What are the key components that lead to the variability in FVF size and thus to less efficient search performance? One explanation for such individual differences might start at the retina. As is well known, the retina is characterized by a systematic decrease in photoreceptor density with eccentricity. Moreover, this decline varies considerably across individuals (
Curcio et al., 1990). Correlations between the accuracy of the target-only condition at the largest eccentricity of the crowding task, on the one hand, and search RTs as well as number of fixations, on the other, suggest that limitations at the retinal level could already be predictive of search performance (at least for the furthest eccentricity). However, when analyzing the average discrimination performance in the target-only condition, we observed no reliable difference in accuracy between eccentricities and well above chance performance. This makes acuity per se an unlikely cause. Perhaps then, the important factor is again processing speed, in that observers feel that they cannot resolve the fringes of their FVF
in time and thus decide to make another eye movement. Another potential source of individual differences lies at the cortical level (which in turn may or may not find its source in retinal differences). A recent neuroimaging study showed that crowding performance related to the size of population receptive fields (pRFs), specifically in V2, where larger pRF sizes came with stronger crowding (
He & Fang, 2019). Receptive field overlap is thought to be a major cause of crowding, leading to mutual suppression of signals, pooling, or both (
Levi, 2008). Important within the present context, given that pRF size can predict the magnitude of crowding across individual subjects, the distinct possibility arises that pRF size will also predict visual search performance. Finally, there may be individual differences in feedback projections that shape the FVF, for example, through covert attention. Several studies have shown that manipulating attention on search and acuity tasks has effects that are most pronounced at peripheral locations (
Carrasco & Yeshurun, 1998;
Carrasco et al., 2002;
Grubb et al., 2013;
Yeshurun & Carrasco, 1999; for a review, see
Anton-Erxleben & Carrasco, 2013). Although to our knowledge, it has never been tested, it is likely that the strength of such attentional modulations differs across individuals, thus contributing to the size and flexibility of the FVF. Future work will be needed to test these possibilities.