To examine whether this facilitation of peripheral target discrimination elicited by the foveal stimulus could also be found with low-level visual features, we tested orientation discrimination of sinusoidal gratings in
Experiments 2a,
2b, and
2c using the same dual task paradigm. No modulatory effect was found when the foveal foil was presented at the fovea 150 ms after the onset of the peripheral target (
Experiment 2a), and the interference effect was observed when the foil and peripheral target gratings were presented simultaneously (
Experiment 2b). Participants' discrimination ability of peripheral target orientation was impaired when the orientation orthogonal to the peripheral target orientation was presented foveally or parafoveally, but this modulatory effect of the foil disappeared when the foil appeared in the periphery (
Experiment 2c). These results in grating experiments indicate that the effect of the foil differed for orientations compared to high-level objects in terms of its effective timing and lack of foveal specificity. Thus, we suggest that our finding in grating experiments is more likely to be explained by the spread of bottom-up feature responses through feature-based attention, which can enhance the processing of a given visual feature across the visual field (Kanai, Tsuchiya, & Verstraten,
2006; Rossi & Paradiso,
1995; Serences & Boynton,
2007; Treue & Martinez-Trujillo,
1999) than by foveal feedback. Perhaps neural activities to the peripheral target were enhanced by the spreading of responses to the feature presented at the fovea. In
Experiment 2b, the disruption effect of the parafoveal foil was not significantly different from that of the foveal foil. These results are consistent with the feature spreading account that attentional facilitation of the features displayed at the fovea can spread to other nonfoveal locations. It should be noted that this account is not suitable for explaining the results found in the object experiments. The modulatory effect of the foveal foil was shown when fine-scale discrimination of complex novel objects, rather than identification of simple visual features, was required. Such discriminations could not be made by attending to one of the objects' constituent features. Therefore, the foveal foil effects with gratings found in
Experiments 2a,
2b, and
2c and with complex objects observed in
Experiments 1a,
1b, and
1c may involve different underlying processes. Whereas foveal feedback contributes to discriminating complex objects in the periphery, feature spreading across the visual field caused by feature-based attention may contribute to the processing of low-level features.