The therapeutic potential of the cerebellum in schizophrenia

The cognitive role of the cerebellum is critically tied to its distributed connections throughout the brain. Accumulating evidence from anatomical, structural and functional imaging, and lesion studies advocate a cognitive network involving indirect connections between the cerebellum and non-motor areas in the prefrontal cortex. Cerebellar stimulation dynamically influences activity in several regions of the frontal cortex and effectively improves cognition in schizophrenia. In this manuscript, we summarize current literature on the cingulocerebellar circuit and we introduce a method to interrogate this circuit combining opotogenetics, neuropharmacology, and electrophysiology in awake-behaving animals while minimizing incidental stimulation of neighboring cerebellar nuclei. We propose the novel hypothesis that optogenetic cerebellar stimulation can restore aberrant frontal activity and rescue impaired cognition in schizophrenia. We focus on how a known cognitive region in the frontal cortex, the anterior cingulate, is influenced by the cerebellum. This circuit is of particular interest because it has been confirmed using tracing studies, neuroimaging reveals its role in cognitive tasks, it is conserved from rodents to humans, and diseases such as schizophrenia and autism appear in its aberrancy. Novel tract tracing results presented here provide support for how these two areas communicate. The primary pathway involves a disynaptic connection between the cerebellar dentate nuclei (DN) and the anterior cingulate cortex. Secondarily, the pathway from cerebellar fastigial nuclei (FN) to the ventral tegmental area, which supplies dopamine to the prefrontal cortex, may play a role as schizophrenia characteristically involves dopamine deficiencies. We hope that the hypothesis described here will inspire new therapeutic strategies targeting currently untreatable cognitive impairments in schizophrenia.


INTRODUCTION
The purpose of this theory and hypothesis manuscript is two fold. First, we review the role of the cerebellum in cognition and schizophrenia, the anatomy of cerebellar projections to frontal cortex, and the available evidence indicating that the cerebellum influences the frontal cortex. Second, we introduce the novel hypothesis that optogenetic cerebellar stimulation may ameliorate cognitive symptoms of schizophrenia by normalizing frontal activity, and we propose testable animal and translational experiments to probe this hypothesis. Studies have shown that connections exist between the frontal cortex and cerebellum and that cerebellar stimulation improves cognitive symptoms of schizophrenia; to our knowledge, this manuscript is the first to propose a method to pharmacologically disrupt frontal function in rodents to mimic abnormalities in schizophrenia, document cognitive dysfunction similar to that reported in schizophrenia, and use cerebellar stimulation to restore frontal activity and rescue cognition. Specifically, we will evaluate the therapeutic potential of the cerebellum by introducing a technique to interrogate cerebellar connections to the frontal cortex using combined opotogenetics, neuropharmacology, and electrophysiology in awake-behaving animals while minimizing incidental stimulation of neighboring cerebellar nuclei.
The cerebellum is a critical component in tasks of both motor and cognitive origin. Thalamic connections between the cerebellum and frontal cortex create the potential for the cerebellum to powerfully influence cognition. In schizophrenia, there are abnormalities in all three regions, which led to the hypothesis that a distributed network involving the cerebellum contributes to cognitive deficits. We, and many of our colleagues, have been investigating this circuit for almost two decades (Leiner et al., 1994;Andreasen et al., 1998;Schmahmann, 1998Schmahmann, , 2010Andreasen and Pierson, 2008). Although studies of functional neuroimaging, anatomy, positron emission tomography (PET), and structural and functional magnetic resonance imaging are extensive, it is unclear how this circuitry is influenced by schizophrenia (Alphs, 2006). The advent of molecular tools and imaging technology such as tractography and diffusion tensor imaging (DTI) that can be used in psychiatric disorders have provided additional clarity to this issue (White et al., 2008).
