Figure 4. The Slow Drift in V4 Activity Is Larger than the Effect of Spatial Attention.

(A) Neural responses (orange and green dots) along the slow drift axis were unrelated to how blocks of trials were cued (black lines versus orange and green lines, |ρ| = 0:06) in an example session. Dot color indicates whether the cued stimulus location was within (“cue-in,” orange) or outside (“cue-out,” green) the receptive fields of the V4 neurons. Black lines indicate the mean activity along the slow drift axis within each cued block. The magnitude of firing rate changes of the slow drift ${\sigma }_{\text{slow}\text{drift }}^2$ was measured as the variance of the vertical levels of the black lines.

(B) For the same session as in (A), responses along an attention axis capturing the largest difference between mean spike counts during cue-in versus cue-out trials. The effect of attention ${\sigma }_{\text{attn }}^2$ was measured as the variance of the mean value of responses averaged within each block, where the variance was taken across blocks (i.e., the spread of the orange and green lines).

(C) Ratios ${\sigma }_{\text{slow}\text{drift }}^2/{\sigma }_{\text{attn }}^2$ across sessions. Medians (“med”) are also shown.