Subjects

Neuropsychological assessments were carried out on 28 patients with benign cerebellar tumors (mean age 50.9 ± 12.1 years; 11 males, 17 females), 17 with right cerebellar lesions (mean age 49.4 ± 13.6 years; 8 schwannomas, 8 meningiomas, and 1 epidermoid) and 11 with left cerebellar lesions (mean age 53.5 ± 8.3 years; 6 schwannomas, 2 meningiomas, 2 epidermoid, and 1 lipoma) (Table 1), as well as on a control group consisting of 23 healthy controls matched for age, sex, and years of education (mean age 53.4 ± 14.1, range 21–72 years; 9 males, 14 females). Regarding clinical histories, one patient (R19) had previously undergone gamma knife radiosurgery, and two patients (L10 and L17) had recurrent tumors. Patients were excluded for the following reasons: age under 20 or over 78 years; lesions involving non-cerebellar cortical or subcortical regions; history of alcohol or drug abuse; or pre-existing psychiatric disease. Neurological examinations of gait, kinetic function-arm, kinetic function-leg, speech, and eye movements were conducted based on the Brief Ataxia Rating Scale [15]. All patients showed normal performance except for two cases (R2 and R7) who walked almost naturally, but were unable to walk with their feet in the tandem position. The locations of the cerebellar tumors are shown in Fig. 1 and Table 2. Notably, the tumor compressed the posterior lateral cerebellum in all patients, especially lobule VI and Crus I (Fig. 1 and Table 2). Lesion size was measured in milliliters on preoperative MRI, according to the formula; a × b × c / 2, where a and b indicate the longest crossed dimension of the horizontal plane, and c indicates the greatest length of the tumor in the coronal plane.

Table 1 Summary of 28 patients examined by neuropsychological assessment Full size table

Fig. 1 Tumor topography of right (middle image, n = 17) and left cerebellar tumors (right image, n = 11). Light blue line indicates the superior posterior fissure; yellow line indicates the horizontal fissure in the left image Full size image

Table 2 Lesion characteristics in patients with cerebellar tumors Full size table

Informed Consent and Approval

All patients and control subjects provided written informed consent for this investigation. The study was approved by the ethical committee of the University of the Ryukyus.

Experimental Design

In the preoperative stage, 28 patients with cerebellar lesions underwent neuropsychological assessments and 19 patients participated in the fMRI examination. The fMRI study was conducted once before surgical treatment. In the postoperative stage, 12 patients with right cerebellar tumors (mean age 45.0 ± 11.5 years, range 21–64 years; 3 males, 9 females) underwent follow-up neuropsychological assessments in order to examine whether surgical intervention had an effect on functional recovery. Detailed individual profiles are shown in Table 1.

Neuropsychological Assessment

The battery consisted of the following tests: (I) mini-mental state examination (MMSE) [16] and modified MMSE (3MS) [17] for global cognitive screening, (II) Trail Making Test (TMT) [18] and Stroop test (ST) [18] for executive function, (III) Wechsler Adult Intelligence Scale-Revised (WAIS-R) digit span subtest (DS) [19] for working memory, (IV) WAIS-R DST [19] for psychomotor speed, (V) partial WAIS-R block design subtest (fifth and ninth items) [19] and the cube-copying test for visuospatial ability. For quantitative assessment of constructional ability in the cube-copying test, the points of connection and plane-orientation errors were evaluated. A point of connection was defined as a point at which three lines met to form a vertex, hence subjects could score up to eight points, since eight points of connection are present in a cube. Each plane with two pairs of parallel lines was evaluated in terms of the number of lines and the extent to which they were parallel. No plane-error points were scored if the cube was copied accurately [20]. We selected brief neuropsychological tests that could be performed within 1 h in order to reduce the burden on patients in the preoperative or postoperative therapeutic stage. As for the duration of patients’ follow-up, we carried out of the assessment within 6 months after resection of the tumor. Patient R2 with a huge schwannoma showed transient neurological symptoms related to the IVth nerve. The double vision by such nerve injury influenced cognitive performance, so we followed up the patient until recovery of its symptom.

