Inhibition of unwanted response is an important function of the executive system. Since the inhibitory system is impaired in patients with dysregulated dopamine system, we examined dopamine neurotransmission in the human brain during processing of a task of executive inhibition. The experiment used a recently developed dynamic molecular imaging technique to detect and map dopamine released during performance of a modified Eriksen's flanker task. In this study, young healthy volunteers received an intravenous injection of a dopamine receptor ligand ( 11 C-raclopride) after they were positioned in the PET camera. After the injection, volunteers performed the flanker task under Congruent and Incongruent conditions in a single scan session. They were required to inhibit competing options to select an appropriate response in the Incongruent but not in the Congruent condition. The PET data were dynamically acquired during the experiment and analyzed using two variants of the simplified reference region model. The analysis included estimation of a number of receptor kinetic parameters before and after initiation of the Incongruent condition. We found increase in the rate of ligand displacement (from receptor sites) and decrease in the ligand binding potential in the Incongruent condition, suggesting dopamine release during task performance. These changes were observed in small areas of the putamen and caudate bilaterally but were most significant on the dorsal aspect of the body of left caudate. The results provide evidence of dopaminergic processing of executive inhibition and demonstrate that neurochemical changes associated with cognitive processing can be detected and mapped in a single scan session using dynamic molecular imaging.

In this experiment we used a newly developed dynamic molecular imaging technique [6] , [7] to detect and map dopamine released during performance of a task of executive inhibition. The technique exploits the competition between dopamine and its ligand for receptor occupancy and detects dopamine released during task performance in a single scan session. We used this technique previously to study dopamine released during performance of a number of cognitive, emotional and behavioral tasks [6] , [7] , [8] , [9] , [10] , [11] , [12] . In the present experiment we detected and mapped dopamine released in the Congruent and Incongruent conditions of a modified Eriksen's flanker task [2] . The task elicited executive inhibition.

Additionally, neuroimaging experiments have consistently reported increased activation in the brain areas that are innervated by dopaminergic neurons. In an fMRI experiment [4] we observed increased BOLD activation in the caudate, anterior cingulate cortex (ACC), and superior and middle frontal gyri during performance of the flanker task. Since these structures are innervated by dopamine, the experiment provides indirect evidence of dopaminergic processing of the inhibition. A number of neurocognitive models of learning (based primarily on the data acquired in laboratory animals) also assume involvement of dopamine in the processing. For example, the actor-critic model of reinforcement learning [5] assumes that dopamine-mediated processes help animals learn the most rewarding action by inhibiting competing options.

Neurochemical control of executive inhibition remains uninvestigated because of the lack of a reliable technique to detect task-induced changes in the brain chemistry. Indirect evidence acquired in cognitive studies suggests that dopamine may be involved in the processing. These studies have found that the patients with dysregulated dopamine neurotransmission show impaired performance in executive inhibition tasks. Thus, poor performance is reported in patients with attention deficit hyperactivity disorder (ADHD), Tourette's syndrome (TS), Parkinson's disease (PD), and schizophrenia [1] . In most of these studies modified Eriksen's flanker task [2] was used to elicit executive inhibition. Involvement of dopamine in the processing is suggested also by the data obtained in laboratory animals. For example, it was shown in monkeys that the number of inhibited neurons reduces significantly after depletion of dopamine [3] . The depletion therefore increases the number of nonspecifically activated neurons and reduces signal to noise ratio. As a result, the depleted monkeys find it extremely difficult to inhibit competing options and select an appropriate response.

Thus, both models found most significant change in the left dorsal caudate. Additionally, both models suggested significant changes in the left and right putamen also. Interestingly, the voxels where maximum change in the rate of ligand displacement (γ) and maximum reduction in the ligand BP occurred, were located within 6 mm of each other in the left caudate and putamen even though these measurements were made using two different receptor kinetic models. In the right putamen these locations were >10 mm apart. It appears that the two models picked up activations from the same neuronal clusters of the left caudate (−10,14,8 and −12,8,10) and left putamen (−22,4,−6 and −26,4,−6). In the right putamen (24,4,2 and 24,8,−8) activations identified by the two models probably came from different clusters. Changes in the left caudate were therefore most significant, consistent and reliable. All parameter values measured in this area were consistent with increased dopamine release in the Incongruent condition in comparison with the Congruent condition.

The t-maps generated using extended reference region tissue model (E-SRTM) show striatal areas where the ligand binding potential decreased significantly in the Incongruent condition in comparison with the Congruent condition. It was most significant in the left caudate and putamen. These areas are located in close proximity to the locations where increased rate of ligand displacement was observed ( Figure 1 ). An agreement in the data computed using two different receptor kinetic models significantly enhances the reliability of results.

