A reference tissue forward model for improved PET accuracy using within-scan displacement studies.

We report a novel forward-model implementation of the full reference tissue model (fFTRM) that addresses the fast-exchange approximation employed by the simplified reference tissue model (SRTM) by incorporating a non-zero dissociation time constant from the specifically bound compartment. The forward computational approach avoided errors associated with noisy and nonorthogonal basis functions using an inverse linear model. Compared to analysis by a multilinear single-compartment reference tissue model (MRTM), fFTRM provided improved accuracy for estimation of binding potentials at early times in the scan, with no worse reproducibility across sessions. To test the model's ability to identify small focal changes in binding potential using a within-scan challenge, we employed a nonhuman primate model of focal dopamine release elicited by deep brain microstimulation remote to ventral striatum (VST) during imaging by simultaneous PET and fMRI. The new model reported an unambiguously lateralized response in VST consistent with fMRI, whereas the MRTM-derived response was not lateralized and was consistent with simulations of model bias. The proposed model enabled better accuracy in PET [11C]raclopride displacement studies and may also facilitate challenges sooner after injection, thereby recovering some sensitivity lost to radioactive decay of the PET tracer.

Stimulation of the ventral tegmental area induces visual cortical plasticity at the neuronal level.

fMRI studies have shown that pairing a task-irrelevant visual feature with electrical micro-stimulation of the ventral tegmental area (VTA-EM) is sufficient to increase the sensory cortical representation of the paired feature and to improve perceptual performance. However, since fMRI provides an indirect measure of neural activity, the neural response changes underlying the fMRI activations are unknown. Here, we pair a task-irrelevant grating orientation with VTA-EM while attention is directed to a difficult orthogonal task. We examine the changes in neural response properties in macaques by recording spiking activity in the posterior inferior temporal cortex, the locus of fMRI-defined plasticity in previous studies. We observe a relative increase in mean spike rate and preference for the VTA-EM paired orientation compared to an unpaired orientation, which is unrelated to attention. These results demonstrate that VTA-EM-stimulus pairing is sufficient to induce sensory cortical plasticity at the spiking level in nonhuman primates.

Electrical stimulation of the macaque ventral tegmental area drives category-selective learning without attention.

Perception improves by repeated practice with visual stimuli, a phenomenon known as visual perceptual learning (VPL). The interplay of attentional and neuromodulatory reward signals is hypothesized to cause these behavioral and associated neuronal changes, although VPL can occur without attention (i.e., task-irrelevant VPL). In addition, task-relevant VPL can be category-selective for simple attended oriented stimuli. Yet, it is unclear whether category-selective task-irrelevant VPL occurs and which brain centers mediate underlying forms of adult cortical plasticity. Here, we show that pairing subliminal complex visual stimuli (faces and bodies) with electrical microstimulation of the ventral tegmental area (VTA-EM) causes category-selective task-irrelevant VPL. These perceptual improvements are accompanied by fMRI signal changes in early and late visual and frontal areas, as well as the cerebellum, hippocampus, claustrum, and putamen. In conclusion, Pavlovian pairing of unattended complex stimuli with VTA-EM causes category-selective learning accompanied by changes of cortical and subcortical neural representations in macaques.

Selective Mesoaccumbal Pathway Inactivation Affects Motivation but Not Reinforcement-Based Learning in Macaques.

Rodent studies have demonstrated the role of the mesoaccumbal circuit in reinforcement-based learning. Importantly, however, while phasic activity of the ventral tegmental area (VTA) contributes to reinforcement learning, rodent evidence suggests that slow changes in tonic VTA activity and associated accumbal dopamine release help regulate motivational behavior. Nonetheless, the consequences of sustained blockage of the mesoaccumbal circuit for motivation and reinforcement learning have not yet been examined in primates. Using a double-infection viral vector technique, we demonstrate that selective, unidirectional, and reversible blockage of the primarily dopaminergic mesoaccumbal circuit in monkeys increased network-level functional connectivity, especially in fronto-temporal cortex. These global network changes were not associated with deficits in reinforcement learning during an object discrimination reversal task. In contrast, sustained mesoaccumbal inactivation greatly reduced motivation for performing a motivation-based decision-making task. Thus, the mesoaccumbal pathway in primates is critical for high-effort motivation but not for all forms of reinforcement-based learning.

Frequency- and State-Dependent Network Effects of Electrical Stimulation Targeting the Ventral Tegmental Area in Macaques.

