Figure 4.

Comparison of position discrimination in higher level visual with that in early visual areas. (A) Precision of physical (blue) and perceived (red) position discrimination is shown for the dorsal visual areas in ascending order. Coding preference is captured in the green bars: they show the within-area comparison of physical versus perceived position discrimination for each visual area, obtained by subtracting the precision of physical coding from that of percept coding −(Z_percept − Z_physical). Positive values in the green bars indicate a preferential coding of perceived position, whereas negative values indicate a preferential coding of retinal position. Although activity in MT+ reflects perceived position more precisely than physical position, in earlier areas this bias is diminished or reversed. A chi-square test revealed that the nature of position coding differed significantly among these dorsal areas (χ²_dorsal = 20.92, p = .0003). Panel B shows a comparison of perceived and physical position discrimination in the ventral visual stream. Here, too, there was significant variability between areas in the bias for representing physical or perceived position (χ²_ventral = 35.29, p < .0001). To test for a systematic progression in the nature of position coding across areas, we ranked all 11 areas according to their locations in the visual processing stream (see Methods) and computed a Spearman rho rank correlation between the visual area ordering and the position coding bias −(Z_percept − Z_physical). The correlation was highly significant (ρ = .80; p = .003). To test this a priori ordering against all other possible rankings of visual areas, we computed a 25,000-sample bootstrapped distribution of rank correlations, with a randomly drawn ranking of areas for each sample. The correlation of .80 obtained with the a priori ranking was larger in absolute value than 99.5% of the bootstrapped samples, indicating that our ranking based on functional anatomy is a good match to the independently measured position discrimination estimates for each area. The strong correlation between an area's position in the visual processing stream and its bias for representing physical or perceived position reinforces the idea that the nature of position coding evolves as information progresses through the visual processing hierarchy, becoming relatively more strongly tied to perception in higher level areas.