Amblyopia, which usually occurs during early childhood and results in poor or blurred vision, is a disorder of the visual system that is characterized by a deficiency in an otherwise physically normal eye or by a deficiency that is out of proportion with the structural or functional abnormalities of the eye. Our previous study demonstrated alterations in the spontaneous activity patterns of some brain regions in individuals with anisometropic amblyopia compared to subjects with normal vision. To date, it remains unknown whether patients with amblyopia show characteristic alterations in the functional connectivity patterns in the visual areas of the brain, particularly the primary visual area. In the present study, we investigated the differences in the functional connectivity of the primary visual area between individuals with amblyopia and normal-sighted subjects using resting functional magnetic resonance imaging. Our findings demonstrated that the cerebellum and the inferior parietal lobule showed altered functional connectivity with the primary visual area in individuals with amblyopia, and this finding provides further evidence for the disruption of the dorsal visual pathway in amblyopic subjects.
Amblyopia is a developmental ocular disorder characterized by a unilateral or bilateral visual deficiency that is out of proportion with any structural abnormalities that are present in the eye [
The primary visual cortex (also known as V1, anatomically equivalent to Brodmann area 17 (BA 17)) is a koniocortex (sensory-type cortex) located in and around the calcarine fissure of the occipital lobe. Each hemisphere of the primary visual cortex receives information directly from its ipsilateral lateral geniculate nucleus and transmits information to the dorsal and ventral streams. Previous studies have observed functional deficits and morphological alterations in the lateral geniculate nucleus in cases of amblyopia [
The two-stream (dorsal and ventral) hypothesis is an influential and widely accepted model of visual information processing. It is generally believed that the dorsal stream (the “how pathway”), which involves areas such as the middle temporal cortex (MT) and the medial superior temporal area, processes spatial location information. The ventral pathway (the “what pathway”) includes area V4 and the inferior temporal lobe and is associated with the processing of object identification and recognition. Interestingly, numerous psychophysical studies have observed that both the ventral and dorsal extrastriate cortical processing functions are disrupted in amblyopia subjects [
Parts of the dataset have been used in our previous study to investigate the regional homogeneity of spontaneous activity patterns in amblyopic subjects [
Written informed consent was obtained from all participants or their legal guardians. This study was approved by the Ethics Committee of Zhong Shan Ophthalmic Center at Sun Yat-sen University and followed the tenets of the Declaration of Helsinki. All participants received detailed eye examinations that included assessments of their visual acuity, intraocular pressure and refraction, slit lamp examination, ophthalmoscopy, binocular alignment, ocular motility, and random-dot butterfly stereograms. In total, fourteen anisometropic amblyopic patients, sixteen mixed (anisometropic and strabismic) amblyopic patients, and twenty-two healthy individuals were enrolled in the study. Three participants (one healthy volunteer and two patients with amblyopia) had excessive head motions during the scanning and were excluded, leaving twenty-one healthy volunteers and twenty-eight patients with amblyopia to be included in the analysis. All of the subjects were right-handed and had no history of other ocular diseases, surgery, neurological disorders, or brain abnormalities based on MRI scans. The volunteers had normal or corrected-to-normal visual acuity in both eyes. Detailed clinical data on the subjects are shown in Table S1 in Supplementary Material available at
The MRI data were obtained using a 3.0 Tesla MR scanner (Trio Tim system; Siemens, Erlangen, Germany). Resting-state fMRI scans were performed with an echo planar imaging sequence with the following scan parameters: repetition time = 2000 ms, echo time = 30 ms, flip angle = 90°, matrix = 64 × 64, field of view = 220 × 220 mm2, slice thickness = 3 mm, and slice gap = 1 mm. Each brain volume was composed of 32 axial slices, and each functional run contained 270 volumes. During the scans, all subjects were instructed to keep their eyes closed, relax, and move as little as possible. Tight but comfortable foam padding was used to minimize head motion, and earplugs were used to reduce scanner noise.
The structural magnetization prepared rapid gradient-echo imaging sequence which was used to acquire structural T1-weighted images in a sagittal orientation. The parameters were as follows: repetition time = 2000 ms, echo time = 2.6 ms, flip angle = 9°, acquisition matrix = 512 × 448, and field of view = 256 × 224 mm2. The scanning time was approximately 5 min, and a total of 192 images with 1 mm thick slices were obtained.