Cognitive impairments in schizophrenia including executive dysfunctions in working and episodic memory, Ragland et al., 2009), attention, reasoning, and timing (Carroll et al., 2009), remain untreatable (Alphs, 2006). These impairments have been described as cognitive dysmetrias, or disruptions in the synchronous coordination of cognitive capacities Schmahmann, 1998). Recently, Demirtas-Tatlidede et al. reported that cerebellar vermal theta burst stimulation (TBS) has been effective at relieving some cognitive symptoms in treatment-resistant schizophrenia patients (Demirtas-Tatlidede et al., 2010). Specifically, eight schizophrenia patients were evaluated using comprehensive neuropsychological testing, Positive and Negative symptoms subscale (PANSS), Clinical Global Impression (CGI), Calgary Depression Scale for schizophrenics (CDSS), Profile of Mood States (POMS), and Visual Analogue Mood Scale (VAS) for dimensions of mood (Happiness, Sadness, Calmness, Anxiety, Wellbeing, Anger, Selfconfidence, Fear, Alertness, and Energy). Subjects received cerebellar vermal TBS twice daily on 5 consecutive days for a total of 10 stimulation sessions. Baseline ratings were compared with ratings immediately following stimulation and 1 week later. The results showed significant improvement in negative symptoms reported by the PANSS both following treatment and at followup, and there was a significant elevation in mood. Cognition also improved as the subject had fewer working memory omissions with no worsening of other measures.
The mechanism underlying the efficacy of cerebellar stimulation in schizophrenia is unknown. Studies have shown that electrically stimulating the cerebellum produces downstream changes in the prefrontal cortex and anterior cingulate cortex (Mittleman et al., 2008;Rogers et al., 2011Rogers et al., , 2013Watson et al., 2014). Connections between the cerebellum and anterior cingulate, comprising a cingulocerebellar circuit, are of particular interest because they have been confirmed to exist by tracing studies, they are conserved between species including humans, primates, and rodents, and structural and functional abnormalities are present in schizophrenia. Here, we present empirical evidence that (1) the deep cerebellar nuclei and anterior cingulate cortex are disynaptically connected via two separate pathways (the thalamus and the ventral tegemental area-VTA) allowing the cerebellum access to the frontal cortex; and (2) the cerebellum may modulate frontal neuron ensembles. This cerebellar modulatory mechanism has the potential to be harnessed to rescue abnormalities in the anterior cingulate, typically associated with schizophrenia. The mechanism underlying the efficacy of cerebellar stimulation needs to be illuminated if cerebellar stimulation is to become a therapeutic treatment strategy for the negative and cognitive symptoms of schizophrenia (Demirtas-Tatlidede et al., 2013).

CINGULOCEREBELLAR INTERACTIONS IN SCHIZOPHRENIA FUNCTIONAL NEUROIMAGING
Cognitive dysmetria has been probed in patients with schizophrenia (Andreasen and Pierson, 2008) concurrently undergoing neuroimaging using tasks such as recall of complex narratives (Andreasen et al., 1995a), episodic memory Ragland et al., 2009), memory for word lists (Andreasen et al., 1995b;Paradiso et al., 1997;Crespo-Facorro et al., 1999), recognition memory (Crespo-Facorro et al., 2001), dichotic listening (O'Leary et al., 1996, eyeblink conditioning (Parker et al., 2013b; Figure 1), and timing (Volz et al., 2001). These data consistently indicate lower blood flow in the cerebellum and frontal cortex  including medial frontal regions such as the anterior cingulate (Adams and David, 2007). FIGURE 1 | During positron emissions tomography (PET) imaging of a task typically thought to be cerebellum dependent, eyeblink conditioning, hypofunction was revealed in the cingulocerebellar circuit. A double subtraction method where the baseline pseudoconditioning phase of unpaired tones and airpuffs was first subtracted and then the rCBF for patients with schizophrenia was subtracted from that of controls, show negative peaks indicating patients with schizophrenia have less rCBF than controls. This representative image shows hypofunction of the anterior cingulate (Talairach coordinates: −1, 30, 14) and cerebellar lobules IV/V, and IX (Talairach coordinates: −3, −67, −17) in patients with schizophrenia in comparison to healthy controls. For each illustration there are three orthogonal views per row with transaxial