Two-switch scanning

Also called Dual-switch scanning, Two-switch step scan, Move and select

One switch moves the highlight, the other chooses. Full control of the pace, at the price of two reliable movements instead of one.

What it is

Two-switch scanning uses one switch to advance the highlight and a second to select. Nothing moves until the person presses; nothing is chosen until they press the other one. There is no timer to race and no pause to wait out.

It is step scanning with the selection given its own button, and for people who have the two movements it is the most controllable form of scanning there is.

Who it can help

People who have two reliable switch sites, for example a left and a right hand, a hand and a head, or a knee and a foot. Children who cannot yet time a press to a moving highlight but can press deliberately. People whose attention drifts, because the highlight waits for them. People with cerebral palsy who find starting a movement hard but repeating it easy.

It is not for people with only one reliable movement, which is many of the people who scan. And because every step costs a press, it is more tiring per selection than auto scan on a large grid. Some people use two switches for auto scan instead: one to select, one to go back or to start the scan, which gives some of the control without the press-per-step cost.

How it works

Switch one advances: through rows, then, after a selection, through cells. Switch two selects. The pattern underneath is still row-column or group scanning; only the driver has changed. Most devices allow a hold on the advance switch to auto-repeat, so a person can hold to skim and release to stop.

The two switches should differ in feel (size, colour, click, position), so the person never has to look to know which is which. Some people run the advance switch on the stronger side and the select switch on the weaker, because selecting happens less often than moving.

Getting started

If the person is already accurate on step scan with one switch and a pause-to-select, add a second switch for select this week and see whether the pause was costing them. If they are on auto scan and always seem to press late, try two switches before slowing the rate again.

Set it up in the tablet's own switch settings first (Switch Control on Apple and Switch Access on Android both take two switches) with a couple of cheap wired or Bluetooth switches. Label them. Keep the grid small for a few days while the two-hand rhythm settles.

Where to get help

Ask for it to be trialled as part of an AAC assessment with an occupational therapist present, since two sites need positioning and mounting. The ACE Centre and CALL Scotland explain two-switch set-ups in their free resources.