Our results suggest that the spatial pattern of both exogenous and endogenous attention does not change significantly across time; only its magnitude is modulated. In other words, the spotlight does not systematically shrink or expand as a function of time nor does it revert its polarity in any specific spatial subregion (e.g., from facilitation to inhibition). This result constrains the neural mechanisms that could support the deployment of visual attention. For example, it is compatible with a simple model composed of two independent neural populations. The first neural population, retinotopically organized, would encode the selected region of the visual field (the shape and the coordinates of the spatial attention pattern) in the form of a “saliency map,” as postulated by Itti and Koch (
2000). The second neural source would represent the magnitude of attentional modulation as a function of time. In this model, the neural population that represents the spatial attention pattern (i.e., the saliency map) would receive global modulatory input from the second neural source, and there would thus be no dependence between the spatial pattern of attention and its magnitude modulation over time. Several putative anatomical localizations have been suggested for each of these two populations. The equivalent of a “saliency map” for attention has been proposed to involve subcortical areas such as the pulvinar (Laberge & Buchsbaum,
1990; Robinson & Petersen,
1992), the lateral geniculate nucleus (Koch & Ullman,
1985), and the superior colliculus (Kustov & Robinson,
1996), as well as cortical locations such as V1 (Li,
2002), V4 (Mazer & Gallant,
2003), the frontal eye field (Thompson & Schall,
2000), and the posterior parietal cortex (Gottlieb,
2007). Neural populations that would gate the temporal modulations of attention have been identified by neuroimaging techniques in different regions such as the intraparietal sulcus, the lateral inferior premotor cortex, the cerebellum (Coull & Nobre,
1998), frontoparietal areas, and the thalamus (Fan, McCandliss, Fossella, Flombaum, & Posner,
2005; Posner, Sheese, Odludaş, & Tang,
2006). Note finally that the region providing the “temporal signal” for attention may not need to activate the saliency map with the precise time course that we have recorded here. Rather, one could also imagine that a simple “trigger” signal is sent to the saliency map. In this case, the intrinsic temporal properties of neuronal circuits within the saliency map would have to match the temporal modulation patterns observed in our experiments.