Different patterns of white matter microstructural alterations between psychotic and non-psychotic bipolar disorder.

This study aimed to investigate alterations in white matter (WM) microstructure in patients with psychotic and non-psychotic bipolar disorder (PBD and NPBD, respectively). We used 3T-magnetic resonance imaging to examine 29 PBD, 23 NPBD, and 65 healthy control (HC) subjects. Using tract-based spatial statistics for diffusion tensor imaging data, we compared fractional anisotropy (FA) and mean diffusion (MD) pairwise among the PBD, NPBD, and HC groups. We found several WM areas of decreased FA or increased MD in the PBD and NPBD groups compared to HC. PBD showed widespread FA decreases in the corpus callosum as well as the bilateral internal capsule and fornix. However, NPBD showed local FA decreases in a part of the corpus callosum body as well as in limited regions within the left cerebral hemisphere, including the anterior and posterior corona radiata and the cingulum. In addition, both PBD and NPBD shared widespread MD increases across the posterior corona radiata, cingulum, and sagittal stratum. These findings suggest that widespread WM microstructural alterations might be a common neuroanatomical characteristic of bipolar disorder, regardless of being psychotic or non-psychotic. Particularly, PBD might involve extensive inter-and intra-hemispheric WM connectivity disruptions.

1. Introduction

Bipolar disorder (BD) is a disabling neuropsychiatric illness characterized by recurrent manic and depressive episodes [1]. Since Kraepelin’s concept of manic-depressive insanity, BD had historically been considered a mood disorder along with major depressive disorder [2]. However, BD is highly heterogeneous in its clinical manifestations, often presenting schizophrenia-like psychotic symptoms as well as mood dysregulation [3]. Moreover, large-scale genetic studies have provided strong evidence for common genetic causes between BD and schizophrenia [4, 5], while neuroimaging studies have shown shared structural abnormalities between the two disorders [6]. Considering this evidence, the concept of a psychosis continuum linking BD and schizophrenia has been recently highlighted [7, 8].

A significant proportion of patients (approximately 70% of patients with BD type I) have experienced frank psychotic symptoms, such as hallucination and delusion, during their illness [9]. Particularly, compared to non-psychotic BD (NPBD), psychotic BD (PBD) is characterized by earlier onset, poorer treatment response, more relapse, and lower cognitive and social function, also common in patients with schizophrenia [10]. Thus, from the point of view of a psychosis continuum, PBD has been expected to be closer to psychotic disorders such as schizophrenia, whereas NPBD might be closer to affective disorders [11]. However, the degree of shared biological underpinnings between PBD and schizophrenia and the differences in neuroanatomical characteristics between PBD and NPBD remain unclear. Accordingly, exploring the neural substrates of the psychotic features in BD could improve our understanding of the neurobiological basis of the psychosis continuum clinical concept.

Diffusion tensor imaging (DTI) studies have revealed extensive white matter (WM) abnormalities not only in schizophrenia [12] but also in BD [13, 14]. Recent comparative studies evidenced an overlap of affected WM areas in BD and schizophrenia [15, 16]. However, few studies have investigated the neural substrates associated with psychotic features in BD, and little is known about the patterns of WM abnormalities in PBD and NPBD. Therefore, this study aimed to concurrently investigate WM alterations in PBD and NPBD using tract-based spatial statistical (TBSS) analyses and explore WM microstructural differences between the two BD subtypes. Particularly, we hypothesized that PBD might involve extensive WM abnormalities, possibly comparable to those of schizophrenia. This is in line with evidence of more extensive gray matter volume deficits in PBD than in NPBD [17, 18].

2. Materials and methods

2.1. Subjects

All subjects were recruited from outpatient clinics of the National Center for Mental Health, Seoul, Korea, and a psychologist interviewed them using the Mini-International Neuropsychiatric Interview (MINI) [19]. The diagnosis of BD was assessed through the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) criteria and confirmed by the clinical consensus of expert psychiatrists. Inclusion criteria were as follows: (1) age 20–50 years old; (2) duration of illness > 1 year; (3) no change in general clinical state and medication for ≥3 months prior to the time of assessment. We excluded patients with a concurrent axis I diagnosis according to the DSM-IV including schizophrenia, schizoaffective disorder, and alcohol use disorder, current or past neurological disease, any contraindication to MRI scan, or a physical condition that would render an MRI scan difficult to administer or interpret. According to the K module in the MINI [19], patients with lifetime experience of psychotic symptoms were assigned into the PBD group, while the others were assigned to the NPBD group. The healthy control (HC) group consisted of volunteers from the local community in the same age range and without history of psychiatric disorders. The same exclusion criteria for the patients was also applied to the HC group with regards to medical, neurological, and physical conditions. We assessed the patients’ overall psychopathology and functioning level using the 18-item Brief Psychiatric Rating Scale (BPRS-18) [20], Clinician-Rated Dimensions of Psychosis Symptom Severity (CRDPSS) [21], and WHO Disability Assessment Schedule 2.0 (WHODAS 2.0) [22].

We first selected 31 PBD, 31 NPBD, and 65 HC, who were recruited in order, except for three patients with BD who were excluded due to poor information or imaging. According to the MINI, among the patients, two PBD and eight NPBD were classified as BD type II, and the remaining were classified as BD type I. In addition, none of the patients had taken any substance that could induce psychosis. For subject homogeneity, we included only 52 patients with BD type I. Finally, 29 PBD, 23 NPBD, and 65 HC were included in this study.

