Schizophrenia: A Disorder of Subcortical Adaptable Neural- Complexity Pacemaker

The most striking feature of schizophrenia is the heterogeneity of clinical manifestation and variety of cognitive disturbances. As evident from this clinical description schizophrenia is related to a widespread cortical whole-brain disturbance. It can be useful to describe schizophrenia as a disorder of neural complexity where a segregated brain organization causes fragmentation of experience leading to disorganized schizophrenia and reality distortion, and where total dependence, overly connected brain systems leads to poverty schizophrenia constringing experiential processing, freezing brain-dynamics, limiting it to few repetitive computations resulting in poverty of thought and perseverations. In order to pinpoint the neuro-pathological origin of schizophrenia it is mandatory to discover what regulates neural complexity in the brain. A regulatory structure for neural complexity must have massive, distributed and parallel connectivity with most, if not all, brain formations. It is argued that the basal ganglia with their striatum and thalamocortical connections are most suitable to regulate cortical neural complexity. They are related to both, to the high mental functions of the brain and schizophrenia. It is proposed that the globus-pallidus and subthalamic-nucleus coupled dynamics is a ‘neural complexity engine’ in the sense that it is in a position to monitor and regulate cortical neural complexity. According to this model schizophrenia is essentially a disorder of un-reached neural complexity levels in adolescence, followed by ineffective neural complexity pacemaker activity that plunges the cortex into gradual progressive deterioration of neural complexity via oscillating dynamics of integration-segregation shifts attributing to schizophrenia its typical clinical heterogeneity and prognosis of cyclic deterioration. A testable prediction from the review and the model forecasts the potential of deep-brain-stimulation to the basal ganglia in controlling the symptoms of schizophrenia. IS THERE A GENERAL BRAIN DISTURBANCE TO EXPLAIN SCHIZOPHRENIA CLINICAL MANIFESTATION? The most striking feature of schizophrenia is the heterogeneity of clinical manifestation and variety of cognitive disturbances (Andreasen 1997). In general this heterogeneity was clustered into three overlapping types of clinical descriptions, 1) disorganized, 2) realitydistortion and 3) poverty schizophrenia (Liddle 1987). Schizophrenia typically evolves as a non-reversible gradually deteriorating disease progressing toward poverty schizophrenia with grave deficits of mental and social functioning. Disorganized schizophrenia is characterized by marked disintegration of conscious experience where the experience of reality is fragmented. In reality-distortion schizophrenia, delusions prevail generated from false reconciliation of experiences and from the collapse of logical inference. Poverty schizophrenia sees the patient incapacitated due to destruction of higher mental functions such as volition and emotions, and constriction of experience and thought (poverty and perseverations of thought). As evident from this clinical description schizophrenia is related to a widespread cortical whole-brain disturbance, in effect years of brain research find schizophrenia patients impaired on a multitude of brain structures and functions. Accordingly, in order to pinpoint the psychopathology of schizophrenia a generalized systembased approach should be adopted, one that takes into account the normal workings of the brain when conscious mental experience is coherent, stable, malleably meaningful and logical. The psychopathology Schizophrenia: A Disorder of Subcortical Adaptable Neural-Complexity Pacemaker 2007 © all rights reserved Avi Peled M.D. ‘NeuroAnalysis’ http://neuroanalysis.googlepages.com/home 2 of schizophrenia probably involves some general characteristic of brain organization. Nervous systems facing complex environments have to balance two seemingly opposing requirements. The need to quickly and reliably extract important features from sensory inputs and the need to generate coherent perceptual and cognitive states allowing an organism to respond to objects and events, which present conjunctions of numerous individual features. The need to quickly and reliably extract important sensory features is accomplished by functionally segregated (specialized) sets of neurons (e.g., those found in different cortical regions), the need to generate coherent perceptual and cognitive states is accomplished by functional integration of the activity of specialized neurons through their dynamic interactions (Tononi and Edelman, 1998). The mathematical concept of “neural complexity” (CN) (Tononi, 1994) captures the important interplay between integration (i.e., functional connectivity) and