This tutorial shows the steps required to attach an optical mouse to an Atari 8-bit home computer
Introduction
How hard can it be to attach a modern optical mouse to an 8-bit home computing relic? Not that hard as it turns out.
There's two sides to this; reading the mouse and then telling the Atari what's going on.
The idea here is that the joystick works by switching on whatever pins correspond to a movement in a particular direction. All we need to do is replicate this with the Arduino using digitalWrite.
Reading the mouse is done by the USB shield.
There is a driver for the mouse at
https://www.instructables.com/id/Atari-800-Mouse-Driver/ 
Supplies:
Arduino Uno
Arduino USB shield
Wires. I'm using Dupont connectors as they plug straight into the USB shield
Optical Mouse
Female 9 pin D connector
Atari 8-bit computer. I'm using an 800 XL but I expect it to work with anything with a suitable joystick port