To ensure validity of this finding we estimated the ligand BP and k 2a ( Table 2 ) in the Congruent and Incongruent conditions using E-SRTM [14] . As compared to the control (Congruent condition), the BP decreased in 3 striatal areas ( Figure 2 ) during task performance (Incongruent condition). It was most significant (t>2.5) in the left caudate (28%) and left putamen (26%). Additionally, relatively small (23%) but significant (t = 2.21) decrease was observed in the right putamen. There was no significant change in any other area. The ligand dissociation coefficient (k 2a ) also increased in all of these 3 areas but it was statistically significant only in the left caudate (t = 2.07).

The striatal areas where rate of ligand displacement increased significantly in the Incongruent condition of the flanker task are shown on the t-maps. The most significant increase was observed on the dorsal aspect of the body of left caudate. The time-activity curves show the ligand concentration (open circles) and least square fit (solid lines) in a striatal area (upper curve) and in the reference region (lower curves). The data on the left of the vertical lines were acquired in the Congruent condition and those on the right were obtained in the Incongruent condition. Significant reduction in the ligand concentration in the Incongruent condition suggests that the rate of ligand displacement increased during task performance. The increase was due to competitive displacement induced by endogenously released dopamine. There was no significant change in the rate of ligand displacement in the reference region (cerebellum). The time-activity curves were drawn using the mean data acquired from the voxels where maximum changes were observed in each area. This analysis used the linear extension of reference region tissue model (LE-SRRM).

The LE-SRRM analysis revealed that the values of γ changed significantly after task initiation in 4 striatal areas located one each in the caudate and putamen of the two hemispheres ( Figure 1 ). It was most significant ( Table 1 ) on the dorsal aspect of anterior part of the body of left caudate (t = 2.56). Stereotactic (MNI) coordinates (x,y,z) of this location were −10,14, and 8 mm. In this area we observed maximum change in the rate of ligand displacement (γ = 0.1). It was 384% higher than the mean striatal value (0.026). The changes were less significant (t<2.1) in the other three striatal areas: the left dorsal putamen (−22,4,−6); right dorsal body of the caudate (16,16,14); and right dorsal putamen (24,4,2). In these areas values of γ were relatively low (<0.08) but significantly higher than the mean striatal value ( Table 1 ).

As described in Materials and Methods section, analysis of the PET data involved measurement of a number of receptor kinetic parameters using two models: linear extension of simplified reference region model or LE-SRRM [12] , [13] ; and extended simplified reference tissue model or E-SRTM [14] . Using the LE-SRRM we dynamically measured changes in the rate of ligand displacement (γ) in the Incongruent condition. This measurement allowed detection and mapping of dopamine released in each voxel at each time point. By comparing the rate of change measured in the Congruent and Incongruent conditions, we located voxels where it increased significantly during task performance (Incongruent condition). To ensure that this measurement reflected endogenously released dopamine and it was not a chance finding, we measured additional receptor kinetic parameters using the E-SRTM [14] . These parameters included the binding potentials (BPs) and dissociation coefficients (k 2a ) of the ligand in the Congruent and Incongruent conditions. Voxel-wise comparison of the parameter values allowed us to locate voxels where the values (ΔBP and Δk 2a ) changed significantly after task initiation (Incongruent condition).

In the modified Eriksen's flanker task performed in the PET camera, volunteers made accurate responses in most trials. In the Congruent condition they made 97.1±2.0% correct response while in the Incongruent condition 91.0±10.9% of responses were accurate. Even though responses were less accurate in the Incongruent condition, the accuracy was not significantly different from that in the Congruent. Similarly, response time was longer in the Incongruent (695±183 msec) as compared to the Congruent (602±151 msec) condition but the difference was not significant statistically. The trend of lower accuracy and greater response time in the Incongruent condition indicated cognitive cost of processing the inhibition. For making a response in this condition volunteers had to inhibit prepotent responses indicated by the direction of flanker arrowheads. This inhibition was not needed in the Congruent condition in which target and flanker arrowheads pointed to the same direction.

Discussion

The results demonstrate increased dopamine release in a number of striatal areas during performance of a flanker task. The increase was most significant on the dorsal aspect of the body of left caudate. All receptor kinetic parameters measured in this area (using two different receptor kinetic models: LE-SRRM and E-SRTM) suggested significant release of endogenous dopamine. In addition, most parameter values suggest dopamine release in three additional striatal locations: dorsal part of the left and right putamen and body of the right caudate (Figure 1).