This study was approved by the Institutional Review Board of the National Center for Mental Health (IRB approval number: 116271-2017-26), and written informed consent obtained from all subjects.

2.2. MRI data acquisition

MRI data were acquired using a 3-Tesla MRI scanner (Ingenia CX; Philips, Erlangen) equipped with a 32-channel head coil at National Center for Mental Health. A diffusion-weighted image was acquired using single-shot echo-planar imaging sequence with the following parameters: acquisition matrix = 128 × 128, voxel size = 1.75 × 1.75 × 2 mm3, axial slices = 72, FOV = 224 × 224 mm2, TE = 90 ms, TR = 9000 ms, flip angle = 90°, slice gap = 0 mm, b-value = 0 and 1000 s/mm2, and diffusion sensitive gradient direction = 64. The baseline image without weighting was used [0, 0, 0].

2.3. DTI data processing

DTI processing was performed using the FMRIB Software Library (FSL) (https://fsl.fmrib.ox.ac.uk/fsl/fslwiki), ver. 6.0. First, motion artifacts and eddy current distortions through affine registration were corrected by considering the B0 volume as a reference using FSL’s Diffusion Toolbox. Then, the diffusion-weighted images were skull stripped using the Brain Extraction Tool within the FSL. Fractional anisotropy (FA) and mean diffusivity (MD) images were obtained from the tensors’ eigenvalues using the DTIFIT program in the FSL. Next, voxel-wise statistical analysis of FA and MD images was performed using the TBSS pipeline [23]. FA is the most widely used scalar in DTI, a measure indicating the overall directionality of water diffusion, higher in organized WM tracts and lower in disorganized fibers [24]. Axon damage or demyelination results in water motion being more isotropic, which may manifest in low FA values. MD describes the rotationally invariant magnitude of water diffusion within brain tissue, independent of tissue directionality [25]. MD is a non-specific, albeit sensitive, measure that can be affected by any disease process affecting the barriers restricting water movement and is usually higher in damaged tissue with edema or necrosis, for instance. FA images were aligned into the standard space (FMRIB58_FA, 1 × 1 × 1 mm MNI 152 space) using the nonlinear registration tool (FNIRT). Afterwards, a mean FA image was created and threshold by an FA value of 0.2 to exclude peripheral tracts and GM regions. Each subject’s aligned FA images were then projected onto the skeleton by filling it with the highest FA values from the nearest relevant center of the fiber tracts. The same transformation and warped-field were applied to MD images.

2.4. Statistical analyses

TBSS analyses were performed using the FSL toolbox (randomize). Pairwise comparisons of FA and MD values were performed at the voxel-level for PBD vs. NPBD, PBD vs. HC, NPBD vs. HC, and BD (PBD + NPBD) vs. HC using a generalized linear model. We adjusted for age, sex, education, and handedness as covariates to compare DTI parameters. To adjust the voxel-wise multiple comparisons, we adopted the family-wise error (FWE) approach. Significance thresholding for the TBSS analysis was determined using 10,000 permutations and threshold-free cluster-enhancement with the 2D parameter settings [26]. Thereafter, to adjust the number of comparisons of two DTI parameters (FA and MD) between four group pairs (PBD vs. NPBD, PBD vs. HC, NPBD vs. HC, and BD vs. HC), we applied the Bonferroni correction to the FWE-corrected P-value. Thus, the threshold of statistical significance for the TBSS analysis was conservatively set at an FWE-corrected P-value < 0.0062 (0.05 / 8) with a cluster size > 100 mm3.

In addition to group comparisons, we conducted exploratory analyses to investigate the association of DTI parameters with severity of psychopathology and functioning level, represented by the BPRS-18 and WHODAS 2.0 total scores, respectively, in each PBD, NPBD, and BD group. For these analyses, we adjusted for sex, age, education, handedness, and chlorpromazine-equivalent dose of antipsychotic drug, applying an FWE-corrected P-value < 0.05.

For comparisons of demographic and clinical data, a P-value < 0.05 was considered statistically significant.

3. Results

3.1. Demographic and clinical characteristics

Table 1 summarizes the demographic and clinical characteristics of the PBD, NPBD, and HC groups. There were no significant differences in age (F = 0.34, P = 0.711), sex (χ2 = 4.36, P = 0.113), level of education (F = 2.55, P = 0.083), and handedness (χ2 = 0.25, P = 0.884) among the three groups, as well as in the duration of illness (t = 1.05, P = 0.298) and chlorpromazine-equivalent dose of antipsychotic drugs (t = 1.70, P = 0.096) between the PBD and NPBD groups. According to the BPRS-18 and WHODAS 2.0 scores, PBD and NPBD were clinically stable without overall vivid psychotic symptoms and their social functioning was also well preserved at the time of assessments. In particular, the scores of CRDPSS domains showed that most PBD and NPBD had been experiencing less than a mild level of manic or depressive symptoms. There were also no significant differences in severity of psychopathology and functioning level between the PBD and NPBD groups. In addition, there were no significant differences in age (t = 0.83, P = 0.409), sex (χ2 = 3.94, P = 0.063), and handedness (χ2 = 0.21, P = 0.771) between the BD group and HC. However, the level of education was significantly lower in the BD group than in HC (t = -2.18, P = 0.032).

Table 1

Demographic and clinical characteristics.

