The allocation of attentional resources across the visual field: Impact of eccentricity and perceptual load

Every second our cognitive system is confronted with complex visual scenes that contain a variety of different objects. As a result of the limited processing capacity of the visual system, they cannot be fully processed at once. Therefore, attentional mechanisms determine the selection of information in favour of behaviourally relevant stimuli for enhanced processing, thereby providing a mechanism for the filtering of irrelevant and distracting information. Attention can be directed to specific parts of the visual field, where information processing is facilitated in contrast to unattended locations. The study of such visual spatial attention has been dominated by spotlight theories , claiming that selective attention can be compared to a spotlight illuminating anything located within the region covered by its beam. Several theories have been proposed using the idea of a spotlight as a metaphor to account for the distribution of attentional resources in visual space: single spotlight , zoom lens , and multiple spotlights models. Recent studies have provided support for an ability to split this attentional focus to selectively process spatially separate locations.Using functional magnetic resonance imaging (fMRI) the signature for the presence of multiple spotlights of attention is the presence of multiple retinotopically specific foci of activation in striate and extrastriate visual areas. This approach was used to investigate the presence of such separable activations as a function of the eccentricity of the spatial foci of attention. Visual stimuli consisted of letters and digits displayed in rapid serial visual presentation (RSVP). Five RSVP streams were presented simultaneously, one in the center of the visual field and one in each visual field quadrant. Subjects had to deploy their attention either to a single peripheral location or two non-contiguous regions performing a match-mismatch judgement.The results show that dividing attention leads to multiple spotlights of attention for central as well as more peripheral locations of the visual field. However, depending on the exact location and width of the attentional spotlights, resulting activation maps might reveal merged activation patterns even in the presence of distinct attentional spotlights.As outlined above, selective attention devotes cognitive resources to behavioural relevant stimuli and events thereby enhancing the processing of attended relative to unattended information. A primary way to determine the importance of a stimulus or event is to evaluate the emotional significance. Emotionally significant stimuli receive enhanced processing either preattentively ( automatically ) or are given priority in the competition for selective attention. The amygdala has been identified to play a crucial role in the processing of emotional stimuli. It provides a coarse, rapid and automatic perception of emotional stimuli based on a subcortical route via the superior colliculus and the pulvinar nucleus of the thalamus, whereas a fine-grained perceptual representation of emotion-laden stimuli involves the occipital and temporal visual neocortex. Recent neuroimaging studies have produced conflicting results regarding the issue whether the processing of emotion-laden stimuli is dependent upon the availability of attentional resources or entirely capacity free. Two main factors have been proposed to be responsible for the discrepancies: the differences in the perceptual attentional demands of the tasks used to divert attentional resources from emotional stimuli and the spatial location of the affective stimuli in the visual field.The effects of variable attentional load as well as different stimulus locations on the processing of emotional faces in the amygdala were investigated using fMRI. The visual display consisted of four RSVP streams located in each visual field quadrant and pairs of faces depicting emotional expression located at three different positions along the horizontal meridian of the visual field. Participants had to either direct their attention to the visual streams performing tasks of high and low attentional load or to the faces matching for gender or expression. The results revealed a strong attenuation of amygdala activity with higher attentional load. The spatial location of the emotional stimuli did not affect the neuronal response in the amygdala. These findings lend support to the concept that the processing of emotional stimuli is modulated by selective attention as the inhibition of amygdala response during attentionally demanding tasks reflects the dependence upon processing resources.