Event-Related fMRI Study

Subjects

Twelve patients with right cerebellar tumors (mean age 46.9 ± 13.3 years, range 17–66 years; 3 males, 9 females), 7 patients with left cerebellar tumors (mean age 53.3 ± 10.1 years, range 38–69 years; 2 males, 5 females), and 30 normal healthy volunteers (mean age 24.0 ± 5.2 years, range 22–35 years; 21 males, 9 females) were enrolled in this study (Table 1). The normal healthy volunteers that participated in the fMRI study were different from those included in the neuropsychological analysis. Standard values in each generation of the correct response rate in fMRI behavioral task were not established. Therefore, to estimate a normal value of the correct response rate, we recruited healthy young subject. None of these patients had any signs or history of neurological or psychological diseases. This study was approved by the ethical committee of the University of the Ryukyus with written informed consent obtained from all participants. Subjects were all right-handed according to the Edinburgh Handedness Laterality Index, with a median score of 100 (range 80–100).

Experimental Paradigm

The fMRI behavioral paradigm used was a rapid event-related fMRI design [14, 21, 22] based on an explicit three-alternative forced choice task including novel (new), repeated (same), and lure (similar) stimuli consisting of color photographs of common objects. A fully randomized functional run consisted of 108 total trials, 16 lure sets, 16 repeat sets, and 44 unrelated novel items (foils) (Fig. 2). Forty-four foil trials, 16 trials first presented from repeat sets, and 16 trials first presented from lure sets were presented as the new stimuli. The same stimuli were 16 trials, which are second presented from repeat sets. The lure stimuli were 16 trials which are second presented from similar sets. Each stimulus was presented for 2,500 ms with a 0–1,000 ms interstimulus interval to prevent adaptive stimulus responses. The number of trials separating similar and identical pairs was randomly varied from 10 to 40 trials. Several photographs were displayed to participants on a goggles display during the session. If the photograph was first presented in the session, participants were required to press the red button indicating a new object. If the photograph had been displayed before in the session, examinees were instructed to press the blue button indicating a repeated object. Finally, if they thought that the photograph was similar to, but not the same as previous stimuli, they were required to press the green button, indicating a similar but not identical object. Responses and reaction times were recorded in a button box (Current Designs, Inc., Philadelphia, Pennsylvania). Visual stimuli were presented to the subjects using 800 × 600 resolution magnet-compatible goggles under computer control (Resonance Technologies, Inc., Salem, Massachusetts) using Presentation® software (Neurobehavioral Systems, Inc., Austin, Texas).

Fig. 2 fMRI behavioral task. Images of single items were presented for 2,500 ms followed by a 0–1,000 ms interstimulus interval. Novel, repeated, and similar lure items were randomly shuffled in the task. Upper left insets show three task buttons for new (red), lure (green), and repeated (blue) stimuli. Lower right insets show examples of lure pairs; Okinawa guardian lions and hibiscuses Full size image

MRI Data Acquisitions

Anatomical and functional images were obtained using a 3-T MRI scanner (Discovery MR750; GE Medical System, Waukesha, Wisconsin, USA) with a 32-channel head coil and high-order manual shimming to the temporal lobes. The array spatial sensitivity encoding technique (a parallel imaging technique) was used to acquire imaging data by reducing geometric distortion for echo planar imaging (EPI). The anatomical three-dimensional (3D) spoiled gradient recalled echo (SPGR) sequence was obtained with a high-resolution 1-mm slice thickness (matrix size 256 × 256, field of view 256 × 256 mm, repetition time 6.9 ms, echo time 3 ms, flip angle 15°). T2*-weighted EPI sequence was used to measure BOLD contrast (repetition time 1,500 ms, echo time 25 ms, flip angle 70°, matrix size 128 × 128, field of view 192 × 192, in-plane resolution 1.5 × 1.5 mm2, 23 slices, 3-mm thickness, 0-mm space). A total of 303 volumes were collected over one session during the experiment in a sequential ascending order. A high-resolution T2 fast spin echo (T2 FSE) sequence (repetition time 4,300 ms, echo time 92 ms, matrix size 512 × 512, field of view 192 × 192, in-plane resolution 0.375 × 0.375 mm2, 23 slices, 3-mm thickness, 0-mm space) was obtained for the co-registration of 3D SPGR and EPI functional images. EPI functional images and T2 FSE structural images were acquired in an oblique coronal plane perpendicular to the long axis of the hippocampus. Almost the entire hippocampus (head, body, and tail) was included in the 23 slices. Functional images were localized in the sagittal plane of the SPGR image to identify the long axis of the hippocampus. Oblique coronal slices were fitted to the principal longitudinal axis of the hippocampus covering the entire bilateral medial temporal lobes. Firstly, distortions of fMRI signals were corrected by array spatial sensitivity encoding techniques, which were used to improve temporal and spatial resolution and reduce artifacts. Secondly, higher order shims were employed to directly compensate for local field distortions. These methods guaranteed homogeneity of the magnetic field.