Variablesa	Psychotic bipolar disorder	Non-psychotic bipolar disorder	Healthy control	Statisticsb
	(N = 29)	(N = 23)	(N = 65)
Age, y	35.90 ± 7.33	35.78 ± 8.93	34.52 ± 8.93	F = 0.34, P = 0.711
Sex (male / female), n	19 / 10	13 / 10	28 / 37	χ2 = 4.36, P = 0.113
Education, y	14.14 ± 1.77	13.61 ± 1.88	14.78 ± 2.55	F = 2.55, P = 0.083
Handedness (right / left), n	26 / 3	21 / 2	57 / 8	χ2 = 0.25, P = 0.884
Duration of illness, y	13.71 ± 8.15	11.65 ± 7.31	-	t = 1.05, P = 0.298
Antipsychotics, n	29	22	-	χ2 = 1.29, P = 0.442
Chlorpromazine-equivalent dose, mg	516.25 ± 315.08	370.23 ± 301.52	-	t = 1.70, P = 0.096
Mood stabilizers, n	20	20	-	χ2 = 2.34, P = 0.188
valproate / lithium / lamotrigine, n	14 / 7 / 3	14 / 7 / 1	-	-
Antidepressants, n	1	4	-	χ2 = 2.87, P = 0.157
BPRS-18 total score	27.24 ± 7.49	27.74 ± 8.17	-	t = -0.23, P = 0.820
BPRS-18 subscale scoresc
Affect	7.76 ± 3.25	8.09 ± 3.94	-	t = -0.33, P = 0.743
Positive symptoms	5.72 ± 3.07	5.48 ± 2.04	-	t = 0.33, P = 0.743
Negative symptoms	5.21 ± 2.16	4.78 ± 2.43	-	t = 0.66, P = 0.509
Resistance	3.66 ± 1.26	4.43 ± 1.93	-	t = -1.68, P = 0.102
Activation	3.90 ± 1.40	4.09 ± 1.50	-	t = -0.47, P = 0.639
CRDPSS total score	3.66 ± 3.24	4.35 ± 3.76	-	t = -0.71, P = 0.479
CRDPSS domain scores
Hallucination	0 (0–0)	0 (0–0)	-	U = 324.50, P = 0.682
Delusion	0 (0–0)	0 (0–1)	-	U = 292.00, P = 0.313
Disorganized speech	0 (0–0)	0 (0–1)	-	U = 306.00, P = 0.477
Abnormal psychomotor behavior	0 (0–0)	0 (0–0)	-	U = 316.00, P = 0.425
Negative symptom	1 (0–2)	0 (0–1)	-	U = 305.00, P = 0.566
Impaired cognition	1 (0–1)	1 (0–2)	-	U = 261.50, P = 0.155
Depression	1 (0–2)	1 (0–2)	-	U = 306.50, P = 0.593
Mania	0 (0–1)	0 (0–1)	-	U = 325.00, P = 0.850
WHODAS 2.0 total score	10.01 ± 7.67	11.61 ± 7.45	-	t = -0.75, P = 0.454
WHODAS 2.0 domain scores
Cognition	10.78 ± 11.44	12.86 ± 10.65	-	t = -0.68, P = 0.500
Mobility	1.21 ± 3.18	3.04 ± 5.98	-	t = -1.42, P = 0.161
Self-care	2.59 ± 3.92	2.72 ± 4.55	-	t = -0.11, P = 0.912
Getting along	21.38 ± 17.57	21.96 ± 16.22	-	t = -0.12, P = 0.904
Life activities	6.47 ± 7.33	9.65 ± 8.53	-	t = -1.45, P = 0.154
Participation	17.67 ± 12.81	19.43 ± 10.23	-	t = -0.54, P = 0.595

a Data are shown as mean ± standard deviation, number, or median (interquartile range).

b ANOVA, Fisher exact test, independent t test, or Mann-Whitney test.

c Factor structures proposed by Shafer (2005) [27].

Abbreviations: BPRS-18, 18-item Brief Psychiatric Rating Scale; CRDPSS, Clinician-Rated Dimensions of Psychosis Symptom Severity; WHODAS 2.0, WHO Disability Assessment Schedule 2.0.

3.2. FA and MD comparison between groups

Comparisons of FA between PBD and NPBD revealed no significant differences. However, compared with HC, PBD showed widespread FA decreases in the body and splenium of the corpus callosum as well as in the bilateral internal capsule and fornix (Fig 1 and Table 2). NPBD showed local FA decreases in a part of the corpus callosum body and limited regions within the left cerebral hemisphere, including the anterior and posterior corona radiata and cingulum.

Fig 1

Tract-based spatial statistics analyses identified white matter areas affected by a significant decrease in fractional anisotropy (FA) in patients with psychotic bipolar disorder (PBD) (N = 29), non-psychotic bipolar disorder (NPBD) (N = 23), and bipolar disorder (BD) (N = 52) in comparisons with healthy controls (HCs) (N = 65).

Family-wise error corrected P < 0.0062.

Table 2

Comparisons of fractional anisotropy among PBD (N = 29), NPBD (N = 23), BD (PBD + NPBD) (N = 52), and HC (N = 65) groups.

