Notation unclear - Radial Basis Funtion

Question

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?

Answer 1

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 $h_i$.