Notation unclear - Radial Basis Funtion In Chapter 6 (Deep Forward Networks) on Page 193 of Deep Learning they talk about the design of Hidden Units.
The Radial Basis Function is introduced as follows:
$$
h_i = exp\big{(}-\frac{1}{\sigma_i^2}||W_{:,i} − x||^2\big{)}
$$
What does the colon as Index for $W$ mean?
According to the notation page, it means the $i$-th column.

$W$ is a template matrix, where the closer $x$ is to $i$-column of $W$, the bigger the value of $h_i$.