function qrat(xpos)
*      function qrat(xpos,gam)
*     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

*     qratio = (1.+a2*x**2+a4*x**4)/(1.+b2*x**2)
 
      common /pawpar/ par(6)

      iz = par(6)

      x0=0

*      ampeak=1.0
      ampeak=par(1)

      gam = sqrt( 5.82 + 0.0824*(iz-0.5))
      gam=par(2)

      del=7.73
*      del =  8.4017 + 0.76942E-02*(iz-0.5)
      del=par(3)
      
*      a= 0.3
      a = 1.1-0.0415*(iz-0.5)
      a=par(4)

      b=0.
*      b=par(5)

*     Define
      x = xpos-x0
      
      if( abs(x).lt.del*0.5 )then
*     The value of the prf function at position 'x' is then given by:
         qrat =  ampeak*qratio(a,b,gam,del,x) 
      else
         qrat = 0.
      endif

      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