You
can use the Channels palette to view any combination of channels
in the document window. For example, you can view an alpha channel
and the composite channel together to see how changes made in the
alpha channel relate to the entire image.
Click
in the eye column next to the channel to show or hide that channel.
(Click the composite channel to view all default color channels.
The composite channel is displayed whenever all the color channels
are visible.)
To show or hide multiple channels, drag
through the eye column in the Channels palette.