Anatomical region	Side	T max	Peak coordinates (MNI)			Cluster size
			x	y	z	(mm3)
PBD vs. HC
Body of corpus callosum		6.873	-10	17	22	30,764
Retrolenticular part of internal capsule	L	4.707	-29	-24	5	1787
Fornix (column and body)		4.300	0	6	6	625
Splenium of corpus callosum		3.960	-2	-34	14	355
Anterior limb of internal capsule	R	2.728	12	5	4	120
NPBD vs. HC
Body of corpus callosum		5.447	-10	19	21	4291
Posterior corona radiata	L	5.532	-17	-53	31	579
Anterior corona radiata	L	4.047	-26	33	6	456
Cingulum	L	5.085	-7	-10	35	346
BD vs. HC
Body of corpus callosum		6.807	-9	17	22	25,656
Posterior thalamic radiation	R	4.880	32	-64	1	3908
Retrolenticular part of internal capsule	L	4.441	-29	-22	3	1451
Posterior corona radiata	R	3.598	37	-57	23	423
Splenium of corpus callosum		3.759	-7	-39	15	365
Sagittal stratum	L	3.813	-49	-24	-17	286
Superior longitudinal fasciculus	R	3.030	42	-43	9	165
Posterior corona radiata	L	3.618	-26	-33	25	115

a Family-wise error corrected P < 0.0062

Note that all MNI coordinates of maximum t values are selected in the significant region.

Abbreviations: PBD, Psychotic Bipolar Disorder; NPBD, Non-Psychotic Bipolar Disorder; BD, Bipolar Disorder; HC, Healthy Control; MNI, Montreal Neurological Institute; L, Left; R, Right.

Compared with HC, all patients with BD also showed widespread FA decreases in the right posterior thalamic radiation, right posterior corona radiata, and left sagittal stratum as well as the corpus callosum, left retrolenticular part of the internal capsule, and the left posterior corona radiata, which were also affected in PBD and NPBD (Fig 1 and Table 2).

Comparisons of MD between PBD and NPBD revealed no significant differences. However, compared with HC, PBD and NPBD showed a widespread MD increase mainly in the right superior corona radiate (Fig 2 and Table 3). In addition, PBD showed MD increases across the left posterior corona radiata, left posterior thalamic radiation, left cingulum, left crus of fornix, and bilateral sagittal stratum. NPBD also showed MD increases across the right retrolenticular part of internal capsule, left superior longitudinal fasciculus, bilateral cingulum, and left sagittal stratum.

Fig 2

Tract-based spatial statistics analyses identified white matter areas affected by a significant increase in mean diffusivity (MD) in patients with psychotic bipolar disorder (PBD) (N = 29), non-psychotic bipolar disorder (NPBD) (N = 23), and bipolar disorder (BD) (N = 52) in comparisons with healthy controls (HCs) (N = 65).

Family-wise error corrected P < 0.0062.

Table 3

Comparisons of mean diffusivity among PBD (N = 29), NPBD (N = 23), BD (PBD + NPBD) (N = 52), and HC (N = 65) groups.

Anatomical region	Side	T max	Peak coordinates (MNI)			Cluster size
			x	y	Z	(mm3)
PBD vs. HC
Superior corona radiata	R	6.704	28	7	27	35,366
Posterior corona radiata	L	5.010	-17	-51	32	604
Posterior thalamic radiation	L	3.911	-36	-52	11	429
Cingulum	L	5.045	-8	-5	33	416
Fornix (crus)	L	5.079	-34	-9	-16	366
Sagittal stratum	R	4.019	41	-38	-10	198
Sagittal stratum	L	3.373	-40	-42	-7	114
Anterior limb of internal capsule	L	3.649	-22	-4	16	107
Retrolenticular part of internal capsule	L	4.019	-29	-23	-1	103
NPBD vs. HC
Superior corona radiata	R	6.222	28	6	29	27,510
Retrolenticular part of internal capsule	R	5.217	41	-29	3	912
Superior longitudinal fasciculus	L	5.466	-44	-1	27	748
Cingulum	L	4.705	-11	-48	23	377
Sagittal stratum	L	3.723	-40	-29	-16	270
External capsule	R	4.480	32	-2	12	169
Cingulum	R	4.436	9	16	28	141
Fornix (crus)	R	5.250	11	-47	25	107
BD vs. HC
Superior corona radiata	R	7.465	28	7	28	40,579
Fornix (column and body)		5.579	3	-11	15	783
Cingulum	L	4.893	-8	-5	33	489
Posterior thalamic radiation	R	4.394	36	-56	-6	339
Retrolenticular part of internal capsule	L	3.189	-29	-23	-1	186
External capsule	R	4.602	35	-5	-1	143
External capsule	L	3.453	-31	-16	12	129
Superior corona radiata	L	3.216	-27	-7	20	102

a Family-wise error corrected P < 0.0062

Note that all MNI coordinates of maximum t values are selected in the significant region.

Abbreviations: PBD, Psychotic Bipolar Disorder; NPBD, Non-Psychotic Bipolar Disorder; BD, Bipolar Disorder; HC, Healthy Control; MNI, Montreal Neurological Institute; L, Left; R, Right.

Compared with HC, all patients with BD also showed widespread MD increases in the column and body of the fornix and right posterior thalamic radiation as well as in the right superior corona radiata and left cingulum, which were also affected in PBD and NPBD (Fig 2 and Table 3).

In addition, we found no significant association between DTI parameters and continuous clinical variables, including the BPRS-18 and WHODAS 2.0 total scores, in all patients with BD as well as in PBD and NPBD.

