function prf(xpos)
*     qratio = (1.+a2*x**2+a4*x**4)/(1.+b2*x**2+b4*x**4)
*     The 6 parameters that could be varied  for best fit are:
*     1) ampeak = prf peak amplitude
*     2) x0 = peak centroid position
*     3) gam = FWHM of the prf
*     4) del = base width
*     5) a and 6) b scale parameters relating the coefficients of the 4th
*     order polynomials to the FWHM and base width del

*      a=1 , b=0
*     qratio = (2.+a2*x**2+x**4 )/(1.+b2*x**2)
 
      common /pawpar/ par(6)

      ampeak=par(2)
      x0=par(1)
      a=1
      b=0
      gam=par(3)
      del=par(4)
*     Define

      x = xpos-x0

*     The value of the prf function at position 'x' is then given by:
      prf =  ampeak*qratio(a,b,gam,del,x)
      return
      end

      function qratio(a,b,gam,del,c)
*     Normalized pad response function

*     Ratio of two symmetric 4th order polynomials
*     The function can approximate a variety of line shapes
*     from a Gaussian to a Lorentzian

      y=gam/2.
      x=del/2.
      a4=a/x**4
      p=-(1.+a)
      a2=p/x**2
      b4=b/y**4
      q=1.+2.*a2*y**2+(2.*a4-b4)*y**4
      b2=q/y**2
      qn=1.+a2*c**2+a4*c**4
      qd=1.+b2*c**2+b4*c**4
      qratio=qn/qd
      return
      end