Driftdemo question

Hello all,

I'm trying to present Gabor patches and i'm using an edited version of
driftdemo to do so. When it gets around to windowing the sinusoid with
a gaussian, it uses the following calculation:

m=exp(-((x/90).^2)-((y/90).^2)).*sin(a*x+b*y+phase);

My question is, what does the "90" refer to? From playing around with
it I realise that it controls the dimensions of the Gaussian, but how
is this quantified? I imagine the units are pixels, but is it the
standard deviation, width-at-half-height, bandwidth, or something else?

Thanks very much in advance, y'all.

Craig