Figure 3

A primate visual system without ipsilateral retinal projections (IRP). This hypothesized visual system demonstrates how the primate visual system is likely to function without IRPs. This type of visual system is present in humans with the rare achiasmatic syndrome [36]. In this case the left hemisphere receives a mixture of visual information from the right and left visual field. Binocular cues to the hand’s position in space will be absent in the directing (left) hemisphere, or delayed due to the need for inter-hemispheric information transfer. In a primate without IRP, both eyes and thereby both hemispheres would “see” the hand when it operates in front of the eyes. However, this architecture would be associated with substantial problems for eye-hand steering. In bimanual operations such as climbing, this architecture will provide the left hemisphere with visual information about the right, as well as the left, hand. The latter information would not be particularly useful since it cannot easily be integrated with tactile information, proprioception, motor programming. To make it useful the brain would need more inter-hemispheric connections, which will increase the volume and weight of the brain. This architecture may also have consequences for oculomotor function in monocular conditions, e.g. when a primate gets sudden, transient problems with one eye. If only one eye is used with this neural architecture, visual feedback is likely to be conducted more slowly to the hemisphere that must rely on interhemispheric transfer. The achiasmatic syndrome, as well as albinism, in which the proportion of IRP is significantly reduced [39], are associated with nystagmus [38]. Reduced oculomotor function due to nystagmus is likely to influence eye-hand coordination, which may be fatal in a tree-climbing species.