The fMRI images were conventionally preprocessed using Statistical Parametric Mapping software (SPM8,
The primary visual cortex of the brain generally refers to Brodmann area 17 (BA 17), and the bilateral primary visual cortices were defined using the method used in a previous study [
Functional connectivity analyses were performed separately for the left and right primary visual cortices. A seed reference time series for each hemisphere of the primary visual cortex was obtained by averaging the fMRI time series of all voxels within the area. A Pearson correlation analysis of the time series was performed between the mean time series and other brain regions in a voxel-wise manner. For further statistical analysis, a Fisher r-to-z transformation was performed to improve the normality of the correlation coefficients.
In this study, we investigated alterations in the connectivity pattern of the visual cortex and other brain areas in amblyopic subjects. A two-sample, two-tailed
To evaluate the alterations in the connectivity pattern of the primary visual area in the amblyopic subjects, all of the regions identified from the two comparisons (anisometropic amblyopic subjects versus normal-sighted and mixed amblyopic subjects versus normal-sighted) were overlapped to investigate the impaired regions in the two patient groups. Only regions larger than 70 voxels were identified as significant.
The demographic and psychological characteristics of the two amblyopic groups (anisometropic amblyopia: 5 males, 8 females, mean age:
Demographic, clinical, and neuropsychological data on normal sighted subjects (NC), anisometropic amblyopia (AA) subjects, and mixed amblyopia (MA) subjects.
NC ( |
AA ( |
MA ( |
|
|
|
---|---|---|---|---|---|
Gender (M/F) | 8/13 | 5/8 | 8/7 | 0.969 | 0.616 |
Age (year) | 23.5 ± 2.1 | 22.3 ± 7.2 | 23.4 ± 7.1 | 0.211 | 0.81 |
Mean head motion | 0.51 ± 0.19 | 0.62 ± 0.33 | 0.52 ± 0.29 | 0.794 | 0.458 |
Mean rotation | 1.48 ± 0.23 | 1.65 ± 0.26 | 1.51 ± 0.30 | 1.868 | 0.166 |
Framewise displacement | 0.11 ± 0.04 | 0.13 ± 0.05 | 0.13 ± 0.08 | 0.559 | 0.575 |
Chi-square analysis was used for gender comparisons, and one-way ANOVA with a Bonferroni post hoc test was used for age and head motion comparisons.
Compared to subjects with normal sight (
The anatomical distribution of the alterations in functional connectivity with the left primary visual cortex (a) and the right primary visual cortex (b) in anisometropic amblyopia are shown in comparison with normal sighted controls, as individually visualized using the Caret v5.61 software (
Compared to the subjects with normal vision (
The anatomical distribution of the alterations in functional connectivity with the left primary visual cortex (a) and the right primary visual cortex (b) in mixed amblyopic subjects is shown in comparison with normal sighted controls, as individually visualized using the Caret v5.61 software (
We also found overlapping brain areas with altered functional connectivity with the primary visual area in anisometropic and mixed amblyopic individuals (70 voxels). The overlapping brain regions that showed altered functional connectivity with the left primary visual area were located in the cerebellum (cerebellum tonsil, vermis 9/vermis 7, and cerebellum crus 1/6) and the conjunction area of the bilateral inferior parietal lobe and the angular lobe (IPL/ANG) (Table
Overlapping brain areas with altered functional connectivity with the primary visual area in amblyopia individuals (cluster size > 70 voxels).
