Fig. 9. Gabor functions fitted to the tuning data shown in Figure 7. To begin, two Gabor functions are fitted, one to each clamp condition; that is, one Gabor for the additional absolute disparity with a positive value and another Gabor for the one with a negative value. The parameters of the two Gabor functions are identical, except for a horizontal translation. The magnitude of the horizontal translation then gives a measure of how consistently the neuron relates to a given experimental variable. Clearly, the relationship to relative disparity (*left*) is not consistent, because there is a substantial horizontal shift in the curve when the absolute disparity is changed. Furthermore, the size of the horizontal shift measured from the fitted curves (−0.363°) is very similar to the measured difference in additional absolute disparity between the two conditions (−0.339°). Consequently, when the data are expressed in terms of absolute disparity and the same comparisons are made, the fitted shift is very small (0.024°). The significance of these shifts can be assessed by comparing the goodness of fit of the linked pair of Gabor functions with a single Gabor that attempts to describe the combined data set. Adding the horizontal shift increases the number of parameters by one. On the *left*, the single Gabor is clearly a poor fit (*dotted line*); the addition of the horizontal shift to create a pair of linked Gabor functions improves the fit (*F*_{(1,86)} = 408, *p* < 0.00001). On the *right*, using the pair of linked Gabor functions does not significantly improve the fit (*F*_{(1,86)} = 3.0, *p* >0.05) compared with a single Gabor. (In fact, the fit with a single Gabor is so similar to the two illustrated curves that it is not shown separately.) It can be concluded that the firing rate bears a consistent relationship to absolute disparity regardless of the added disparity clamp.