4. Discussion

This study investigated WM microstructural alterations in PBD and NPBD using TBSS analyses. We observed no significant difference in diffusion measures between PBD and NPBD. However, in comparison with HC, patients with PBD and NPBD exhibited widespread alterations in the WM microstructure, with possible differences between the two BD subtypes. PBD showed widespread decreases in FA, particularly in the body and splenium of the corpus callosum, as well as in the bilateral internal capsule and fornix. Conversely, NPBD showed local decreases in FA only in a part of the corpus callosum body and some WM areas within the left cerebral hemisphere. In contrast to decreased FA, both PBD and NPBD showed widespread increases in MD across the whole-brain WM skeleton.

4.1. Comparisons of diffusion measures between PBD and NPBD

In this study, we first compared diffusional measures between PBD and NPBD to explore WM substrates that might be unique to PBD contributing to psychotic features and poorer clinical course. However, there was no significant difference in FA and MD between PBD and NPBD. Similar to our finding, recent studies have shown minimal differences in DTI parameters between PBD and NPBD. Ji et al. found a significant FA difference between the two BD subtypes only in the left uncinate fasciculus [28]. Furthermore, Brown et al. could not find any difference in diffusion measures between PBD and NPBD [29]. The sample size of these studies, and our study, might have been too small to detect subtle differences between the two BD subtypes. In addition, these studies revealed significant diffusional alterations in patients with PBD and NPBD compared with HC [28, 29]. These microstructural alterations have mainly been interpreted as the WM pathology associated with affective disturbance in BD. However, we supposed that the distribution pattern of WM abnormalities might be different between PBD and NPBD, which might be a neuroanatomical characteristic underlying the different clinical manifestations between the two BD subtypes.

4.2. Widespread compromise of WM integrity and connectivity in PBD

In the present study, compared with HC, PBD showed a pronounced FA decrease in the corpus callosum, with widespread FA decreases across the internal capsule and fornix. PBD also showed widespread WM areas affected by MD increase across the right superior corona radiata, left posterior thalamic radiation, and left cingulum. Considering that either a decrease in FA or an increase in MD reflects disruptions to water diffusion coherence due to axonal degradation, demyelination, or neurodegeneration [25], these findings suggest that PBD might involve widespread alterations in the WM microstructure across cortico-cortical, cortico-limbic, thalamocortical, and callosal connections. These patterns of widespread WM abnormalities are also largely in agreement with the results of previously published large DTI analyses and meta-analyses of schizophrenia, which provided evidence of extensive disruptions of WM integrity and connectivity across frontotemporal, frontosubcortical, and callosal networks in patients of schizophrenia [30-32]. In particular, given that the corpus callosum contains axon fibers connecting the bilateral frontal cortices and that WM integrity is related to cognitive performance in various domains including sustained attention, processing speed, and problem solving abilities [33], extensive disruptions of WM connectivity in the corpus callosum might be the underlying mechanism of the more severe cognitive decline in PBD compared to that in NPBD [10]. Therefore, this study’s findings suggest that extensive WM integrity and connectivity disruptions might be the neural substrates underlying the clinical manifestations of PBD, such as concurrent psychotic features, cognitive impairment, and poor clinical outcome, overlapping with those of patients with schizophrenia, largely in line with our prior hypothesis that PBD might be close to schizophrenia on a psychosis continuum.

4.3. Local disruptions of WM connectivity along with widespread microstructural alterations in NPBD

We observed that, compared with HC, NPBD showed local decreases in FA in a part of the corpus callosum and some WM areas within the left cerebral hemisphere, including the anterior and posterior corona radiata and cingulum. However, the increase in MD was more widely distributed across both cerebral hemispheres, including the right superior corona radiata, right retrolenticular part of internal capsule, left superior longitudinal fasciculus, and left cingulum, which partially overlapped with the affected WM areas in PBD. FA measures tract directional coherence, and a decrease in FA indicates impaired WM connectivity [34]. However, MD is directionally averaged and consequently less influenced by directional coherence [35]. Thus, our findings suggest that NPBD might involve widespread WM microstructural alterations, but WM connectivity might be compromised only in a part of the corpus callosum body and limited WM areas within the left cerebral hemisphere. In addition, major depression has been reportedly associated with microstructural alterations in the affected WM areas in NPBD in this study, particularly, the corpus callosum and corona radiata [36, 37]. There is also evidence of brain asymmetry, such as left hemispheric hypo-activation or right hemispheric hyper-activation, in patients with depression or mania [38-40]. In this respect, impairments of inter- and intra-hemispheric connectivity within the left cerebral hemisphere could be interpreted as a possible neural mechanism underlying the affective symptoms in patients with BD. Further studies to investigate hemispheric differences in the pathophysiology of BD are warranted.

4.4. Widespread compromise of WM integrity in BD

In this study, comparisons of HC with total BD patients, including both PBD and NPBD, showed widespread alterations of DTI parameters in major WM tracts. This is largely consistent with the results of previous DTI studies reporting extensive impairment of WM integrity across the cortico-cortical, cortico-subcortical, and callosal networks in BD [13, 14]. Thus, these findings implicate BD as a brain network disorder involving widespread compromise of WM integrity. However, it has not been elucidated whether the disruptions of WM integrity are common to both PBD and NPBD or whether they are more pronounced in PBD. Given that most patients with BD experience psychotic features in their lifetime [9], previous BD studies might have included a high proportion of patients with PBD. Thus, we supposed that the extensive WM integrity impairment reported in the previous studies might have been associated with psychotic features in patients with BD as well as their affective disturbance. In this study, comparisons between patients with PBD and HC revealed FA decrease, widely distributed across both cerebral hemispheres and a large portion of the corpus callosum. In contrast, in NPBD, this decrease was limited only to local regions within the left cerebral hemisphere and corpus callosum body. Considering that a decrease in FA reflects decreased axonal connectivity [25, 34], these findings suggest that PBD might involve extensive disruptions of inter-and intra-hemispheric WM connectivity, contributing to its psychotic features and poor clinical outcomes. However, in contrast to FA, PBD and NPBD shared increases in MD across both cerebral hemispheres, reflecting widespread alterations in axonal water diffusion in both BD subtypes. Therefore, the patterns of WM microstructural alterations in PBD and NPBD in this study should be interpreted carefully. In addition, future studies need to investigate the neurobiological substrates underlying BD by dividing the patients according to the presence or absence of psychotic features.

