Visual working memory and attention in early visual cortex

Abstract

Objective: Does early visual cortex (V1–V3) play a role in maintaining working memory and attention? Methods: Subjects were scanned (3T fMRI, BOLD) while performing each of four tasks designed to probe visual working memory, attention, or both. Delayed comparison: A high-contrast grating (randomized orientation and spatial frequency) was presented within an annulus (1–3°) around fixation followed, after a variable delay (1–16s), by a second high-contrast grating with near-threshold change in orientation and spatial frequency. Fixation point color then cued subjects to indicate either the orientation or spatial frequency change. Cued detection: Stimuli were nearly identical except the contrast of the final target grating was at detection threshold and subjects indicated its presence or absence. Detection: Same as cued detection except the probe and test orientations were chosen independently. Discrimination: Stimuli were identical to the detection task except the target had one of two orientations (tilted slightly right or left of vertical), and subjects indicated its orientation. Results: Despite the similarity between the stimuli and tasks, we found a striking difference in cortical activity. Areas V1–V3 exhibited no sustained activity during the delay period for the delayed comparison task (which required working memory for easily visible stimuli), but robust sustained activity for the other three tasks (which all had barely visible targets). Conclusions: The results imply a dissociation between working memory and attention. We hypothesize that attentional selection, critical for scrutinizing stimuli that are barely visible, is implemented by sustained boosting of relevant neuronal signals in early visual cortex.