on the top, sagittal in the middle, and coronal on the bottom. Green crosshairs are used to show the location of the slice. Images follow radiological convention and show location as if facing the patient where the left side of the image represents the patient's right side. The statistical maps of the PET, showing the regions where the two groups differed significantly at the 0.005 level, are superimposed on a composite magnetic resonance image (MRI) derived by averaging the MR scans from the subjects. Regions in red/yellow tones indicate positive peaks (greater activation in patients than in controls) and regions with blue/purple indicate negative peaks (less activity in patients than controls). The statistical results are portrayed using the value of the associated t-statistic, which is shown on the color bar on the right. The images are referred to as "t maps" showing all voxels in the image which exceed a display threshold. The correlation of these cognitive deficits with abnormal regional cerebral blood flow, led to the mechanistic explanation that abnormalities in a distributed cerebellar neural circuit may underlie cognitive impairments in schizophrenia . Aberrant connections from the cerebellum to the cerebral cortex may influence misconnections between percepts and their meanings, in turn causing errors in perceptual binding and misinterpretations of many kinds (e.g., delusions, hallucinations); they may also lead to inefficient or inaccurate information processing, forming the basis for the multiple types of cognitive impairments observed in schizophrenia Wiser et al., 1998;Schmahmann, 2004).

STRUCTURAL ABNORMALITIES
In addition to hypofunction, studies report structural abnormalities in the anterior cingulate, thalamus, and cerebellum in schizophrenia. Specifically, voxel based morphometry (Mouchet-Mages et al., 2011) and DTI (Wiser et al., 1998) reveal abnormalities in white matter connectivity between the nodes of the cingulocerebellar circuit in schizophrenia. The presence of structural abnormalities in these structures supports abnormalities in the distributed cerebellar network.
The cingulum bundle, including the anterior cingulate, has consistently shown abnormalities in schizophrenia (White et al., 2008). Patients with schizophrenia have an overall reduction of gray matter in the anterior cingulate (Brodmann Area 32) (Glahn et al., 2008;Takayanagi et al., 2013;Salgado-Pineda et al., 2014). Post mortem studies revealed a reduction in laminar thickness in the anterior cingulate (dorsal and subcullosal regions) (Fornito et al., 2009). Mitelman et al. report that schizophrenia patients with better outcomes have correspondingly higher fractional anisotropy (FA) than normal in frontal white matter areas including bilateral cingulate gyri; emphasizing the integral role of the anterior cingulate, he proposed that this increased FA in cingulate white matter may serve a neuroprotective role as indicated by a better outcome for patients (Mitelman et al., 2006). Our data support this hypothesis, as our patients with larger anterior cingulate volumes reported greater psychotic symptom improvement overtime (McCormick et al., 2005). In addition, functional and structural imaging data found convergent abnormalities in the medial frontal cortex including the anterior cingulate (Fornito et al., 2009;Pomarol-Clotet et al., 2010).
Structural abnormalities have also been revealed in thalamic nuclei and their projections in schizophrenia (Andreasen et al., 1994;Magnotta et al., 2008). Specifically, the thalamus has been shown to be reduced in size in schizophrenia (Andreasen et al., 1994;Buschman and Miller, 2007). In addition, FA was decreased in the internal capsule connecting the thalamus to the anterior cingulate (Oh et al., 2009). DTI reveals reduced FA in the white matter fiber tracts located between the thalamus and cerebellum in patients with schizophrenia compared to normal controls. Specifically, there was reduced FA within the superior cerebellar peduncle but not along the tract from the cerebellum to the thalamus (Magnotta et al., 2008). Little is known about whether the thalamus is essentially involved in schizophrenia or whether it is simply a convergence point and relay station for signals from other parts of the brain (Saalmann and Kastner, 2011). However, medial thalamic nuclei have been shown to project to the anterior cingulate cortex and inactivating these projections impairs working memory (Hsu and Shyu, 1997;Parnaudeau et al., 2013).