segregation (i.e., functional specialization of distinct neural subsystems). CN is low for systems whose components are characterized either by total independence or by total dependence. CN is high for systems whose components show simultaneous evidence of independence in small subsets, and increasing dependence in subsets of increasing size. Different neural groups are functionally segregated if their activities tend to be statistically independent. Conversely, groups are functionally integrated if they show a high degree of statistical dependence. It can be useful to describe schizophrenia as a disorder of CN where a segregated brain organization (i.e., total independence among cortical systems) causes fragmentation of experience leading to disorganized schizophrenia and reality distortion (Peled 1999; Peled 2004), and where total dependence (overly connected systems) leads to poverty schizophrenia constringing experiential processing, freezing brain-dynamics, limiting it to few repetitive computations (i.e., poverty of thought and perseverations). IS THERE ANY EVIDENCE THAT NEURAL COMPLEXITY DYNAMICS IS DISTURBED IN SCHIZOPHRENIA In the past much has been written to support a ‘disconnection syndrome’ for schizophrenia (Peled 1999), recently more evidence has been added to support this hypothesis. Whitford et al (2007) found that firstepisode schizophrenia patients exhibited volumetric deficits in the white matter of the frontal and temporal lobes at baseline, as well as volumetric increases in the white matter of the frontoparietal junction bilaterally. Furthermore, these first-episode schizophrenia patients lost considerably more white matter over the follow-up interval relative to comparison subjects in the middle and inferior temporal cortex bilaterally. Buchsbaum et al (2007) found that compared with normal volunteers, Schizophrenia patients showed higher relative metabolic rates in the frontal white matter, corpus callosum, superior longitudinal fasciculus, and white matter core of the temporal lobe. Elevated activity in white matter was most pronounced in the center of large white matter tracts, especially the frontal parts of the brain and the internal capsule. In another work Whitford et al (2007) found that although grey matter volume decreased longitudinally in schizophrenia patients, particularly fronto-parietally, electroencephalographic power increased in the slow-wave and beta-frequency bands , suggesting abnormally elevated levels of neural synchrony. Andreone et al (2007) found cortical whitematter microstructure disruption in frontal and temporooccipital lobes in a large sample of sixty-eight patients with schizophrenia and 64 healthy controls. Brambilla and Tansella (2007) conclude that diffusion weighted imaging (DWI) studies in schizophrenia strongly suggest that white matter communication is disrupted. This supports the hypothesis that there is a cortico-cortical and transcallosal altered connectivity in schizophrenia, which may be relevant for the pathophysiology and the cognitive disturbances of the disorder. While these findings can support the idea of reduced dependence among brain systems, is there also evidence for disturbed integration? Recent descriptions of a ‘default network’ organization in the brain may provide such evidence. Recent blood oxygenation level dependent functional MRI (BOLD fMRI) studies of the human brain have shown that in the absence of external stimuli, activity persists in the form of distinct patterns of temporally correlated signal fluctuations (Horovitz et al 2007; Sonuga-Barke and Castellanos 2007). This Schizophrenia: A Disorder of Subcortical Adaptable Neural-Complexity Pacemaker 2007 © all rights reserved Avi Peled M.D. ‘NeuroAnalysis’ http://neuroanalysis.googlepages.com/home 3 spontaneous low-frequency fluctuations (<0.1Hz) in the blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (MRI) signal have been shown to reflect neural synchrony between brain regions. This phenomenon of synchrony between brain regions is also called the "default network." The default network of spontaneous low-frequency fluctuations has been described in healthy volunteers during stimulusindependent thought and was found to be negatively correlated with regions activated during attentiondemanding tasks (Bluhm et al 2007). This condition was also formulated as two anti-correlated networks taskrelated and stimulus-independent; the stimulusindependent network was proposed to have a role in self monitoring while the task related network had the role of cognitive performance (Williamson 2007). Buckner and Carroll (2007) speculate that the default network is relevant to a set of processes by which past experiences are used adaptively to imagine perspectives and events beyond those that emerge from the immediate environment. Within this default mode network that is engaged during