Brain Region | Cluster Size | MNI Coordinates |
---|---|---|
Left primary visual cortex | ||
20 −38 −52 | ||
Cerebellum Tonsil | 85 | 28 −42 −48 |
26 −34 −48 | ||
8 −50 −44 | ||
Cerebellum Vermis_9 | 155 | −2 −56 −40 |
8 −60 −38 | ||
12 −82 −42 | ||
Cerebellum Crus2/Vermis_7 | 182 | 18 −82 −36 |
6 −84 −34 | ||
−38 −58 −42 | ||
Cerebellum_6 | 178 | −38 −48 −40 |
−32 −42 −38 | ||
36 −46 −40 | ||
Cerebellum Crus1/6 | 523 | 16 −72 −38 |
40 −64 −38 | ||
−42 −56 36 | ||
IPL/ANG.L | 269 | −36 −60 40 |
−44 −60 44 | ||
34 −48 40 | ||
IPL/ANG.R | 179 | 40 −54 42 |
32 −56 44 | ||
| ||
Right primary visual cortex | ||
4 −74 −16 | ||
Lingual/Vermis_6 | 79 | −10 −90 −12 |
−2 −70 −12 | ||
−42 −60 34 | ||
IPL/ANG.L | 117 | −48 −66 38 |
−36 −56 38 |
IPL: inferior parietal lobe, ANG: angular lobe, L: left, R: right, MNI Coordinates: Montreal Neurological Institute Coordinates [
Overlapping brain areas with alterations in functional connectivity with the left primary visual cortex (a) and the right primary visual cortex (b) are shown for amblyopic individuals (cluster size larger than 70 voxels). The details of the regions can be found in Table
Left primary visual cortex
Right primary visual cortex
Compared to the patients with anisometropic amblyopia, patients with mixed amblyopia showed increased functional connectivity between the medial/inferior temporal gyri and the left primary visual area and decreased functional connectivity between cerebellar crus 1/6/8 and the right primary visual area (Figure
Alterations in functional connectivity with the primary visual area between anisometropic amblyopic subjects and mixed amblyopic (anisometropic and strabismic) individuals (
Brain Region | Cluster Size |
|
|
MNI Coordinates ( |
---|---|---|---|---|
Left primary visual cortex | ||||
MTG/ITG | 136 | 5.28 | 4.26 | −52 −36 −18 |
3.99 | 3.46 | −62 −44 −18 | ||
| ||||
Right primary visual cortex | ||||
Cerebellum Crus 8 |
180 | −4.52 | −3.81 | −18 −60 −52 |
−3.17 | −2.87 | −30 −58 −50 | ||
−3.69 | −3.25 | −26 −74 −32 | ||
Cerebellum Crus 1/6 | 200 | 3.56 | −3.16 | −22 −80 −42 |
3.37 | −3.02 | −16 −68 −22 |
ITG: inferior temporal guys, MTG: middle temporal guys, L: left, R: right, MNI: Montreal Neurological Institute.
Alterations in functional connectivity with the left primary visual cortex (a) and the right primary visual cortex (b) between anisometropic amblyopic subjects and mixed amblyopic individuals are shown (
Left primary visual cortex
Right primary visual cortex
In the present study, we investigated the functional connectivity between the primary visual cortex and other brain areas in amblyopic individuals using a resting-state functional connectivity technique. From our results, we mainly find significant decreases in functional connectivity with the primary visual area in the inferior parietal lobule and the posterior cerebellum in both anisometropic amblyopia and mixed amblyopia.
The dorsal stream, sometimes called the “where pathway” or the “how pathway”, originates from the V1 area, passes through the V2 and MT (also known as V5) areas, and arrives at the inferior parietal lobule. This pathway primarily participates in the detection of motion, the representation of object locations, and the control of the eyes and arms, especially when visual information is used to guide saccades or reaching behaviors [
We also found a decrease in the functional connectivity between the primary visual area and the cerebellum (cerebellum tonsil, vermis 9, cerebellum crus 2/vermis 7, and cerebellum crus 1/6). The cerebellum, which functionally interacts with the frontal eye fields [
In some strabismic subjects, the brain ignores input from the deviated eye. We have found altered functional connectivity between the MTG and the left primary visual cortex and between the cerebellum crus and the right primary cortex in mixed amblyopic subjects compared to anisometropic amblyopic subjects. This might occur because the amblyopic subjects with strabismus would have severely affected gaze judgment and information interaction between the sensory motor and visual areas (Table
We found increased functional connectivity between the right primary visual area and the left PostCG in cases of anisometropic amblyopia. This corresponds to our previous finding of increased spontaneous activity in the PostCG and PreCG, which may reflect the compensatory plasticity that compensates for amblyopia-related deficits [
In the initial experimental design of the present study, we only wanted to determine the alteration of spontaneous activity and the functional connectivity pattern in the amblyopic individuals in the resting state. In fact, stereopsis-related changes may provide deeper insight into the neural substrate of the impaired binocular perception in the patient groups. Unfortunately, most of our participants did not have stereopsis scores. Meanwhile, we did not find a statistically significant correlation between altered functional connectivity and disease severity (visual acuity of bilateral eyes) in the patient groups. Furthermore, our results should be interpreted carefully because we did not consider the side of the eye impairments due to the small sample size. In the future, a larger sample neurophysiological and neuroimaging study is required to distinguish the differences among the affected brain regions in the different types of amblyopia.
The authors have declared that they have no conflict of interests.
This work was partially supported by the National Key Basic Research and Development Program (973), Grant no. 2011CB707800; the National Natural Science Foundation of China, Grant nos. 81270020 and 60831004; and the Research Foundation of Science and Technology Plan Project, Guangdong, China, Grant nos. 2011B061300067.