The meaning of 'digit' and a question on agreement

the meaning of 'digit' and a question on agreement

The passage below is from ‘the Blank Slate’ by Steven Pinker.

http://evolbiol.ru/blankslate/blankslate.htm

Most demonstrations of plasticity involve remappings within primary sensory cortex. A brain area for an amputated or immobilized finger may be taken over by an adjacent finger, or a brain area for a stimulated finger expands its borders at the expense of a neighbor. The brain's ability to reweight its inputs is indeed remarkable, but the kind of information processing done by the taken-over cortex has not fundamentally changed: the cortex is still processing information about the surface of the skin and the angles of the joints. And the representation of a digit or part of the visual field cannot grow indefinitely, no matter how much it is stimulated; the intrinsic wiring of the brain would prevent it.

I'd like to ask two questions regarding the underlined 'digit'.
First, what does the underlined 'digit' mean?
It seems to be able to mean 'a finger' or 'a number', but I am not sure which in this context.

Second question is about agreement.
I think there is two possible options as follows:
?the representation of a [?digit or ?part] of the visual field
?the representation of [?a digit] or [?part of the visual field]

Which is right? #1, or #2?
(Or is there another agreement that eludes my thought?)

Regards.