rest and disengaged during cognitive tasks Hampson et al (2007) investigated the posterior cingulate cortex and a medial frontal region incorporating portions of the medial frontal gyrus and ventral anterior cingulate cortex during a working memory task. They found that the two regions were functionally connected in both conditions. In addition, performance on the working memory task was positively correlated with the strength of this functional connection not only during the working memory task, but also at rest. Thus, it appears these regions are components of a network that may facilitate or monitor cognitive performance, rather than becoming disengaged during cognitive tasks. In addition, these data raise the possibility that the individual differences in coupling strength between these two regions at rest predict differences in cognitive abilities important for this working memory task (Hampson et al 2007). The biggest study to date of the default network in schizophrenia was performed by Garrity and colleagues (2007) on 22 patients and 21 controls. Healthy comparison subjects and patients had significant spatial differences in the default mode network, most notably in the frontal, anterior cingulate, and parahippocampal gyri. In addition, activity in patients in the medial frontal, temporal, and cingulate gyri correlated with severity of positive symptoms. The patients also showed significantly higher frequency fluctuations in the temporal evolution of the default mode. They conclude that schizophrenia is associated with altered temporal frequency and spatial location of the default mode network. They hypothesize that this network may be underor overmodulated by key regions, including the anterior and posterior cingulate cortex. In addition, the altered temporal fluctuations in patients may result from a change in the connectivity of these regions with other brain networks (Garrity et al 2007). A similar study by Bluhm et al (2007) with a slightly smaller sample (17 patients and controls) found that Healthy volunteers demonstrated correlation between spontaneous lowfrequency fluctuations of the BOLD signal in the posterior cingulate and fluctuations in the lateral parietal, medial prefrontal, and cerebellar regions, similar to previous reports. Schizophrenic patients had significantly less correlation between spontaneous slow activity in the posterior cingulate and that in the lateral parietal, medial prefrontal, and cerebellar regions. Connectivity of the posterior cingulate was found to vary with both positive and negative symptoms in schizophrenic patients. They indicate that these data suggest significant abnormalities in resting-state neural networks in schizophrenia (Bluhm et al 2007). Using a similar number of controls and patients Zhon et al (2007) found that the bilateral DLPFC showed reduced functional connectivities to the parietal lobe, posterior cingulate cortex, thalamus and striatum in FES patients. They also found enhanced functional connectivity between the left DLPFC and the left mid-posterior temporal lobe and the paralimbic regions in first-episode schizophrenia patients. They suggest that functional dysconnectivity associated with the DLPFC exists in schizophrenia during rest. Schizophrenia: A Disorder of Subcortical Adaptable Neural-Complexity Pacemaker 2007 © all rights reserved Avi Peled M.D. ‘NeuroAnalysis’ http://neuroanalysis.googlepages.com/home 4 WHICH BRAIN STRUCTURES ARE BEST CANDIDATES TO REGULATE AND CONTROL NEURAL COMPLEXITY IN THE BRAIN? In order to pinpoint the neuro-pathological origin of schizophrenia it is mandatory to discover what regulates neural complexity (i.e., connectivity balance) in the brain. A regulatory structure for neural complexity must have massive, distributed and parallel connectivity with most, if not all, brain formations. Since the brain cortex performs the higher mental functions then neuralcomplexity regulators would require massive connectivity spread to the cortex. Since neural complexity regulation will require response monitoring, then massive cortical afferents (inputs) would be required to feedback to the CN regulatory systems. Based on these intuitions, it seems that the basal ganglia with their striatum and thalamocortical connections are most suitable to regulate cortical neural complexity. The main structures of interest are the ‘striatum,’ the ‘globus pallidus’ (external segment GPe and internal segment GPi), the ‘subthalamic nucleus’ (STN), the ‘substantia nigra’ (divided into pars reticulata SNr and pars compacta SNc) and the thalamus itself. The whole system starts as a major output of the cortex, almost every part of the cortex, except for the primary olfactory, visual and auditory cortices, sends axons to the striatum. The origin of the connection is in the pyramidal neurons of layer V of the cortex. The corticostriatal connection is