These findings are interesting because in an earlier fMRI experiment [4] we found increased activation in the same region of the left caudate. Further, the maxima of BOLD response observed in the fMRI experiment and change in the rate of ligand displacement observed in the present experiment were located only 4 mm apart (MNI coordinates: −14,16,10 and −10,14,8). Additionally, maxima of the fMRI activation were located within 8 mm of the location where maximum decrease in the ligand BP (−12,8,10) was observed during task performance. Finding of activation in the same location in experiments that used different techniques validates the observation and underscores significance of the left caudate in processing of executive inhibition.

This observation of dopamine release in the left caudate is consistent with the observation of a number of fMRI studies [15], [16], [17], [18]. These studies however, have implicated other striatal areas also in the processing of executive inhibition tasks, and it was suggested that different striatal structures process different aspects of the task. Thus, caudate and putamen of the right hemisphere are associated with the preparatory phase of response execution [19], [20] and those of the left side with inhibition and interference resolution [18]. In a recent study [18] the caudate and putamen of both hemispheres were activated in a flanker task that involved response selection and interference suppression. When the task was modified to require only response selection without interference (in a stimulus-response compatibility task) only the caudate was activated. Further, requirement of inhibition without selection (in a go-no-go task) activated the right putamen. This finding is supported by another recent experiment in which a negative correlation was observed between the volume of left putamen and the degree of interference. This study also found a positive correlation between the right putamen volume and the accuracy of response [21].

Thus, it appears that different striatal areas process different aspects of the task. The location of striatal activity in an experiment therefore depends on the degree to which these aspects/components are expressed. Thus, in the present experiment interference suppression was the most prominent component and the most significant activation was observed in the left caudate. It therefore suggests that the dopamine system of left caudate is involved in the processing associated with the inhibition of unwanted response.

This suggestion is consistent with the observation of hyperactivity, agitation and inattention (due to loss of inhibitory control) following lesion, destruction or shrinkage of the caudate head [22]. In a recent study impaired executive function in patients with temporal lobe epilepsy has been attributed to the atrophy of left caudate in vicinity of the area where we found dopamine release [23]. It appears that the caudate is able to exert inhibitory control due to its functional connection with the dorsolateral prefrontal cortex (DLPFC) and ACC [24]. In animals dorsal caudate receives cortico-caudate projections from the dorsolateral frontal area and the cingulate [25]. Functional connection between these areas in the human brain has been recently demonstrated in an fMRI experiment [26]. In this experiment simultaneous activation of these areas (the left dorsal caudate, DLPFC and ACC) was observed when attention was focused on a target. Since focused attention is required to resolve interference, the DLPFC and ACC are most consistently activated during resolution of interference caused by multiple response options [27]. In addition to interference suppression, the caudate and its functional connection to the DLPFC are needed to inhibit irrelevant options. It appears that the same frontal areas, located in different hemispheres are activated when the emphasis of task is shifted from response selection to inhibition. These activations are lateralized on the left hemisphere when a task requires response selection and on the right, when the emphasis shifts to inhibition [28]. Further, clinical evidence suggests that the activations associated with inhibition are dependent on dopamine neurotransmission. That is why unmedicated PD patients have difficulty ignoring non-essential stimuli [29], [30], [31].

The neural mechanism that allows dopamine to control inhibition in the human brain is not known but animal studies suggest a possible cellular mechanism. For example, after dopamine neurons are depleted in monkeys, the number of inhibited neurons reduces and the number of nonspecifically activated cells increases significantly [3]. As a result these monkeys find it difficult to select an appropriate response and focus attention on a stimulus. This dopaminergic effect on focused attention is validated in hyper-dopaminergic psychiatric conditions like schizophrenia. These patients are generally hyper-attentive [32], [33] and have difficulty in shifting attention away from irrelevant stimuli. Thus, it appears the inhibitory system works most efficiently when dopaminergic activity is optimal. Both high and low levels disrupt inhibition. This effect is similar to the dopaminergic effect on cognitive functions, which are impaired at both high and low levels of dopaminergic activity [34].

The other striatal areas where changes in dopamine release were relatively small, process aspects of the flanker task that were inadequately expressed in the current experiment. These aspects include selection of an appropriate response. The response selection is an important aspect of not only flanker task but also those of learning and reward systems. The dopamine system is believed to facilitate learning of the outcome of a response and therefore, help us select the most rewarding response [35]. Therefore, dopaminergic agents alter outcome-based selection in PD patients and change their bias for learning from negative outcomes in favor of positive outcome [36]. This observation is consistent with the actor-critic model of reward and reinforcement. The model assumes that the dopamine system learns to select the action that is most rewarding [5].