The ventral tegmental area (VTA) is a midbrain structure at the heart of the dopaminergic system underlying adaptive behavior. Endogenous firing rates of dopamine cells in the VTA vary from fast phasic bursts to slow tonic activity. Artificial perturbations of the VTA, through electrical or optogenetic stimulation methods, generate different and sometimes even contrasting behavioral outcomes depending on stimulation parameters such as frequency, amplitude, and pulse width. Here, we investigate the global functional effects of electrical stimulation frequency (10, 20, 50, and 100 Hz) of the VTA in rhesus monkeys. We stimulated 2 animals with chronic electrodes, either awake or anesthetized, while concurrently acquiring whole-brain functional magnetic resonance imaging (fMRI) signals. In the awake state, activity as a function of stimulation frequency followed an inverted U-shape in many cortical and subcortical structures, with highest activity observed at 20 and 50 Hz and lower activity at 10 and 100 Hz. Under anesthesia, the hemodynamic responses in connected brain areas were slightly positive at 10 Hz stimulation, but decreased linearly as a function of higher stimulation frequencies. A speculative explanation for the remarkable frequency dependence of stimulation-induced fMRI activity is that the VTA makes use of different frequency channels to communicate with different postsynaptic sites.

Ventral midbrain stimulation induces perceptual learning and cortical plasticity in primates.

Practice improves perception and enhances neural representations of trained visual stimuli, a phenomenon known as visual perceptual learning (VPL). While attention to task-relevant stimuli plays an important role in such learning, Pavlovian stimulus-reinforcer associations are sufficient to drive VPL, even subconsciously. It has been proposed that reinforcement facilitates perceptual learning through the activation of neuromodulatory centers, but this has not been directly confirmed in primates. Here, we paired task-irrelevant visual stimuli with microstimulation of a dopaminergic center, the ventral tegmental area (VTA), in macaques. Pairing VTA microstimulation with a task-irrelevant visual stimulus increased fMRI activity and improved classification of fMRI activity patterns selectively for the microstimulation-paired stimulus. Moreover, pairing VTA microstimulation with a task-irrelevant visual stimulus improved the subject's capacity to discriminate that stimulus. This is the first causal demonstration of the role of neuromodulatory centers in VPL in primates.

In Vivo Identification of Thick, Thin, and Pale Stripes of Macaque Area V2 Using Submillimeter Resolution (f)MRI at 3 T.

Primate area V2 contains a repetitive pattern of thick, thin and pale cytochrome oxidase stripes that are characterized by largely discrete in- and output channels, as well as differences in function, and myelo- and cytoarchitecture. Stripes have been identified mainly using microscope-based imaging of tiny portions of superficially located V2, or by postmortem methods, hence, the quest for (quasi) noninvasive tools to study these mesoscale functional units. Only recently, stripe-like V2 patterns have been demonstrated in humans with high-field (functional) magnetic resonance imaging (f)MRI, but in both such studies only 2 stripe compartments could be identified. Although interstripe distances in monkeys are ~half of those in humans, we show that all 3 V2 stripe classes can be reliably separated using submillimeter (f)MRI (0.6 mm isotropic voxels) on regular 3 T scanners by combining contrast agents and implanted phased-array coils. Specifically, we show highly reproducible fMRI patterns, both within and across subjects, of color-selective thin and disparity-selective thick stripes. Furthermore, reliable MRI-based higher myelin-density was observed in pale stripes. Hence, this is the first study showing segregation of columns using (f)MRI-based methods in macaque cortex, which opens the possibility of studying these elementary building blocks of the visual system noninvasively on a large scale.

Attention Shifts Recruit the Monkey Default Mode Network.

A unifying function associated with the default mode network (DMN), which is more active during rest than under active task conditions, has been difficult to define. The DMN is activated during monitoring the external world for unexpected events, as a sentinel, and when humans are engaged in high-level internally focused tasks. The existence of DMN correlates in other species, such as mice, challenge the idea that internally focused, high-level cognitive operations, such as introspection, autobiographical memory retrieval, planning the future, and predicting someone else's thoughts, are evolutionarily preserved defining properties of the DMN. A recent human study demonstrated that demanding cognitive shifts could recruit the DMN, yet it is unknown whether this holds for nonhuman species. Therefore, we tested whether large changes in cognitive context would recruit DMN regions in female and male nonhuman primates. Such changes were measured as displacements of spatial attentional weights based on internal rules of relevance (spatial shifts) compared with maintaining attentional weights at the same location (stay events). Using fMRI in macaques, we detected that a cortical network, activated during shifts, largely overlapped with the DMN. Moreover, fMRI time courses sampled from independently defined DMN foci showed significant shift selectivity during the demanding attention task. Finally, functional clustering based on independent resting state data revealed that DMN and shift regions clustered conjointly, whereas regions activated during the stay events clustered apart. We therefore propose that cognitive shifting in primates generally recruits DMN regions. This might explain a breakdown of the DMN in many neurological diseases characterized by declined cognitive flexibility.SIGNIFICANCE STATEMENT Activation of the human default mode network (DMN) can be measured with fMRI when subjects shift thoughts between high-level internally directed cognitive states, when thinking about the self, the perspective of others, when imagining future and past events, and during mind wandering. Furthermore, the DMN is activated as a sentinel, monitoring the environment for unexpected events. Arguably, these cognitive processes have in common fast and substantial changes in cognitive context. As DMN activity has also been reported in nonhuman species, we tested whether shifts in spatial attention activated the monkey DMN. Core monkey DMN and shift-selective regions shared several functional properties, indicating that cognitive shifting, in general, might constitute one of the evolutionarily preserved functions of the DMN.