Preprocessing and Estimations

Functional and structural MR images of the brain were preprocessed using the methods of realignment, temporal correlation, spatial normalization, and spatial smoothing. The data were analyzed using SPM8 software (Wellcome Trust Centre for Neuroimaging, University College London, London, UK). The first five volumes in each data set were removed to ensure that the signal reached a steady state. EPI functional images were corrected to account for the differences in slice acquisition times by interpolating the voxel time series using sinc interpolation and resampling the time series using the center slice as the reference point. The EPI functional images were then corrected for motion artifacts by realignment to the first volume. A mean EPI functional image was constructed during realignment. Co-registration was performed through two processes. Both the mean EPI functional image and the motion-corrected EPI functional images were co-registered to the T2 FSE structural image. The co-registered T2 FSE structural image was then co-registered to the structural SPGR image. Next, the registration points of the anterior and posterior horns of the lateral ventricle, top surface of the paracentral lobule, and bottom surface of the inferior temporal gyrus were checked in the T2 FSE structural, structural SPGR, and EPI functional images. Before spatial normalization, a parameter was produced by the segmentation process from the structural SPGR image. The structural SPGR image and EPI functional images were spatially normalized (1 × 1 × 1 mm) using the Montreal Neurological Institute space. Finally, the images were spatially smoothed using a Gaussian kernel with a full width at a half maximum of 3 mm. To detect the brain activation associated with a specific task while simultaneously reducing noise, the size of the smoothing kernel was kept at a recommended 2 to 3 times the voxel size [23]. A high-pass filter regressor (200 s) was included in the design matrix to exclude low-frequency noise and artifacts. To identify the correct activation spots of the brain, movement effects were discounted in a number of rows (298) and columns (3 translations and 3 rotations). For each subject, the three (new, lure, and repeated) regressors were estimated by a general linear model calculated by applying a canonical hemodynamic response function combined with time and dispersion derivatives. To assess the main effect of the lure images, as characterized by both the hemodynamic response function and these derivatives, an F-contrast obtained by the F test was required. Intra-individual activation maps were calculated by F tests. We calculated second-level group contrasts using a one-sample t test for each regressor (new, lure, and same) from the response of the canonical hemodynamic function. Differences in the intensities of the activation between task conditions were confirmed by a voxel-level threshold of p < 0.001 uncorrected, and a cluster-level threshold of FWE (family-wise error)-corrected p < 0.05. We extracted the average percent signal change values of the regions of interest (ROIs) from the anatomically defined AAL ROI atlas [24] and established 3D MRI atlases [25–27] for each subject and type of task stimuli using the MarsBar toolbox [28].

Global Brain Connectivity Analysis

Subjects

Twelve patients with right cerebellar tumors (mean age 46.9 ± 13.3 years, range 17–66 years; 3 males, 9 females), 7 patients with left cerebellar tumors (mean age 53.3 ± 10.1 years, range 38–69 years; 2 males, 5 females), and 15 right-handed healthy volunteers (mean age 27.6 ± 6.5 years, range 20–44 years; 5 males, 10 females) were enrolled in this study. The normal healthy volunteers participating in the resting-state fMRI study were different from those included in the neuropsychological analysis and the event-related fMRI experiment. No participants had any signs or history of neurological or psychological diseases. This research was approved by the ethical committee of the University of the Ryukyus, and written informed consent was obtained from all participants.

