 neurons signal the deviation. One potential source for conveying the long range predictions in the case of an extended bar could be feedback from higher visual areas with larger receptive fields. This hypothesis was elegantly tested in Nassi et al. [@Nassi_2013] using reversible inactivation of V2 paired with V1 recordings in the macaque. As illustrated in the left side of Fig. \[length\_suppression\], cryoloop cooling of V2 led to a significant reduction in length suppression, indicating that feedback from V2 to V1 is essential for the effect.

![Length suppression. Left: Responses of example macaque V1 units to bars of different lengths before (red), during (blue), and after (green) inactivation of V2 via cryoloop cooling. Reproduced with permission from [@Nassi_2013]. Right: PredNet after training on the KITTI dataset – average $E_1$ and $A_1$, and examples. Red: Original network. Blue: Feedback weights from $R_2$ to $R_1$ set to zero.[]{data-label="length_suppression"}](length_suppression_relu.pdf){width="100.00000%"}

The right side of Fig. \[length\_suppression\] demonstrates that length suppression, and its mediation through top-down feedback, are also present in the PredNet. The upper left and right panels contain the mean normalized response for units in the $E_1$ and $A_1$ layers, respectively, to bars of different lengths. The red curves correspond to the original network (trained on KITTI) and the blue curves correspond to zero-ing the feedback from $R_2$ to $R_1$. For each filter channel, a set of 2D Gabor wavelet stimuli was first used to determine the optimal orientation. Responses to bars of different length at this orientation were then measured, as a sum of the activations over stimulus duration ($10$ time steps). The bottom row contains exemplar $E_1$ and $A_1$ units. Quantifying percent length suppression (%LS) as $100*\frac{R_{max} - R_{longest \: bar}}{R_{max}}$, the median decrease in %LS upon removing top-down signaling was $16$% for $E_1$ units ($p < 0.05$, Wilcoxon signed rank test)