Spatial attention and lateralized pre-stimulus alpha shape iconic memory via inhibition

Abstract

The earliest stage of visual memory is often characterized as pre-attentive, yet recent work suggests that spatial attention and pre-stimulus alpha oscillations both shape iconic memory at its onset. Whether these influences reflect a common inhibitory mechanism or contribute independently is unknown. We combined an endogenous spatial pre-cue (valid, invalid, neutral) with an analysis of pre-stimulus alpha power in a partial-report paradigm, at target-to-post-cue onset asynchronies (SOAs) of 0, 120, and 1240 ms, while recording EEG from 45 participants. Behaviorally, perceptual sensitivity (d') showed a cost-dominated signature: invalid cues impaired performance relative to both neutral and valid cues at the 0 and 120 ms SOAs, with no benefit of valid over neutral cues. The pre-stimulus EEG mirrored this asymmetry: at 120 ms, stronger alpha power ipsilateral to the target predicted higher d', whereas contralateral alpha had no effect at any SOA, indicating suppression of the irrelevant hemifield rather than facilitation of the target. This ipsilateral effect was additive with cue validity rather than interactive, and it persisted in neutral trials without any directional cue. Iconic memory is thus continuously biased by the inhibitory state of early visual cortex, to which endogenous attention and spontaneous excitability fluctuations contribute through separable channels. Significance StatementWhether the earliest stage of visual memory depends on attention has long been debated. We show that the same asymmetry with which spatial attention shapes iconic memory--misdirecting attention impairs memory, but correctly directing it yields no benefit--also appears in EEG alpha power. Stronger prestimulus alpha power over the hemisphere ipsilateral to the target improves memory, while activity over the target-processing hemisphere does not. This alpha effect occurs even without any spatial cue, showing that it does not depend on deliberate attention. Because the cue-driven and spontaneous alpha effects add together rather than interact, they reflect two separable routes to the same outcome: suppression of irrelevant input. Early visual memory is thus continuously gated by the inhibitory state of visual cortex.