Acquisition of Resting-State fMRI Data

Functional and anatomical images were obtained using a GE Discovery MR750 3.0 Tesla MRI scanner (GE Medical System) with a 32-channel head coil. In order to minimize head movement, the heads of each of the participants were fixed using foam pads. In order to reduce geometric distortion in EPI, a parallel imaging technique known as the array spatial sensitivity encoding technique was used during imaging data acquisition. T2*-weighted EPI images were used to measure BOLD contrast (repetition time 2,000 ms, echo time 30 ms, flip angle 70°, matrix size 64 × 64, field of view 256 × 256, in-plane resolution 4 × 4 mm, 42 slices, 4-mm thickness, 0-mm space). During EPI image scanning, participants were instructed to remain motionless, remain awake, relax with their eyes closed, and to try not to think about anything in particular. A total of 150 volumes were collected over one session in a sequential ascending manner (plus 5 initial discarded volumes). An anatomical three-dimensional spoiled gradient recalled echo (3D SPGR) sequence was obtained with high-resolution 1-mm slice thickness (matrix size 256 × 256, field of view 256 × 256 mm, repetition time 6.9 ms, echo time 3 ms, flip angle 15°). A high-resolution T2 fast spin echo (T2 FSE) sequence (repetition time 4,300 ms, echo time 92 ms, matrix size 256 × 256, field of view 192 × 192, in-plane resolution 1.33 × 1.33 mm, 42 slices, 4-mm thickness, 0-mm space) was obtained for the co-registration of 3D SPGR images and EPI functional images. EPI functional and T2 FSE structural images were acquired in an oblique axial transverse plane (tilted 30° anterior relative to the intercommissural plane).

Preprocessing and Analysis of Resting-State fMRI Data

Following this step, fMRI preprocessing, analysis, and visualization methods were conducted as implemented in SPM (8 package, http://www.fil.ion.ucl.ac.uk/spm8/) and the “conn” toolbox (www.nitrc.org/projects/conn). Images were corrected for slice acquisition time within each volume, motion corrected with realignment to the first volume, spatially normalized to the standard MNI EPI template, and spatially smoothed using a Gaussian kernel with a full width at half maximum of 8 mm. 3D SPGR images were co-registered with each mean EPI and T2 FSE image, and averaged together to permit anatomical localization of the functional connectivity at the group level. The transformed structural images were segmented into gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF) using a unified segmentation algorithm.

In addition to removing noise correlations present in WM and CSF, the addition of six motion regressors (six realignment parameters and first derivatives) controlled for correlations due to movement. Data were filtered between 0.009 and 0.08 Hz.

Correlation Analysis with Global Brain Connectivity and the Lure Task

A map of GBC was computed from resting-state fMRI data using the “conn” toolbox (www.nitrc.org/projects/conn) [29, 30]. In the “conn” toolbox, correlation maps were calculated on the basis of seed-based correlation analysis using the AAL ROI atlas [24]. When the population correlation coefficient is zero, the distribution of correlation coefficient is consistent with the normal distribution. However, normal distribution of the correlation coefficient is lost when the correlation coefficient approximates to 1 [31]. Each ROI’s correlation coefficient map was transformed by Fisher’s Z transformation to Z value maps in order to normalize the correlation coefficient.

These Z value maps were averaged together across each subject in order to calculate GBC values. For correlation analysis of the GBC and score of the lure task, GBC values were extracted from the ROIs that were activated by the lure stimulus in the event-related fMRI experiment. The Pearson product–moment correlation coefficient was used to calculate correlations between GBC values and the correct response rates in the lure task. When the correlation coefficient was close to +1, the r value indicated a proportional connection between GBC values and the scores in the lure task (positive correlation). Conversely, when the correlation coefficient was close to −1, the r value showed an inverse proportion (negative correlation). We estimated the strength of the correlation in five categories: negligible correlation (0.00 to 0.30 or 0.00 to −0.30), low correlation (0.30 to 0.50 or −0.30 to −0.50), moderate correlation (0.50 to 0.70 or −0.50 to −0.70), high correlation (0.70 to 0.90 or −0.70 to −0.90), and very high correlation (0.90 to 1.00 or −0.90 to −1.00) [32].

Statistical Analysis

The Kruskal-Wallis test for three independent samples or the Mann–Whitney U test for two independent samples was used to evaluate statistical significance in the neuropsychological assessments and fMRI behavioral tasks, since a normally distributed population could not be assumed. Preoperative and postoperative neuropsychological data were compared using the Wilcoxon signed rank test. Statistical significance was accepted at p < 0.05. The chi-square test was used to evaluate the performance in the block design subtest, since only two items of the block design subtest were evaluated as pass/fail.