glutamatergic and excitatory. The corticostriate connection is the first in a chain of strong reduction in numbers between emitter and receiver neurons (Percheron et al. 1987), i.e. a numerical convergence exists. The effect of this is that if each striatocortical neurons has its own message, this will be mixed or compressed, in the input map. In primates, the striatum has 96% spiny neurons indicating its late evolutionary development, thus relevance to higher mental functions. Striatal neurons are activated by cortical stimulation. At rest, spiny neurons are left in a state of low excitability by two types of potassium conductance that hyperpolarize the cell. Striatal neurons need strong synchronized input from their excitatory cortical afferences in order to activate downstream formations. These are the striato-pallidonigral bundle: the two nuclei of the pallidum and the substantia nigra. Inhibitory neurons in the striatum send long axons to their targets in the pallidum and the substantia nigra. The pallidonigral set comprises the direct targets of the striatal axons: the two nuclei of the pallidum and the pars lateralis and pars reticulata of the "substantia" nigra". One character of this ensemble is given by the very dense striato-pallidonigral bundle. In addition to the striato-pallidal afference, the lateral pallidum receives a major connection from the subthalamic nucleus. It also receives dopaminegic afferences from the nigra compacta. Contrary to two other elements of the basal ganglia core, the lateral pallidum is not a source of output to the thalamus as it sends its axons essentially to other basal ganglia elements (intra systemic connections). The medial pallidum, though absolutely similar to the lateral, is phylogenetically younger, as it appears only in primates. The medial pallidum is a "fast-spiking pacemaker" with spontaneous discharges in awake monkeys at about 90 Hz (Mink and Thach, 1991), 70 to 80 for Fillion and Tremblay (1991). In opposition to that of the lateral pallidum, the activity is continuous (DeLong, 1971) devoid of long intervals of silence (DeLong, 1971). In addition to the massive striatopallidal connection, the medial pallidum receives a dopaminergic innervation from the nigra compacta. Contrary to the lateral pallidum, it is a major source of basal ganglia outputs. As mentioned above the substantia nigra is divided to pars compacta and pars reticulata The pars reticulata differs from the compacta by that it sends axons to the superior colliculus (Beckstead and Franckfurter, 1982, François et al. 1984). The border between the two basins is not clear cut but their difference in the participation of distinct subsystems is a sufficient reason for considering the two apart. The neurons that send axons to the superior colliculus have high discharge rates (80 to 100) (hence also a fast spiking pacemaker) and "the signal conveyed by the cells is a decrease in discharge rate" (Hikosaka and Wurtz, 1989). These neurons are involved in occular saccades Schizophrenia: A Disorder of Subcortical Adaptable Neural-Complexity Pacemaker 2007 © all rights reserved Avi Peled M.D. ‘NeuroAnalysis’ http://neuroanalysis.googlepages.com/home 5 The pars reticulata is most often considered as a single entity. It is another "fast-spiking pacemaker" (Surmeier et al. 2005) and shows responses that may be related to memory, attention or movement preparation" (Wicheman and Kliem, 2004) In addition to the massive striatopallidal connection, the nigra reticulata receives a dopamine innervation from the nigra compacta and glutamatergic axons from the pars parafascicularis of the central complex. One of the most important recent discoveries is that the basal ganglia machinery is not simply set in motion from the outside (from afferent inputs). It has indeed several "autonomous pacemakers", defined as sets of "neurons capable of periodic spiking in the absence of synaptic input" (Surmeier et al. 2005) i.e. able of producing an own activity. Among autonomous pacemakers, the pallidonigral ensemble belongs to the "fast-spiking pacemakers" "capable of discharges rates in excess of 200 Hz for sustained periods" (Surmeier et al., 2005). The regularity and frequency of the pacemaker is linked to cyclic nucleotide-gated channels (HCN2 and HCN1, Chan et al. 2004) present on the dendrites of pallidal neurons. Pacemakers are oscillatory systems they are indeed chaotic oscillators. In a recent paper in humans, (Rasouli et al. 2006) wrote that "robust fractal dynamics (could be) observed in single neurons...the neuronal dynamics of the internal segment of the globus pallidus are essentially a nonlinear and nonequilibrium process". Such systems were found relevant to a double need: regularity and adaptability functions. The pallidonigral pacemaker is modulated by striatopallidonigral inputs. As mentioned above, after the huge