replace values in columns of a matrix

I have a matrix of random numbers, with some repeating. For example:

OV =

 29.5  -29.3  82.2  -49.1
 12.6  -62.2  82.2  -72.8
 114.8  -29.3  -6.9  156
 29.5  -97.1  -42.5  71.1
 -63.8  119  33.3  -49.1

I want to do some columnwise operations based on the values of the first row. So far I have

 idxo             = bsxfun(@ne,OV,OV(1,:))*2.5; % =0 for OV(1,:)=OV and 2.5 otherwise
 idxo(sec_ovec<0) = 1;                          % for OV<0, =1

which gives me a new matrix idxo as follows:

 0  1  0  1
 2.5  1  0  1
 2.5  1  1  2.5
 0  1  1  2.5
 1  2.5  2.5  1

The last thing I need is to, for all OV rows which have a negative number in the first row: make all corresponding idxo values =0 if OV is negative, and =1 otherwise.

I can do this, but my solution is inefficient. What is a fast way to do this? Speed is important because it is a big bottleneck in my code.

The end result should modify column 2 and 4 to give

idxo =

 0  0  0  0
 2.5  0  0  0
 2.5  0  1  1
 0  0  1  1
 1  1  2.5  0

Answer 1

[Matt J]

j=OV(1,:)<0;
idxo(:,j)=OV(:,j)>=0;

[Christopher]

Excellent solution, thanks!