We have also reported that our schizophrenia patients have decreased overall cerebellar volume and, more specifically, decreased volume of the anterior lobe of the cerebellar vermis. Decreased cerebellar volume correlated with longer duration of psychotic and negative symptoms, and greater psychosocial impairment Wassink et al., 1999). Post mortem morphometric analyses of patients with schizophrenia confirm decreased anterior cerebellar vermis volume (Weinberger et al., 1980). A reduction of gray matter in Crus I and II of lobule VII has also been reported (Kühn et al., 2012). There may also be reduced FA in the vermis and middle cerebellar peduncles of schizophrenia patients (Okugawa et al., 2003(Okugawa et al., , 2005. Human lesion studies provide support for cingulocerebellar involvement in cognition. Although cerebellar lesions predominantly reveal motor impairments (Groiss and Ugawa, 2013), patients also suffer from comorbid cognitive impairments (Gottwald et al., 2004;Grimaldi and Manto, 2012). These dysfunctions include impaired timing, attention, memory, and language, all of which classically rely on the frontal lobes (Akshoomoff and Courchesne, 1992;Fiez et al., 1992;Grafman et al., 1992;Courchesne et al., 1994;Stoodley and Schmahmann, 2009). Schmahmann's cerebellar cognitive affective syndrome has shown that human cerebellar lesions cause cognitive impairments (Schmahmann, 2004). Inducing transient impairments using repetitive transcranial cerebellar stimulation has recently been shown to impair cognitive tasks such as language, emotion, learning, memory, perception, and timing (Fierro et al., 2007;Koch et al., 2007;Oliveri et al., 2007;Grube et al., 2010;Bijsterbosch et al., 2011;Boehringer et al., 2013;Tomlinson et al., 2013;Grimaldi et al., 2014). Similar deficits are reported following lesions to the anterior cingulate (Devinsky et al., 1995). Specifically, timing on the go-nogo task, which probes response inhibition, has been shown to be impaired (Picton et al., 2006).

ANATOMICAL CONNECTIONS
Neuronal tract-tracing has made giant strides to link the cerebellum to prefrontal networks, but many have focused exclusively on the dorsal lateral prefrontal cortex, which rodents lack (Uylings et al., 2003). The dorsal lateral prefrontal cortex (Brodmann Areas 9/46) has been the subject of extensive tracing studies in primates, which establish a "closed-loop" linking cerebellar nuclei (specifically the dentate) and the dorsal lateral prefrontal cortex via the contralateral thalamus. Retrograde tracer infusions in the dorsolateral prefrontal cortex and DTI reveal projections back to the deep cerebellar nuclei and cerebellar cortex via the pontine nuclei (PN; Middleton and Strick, 2001;Kamali et al., 2010;Schulz et al., 2014; Figure 2). These studies establish the potential for cerebellar-prefrontal interactions in primates but additional research is necessary (Uylings et al., 2003).
As we have argued, the anterior cingulate cortex is essential for normal cognition and shows impairments in schizophrenia. In humans, the anterior cingulate is also known as Brodmann areas 24/32 or the prelimbic cortices. Although homogenous in rodents, nomenclature is inconsistent. In rodents, the anterior cingulate cortex (Cg3/prelimbic) is commonly known as the medial prefrontal cortex and is described based on four cytoarchitecturally unique regions, Cg1, Cg2, Cg3/prelimbic, and infralimbic (Bostan et al., 2013;Vogt et al., 2013;Vogt and Paxinos, 2014). Although it remains unclear if a similar "closed-loop" exists in the cingulocerebellar circuit, we hope to elucidate these connections.