reduction in number of neurons between the cortex and the striatum the striatopallido-nigral connection show further reduction in the number of transmitting compared to receiving neurons. This represents a huge reduction in neuronal connections. The consecutive compression of maps cannot preserve finely distributed maps as in the case for instance of sensory systems. The very particular and contrasted geometry of the connection between striatal axons and pallidonigral dendrites offers particular conditions (the possibility for a very large number of combinations through local additions of simultaneous inputs to one tree or to several distant foci. Pallidum is precisely one cerebral place where there is a dramatic change between one afferent geometry and a completely different efferent one. The inmap, and outmap are totally different. This is an indication of the fundamental role of the pallidonigral set: the spatial reorganisation of information for a particular "function", which is predictably a particular reorganisation within the thalamus preparing a distribution to the cortex. It is predicted that this convergence of stirato-pallidonigral transmission may serve an ‘assessment’ of relationships among large spread cortical connectivity states, and via a particular reorganisation within the thalamus, prepares a neural-complexity-inducing efferent distribution to the cortex. Contrarily to the neurons of the pars reticulata-lateralis, dopaminergic neurons are "low-spiking pacemakers" (Surmeier et al. 2005), spiking at low frequency (0,2 to 10 Hz). Their activity is linked to reward and prediction of reward. Due to its widespread distribution, the dopaminergic system may regulate the basal ganglia system in many places. As indicated by its name, the subthalamic nucleus is located below the thalamus; dorsally to the substantia nigra and medial to the internal capsule. The subthalamic nucleus is lenticular in form and of homogeneous aspect. The subthalamic neurons are "fast-spiking pacemakers" (Surmeier et al. 2005) spiking at 80 to 90 Hz. The subthalamic nucleus receives its main afference from the lateral pallidum. Another afference comes from the cerebral cortex (glutamatergic), particularly from the motor cortex, subthalamic axons leave the nucleus dorsally, most are afferent neurons with multi-targets to the other elements of the core of the basal ganglia (Sato et al. 2000). The subthalamic nucleus and lateral pallidum are both fast-firing pacemakers (Surmeier et al.2005). Together they constitute the "central pacemaker of the basal ganglia" (Plenz and Kitai,1999) with synchronous bursts. The pallido-subthalamic connection is inhibitory; the subthalamo-pallidal is excitatory. They are coupled regulators or in other words, coupled autonomous oscillators. The lateral pallidum receives a lot of striatal axons, the subthalamic nucleus not. The subthalamic nucleus receives cortical axons, the pallidum not. The subsystem they make with their inputs and outputs corresponds to a classical systemic feedback circuit. It will be proposed below (Figure 1) that having converging striatal inputs, the globus pallidus monitors cortical integration while the subthalamic nucleus with its direct cortical afferents, monitors cortical segregation, Schizophrenia: A Disorder of Subcortical Adaptable Neural-Complexity Pacemaker 2007 © all rights reserved Avi Peled M.D. ‘NeuroAnalysis’ http://neuroanalysis.googlepages.com/home 6 coupled together they are in a potion to balance cortical segregation-integration equilibrium, thus optimizing cortical neural-complexity. Efferents from pallidum, subthalamic nucleus and substantia nigra (only the SNr) reach the thalamus and from there ascend distributing to the cortex via excitatory glutamatergic pathways. This completes the cortical-striatal-pallidal-subthalamic-nigral-thalamiccortical feedback and interconnected circuitry. ARE THE BASAL-GANGLIA FORMATIONS RELEVANT TO HIGHER MENTAL FUNCTIONS? If the basal-ganglia formation, as explained so far, are really relevant to the regulation of neural-complexity and dynamics of task-related (and default networks) brain activations, then research evidence should link these structures to higher-mental cognitive functions such as coherent adaptable conscious experience. As early as the beginning of the eighties Creutzfeld (1979) realized that in the neocortex various aspects of the world and of the physical and social relationships of the individum to the world are represented through thalamocortical projection systems. There is no unified representation of the world in any single cortical area. All neocortical outputs feed into action systems of the brain. The synthesis of the distributed cortical representations of the world is thus realized through the action elicited by their combination. The action systems of the midbrain-cerebellum and the basal ganglia