  The human brain prioritises relevant sensory information to perform different
tasks. Enhancement of task-relevant information requires flexible allocation of
attentional resources, but it is still a mystery how this is operationalised in
the brain. We investigated how attentional mechanisms operate in situations
where multiple stimuli are presented in the same location and at the same time.
In two experiments, participants performed a challenging two-back task on
different types of visual stimuli that were presented simultaneously and
superimposed over each other. Using electroencephalography and multivariate
decoding, we analysed the effect of attention on the neural responses to each
individual stimulus. Whole brain neural responses contained considerable
information about both the attended and unattended stimuli, even though they
were presented simultaneously and represented in overlapping receptive fields.
As expected, attention increased the decodability of stimulus-related
information contained in the neural responses, but this effect was evident
earlier for stimuli that were presented at smaller sizes. Our results show that
early neural responses to stimuli in fast-changing displays contain remarkable
information about the sensory environment but are also modulated by attention
in a manner dependent on perceptual characteristics of the relevant stimuli.
Stimuli, code, and data for this study can be found at https://osf.io/7zhwp/.