4.5. Limitations

This study has some methodological limitations. First, study participants had a wide range of illness duration and had received various types and doses of antipsychotics, mood stabilizers, or antidepressants. Considering the evidence on the effects of aging, disease severity, and medications on WM integrity [41], we cannot exclude the possibility that subject heterogeneity might have influenced the results. Second, the sample size for the PBD and NPBD groups was small and was not matched; therefore, the statistical power may not be sufficient to detect differences in the direct comparison of the two groups. Third, we could not find any significant association between illness severity and diffusion measures, probably because most patients were clinically stable with less than a mild level of mood symptoms and functional impairment.

4.6 Conclusion

In conclusion, widespread alterations in the WM microstructure might be a common neuroanatomical characteristic of BD, regardless of PBD and NPBD. In particular, PBD might involve extensive disruptions of inter- and intra-hemispheric WM connectivity, suggesting that psychotic features in patients with BD might be attributed to severe impairment in brain networks. Furthermore, these findings could improve our understanding of the pathophysiological basis underlying the clinical and neurobiological continuum of psychosis. Future studies to investigate the underlying neural mechanism of psychotic features in patients with BD are warranted.

11 Nov 2021

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: No

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: No

Reviewer #2: No

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: No

Reviewer #2: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

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Reviewer #1: Yes

Reviewer #2: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Interesting study.

Comments, please:

1- present first the analyses by comparing BD vs HC before separating them for psychosis;

2- correct the multiple comparisons for post-hoc test (Bonferroni, etc);

3- implement the analyses including ONLY right-handed BD type I patients (therefore excluding BD type II and left-handers);

4-the impact of sociodemographic (age, gender, handedness) and clinical variables (duration of illness, CPZ-equivalents, BPRS) on DTI measures has to be performed and shown ---> -correlations or regression should be considered;

5- BPRS can be shown in sub-clusters/sub-scales, including psychotic symptoms as well as depression-anxiety, and mania; please report them in the related table;

6- I see there are some BD type II in the PBD patients, this is strange since BD type II in general should not have any history of psychosis, how come?;

7- in general I suggest to exclude BD type II from the study and all the analyses

Reviewer #2: In this manuscript, Lee et al. conducted a DWI study that compared FA and MD measures in individuals with bipolar disorder with psychotic features, those without psychotic features and healthy controls (HC). They concluded that both patient populations significantly differed from HC in both measures, but disruption of white matter (WM) integrity was greater in PBD.

While the question of understanding the differences between the psychotic and non-psychotic BD is critically important, I am not sure that the study adequately addressed this question. First, the acquisition parameters were sub-optimal with single-shell acquisition and b=600. I am surprised that the authors were able to get some significant differences given this low b-value.

Second, considering that there were 6 contrasts of interest, the p-values had to be corrected for 6 contrasts (0.05/6=0.00833), so p<0.01 is not conservative (as the authors stated), but rather not sufficient. This correction had to be applied after the data were corrected for multiple comparisons using TFCE (or other methods). Unfortunately, the authors do not specify whether that was the case.

Third, the group comparison based on the % of voxels showing the differences with HC does not have any statistical basis and cannot serve as evidence for "disruptions of intra- and inter- hemispheric WM connectivity, in terms of FA, seem more pronounced in PBD compared to NPBD". From this perspective, the conclusions are highly overstated.

One strategy to analyze these data could 1) to compare all BD vs. HC to identify the regions affected by the disorder, and 2) use the identified regions as a mask to further compare PBD vs. NPBD.

The inclusion of BD-II to the data analyses is of concern due to a greater number of those participants in NPBD group. BD-II is even more versatile disorder than BD-I and can dilute the effects. Two resent reports on white matter in BD-II support this view by reporting different results despite using similar populations (Manelis et a., 2021 DOI: 10.1038/s41598-021-87069-2; Mak et al., 2021 DOI: 10.1038/s41598-021-81355-9). I suggest re-running the analyses using only BD-I patients.

Minor concerns include:

- please confirm that participants who experienced psychotic symptoms due to substance use were excluded

- please add the information about the data quality assurance and how many participants were removed due to poor data quality, etc.

- please add the information about whether the acquisition was multi-band and in what direction the data were collected

- please add citations for all FSL tools you used in the study

- please correct the p-values as suggested above

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Reviewer #1: Yes: Paolo Brambilla

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

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15 Dec 2021

Response to the Reviewers' comments:

Reviewer #1: Interesting study.

- Response: We would like to thank the reviewer #1 for their careful and thoughtful suggestions.

Comments, please:

1- present first the analyses by comparing BD vs HC before separating them for psychosis;

- Response: Following the reviewer’s recommendation, we compared HCs and BD patients, combining PBD and NPBD. We have described the new findings and rewritten our discussion in sections 3.2 and 4.4 (highlighted, page 11 line 146–149, page 13 line 164‒167, page 18 line 261–275) as well as tables and figures.