The cerebellar afferent pathway in the cingulocerebellar circuit involves direct anterior cingulate projections to the PN (Vilensky and Van Hoesen, 1981;Glickstein et al., 1985;Legg et al., 1989;Dembrow et al., 2010). Specifically, Vilensky et al. report that the rostral cingulate projects to the medial PN while the caudal cingulate regions project more laterally (Vilensky and Van Hoesen, 1981). These corticopontine fibers form the middle cerebellar peduncle and project to the deep cerebellar nuclei and cerebellar cortex. There is evidence for two efferent pathways through which deep cerebellar nuclei communicate with the anterior cingulate. The first efferent pathway involves a disynaptic connection between the deep cerebellar nuclei and contralateral thalamus (Magnotta et al., 2008;Strick et al., 2009). Medial thalamic nuclei have been shown to project to the anterior cingulate cortex (Hsu and Shyu, 1997;Parnaudeau et al., 2013) although there is evidence for projections from all thalamic nuclei depending on the precise location of the anterior cingulate/prelimbic area in question (Condé et al., 1990). Therefore, it is likely that cerebellar projections to the thalamus are capable of transmitting information to broad regions of the anterior cingulate cortex. These connections are highly topographic and the labeled neurons are highly dependent on the precise location of the tracer injection.
The second efferent pathway includes deep cerebellar nuclei (fastigial) projections to the VTA (Snider et al., 1976). The VTA is known to supply dopaminergic input to the anterior cingulate and is able to affect neuronal activity when pharmacologically manipulated (Williams and Goldman-Rakic, 1998). Studies have corroborated this efferent pathway by showing that electrically stimulating cerebellar dentate nuclei (DN; Mittleman et al., 2008) and fastigial nuclei (FN; Watson et al., 2009) influences medial prefrontal dopamine signaling, which is impaired in schizophrenia (Hadley et al., 2014). Conversely, electrically stimulating the medial prefrontal cortex elicited neuronal firing in cerebellar lobule VII which projects back to the deep nuclei (Watson et al., 2009). A similar pathway and mechanistic approach to target the dento-VTA-PFC tract has been suggested by Rogers et al. in autism (Rogers et al., 2013) (see section on applications for neuropsychiatric illnesses). One challenge of electrical stimulation in mapping these circuits is that the influence of fibers of passage and indirect stimulation of neighboring nuclei confounds the results. Future studies using optogenetics may achieve greater specificity.
Although these studies show connections exist, the precise projections remain elusive. To illuminate the cingulocerebellar circuit, we infused retrograde tracer (RetroBeads, Lumaflour) in the left anterior cingulate/medial frontal cortex and anterograde tracer (Phaseolus vulgaris leucoagglutinin, Life Technologies) in the right cerebellar dentate deep nuclei of Long Evans rats and found these structures to be disynaptically connected via two distinct routes as previously reported. Single synapse red prefrontal beads and green cerebellar tracer colocalized on left ventrolateral thalamic nuclei and on VTA neurons nuclei (Figures 3A,B). Convergence of information in these networks provides two avenues through which the cerebellum may influence neurons in the anterior cingulate cortex. However, the cerebellum may influence cortical function indirectly through other thalamic inputs. More analyses are needed to identify or rule out colocalization in other thalamic nuclei.
This anatomical connection opens up an avenue for the cerebellum to modulate aberrant prefrontal networks in schizophrenia. We are currently analyzing longitudinal DTI and tractography data from patients at intake and throughout the disease course, which will allow us to choose regions of interest in the anterior cingulate and the deep cerebellar nuclei to more precisely document abnormalities in the cingulocerebellar circuit. It is important to note that as deep cerebellar nuclei are the sole output of the cerebellum, abnormalities in any area of the cerebellum have the potential to influence frontal function through the cinglocerebellar circuit.

