The Neural Mechanisms of Visual Attention
Attentional selection mechanisms in visual cortex involve changes in oscillatory activity in the EEG alpha band (8 to 12 Hz) – decreased alpha indicating focal cortical enhancement and increased alpha indicating suppression. This has been observed for spatial selective attention and attention to stimulus features such as color and motion. Theoretical work suggests that the control of oscillatory neural activity may be a crucial component of the mechanisms of attentional signal propagation through cortex, and in particular, the modulation of alpha band activity might constitute an information routing mechanism across all of visual cortex. This hypothesis leads to the prediction that alpha band modulation should also accompany attention to high-level visual features, such as object categories. To test this prediction, we investigated whether attention to objects involves alpha-mediated changes in focal cortical excitability. We conducted three experiments to look for systematic differences in the pattern of EEG alpha power across the scalp, and assess whether any such systematic differences were driven by the engagement of object-based attention to different object categories. In Experiment 1, twenty volunteers (8 males; 12 females) were cued (80% predictive) on a trial-by-trial basis to different objects (faces, scenes, or tools).¬ Support vector machine decoding of alpha power patterns revealed that late (>500 msec latency) in the cue-to-target foreperiod, only EEG alpha differed with the to-be-attended object category. In Experiment 2, to eliminate the possibility that decoding of physical features of the cues led to our results, twenty-five participants (9 males; 16 females) performed a similar task where cues were non-predictive of the object category. Alpha decoding was now only significant in the early (<200 msec) foreperiod. In Experiment 3, to eliminate the possibility that task set differences between the different object categories led to our Experiment 1 results, twenty participants (9 males; 11 females) performed a predictive cuing task where the discrimination task for different objects was identical across object categories. The results replicated Experiment 1. Together, these findings support the hypothesis that the neural mechanisms of visual selective attention involve focal cortical changes in alpha power for not only simple spatial and feature attention, but also high-level object attention in humans. These findings support the Specificity of Control model of attention, according to which a top-down source of attentional control signals issues instructions over individualized channels to sensory sites according to the particular receptive properties of those sites, and an identical mechanism to elaborate information processing operates at the sensory sites. We followed up the planned analyses with a series of ERP decoding analyses to corroborate our theoretical interpretation, and to provide evidence that alpha band modulation is directly related to an immediate perceptual benefit afforded by attention. Altogether, our findings suggest that the modulation of oscillatory neural activity in the alpha frequency band is an essential component of the enhancement mechanism of attention, operating throughout visual cortex.