feed back into neocortical areas (internal loops). The role of the basal ganglia is thus relevant to a unified conscious experience of the world. Exactly the experience that vanishes in schizophrenia patients replaced by disorganized delusional experience. During various states of vigilance, brain oscillations are grouped together through reciprocal connections between the neocortex and thalamus (Steriade 2000). During behavioral states associated with brain disconnection from the external world, the large-scale synchronization of low-frequency oscillations is accompanied by the inhibition of synaptic transmission through thalamocortical neurons. Sustained fast oscillations that characterize alert states are synchronized over restricted territories and are associated with discrete and differentiated patterns of conscious events. This description relates to increase of neural complexity were subsets have increasing statistical dependence (Tononi 2000). Recent studies have revealed that the brain as a whole is not affected to the same degree by anesthetics, but that specific brain regions (and particular cognitive processes mediated by these regions) are more sensitive to anesthesia and sedation than others. Inhibition of activity in multimodal association cortices (such as parietal and prefrontal association cortices) by sedative concentrations of anesthetics produces amnesia and attention deficits, whereas activity in unimodal cortices and in the thalamus remains largely unaffected by low doses of anesthetics (Heinke et al 2005). This shows the relevance of cortical-cortical connectivity for attention and memory. Activity in the midbrain reticular formation, thalamus, and unimodal cortices appears to be suppressed only by anesthetic concentrations causing unconsciousness. Besides those regional suppressive effects, anesthetics impair functional connections between neurons in distributed cortical and thalamocortical networks, indicating their relevance for emergence of consciousness. During behavioral quiescence, the neocortex generates spontaneous slow oscillations that consist of Up and Down states. Up states are short epochs of persistent activity that resemble the activated neocortex during arousal and cognition. Using thalamocortical slices, Rigas and colleagues (2007) found that the persistent cortical activity during spontaneous Up states effectively drives thalamocortical relay cells through corticothalamic connections. However, thalamic activity can also precede the onset of cortical Up states, which suggested a role of thalamic activity in triggering cortical Up states through thalamocortical connections. In support of this hypothesis, they found that cutting the connections between thalamus and cortex reduced the incidence of spontaneous Up states in the cortex. There is resemblance between the description of Up states and task-related networks anti-correlated with the default network (see above), thus providing support for Schizophrenia: A Disorder of Subcortical Adaptable Neural-Complexity Pacemaker 2007 © all rights reserved Avi Peled M.D. ‘NeuroAnalysis’ http://neuroanalysis.googlepages.com/home 7 involvement of thalamocortical involvement in the brain organizations transiting from non-task to task related networks. According to Huguenard and McCormick (2007) feedforward and feedback connections between cortex and thalamus reinforce the thalamic oscillatory activity into larger thalamocortical networks to generate sleep spindles and spike-wave discharge of generalized absence epilepsy. The degree of synchrony within the thalamic network seems to be crucial in determining whether normal (spindle) or pathological (spike-wave) oscillations occur. Ferrarelli and colleagues (2007) found deficit in sleep spindles in schizophrenia subjects and have related them to dysfunction in thalamicreticular and thalamocortical mechanisms in these patients. They propose that these findings could represent a biological marker of illness. ARE THE BASAL-GANGLIA FORMATIONS RELEVANT TO SCHIZOPHRENIA? To link neural-complexity disturbances to schizophrenia via the basal-ganglia formation there is a need to show alterations of basal-ganglia formations and basalganglia-related cognitive dysfunction in schizophrenia. Basal ganglia volumes in drug-naive first-episode schizophrenic patients before and after treatment were measured using high-resolution magnetic resonance imaging (MRI) scans. Compared with controls, absolute volumes of interest of caudate nucleus, nucleus accumbens, and putamen volumes were smaller in patients at baseline and increased after treatment. Additionally altered asymmetry in caudate volume in patients suggests intrinsic basal ganglia pathology in schizophrenia (Glenthoj et al 2007). The total volume and shape of several basal ganglia structures were compared in subjects with and without schizophrenia (Mamah