Table 1.

Brief summary of brain magnetic resonance imaging results in AA and SA in the present systematic review

Common brain alterations in AA and SA patients
• AA and SA patients both demonstrated reduced functional activations in the striate and extrastriate cortices (such as V1, V2, and V3)* observed during the processing of spatial-frequency stimuli, and the topological cortical activated maps in V1, V2, and V3 were more distorted under retinotopic representations, which might result from abnormal visual experience. Evidence also indicated increased spontaneous brain function during the resting state in these regions in the two conditions, the causes and mechanisms of which require further investigation.
• AA and SA patients both showed reduced FC and GMV in the dorsal pathway and aberrant white matter microstructure in the ventral pathway, thus suggesting attenuated functional and structural connections in the dorsal and ventral pathways, respectively.
• The dysfunction of AA and SA patients was also characterized by reduced spontaneous brain activity in the oculomotor cortex relative to controls, mainly involving the frontal and parietal eye fields and the cerebellum, which might underlie the neural mechanism of fixation instability and anomalous saccades in amblyopia.
Distinct brain alterations in AA and SA patients
• AA has a greater impact than SA in the precortical pathway as AA patients demonstrate structural impairments from the retina to V1, while SA patients’ alterations were more prominent in the lateral geniculate nucleus and optic radiation.
• In the ventral pathway, AA patients showed reduced functional activation while viewing high-frequency stimuli, reduced FC relative to controls, and greater reductions in GMV than SA patients.
• SA patients showed more attenuated functional activation in the extrastriate cortex than in the striate cortex during visual tasks compared to AA patients, suggesting abnormal visual (bottom-up) correlated with disrupted binocular interaction and regulated by a top-down modulation of attentional regulation mechanisms.
Differences in amblyopic children and adults
• Reduced cortical thickness tended to be more lateralized in adult AA patients than in pediatric AA patients.
• Brain regions with reduced GMV and altered spontaneous brain activity tended to be more limited in adult amblyopia patients than in amblyopic children.

AA: Anisometropic amblyopia; FC: functional connectivity; GMV: gray matter volume; SA: strabismic amblyopia. *Note: the visual cortex includes the primary visual cortex (also called striate cortex or V1) and extrastriate visual cortex (including, for example, visual area two [V2], three [V3], four [V4], and five (V5]).