Dysfunction of the frontal-striatal-thalamic circuit is considered a pathogenesis of APS as well as schizophrenia (Mishara et al., 2016; Pantelis et al., 1997; Dandash et al., 2017; Howes et al., 2009a,b; Steullet et al., 2020). We previously reported a reduction in the longitudinal volume of the right putamen in the striatum in individuals with ARMS, most of whom were individuals with APS (Katagiri et al., 2019). As the putamen connects to the premotor cortex through frontal-striatal-thalamic subcircuit originating from the premotor cortex, we hypothesized that biological changes in the frontal-striatal-thalamic subcircuit derived from the premotor cortex in individuals with APS. In the current study, we employed a novel method, TractSeg, to analyse the FA values between ST-PREM and T-PREM and examined the effects of age, sex, handedness, and the use of antipsychotic medication on these FA values. Individuals with APS showed a reduction in FA values in the right T-PREM, suggesting the potential for biological alterations in a segment of the frontal-striatal-thalamic circuit, originating from the premotor cortex in APS.
4. 1. FA value changes in the frontal-striatal-thalamic circuit originate from the premotor cortex in APS
In the first step, logistic regression analysis with adjustments for potential confounding factors, such as age and sex, revealed a significantly lower FA value in the right T-PREM in the APS-W group than that in the Control-W group. In the second step, we excluded the influence of handedness and antipsychotic medication. Although the number of participants of both groups were relatively low, there were no significant differences in terms of sex and age between the groups. The logistic regression analysis revealed a significantly lower FA value in the right T-PREM in the APS-R-N group than in the Control-R group, consistent with results of the first step. These results suggest that there are biological changes of white matter fiber bundle, which connects and relays information among brain regions, between the right premotor cortex and thalamus in individuals with APS, regardless of sex, age, handedness, and medication.
Lesions of the premotor cortex cause apraxia (Martin, 2012), and dysfunction of the premotor cortex is supposed to underlie catatonia that is observed in several psychiatric diseases, including schizophrenia (Northoff, 2002; Walther et al., 2019). Additionally, the premotor cortex is a major part of the mirror neuron network (Gallese et al., 1996; Burns, 2006), and an aberrant mirror neuron network is related to affective echo-phenomena symptoms, ego-boundary disturbances, negative symptoms, social cognition impairments, and catatonia (Pridmore et al., 2008; Mehta et al., 2014). Recent studies suggest that pathological changes in the premotor cortex, the lowest layer of the lateral frontal cortex, underlie impairment of a broad range of the lateral frontal cortex, including the prefrontal cortex, which is in the upper layer (Abe and Hanakawa, 2009; Koechlin and Summerfield, 2007; Yeo et al., 2011; Fine and Hayden, 2022). While many studies have corroborated that impairment in the premotor cortex relates to schizophrenia, our report, indicating that the impairment of neural network originating from the premotor cortex underlie APS, provides a new perspective.
4.2. Reduction of the FA value of the T-PREM in APS
Few studies have investigated whether impairments in frontal-thalamus and frontal-striatum emerge simultaneously and within the same subcircuit in APS (Cho et al., 2016). In the present study, we comprehensively and simultaneously investigated the structural changes in the frontal-striatal-thalamic subcircuit that originate from the premotor cortex in individuals with APS. Wasserthal et al. (2020) reported FA value reduction in both ST-PREM and T-PREM using TractSeg in individuals with schizophrenia. However, in the present study, the alteration in the FA value was limited to T-PREM in APS, suggesting that ST-PREM impairment is not predominant in APS but emerges in schizophrenia (Walther et al., 2019). It has been long hypothesized that impairment of the thalamus-frontal cortex could secondly provoke an adverse hyperdopaminergic state in the striatum, resulting in the onset of schizophrenia (Davis et al., 1991; Steullet, 2020; Dorph-Petersen and Lewis, 2017; Meyer-Lindenberg et al., 2002; Shin et al., 2022). Therefore, it is possible that biological changes in the T-PREM rather than the ST-PREM may be related to the emergence of subthreshold psychotic symptoms in APS.
4.3. Pathophysiology of aberrant frontal-striatal-thalamic circuit
The longitudinal volume reduction of the right putamen observed in the study by Katagiri et al. (2019), coupled with the reduction of FA value of right T-PREM, suggests a biological change in the right side of a subcircuit of the frontal-striatal-thalamic circuit. This subcircuit, originating from the premotor cortex, is referred to as the “skeletomotor loop.” Previous animal studies have suggested that these loops not only control the motor movement of skeletal muscles but also integrates voluntary motor information with reward information via dopaminergic and other signals (Isomura et al., 2013). However, most previous studies on frontal-striatal-thalamic subcircuits were conducted on animals (Hunnicutt et al., 2016; Haber et al., 2009). Human studies have revealed a more complex and numerous sets of frontal-striatal-thalamic subcircuits, which contain features of multiple conventional subcircuits rather than corresponding precisely to just one (Cocchi et al., 2014; Draganski et al., 2008).
More recent evidence suggests that impairments relate to frontal-striatal-thalamic circuits underlie several psychotic disorders, such as depressive disorder, attention deficit hyperactivity disorder, substance misuse, as well as schizophrenia (Peters et al., 2016; Hong, 2023). Meanwhile, an increasing number of epidemiological studies have shown that APS, previously referred to as the “schizophrenia prodrome,” does not necessarily progress to schizophrenia exclusively, but to other several psychiatric disorders, such as depressive disorder, personality disorder and substance misuse. This has led to the proposal of the concept of transdiagnostic psychiatry (McGorry et al., 2018). Altogether, these findings raise the possibility that disruption of the frontal-striatal-thalamic circuit may be one of the biological bases of APS which then transitions to various diseases. Further follow-up studies, considering transdiagnostic psychiatry, are needed to clarify differences in the changes in the frontal-striatal-thalamic circuits during the transition from APS to various psychiatric disorders.
4.4. Limitations
Our study had certain limitations. First, the sample size was small, limiting the generalizability of our findings; additional large-scale studies with age- and sex-matched participants are warranted to further validate them. Moreover, our investigation was limited to the frontal-striatal-thalamic circuit derived from the premotor cortex. Previous studies on schizophrenia have repeatedly reported functional and structural changes in the prefrontal cortex rather than in the premotor cortex. Further studies are needed to investigate the relationship between the frontal-striatal-thalamic subcircuits derived from other frontal regions, such as the prefrontal cortex.