ERP INDICES OF THE SIZE OF ATTENTIONAL FOCUS IN SPATIAL ORIENTING

While many studies on visuo-spatial attention have suggested that the attentional focus can be directed toward a single spatial location [1], other studies have provided evidence that attention may be allocated to non-contiguous locations [2]. The aim goal of this research is to further investigate the effects of spatial attention on visual processing. The data demonstrate that the distribution of attentional resources may vary from a stage in which they are highly concentrated or focused on an area of about 1° 30' of visual angle (at P1 level) to a stage in which they are uniformly spread over the entire visual field (at P3 level). More in detail, at the earliest sensory stage, over striate and extra-striate visual cortex the focus of spatial attention seemed to be delimitated to a rather restricted area. In this way, at the lowest level, selective attention acts like an early filter able to confine the irrelevant information outside the attentional focus [6]. This result can be also interpreted in term of 'spotlight' analogy. At a later level, the P3 component evidenced a benefit for stimuli falling to non-target space positions not-far form the target location. This suggests the spreading of spatial attentional benefits within the attended hemi-field. Actually, this effect might reflect a certain difficulty in spatial discriminability between target and non target locations at a decision making stage. In conclusion, our results indicate that the size of attentional focus may vary as a function of processing stage, being it stricter at the earliest sensory levels.