Does spatial attention modulate the earliest component of the visual evoked potential?

Whether visual spatial attention can modulate feedforward input to human primary visual cortex (V1) is debated. A prominent and long-standing hypothesis is that visual spatial attention can influence processing in V1, but only at delayed latencies suggesting a feedback-mediated mechanism and a lack of modulation during the initial afferent volley. The most promising challenge to this hypothesis comes from an event-related potential (ERP) study that showed an amplitude enhancement of the earliest visual ERP component, called the C1', in response to spatially attended relative to spatially unattended stimuli. In the Kelly et al. study, several important experimental design modifications were introduced, including tailoring the stimulus locations and recording electrodes to each individual subject. In the current study, we employed the same methodological procedures and tested for attentional enhancements of the C1 component in each quadrant of the visual field. Using the same analysis strategies as Kelly et al., we found no evidence for an attention-based modulation of the C1 (measured from 50-80ms). Attention-based amplitude enhancements were clear and robust for the subsequent P1 component (90-140ms). Thus, despite using methods specifically designed to reveal C1 attention effects, the current study provided no confirmatory evidence for such effects.