A testable hypothesis: cerebellar stimulation restores prefrontal function and rescues cognition in schizophrenia
It is through the convergence of cerebellar deep nuclei and anterior cingulate projections on thalamic neurons that we propose the cerebellum can be harnessed to rescue aberrant prefrontal circuits in schizophrenia. The implications of this efferent, disynaptic pathway are numerous. Cerebellar stimulation may have the ability to restore prefrontal neuronal activity and firing patterns, allowing relief from some of the cognitive symptoms of schizophrenia. Several studies provide support for the efficacy of cerebellar stimulation in neuropsychiatry (Grimaldi et al., 2014). A classic study electrically stimulated the cerebellum and reported improved cognition based on increased alertness, improvement in thinking, and fluency of speech in addition to many enriched emotional characteristics in patients with epilepsy (Cooper et al., 1976). Recently, cerebellar vermal TBS has been effective at relieving some cognitive symptoms in treatment-resistant schizophrenia patients (Demirtas-Tatlidede et al., 2010). In addition, Schutter et al. showed that cerebellar vermal TBS produced downstream changes in neuronal activity in the frontal cortex (Schutter et al., 2003). The exact cerebellar circuitry needs to be explicitly mapped prior to pursuing translational research.
How the cerebellum normally influences the prefrontal cortex and anterior cingulate is an important question that can be investigated using neuronal ensemble recordings of these areas in animals exhibiting phenotypes of schizophrenia. The cingulocerebellar circuit is likely essential for a variety of cognitive tasks as is documented by the previously described neuroimaging results. Correlating neuronal recordings will provide clarity as to how the cerebellum and prefrontal cortex work in synchrony during cognitive tasks such as learning, timing, and attention. Task-specific modulation will indicate if these areas are necessary.
Several groups have made progress deciphering the role of nodes in the cingulocerebellar circuit using electrical stimulation (Mittleman et al., 2008;Watson et al., 2009;Rogers et al., 2011Rogers et al., , 2013. However, results from these studies are limited due to the unwanted spread of electrical stimulation. To circumvent this issue, we propose using optogenetic stimulation of select, isolated pools of neurons in the cingulocerebellar circuit. Optogenetic stimulation of Purkinje cells has been shown to be an effective way to modulate cerebellar output (Tsubota et al., 2011). Using our tract-tracing results to target specific deep nuclear projections in the cingluocerebellar circuit, channelrhodopsin, a light-activated channel, can be infused into cerebellar neurons. Once expressed, these proteins render their projections photoexcitable. Optical fibers can be placed in the thalamus to selectively stimulate the cerebellar neuronal projections to the anterior cingulate while not affecting other cerebellar neuronal populations. Optogenetically stimulating cerebellar dentate projections in the thalamus could influence the prefrontal cortex (Figure 4). This optogenetic paradigm could be combined with neuronal ensemble recordings to probe the dynamic relationship between the prefrontal cortex and cerebellum while analyzing task-dependent modulation. Based on evidence from cerebellar stimulation studies, cerebellar optogenetic stimulation has the potential to enhance prefrontal neuronal modulation and show correlates of behavioral improvement.