2- correct the multiple comparisons for post-hoc test (Bonferroni, etc);

- Response: We have described in more detail the Bonferroni correction in the FWE-corrected P-value to adjust for the number of multiple comparisons in section 2.4 (highlighted, page 7 line 103–108).

3- implement the analyses including ONLY right-handed BD type I patients (therefore excluding BD type II and left-handers);

- Response: Following the reviewer’s recommendation, we re-analyzed only patients with BD type I, including 29 patients PBD and 23 NPBD. However, we had to include right-handed subjects in the study due to a significant loss of sample size when excluding right-handed ones. Instead, we statistically adjusted for handedness as a covariate to compare DTI parameters between groups. We have added more clear descriptions on the study population in sections 2.1 and 2.4 (highlighted, page 5 line 55–60, page 7 line 97–100).

4-the impact of sociodemographic (age, gender, handedness) and clinical variables (duration of illness, CPZ-equivalents, BPRS) on DTI measures has to be performed and shown ---> -correlations or regression should be considered;

- Response: We adjusted for age, sex, education, and handedness as covariates to compare DTI parameters between group pairs (PBD vs. HC, NPBD vs. HC, PBD + NPBD vs. HC, and PBD vs. NPBD). However, we did not adjust for clinical variables in group comparisons because clinical variables were not available in HC (highlighted, page 7 line 97–100).

As shown in section 3.1, there were no significant differences in age, sex, level of education, and handedness between patient groups and HC or in the duration of illness, medication, symptom severity, and social functioning between the PBD and NPBD groups (highlighted, page 8 line 121–125).

In addition to group comparisons, we examined the association of DTI parameters with clinical variables including BPRS-18 and WHODAS 2.0 total scores in each PBD and NPBD group, adjusting for the chlorpromazine equivalent dose of antipsychotic drug as well as other demographic characteristics. However, we did not find any significant association with clinical variables (highlighted, page 7 line 109–114, page 14 line 175–177, page 19 line 284–287).

Nevertheless, we agree that this study could not exclude the possibility that the demographic and clinical characteristics might have influenced the results and have mentioned this point in the Limitations section (highlighted, page 19 line 280–282).

5- BPRS can be shown in sub-clusters/sub-scales, including psychotic symptoms as well as depression-anxiety, and mania; please report them in the related table;

- Response: Following the reviewer’s recommendation, we have presented the BPRS-18 subscale scores; the ‘affect,’ ‘positive symptoms,’ ‘negative symptoms,’ ‘resistance,’ and ‘activation’ proposed by Shafer (2005); as well as the WHODAS 2.0 domain scores in Table 1.

6- I see there are some BD type II in the PBD patients, this is strange since BD type II in general should not have any history of psychosis, how come?;

- Response: Using the Mini-International Neuropsychiatric Interview (MINI), we determined that eight patients with BD type II had experienced frank psychotic symptoms mainly during their depressive episode and classified them into the PBD group. According to previous studies of psychopathology on patients with BD, a significant proportion (approximately 20–40%) of patients with BP type II have experienced psychotic symptoms such as hallucination and delusion during their illness. However, following the reviewer’s recommendation, to maintain subject homogeneity, we re-analyzed only patients with BD type I (highlighted, page 5 line 55–60).

7- in general I suggest to exclude BD type II from the study and all the analyses

- Response: Following the reviewer’s recommendation, we re-analyzed only patients with BD type I, including 29 PBD and 23 NPBD (highlighted, page 5 line 55–60). Accordingly, we have rewritten our results and discussion in the Results and Discussion section.

Reviewer #2: In this manuscript, Lee et al. conducted a DWI study that compared FA and MD measures in individuals with bipolar disorder with psychotic features, those without psychotic features and healthy controls (HC). They concluded that both patient populations significantly differed from HC in both measures, but disruption of white matter (WM) integrity was greater in PBD.

- Response: We would like to thank reviewer #2 for their careful and thoughtful suggestions.

While the question of understanding the differences between the psychotic and non-psychotic BD is critically important, I am not sure that the study adequately addressed this question. First, the acquisition parameters were sub-optimal with single-shell acquisition and b=600. I am surprised that the authors were able to get some significant differences given this low b-value.

- Response: We realized that the first submitted manuscript had some errors in the description on DTI acquisition parameters. In fact, we employed a b-value of 1000 s/mm2 in this study, not a value of 600 s/mm2. We have corrected this error in section 2.2 ‘MRI data acquisition’ (highlighted, page 5 line 68–71).

Second, considering that there were 6 contrasts of interest, the p-values had to be corrected for 6 contrasts (0.05/6=0.00833), so p<0.01 is not conservative (as the authors stated), but rather not sufficient. This correction had to be applied after the data were corrected for multiple comparisons using TFCE (or other methods). Unfortunately, the authors do not specify whether that was the case.

- Response: We have described in more detail the FWE correction for the voxel-wise multiple testing and the additional Bonferroni correction in the FWE-corrected P-value after adjusting the number of comparisons of two DTI parameters (FA and MD) between four group pairs (PBD vs. HC, NPBD vs. HC, PBD + NPBD vs. HC, and PBD vs. NPBD) in section 2.4 ‘Statistical analyses’ (highlighted, page 7 line 103–108).