et al 2007). Left and right volumes of the caudate, putamen and right globus pallidus volume were significantly increased in subjects with schizophrenia as compared to comparison subjects after total brain volume was included as a covariate. Significant differences in shape accompanied these volume changes. There were few significant correlations between volume or shape measures and either cognitive function or clinical symptoms. For example a positive correlation between an attention-vigilance cognitive dimension and the volume of the caudate and putamen, and a negative correlation between nucleus accumbens volume and delusions was found. Mamah and colleagues (2007) conclude that basal ganglia volumes relative to total brain volume were larger in schizophrenia subjects than healthy comparison subjects. While responding to the embedded sequence within the serial reaction time task schizophrenia patients did not activate frontal or parietal areas, but had greater activation in the right anterior cingulate, left globus pallidus and the right superior temporal gyrus (Zedkova et al 2006).Paucity of activity in bilateral frontal cortex, left parietal cortex and bilateral caudate nucleus was found in patients and may represent cerebral dysfunction associated with schizophrenia, whereas the hyperactivation of the right superior temporal gyrus, the right anterior cingulate cortex and the left globus pallidus may represent a compensatory cerebral action capable of facilitating near-normal task performance (Zedkova et al 2006). This description supports the idea of dynamic regulatory control from basal ganglia trying to compensate reorganizing cortical efforts to compute the task. Comparison of globus pallidus volume between neuroleptics-naive patients with schizophrenia and healthy controls using structural MRI found that the volume of the external segment of the globus pallidus was positively correlated with the severity of global symptoms, as measured by the scale for the assessment of negative and positive symptoms (Spinks et al 2005). Using SPM (Statistical Parametric Mapping) of brain PETs obtained in resting conditions from severely affected schizophrenia patients, increased activity in: globus pallidus, insular cortex, cuneus, claustrum, postcentral gyrus and pre-central gyrus; decreased activity in Schizophrenia: A Disorder of Subcortical Adaptable Neural-Complexity Pacemaker 2007 © all rights reserved Avi Peled M.D. ‘NeuroAnalysis’ http://neuroanalysis.googlepages.com/home 8 fusiform gyrus and superior temporal gyri, were demonstrated (Galeno et al 2004). These results permit correlation of negative symptomatology with abnormalities in the cortico-striato-pallido-thalamic neural circuit. According to Galeno and colleagues severity of negative symptoms is clearly correlated to abnormal left external pallidal activation, evidencing the relevance of this nucleus for cognitive, planning and social capabilities. PET data obtained from schizophrenic subjects and controls while performing cognitive tasks show a change in the functional interactions among distributed brain areas in schizophrenics, Tononi and Edelman (2000) raised the possibility that disruption of re-entrant interactions among cortical areas may contribute to the pathophysiology of schizophrenia. Their simulations show that an altered dynamics of corticothalamic and corticocortical re-entrant circuits may participate in this disturbance of cortical organization in schizophrenia. A MODEL FOR DISORDERED SUBCORTICAL ADAPTABLE NEURAL-COMPLEXITY PACEMAKER IN SCHIZOPHRENIA. If we summarize the relevant findings of the above minireview, we find that schizophrenia, with its homogeneous manifestations, can be attributed to a disorder of neural complexity in the brain. Potetnially neural complexity could be regulated by basal ganglia via a feedback network involving cortex-basal-thalamiccortical pathways In the basal ganglia the STN and GP are both fast-firing pacemakers (Surmeier et al. 2005). Together they constitute the "central pacemaker of the basal ganglia" (Plenz and Kitai, 1999) with synchronous bursts. The pallido-subthalamic connection is inhibitory; the subthalamo-pallidal is excitatory. They are coupled regulators or in other words, coupled autonomous oscillators. The GP receives a lot of striatal axons, the STN not. The STN receives cortical axons, the pallidum not (left part of figure 1). The subsystem they make with their inputs and outputs corresponds to a classical systemic feedback circuit. The neuronal dynamics of these oscillators are essentially a nonlinear and nonequilibrium process. Such systems were found relevant to a double need: regularity and adaptability functions. Schizophrenia: A Disorder of Subcortical Adaptable Neural-Complexity Pacemaker 2007 © all rights reserved Avi Peled M.D. ‘NeuroAnalysis’ http://neuroanalysis.googlepages.com/home