the N1 component for DARPP-32 A homozygotes (left) and DARPP-32 any G carriers (right) separately for focused-right (FR, upper panel) and focused-left (FL, lower panel) conditions. ERPs are shown as a function of inter-aural intensity difference: right ear>left ear and left ear>right ear. Figure options Grand averages of the stimulus-locked event-related potential (ERP) waveforms at ... Fig. 6. Grand averages of the stimulus-locked event-related potential (ERP) waveforms at parietal electrodes highlighting the N450 modulation effect for DARPP-32 A homozygotes (left) and DARPP-32 any G carriers (right) separately for focused-right (FR, upper panel) and focused-left (FL, lower panel) conditions. ERPs are shown as a function of inter-aural intensity difference: right ear>left ear and left ear>right ear. Insets indicate mean ERP amplitude separately for each genotype group across conflict and no conflict conditions in FR and FL conditions. Error bars indicate 1 SE of the mean.