Pigmentation predicts the shift in the line of decussation in humans with albinism

In albinism a large proportion of nerve fibres originating in temporal retina cross the midline at the chiasm and project to the contralateral hemisphere. Studies in rodents with albinism have suggested that the extent of this misrouting at the chiasm is inversely related to pigmentation levels. Here, we examine whether there is evidence for a similar relationship in humans with albinism. Functional MRI was performed on 18 subjects with albinism, 17 control subjects and six controls with nystagmus as they underwent hemifield visual stimulation of nasal or temporal retina. Functional activation in 16 coronal slices beginning at the posterior occipital lobes were analysed and the extent of hemispheric response lateralization at each slice position was determined. During temporal retina stimulation, the control response was lateralized to the hemisphere ipsilateral to the stimulated eye for all slices. In albinos, the response in posterior slices was predominantly in the contralateral hemisphere, consistent with misrouting of temporal retina fibres. However, as slice location became progressively anterior, response lateralization reverted to the ipsilateral hemisphere. The slice location at which the transition from contra- to ipsilateralization occurred provided an estimate of the extent of fibre misrouting in the individual. The slice transition location correlated negatively with pigmentation level, providing the first evidence for a relationship between pigmentation and the extent of misrouting in humans with albinism.