V5) or Brodmann areas 18 and 19. Areas V2 and V3 are divided into dorsal and ventral (upper and lower) halves that wrap round V1.

The superior colliculus is situated in the midbrain. It has a role in multisensory integration of perception, as was discussed in Section 2.1.4. It comprises seven alternating cellular and fibrous layers and is generally divided into two parts, consisting of the upper three and the four deeper layers. It is only the deeper layers that receive inputs from different sensory modalities, as well as from motor related structures. It is also the deeper layers that are most involved in using sensory information to affect motor behaviour. Visual organisation in both the superficial and deep layers of the superior colliculus is map-like or visuotopic, but the deep layer map is coarser than the superficial one and also includes the far periphery of the retina.

The functions of the superior colliculus include the control of eye movements and the direction of the gaze, using saccades and sometimes also head movements. In particular, it seems to be involved in the mechanisms that direct the gaze towards a target and ensure that the image is placed on the macula.

2.3.7 Visual Pathways

It is generally believed that there are two main visual pathways or information streams, which are responsible for fine detail, colour and form; and motion and spatial location processing. The fine detail pathway is referred to variously as the parietal, ventral, P or parvocellular pathway and sometimes also as the What pathway. It connects the smaller P ganglion cells in the retina to the parvocellular layers in the lateral geniculate nucleus, continues to the striate cortex and terminates in the parietal lobe after passing through the visual areas V2 and V4 of the extrastriate cortex. It is believed to be involved in form recognition, object representation and fine detail and colour perception. It has been suggested, but not confirmed, that visual area V4 is the colour-processing centre of the brain.

The motion pathway is referred to as the parietal, dorsal, M or magnocellular pathway and sometimes as the Where pathway. It connects the larger retinal ganglion cells via larger diameter axons to the ventral magnocellular layers of the lateral geniculate body and then continues to the primary visual cortex to terminate in the parietal lobe after passing the visual areas V2, V3 and V5 or MT (middle temporal) region of the extrastriate cortex. It is sensitive to changes in luminance at low light levels and motion. It is believed to be involved in spatial processing, including motion, representing object locations and controlling the eyes and arms, particularly when visual information is used to guide saccades or reaching movements. There is a reasonable body of evidence to suggest that the visual area MT is the motion-processing centre of the brain.

The terms M and P, or magnocellular and parvocellular pathway, are generally used for the first part of the pathway from the retinal ganglion cells to the primary visual cortex, whereas the terms ventral or parietal and dorsal or temporal stream or pathway are generally used for the part of the pathway from the primary visual cortex to the parietal or inferior temporal lobe.

This division of the functions of these two pathways is consistent with clinical observations that lesions in the parietal lobe affect perceptions of spatial relation-