Eye leads placed at the outer canthus of each eye will predominantly record:

When eye leads are placed at the outer canthus of each eye, they predominantly record electrical activity related to horizontal eye movements. This positioning helps in capturing the eye's lateral movements, as the outer canthus aligns with the major muscle groups responsible for moving the eyes horizontally.

The recording focuses on the changes in electrical potential generated by the ocular muscles during these horizontal movements, such as looking left or right. While some minimal vertical movements may be recorded due to general eye activity, the primary function of these leads is to monitor horizontal eye shifts.

Options that suggest all eye movements or vertical eye movements do not accurately reflect the specific function of the outer canthus lead placement, as the primary data captured would exclude significant vertical eye activity. Hence, while some degree of vertical motion may be detectable, the emphasis remains on horizontal movements, rendering the assertion that they mainly record horizontal eye movements as the most accurate.