Why are mouse sensors in the middle of the mouse?

(When they should be at the front)

In 1987, Microsoft commissioned Matrix Product Design to design a mouse. A mouse that would beat anything that had proceeded it. It was to be clearly different and superior to the clutter of beige mice that existed at the time.

To complete the brief, Matrix found partners in ID TWO and David Kelley Design.

In a GUI, the mouse controlling the cursor is the fundamental link between the user and the computer. A strong connection is created for the user when the mouse performs well. The emotional attachment created between operator and system is strongest when the system does precisely what the operator requests. Imprecise controls make the operator feel that they are not in control: and so there’s a compelling case to get the design of the mouse right.

As part of the design process, Matrix made a series of prototypes to find the right form for the mouse; a form that looked right, felt right, and above all, gave the user real control and precision of the mouse and cursor.

Matrix conducted trials of these prototypical forms, using a series of tests written as HyperCard stacks. They tested how quickly users could home in to a marked position, how precisely they could click on a specific target, and how accurately they could move a cursor through a maze.

They found that the form of the mouse — how it felt in the hand — had a direct impact on how well the user could control the mouse.

But even more important, particularly for the precision and maze tests, was the location of the mouse ball.

Matrix found that the best place for the ball was up front as far as possible between the users thumb and forefinger. The forefinger can be controlled very precisely — much more so than the wrist and forearm. Matrix found that users would move their wrist and arm to move the cursor are large distance, but for fine control relied on the thumb and forefinger.

At this time, mouse manufacturers tended to position their balls far to the back of the mouse. It hadn’t been placed there for the benefit of the users, but for the ease and cost-efficiency of manufacturing. The ball and motion sensors were large and took up a lot of room, and the front of the mouse is taken up with buttons and the cord: simpler by far to push it to the back.

And so despite the extra costs of manufacturing, the final design of the Microsoft Mouse had the ball quite far forward. Matrix published their findings, and soon all the other manufacturers followed suit.

Nowadays, however, all the mice I own have their sensors placed roughly in the middle of the longitudinal axis, and randomly on the transverse axis. Including mice from Microsoft.

At first blush, the position of the ball doesn’t seem to make much difference to this fine control. After all, the user can move the mouse with their thumb and forefinger wherever the ball is placed. Push up and the cursor moves up; left and it moves left.

But moving just the forefinger — not the whole hand, or bunching all the fingers, but just the forefinger — actually makes the mouse rotate, particularly for left- and rightward movements. If the sensor is up the front of the mouse, that rotation of the mouse will give the user the desired outcome: the cursor will accurately move to the left or right, or back and forth. But if the sensor is in the middle of the mouse, then all of that fine control is lost: no or little movement will be registered. Worse, if the sensor is towards the back of the mouse, the direction of the cursor’s movement will be reversed.

I don’t know why this change has happened. I feel that an important understanding has been lost: that we have taken a step back.

The cost basis doesn’t seem to be there: laser diodes and the associated componentry are much smaller now than the old rubber-covered steel balls and the sensors to track their movement. True, up at the front of the mouse there’s a scroll wheel to get in the way, but the sensor can easily be placed to the left of that.

It simply seems that designers and manufacturers have, over the years, forgotten about the benefit of putting the sensor up front, or have placed precision and control further down their list of priorities. I hope that this isn’t the case: that newer research has shown that the current placement is the correct placement, or that something else has changed over time. But if that’s not the case, then I hope that some design team will rediscover either the principle, or the findings — so that we can continue to strengthen the connection between the user and the computer.

Further reading

Bill Moggridge, Designing Interactions: a fantastic book that looks at how designers design how we use computers, systems, services, and gadgets.

The earlier development of the Macintosh Mouse; also by the partnership that would become IDEO.