Third, the group comparison based on the % of voxels showing the differences with HC does not have any statistical basis and cannot serve as evidence for "disruptions of intra- and inter- hemispheric WM connectivity, in terms of FA, seem more pronounced in PBD compared to NPBD". From this perspective, the conclusions are highly overstated.

- Response: We agree with the reviewer’s comment. In comparison with HC, the affected WM areas appear more widespread in PBD than in NPBD, but we could not determine the statistical differences in the area. Moreover, direct comparisons of DTI parameters between PBD and NPBD showed no significant differences. We have added descriptions on this point to section 4.3 and 4.5 (highlighted, page 17 line 253–page 19 line 256, page 19 line 282–284). In addition, we have re-written our conclusion to avoid overstating our findings (highlighted, page 15 line 191–192, page 19 line 291–294).

One strategy to analyze these data could 1) to compare all BD vs. HC to identify the regions affected by the disorder, and 2) use the identified regions as a mask to further compare PBD vs. NPBD.

- Response: Following the reviewer’s recommendation, we compared HCs and BD patients, including PBD and NPBD. We have described the new findings and rewritten our discussion in sections 3.2 and 4.4 (highlighted, page 11 line 146–149, page 13 line 164‒167, page 18 line 261–275) as well as tables and figures.

The inclusion of BD-II to the data analyses is of concern due to a greater number of those participants in NPBD group. BD-II is even more versatile disorder than BD-I and can dilute the effects. Two resent reports on white matter in BD-II support this view by reporting different results despite using similar populations (Manelis et a., 2021 DOI: 10.1038/s41598-021-87069-2; Mak et al., 2021 DOI: 10.1038/s41598-021-81355-9). I suggest re-running the analyses using only BD-I patients.

- Response: Accordingly, we re-analyzed only patients with BD type I, including 29 PBD and 23 NPBD. We have described the study population more clearly in section 2.1 ‘Subjects’ (highlighted, page 5 line 55–60) and have rewritten our results and discussion in both sections.

Minor concerns include:

- please confirm that participants who experienced psychotic symptoms due to substance use were excluded

- Response: We confirmed that, according to the MINI, no patients had taken any kind of substance among the study subjects. We have added descriptions on this point to section 2.1 ‘Subjects’ (highlighted, page 5 line 58–59).

- please add the information about the data quality assurance and how many participants were removed due to poor data quality, etc.

- Response: We have added descriptions on this point to section 2.1 ‘Subjects’ (highlighted, page 5 line 55–56).

- please add the information about whether the acquisition was multi-band and in what direction the data were collected

- Response: In this study, we acquired diffusion-weighted images using single-shot EPI sequence. We have presented the imaging acquisition parameters in section 2.2 (highlighted, page 5 line 67–71).

- please add citations for all FSL tools you used in the study

- Response: We have added a citation for the FSL tool in section 2.3 (highlighted, page 6 line 75–76).

- please correct the p-values as suggested above

- Response: We have described in more detail the Bonferroni correction in the FWE-corrected P-value to adjust for the number of multiple comparisons in section 2.4 ‘Statistical analyses’ (highlighted, page 7 line 105–108).

Submitted filename: Response to reviewers.docx

Click here for additional data file.

26 Jan 2022

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If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

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Lubin Wang, Ph.D.

Academic Editor

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: (No Response)

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: No

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: amended in accordance to reviewer's comments. the manuscript can be accepted as it is in my opinion

Reviewer #2: The authors addressed most of my concerned. However, they still present the qualitative differences between the PBD vs. NPBD as statistically significant. On p.10 of the revised manuscript, the authors clearly state that "Direct comparisons of FA and MD between PBD and NPBD revealed no significant difference". Therefore, discussing "different patterns of WM connectivity disruptions between PBD and NPBD" does not make any sense. Moreover, the Discussion section should state and discuss the fact that there were NO significant differences between psychotic and non-psychotic groups. Considering that the BD groups did not differ from each other, the authors could focus on the BD vs. HC comparison. The results are also interesting and can be discussed in the context of existing literature. These results should be reported and discussed right after the non-significant results are reported and discussed. The findings regarding the HC vs. PBD and HC vs. NPBD differences can be reported, but should not be compared as the differences are not statistically significant. The statements like "In particular, disruptions of inter- and intra- hemispheric WM connectivity, in terms of FA, might be pronounced in PBD" are not statistically supported.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: paolo brambilla

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

9 Feb 2022

- Response: We would like to thank reviewer #2 for the careful and thoughtful suggestions. We agree with the comments and have revised our abstract and manuscript more modestly (highlighted, page 2 line 13–16, page 14 line 177‒178). First, we have added the negative findings of direct comparisons between PBD and NPBD in section 4.1 (highlighted, page 15 line 187–200). We have also discussed the results of comparisons with HC in sections 4.2 and 4.3. Finally, we have mentioned the implication of WM alterations in PBD, NPBD, and BD in section 4.4 (highlighted, page 17 line 187 – page 18 line 270). In addition, we have deleted the section on “different patterns of WM connectivity disruptions between PBD and NPBD.”

Submitted filename: Response to Reviewer.docx

Click here for additional data file.

7 Mar 2022

Different patterns of white matter microstructural alterations between psychotic and non-psychotic bipolar disorder

PONE-D-21-31335R2

Dear Dr. Ryu,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

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Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: (No Response)

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Reviewer #2: No

10 Mar 2022

PONE-D-21-31335R2

Different patterns of white matter microstructural alterations between psychotic and non-psychotic bipolar disorder

Dear Dr. Ryu:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

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