B.3.6.1.2. Relevance

Beside this exogenous biasing, competition could also be modulated endogenously, on the basis of prior information available in working memory. One could select a feature, dimension or template, within the visual field, so as to improve its processing relative to others. It is well known that orienting attention to a spatial location could improve the processing of items lying there (Jonides, 1981), but non-spatial features could also be selected attentionally. For instance, Von Wright (1968), using the partial report methodology proposed by Sperling (1960), showed that one can select some items efficiently on the basis of their colour, location or size. The biased competition hypothesis considered that the endogenous influences affected the perceptual processing by the top. Indeed, some representations in working memory –called templates– are thought to modulate, or bias, the competition in the perceptual system in the favour of the relevant items, features or dimensions (Desimone & Duncan, 1995; Duncan, 1996, 2006).

Many neurophysiological data confirmed this possibility, and showed that attending a feature enhanced the activation of the areas particularly involved in this feature's processing (see Maunsell & Treue, 2006 for a review). Let us consider the example of colour. Several results showed that orienting attention to colour induced modulation in a brain area involved in processing colour, the inferior occipital area V4 (Bartles & Zeki, 2000). For instance, Schoenfeld et al. (2007) nicely evidenced that attending colour modulated V4 activity. They obtained congruent results from EEG, MEG and fMRI and showed that these modulations began at 90-120 ms post-stimulus. These results confirmed and extended older results (e.g. Anlle-Vento, Luck, & Hillyard, 1998; Corbetta et al. 1991; Clark et al. 1997; Chawla, Rees, & Friston, 1999). Similarly, attending a feature could modulate activities in other cerebral areas according to their perceptual specialization, for instance for motion (Treue & Martinez-Trujillo, 1999; Treue & Maunsell, 1996), or orientation (Haenny, Maunsell, & Schiller, 1988).

These endogenous influences are not only "late" top-down modulations, but could act by biasing the baseline of neuronal response from perceptual areas (V1, V2 and V4), thus before the appearance of the effective stimuli. Indeed, both selecting a spatial location (Kastner, Pinsk, DeWeerd, Desimone, & Ungerleider, 1999; Luck, Chelazzi, Hillyard, and Desimone, 1997) and a non-spatial feature or dimension (Bichot, Rossi, & Desimone, 2005; Serences & Boynton, 2007) could enhance the baseline neuronal response within the visual system. This also induced rather fast modulation of neuronal response after stimulus presentation (Schoenfeld et al., 2007). Recently, Zhang and Luck (2009) even showed that attending a colour could modulate the feed-forward flow of sensory processing within 100 ms of stimulus onset, even at unattended locations. These influences are likely to be mediated by feedback projections from frontal or parietal afferences that triggered neurons responding to the relevant feature at hand (Corbetta & Shulman, 2002; Miller & Cohen, 2001).