Another way to investigate the cingulocerebellar circuit is to induce abnormalities similar to those reported during schizophrenia and attempt to rescue impairments using cerebellar stimulation. Targeting neurotransmitter cascades such as dopamine (Abi-Dargham et al., 2002;Goldman-Rakic et al., 2004), glutamate (Olney and Farber, 1995;Chaki and Hikichi, 2011;Moghaddam and Javitt, 2012;Anticevic et al., 2013), and GABA in prefrontal pyramidal neurons, it is possible to mimic schizophrenia phenotypes by direct pharmacological microinjections (Jones et al., 2011). These manipulations may induce changes in neuronal activity that resemble abnormalities in schizophrenia and can be correlated with behavioral abnormalities. For example, frontal dopamine has been implicated in schizophrenia (Abi-Dargham et al., 2002;Goldman-Rakic et al., 2004;Stahl and Buckley, 2007). The level of frontal dopamine determines the type and severity of associated symptoms (Goldman-Rakic et al., 2004;Kuepper et al., 2012). Excess dopamine contributes to positive symptoms such as hallucinations and delusions, while depleted dopamine is responsible for negative symptoms such as anhedonia, avolition, and cognitive deficits such as impaired timing (Andreasen and Olsen, 1982;Seeman, 1987;Kuepper et al., 2012). We, and others, have shown that temporal processing depends on the cerebellum for short, sub-second intervals (Ivry and Spencer, 2004;Buhusi and Meck, 2005;Bracha et al., 2009;Parker et al., 2009) while D1 dopamine signaling in the anterior cingulate of the prefrontal cortex is essential for longer intervals (Deutch, 1993;Narayanan et al., 2012;Parker et al., 2013a,c). In schizophrenia, the prefrontal cortex shows abnormal D1 dopamine (Weinberger et al., 1986;Okubo et al., 1997;Goldman-Rakic et al., 2004) and patients inaccurately estimate a discrete interval of time (Elvevåg et al., 2003;Bonnot et al., 2011). In animals, it is possible to model these concepts by locally infusing pharmacological agents into the frontal cortex to disrupt prefrontal D1 dopamine signaling and mimic impaired timing (Narayanan et al., 2012). If the cingulocerebellar circuit is necessary for temporal processing and is sensitive to levels of prefrontal dopamine, pharmacologically manipulating prefrontal dopamine will disrupt neuronal activity and impair timing. Mittleman et al. report efflux of dopamine in the prelimibic cortex (anterior cingulate) following electrical stimulation of the dentate nuclei (Mittleman et al., 2008). Therefore, optogenetically stimulating cerebellar nuclei has the potential to rescue aberrant contralateral prefrontal neuronal ensembles, recovering elementary cognitive tasks (Boyden et al., 2005;Narayanan et al., 2012). An example of a task that has been used to probe the role of the cingulocerebellar circuit is eyeblink conditioning as it is abnormal in schizophrenia (Brown et al., 2005;Bolbecker et al., 2009;Forsyth et al., 2012;Parker et al., 2013b). Specifically, trace eyeblink conditioning, where a conditioning stimulus and an unconditioned stimulus are separated by a trace interval, requires integration of the cerebellum and frontal cortex (Weiss and Disterhoft, 1996). Siegel et al. have explored this afferent pathway and the influence of the dorsal anterior cingulate in layers V and VII (secondary motor cortices) in rabbits (Siegel et al., 2012). They found direct projections to the ipsilateral rostral PN via the corticospinal pyramidal tract. It has also been reported that prefrontal neurons are consistently active in the trace interstimulus interval, likely encoding the elapsed time between the stimulus and providing the cerebellum with the timing information necessary to accurately execute the eyeblink response (Siegel et al., 2012;Chen et al., 2014). It is possible to pharmacologically mimic psychiatric abnormalities by inactivating various areas of the prefrontal cortex to produce disruptions in trace eyeblink conditioning. Following expression of channelrhodopsin infusions in deep cerebellar nuclei, optogenetic stimulation of cerebellar projections to the thalamus have the potential to recover prefrontal neuronal activity and rescue eyeblink conditioning.
Optogenetic stimulation of the cingulocerebellar pathway can also be explored in genetically modified animals exhibiting schizophrenia phenotypes. Currently, several animals exist that exhibit some phenotypes of schizophrenia such as G72/G30, Df(16)A KO model, and Disrupted in Schizophrenia 1 (DISC1; Shevelkin et al., 2014). Each of these animals has cerebellar abnormalities similar to those consistently detected in schizophrenia making them ideal models to study the cingulocerebellar circuit (Shevelkin et al., 2014).
There are many additional aspects of this circuit that can be explored. Although we have focused on a very specific circuit involving the anterior cingulate and cerebellum, it is possible that a more widespread activity is sufficient for normal functioning. There are known connections throughout frontal lobes and normalized neuronal firing throughout the brain may prove essential rather than the firing patterns of one particular structure. This theory is supported by the more diffuse type of stimulation used by transcranial magnetic stimulation (Grimaldi et al., 2014) and thalamic stimulation (Klein et al., 2013).

