Rapid enhancement of visual cortical response discriminability by microstimulation of the frontal eye field

Armstrong, Katherine M.; Moore, Tirin · 2007 · Crossref

DOI: 10.1073/pnas.0701104104

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Summary

This study investigates whether subthreshold microstimulation of the frontal eye field (FEF) enhances the discriminability of visual cortical responses, a hallmark of voluntary spatial attention. While previous research established that FEF microstimulation improves behavioral attention and increases response magnitude in area V4, it remained unclear if this manipulation also improves the ability of V4 neurons to discriminate between different visual stimuli. The authors sought to determine if FEF stimulation mimics the neural mechanisms of voluntary attention, specifically by enhancing signal discriminability without altering response reliability. The researchers recorded single-unit activity from V4 neurons in monkeys performing a passive fixation task. They presented stable visual stimuli (oriented bars) within the neurons' receptive fields and applied brief (20–50 ms) subthreshold microstimulation to FEF sites whose saccade vectors were spatially aligned with the V4 receptive fields. To quantify discriminability, they used receiver-operating characteristic (ROC) analysis to calculate the area under the curve (AROC), which measures how well neuronal responses distinguish between stimuli. They compared discriminability during control trials versus stimulation trials, examining both the magnitude of the effect and its timing relative to stimulation onset. Additionally, they analyzed the relationship between response magnitude and variance to assess changes in response reliability. The results demonstrated that visual response discriminability in V4 neurons naturally decayed over time during the presentation of stable stimuli. However, FEF microstimulation transiently restored this discriminability. This enhancement was spatially specific, occurring only for stimuli aligned with the saccade vector evoked by the stimulation site, not for misaligned stimuli. Crucially, the improvement in discriminability resulted solely from an increase in the magnitude of responses to preferred stimuli, with no change in response reliability (the relationship between mean response and variance). Timing analysis revealed that the enhancement in discriminability appeared immediately after the offset of the microstimulation train, within 40 ms of the first pulse. In contrast, a simulated visual phosphene disrupted discriminability, confirming that the FEF effect was not due to masking or indirect visual cues. These findings indicate that neural circuits involved in voluntary saccade production directly modulate visual representations in the cortex during covert attention. The rapid onset of the effect suggests a direct influence of FEF signals on V4 processing rather than an indirect effect mediated by a subjective visual experience. The study provides strong evidence that saccade-related signals serve as a source of spatial attentive selection, mirroring the neural signatures of voluntary attention by enhancing signal magnitude without compromising response reliability.

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