When spatial attention cannot be divided: Quadrantic enhancement of early visual processing across task–relevant and irrelevant locations

Özkan, Mert; Störmer, Viola · 2024 · Crossref

DOI: 10.1101/2024.03.19.585785

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Summary

This study investigates how spatial attention is distributed within a single visual quadrant when participants are instructed to attend to multiple non-contiguous locations, a scenario where previous research suggests attentional resources cannot be effectively split. The authors hypothesized that if attention cannot be divided, the distribution of resources in a "split-focus" condition (attending to two separate locations) would resemble that of a "broad-focus" condition (attending to a contiguous region), specifically resulting in enhanced processing at the intermediate, uncued location. To test this, the researchers conducted three experiments using behavioral tasks and steady-state visual-evoked potentials (SSVEPs) to measure early visual-cortical processing with high spatial resolution. In Experiment 1, 30 participants performed a probabilistic cueing task involving narrow, broad, and split-focus conditions. Behavioral results showed a significant main effect of cue validity, with better performance at cued locations, but no significant difference in overall performance between the broad and split-focus conditions. Critically, performance at the intermediate location did not differ significantly between being cued (broad-focus) and uncued (split-focus), suggesting that the intermediate location was processed similarly regardless of task instructions. Experiments 2 and 3 utilized EEG to record SSVEPs from 31 participants, allowing for the direct measurement of cortical processing at distinct locations flickering at different frequencies (~10.91, 12, and ~13.33 Hz). The results revealed that attentional modulation in the early visual cortex was strongest at the intermediate location in both the split-focus and broad-focus conditions. This "quadrantic enhancement" indicated that when participants attempted to split attention across two non-contiguous locations within a hemifield, the intermediate location received the highest boost in visual-cortical processing, even though it was an uncued, to-be-ignored location. This pattern was indistinguishable from the broad-focus condition, where the intermediate location was part of the attended region. The findings demonstrate that sustained spatial attention within a visual quadrant cannot be effectively divided among non-contiguous locations. Instead, attentional resources are distributed in a manner that inadvertently enhances processing at the intermediate location, regardless of whether that location is task-relevant or irrelevant. This suggests a fundamental constraint in the architecture of early visual processing, where the spatial spread of attention within a hemifield leads to a uniform enhancement across the cued region and the intervening space, rather than a selective focus on discrete targets. These results provide neural evidence for the inefficiency of divided attention within a single visual half-field and highlight the role of intermediate locations in shaping early visual processing.

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Topics

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• attention selective divided
• attention allocation
• useful field of view
• peripheral attention