Functional Similarity of Medial Superior Parietal Areas for Shift-Selective Attention Signals in Humans and Monkeys.

We continually shift our attention between items in the visual environment. These attention shifts are usually based on task relevance (top-down) or the saliency of a sudden, unexpected stimulus (bottom-up), and are typically followed by goal-directed actions. It could be argued that any species that can covertly shift its focus of attention will rely on similar, evolutionarily conserved neural substrates for processing such shift-signals. To address this possibility, we performed comparative fMRI experiments in humans and monkeys, combining traditional, and novel, data-driven analytical approaches. Specifically, we examined correspondences between monkey and human brain areas activated during covert attention shifts. When "shift" events were compared with "stay" events, the medial (superior) parietal lobe (mSPL) and inferior parietal lobes showed similar shift sensitivities across species, whereas frontal activations were stronger in monkeys. To identify, in a data-driven manner, monkey regions that corresponded with human shift-selective SPL, we used a novel interspecies beta-correlation strategy whereby task-related beta-values were correlated across voxels or regions-of-interest in the 2 species. Monkey medial parietal areas V6/V6A most consistently correlated with shift-selective human mSPL. Our results indicate that both species recruit corresponding, evolutionarily conserved regions within the medial superior parietal lobe for shifting spatial attention.

Should I Stay or Should I Go?

In this issue of Neuron, Lee et al. (2016) assessed the brain-wide effects of stimulating the direct and indirect pathway by optogenetic activation of D1 and D2 striatal neurons. This work demonstrates the exquisite power of combining cell-type-specific perturbation methods with focal and whole-brain measurements of brain activity.

Noisy neurons, neat networks.

In this issue of Neuron, Kiani et al. (2015) show that the correlated activity of multiple simultaneously recorded neurons can be used to identify, in a completely un-biased manner, distinct functional domains within prefrontal and (pre)motor cortex of macaque monkeys.

Role of the primate ventral tegmental area in reinforcement and motivation.

Monkey electrophysiology suggests that the activity of the ventral tegmental area (VTA) helps regulate reinforcement learning and motivated behavior, in part by broadcasting prediction error signals throughout the reward system. However, electrophysiological studies do not allow causal inferences regarding the activity of VTA neurons with respect to these processes because they require artificial manipulation of neuronal firing. Rodent studies fulfilled this requirement by demonstrating that electrical and optogenetic VTA stimulation can induce learning and modulate downstream structures. Still, the primate dopamine system has diverged significantly from that of rodents, exhibiting greatly expanded and uniquely distributed cortical and subcortical innervation patterns. Here, we bridge the gap between rodent perturbation studies and monkey electrophysiology using chronic electrical microstimulation of macaque VTA (VTA-EM). VTA-EM was found to reinforce cue selection in an operant task and to motivate future cue selection using a Pavlovian paradigm. Moreover, by combining VTA-EM with concurrent fMRI, we demonstrated that VTA-EM increased fMRI activity throughout most of the dopaminergic reward system. These results establish a causative role for primate VTA in regulating stimulus-specific reinforcement and motivation as well as in modulating activity throughout the reward system.

Dopaminergic reward signals selectively decrease fMRI activity in primate visual cortex.

Stimulus-reward coupling without attention can induce highly specific perceptual learning effects, suggesting that reward triggers selective plasticity within visual cortex. Additionally, dopamine-releasing events-temporally surrounding stimulus-reward associations-selectively enhance memory. These forms of plasticity may be evoked by selective modulation of stimulus representations during dopamine-inducing events. However, it remains to be shown whether dopaminergic signals can selectively modulate visual cortical activity. We measured fMRI activity in monkey visual cortex during reward-only trials apart from intermixed cue-reward trials. Reward without visual stimulation selectively decreased fMRI activity within the cue representations that had been paired with reward during other trials. Behavioral tests indicated that these same uncued reward trials strengthened cue-reward associations. Furthermore, such spatially-specific activity modulations depended on prediction error, as shown by manipulations of reward magnitude, cue-reward probability, cue-reward familiarity, and dopamine signaling. This cue-selective negative reward signal offers a mechanism for selectively gating sensory cortical plasticity.

