In order to assess in which FOR the BOLD activity associated with covert search in the anterior insula and the SEF was modulated, we calculated the percentage signal change for the ROIs in these two areas in both hemispheres (see Supporting
Information Fig. S1). These ROIs were defined by comparing covert search with the control condition (see ‘Materials and methods’). In all four ROIs, covert search seemed to evoke larger higher BOLD responses than the control condition (see Supporting Information Fig. S1). However, one-way anova across the different search conditions did not yield a significant modulation of the signal change (P > 0.1 for all MI-503 mouse four ROIs). Hence, the search related BOLD response in both the anterior insula and the SEF does not encode the FOR in which covert search operates. We tried to identify the FOR for covert serial search by studying the dependence of cortical BOLD activity, evoked by visual search, on eye-gaze position. Our key observation was that specific parts of the IPS and the right FEF showed a higher BOLD response during covert search to eye-centred contralateral locations, independent of
eye position. In other words, objects singled out in a search array by covert serial search are represented in an eye-centred or retinal coordinate system. However, compared with the left IPS, this effect was weaker in the right pIPS. Early and later Dabrafenib in vivo visual regions similarly exhibited stronger responses for covert
search directed to contralateral eye-centred locations, as expected from their known retinotopic organization. The anterior insula and the SEF did not show the above-mentioned eye-centred modulation of their search related response. Although not very likely, we admit that with the paradigm used we cannot exclude that a modification related to an effector such as the hand or the head could have had a modulatory effect on the clearly eye-centred BOLD responses, which we observed. However, in our paradigm the non-eye-centred search array location did not have any influence on the results, so we think that it is unlikely, though in principle possible to expect different results by changing the head or body position in our experiment. In the following discussion we will first address the question: can our findings based on BOLD responses be reconciled with single-unit studies before on covert visual search? We will then discuss how the evidence for eye-centred coding of covert search provided by our study fits with previous fMRI studies that addressed the reference frame for the encoding of covert as well as overt shifts of attention, i.e. saccades. This comparison seems pertinent, given the fact that overt and covert shifts of attention are tightly coupled and, moreover, usually assumed to share most of their cortical (Rizzolatti et al., 1987; Corbetta et al., 1998) and subcortical (Ignashchenkova et al., 2004) substrates.