CEREBELLAR STIMULATION IN PARKINSON'S DISEASE AND AUTISM
Although schizophrenia is not considered to be a motor disease, many indicators of motor dysfunction are present, suggesting that the basic abnormality in the disorder could be a brain system that mediates both motor and cognitive functions. Kraepelin described a variety of motor abnormalities in schizophrenia in his classic textbook (1919). Slowed reaction time is one of the oldest and most robust tests showing differences between schizophrenic patients and normal controls. More recent studies of premorbid indicators and neurological soft signs also implicate the motor system (Walker and Shaye, 1982;Gupta et al., 1995;Flashman et al., 1996;Mouchet-Mages et al., 2011). These mild indicators of poor coordination provide a clue that the underlying mechanism may reflect an abnormality in a basic process that regulates the synchrony of both thought and motor activity. Therefore, schizophrenia may be conceptualized as a disease that is characterized by poor coordination, or dysmetria, in all domains of functioning, including movement and cognition. In addition to its role in cognition, the cerebellum has traditionally been associated with movement and gait. Therefore, the circuit between the prefrontal cortex and cerebellum may facilitate the smooth execution, or synchrony of both motor and cognitive function. As we have proposed optogenetic stimulation of cerebellar projections for the treatment of the cognitive symptoms of schizophrenia, the motor impairments may actually benefit as well.
Parkinson's disease (PD) also causes impaired gait and cognition and involves abnormalities in the cerebellum, prefrontal cortex, and the basal ganglia. Cerebellar circuitry involves efferent connections with the basal ganglia which project back to the cerebellum via the PN (similar to the anterior cingulate projections) . However, similar to schizophrenia, the role of the cerebellum in PD is unknown. Recently, Wu et al. published a thorough review of the role of the cerebellum in PD (Wu and Hallett, 2013). They speculated that the cerebellum plays either a compensatory role or a pathological role (Wu and Hallett, 2013). Using optogenetic stimulation of cerebellar projections in the striatum, it may be possible to modulate cerebellar activity and repair movement, gait, and cognition in PD patients. How cerebellar circuitry interacts with the anterior cingulate via the dopaminergic VTA projections should be pursued, as cerebellar stimulation could represent novel therapeutic opportunities for both the cognitive and motor impairments in PD.
Courchesne et al. have documented cerebellar abnormalities in autism. In addition to decrease in cerebellar vermal volume, there appears to be aberrancies in cerebellar-prefrontal interactions (Carper and Courchesne, 2000;Pierce and Courchesne, 2001;Courchesne et al., 2011). Recently, Purkinje cell abnormalities have been linked to behavioral deficits similar to those exhibited in autism (Sudarov, 2013). The middle cerebellar peduncle has also shown structural impairments in autism and as the output pathway for cerebellar projections, this altered microstructure could have dire consequences for cerebellar-frontal lobe communications (Sivaswamy et al., 2010;Hanaie et al., 2013). More research needs to be done to understand the behavioral and cognitive symptoms in autism which depend on the cingulocerebellar pathway (Heck and Howell, 2013). Using optogenetics, this circuit should be parsed out and pursued to repair brain circuitry in autism. There are currently several animal models that exhibit phenotypes of autism, which are ripe for investigation.

CONCLUSIONS
The ideas described here have the potential to map the influence of cerebellar circuitry on the frontal cortex and lead to new insights about cingulocerebellar interactions. Through the cingulocerebellar circuit, cerebellar stimulation may recover aberrancies in the anterior cingulate cortex, rescuing cognition in schizophrenia. To our knowledge, this idea has not been proposed before, and we hope systematically applying the techniques described here to the many cognitive tasks impaired in schizophrenia such as attention and working memory, will encourage the development of new, targeted treatments for treatment-resistant patients.