A backward progression of attentional effects in the ventral stream

Abstract

The visual processing of behaviorally relevant stimuli is enhanced through top-down attentional feedback. One possibility is that feedback targets early visual areas first and the attentional enhancement builds up at progressively later stages of the visual hierarchy. An alternative possibility is that the feedback targets the higher-order areas first and the attentional effects are communicated "backward" to early visual areas. Here, we compared the magnitude and latency of attentional enhancement of firing rates in V1, V2, and V4 in the same animals performing the same task. We found a reverse order of attentional effects, such that attentional enhancement was larger and earlier in V4 and smaller and later in V1, with intermediate results in V2. These results suggest that attentional mechanisms operate via feedback from higher-order areas to lower-order ones.

Fig. 1.

Latency of attentional modulation of firing rate in areas V1, V2, and V4. (A–C) Red traces represent spike density plots of the average response in each area with attention directed INTO the neuron’s RF (as illustrated by the “spotlight” of attention in the drawing in the top panel of F). Blue traces represent responses with attention directed OUT of the RF (F, Bottom). Responses are shown for area V1 (A), V2 (B), and V4 (C). Responses were aligned to the stimulus onset (0) and were smoothed with a Gaussian window of 30 ms. Shaded areas represent SEM. Vertical black lines represent the onset of the attentional modulation in each area. (D) Distributions of the latency of the attentional modulation are shown for each area. Red: V4; Blue: V2; Green: V1. Arrows denote median latencies for each visual area. (E) Cumulative distribution plot of the latency of attentional modulation in each area; colors represent the three areas as in the previous plot. (F) Cartoon depicting the stimuli in the blocked design task.

Fig. 2.

Magnitude of attentional modulation. Distributions of the magnitude of the attentional effect (1,000–3,000 ms after stimulus onset) are shown for area V4 (A), V2 (B), and V1 (C). Black bars denote cells with significant attentional modulation.

Fig. 3.

Relationship between the magnitude of the attentional effect and the latency of the attention modulation. Scatter plot showing mean magnitude of the attentional effect (average contrast between attention IN and attention OUT from 1 to 2 s after stimulus onset) vs. the latency of attentional modulation for all neurons recorded from area V4 (red), area V2 (blue), and area V1 (green).