Neural correlates of the formation and retention of cocaine-induced stimulus-reward associations.

BACKGROUND: Cocaine can elicit drug-seeking behavior for drug-predicting stimuli, even after a single stimulus-cocaine pairing. Although orbitofrontal cortex is thought to be important during encoding and maintenance of stimulus-reward value, we still lack a comprehensive model of the neural circuitry underlying this cognitive process. METHODS: We studied the conditioned effects of cocaine with monkey functional magnetic resonance imaging and classical conditioning by pairing a visual shape (conditioning stimulus [CS+]) with a noncontingent cocaine infusion; a control stimulus was never paired. We correlated the behavioral preference of the monkey for the CS+, as measured offline, with the activity induced by the CS+ relative to the control stimulus as function of time. RESULTS: We observed that during formation of stimulus-cocaine associations strong CS+-induced functional magnetic resonance imaging activations emerged in frontal cortex that correlated significantly with behavioral CS+ preference. Afterward, CS+ preference correlated only with activity in early visual cortex. Control experiments suggest that these findings cannot be explained by increased familiarity for the CS+. CONCLUSIONS: Our findings suggest a complex interaction between frontal and occipital cortex during cocaine conditioning. Frontal cortex is important for establishing novel representations of stimulus valence when cocaine is used as reinforcer, whereas early visual cortex is involved in retaining these cocaine-stimulus associations.

Modulation of the contrast response function by electrical microstimulation of the macaque frontal eye field.

Spatial attention influences representations in visual cortical areas as well as perception. Some models predict a contrast gain, whereas others a response or activity gain when attention is directed to a contrast-varying stimulus. Recent evidence has indicated that microstimulating the frontal eye field (FEF) can produce modulations of cortical area V4 neuronal firing rates that resemble spatial attention-like effects, and we have shown similar modulations of functional magnetic resonance imaging (fMRI) activity throughout the visual system. Here, we used fMRI in awake, fixating monkeys to first measure the response in 12 visual cortical areas to stimuli of varying luminance contrast. Next, we simultaneously microstimulated subregions of the FEF with movement fields that overlapped the stimulus locations and measured how microstimulation modulated these contrast response functions (CRFs) throughout visual cortex. In general, we found evidence for a nonproportional scaling of the CRF under these conditions, resembling a contrast gain effect. Representations of low-contrast stimuli were enhanced by stimulation of the FEF below the threshold needed to evoke saccades, whereas high-contrast stimuli were unaffected or in some areas even suppressed. Furthermore, we measured a characteristic spatial pattern of enhancement and suppression across the cortical surface, from which we propose a simple schematic of this contrast-dependent fMRI response.

Visual field map clusters in macaque extrastriate visual cortex.

The macaque visual cortex contains >30 different functional visual areas, yet surprisingly little is known about the underlying organizational principles that structure its components into a complete "visual" unit. A recent model of visual cortical organization in humans suggests that visual field maps are organized as clusters. Clusters minimize axonal connections between individual field maps that represent common visual percepts, with different clusters thought to carry out different functions. Experimental support for this hypothesis, however, is lacking in macaques, leaving open the question of whether it is unique to humans or a more general model for primate vision. Here we show, using high-resolution blood oxygen level-dependent functional magnetic resonance imaging data in the awake monkey at 7 T, that the middle temporal area (area MT/V5) and its neighbors are organized as a cluster with a common foveal representation and a circular eccentricity map. This novel view on the functional topography of area MT/V5 and satellites indicates that field map clusters are evolutionarily preserved and may be a fundamental organizational principle of the Old World primate visual cortex.

Bottom-up dependent gating of frontal signals in early visual cortex.

The frontal eye field (FEF) is one of several cortical regions thought to modulate sensory inputs. Moreover, several hypotheses suggest that the FEF can only modulate early visual areas in the presence of a visual stimulus. To test for bottom-up gating of frontal signals, we microstimulated subregions in the FEF of two monkeys and measured the effects throughout the brain with functional magnetic resonance imaging. The activity of higher-order visual areas was strongly modulated by FEF stimulation, independent of visual stimulation. In contrast, FEF stimulation induced a topographically specific pattern of enhancement and suppression in early visual areas, but only in the presence of a visual stimulus. Modulation strength depended on stimulus contrast and on the presence of distractors. We conclude that bottom-up activation is needed to enable top-down modulation of early visual cortex and that stimulus saliency determines the strength of this modulation.