The question whether unconscious stimuli can direct spatial attention is theoretically meaningful because consciousness has been linked to several executive control functions one of which is the voluntary direction of spatial attention (Baars, 1993). Mulckhuyse &
Theeuwes (2010) reviewed several studies on this topic. However, thus far effects have only been reported for peripheral cues (Kentridge, Heywood, & Weiskrantz, 1999; Lambert, Naikar, McLachlan, & Aitken, 1999; McCormick 1997, Mulckhuyse, Talsma & Theeuwes, 2007, Scharlau & Ansorge, 2003) and spatially compatible and socially relevant central cues like arrows and eye gaze (Cole & Kuhn, 2010; Reuss, Pohl, Kiesel & Kunde, 2011; Sato, Okada & Toichi, 2007).
1.3.1 Peripheral cues
Scharlau and Ansorge (2003) provided evidence that subliminal peripheral cues at the target location can attract spatial attention. In their experiments, metacontrast masked primes were presented before targets in a temporal order judgment task. The authors found that primes which matched the shape of potential targets and were thus consistent with participants’ search settings facilitated the perception of targets at the same location relative to perception of targets at other locations; a phenomenon termed “perceptual latency priming”.
This effect can be explained by the idea that primes with task relevant shapes capture attention like an exogenous cue even when they are masked. Mulckhuyse and colleagues (2007) also found effects of masked peripheral cues that consisted of one of three dots that appeared slightly earlier than the other two in a detection task. They concluded that subliminal sudden onsets can capture attention independently of participants’ control settings.
These studies suggest that exogenously controlled shifts of attention do not require conscious
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perception of cue stimuli, because peripheral cues can be effective even when participants are unaware of them.
McCormick (1997) reported evidence for a role of awareness in spatial orienting. In his experiment, cues were presented either above or below a subjective threshold of awareness and predicted a target at the opposite side of the screen. With invisible cues participants involuntarily shifted their attention to the location of the dimmed cue. In contrast, when the cue was visible, they were able to use the information conveyed about the likely target location to shift their attention voluntarily away from the cue. The author concluded from this that endogenous orienting requires executive control which depends on the awareness of cue stimuli.
1.3.2 Central cues
More recently, three studies reported effects of masked centrally presented cues (Cole
& Kuhn, 2010; Reuss, Pohl et al., 2011; Sato, Okada & Toichi, 2007). Sato and colleagues (2007) presented masked face stimuli at fixation assuming that gaze direction operates as a spatial cue. Even though cues were not predictive for target locations, participants localized targets more rapidly when they appeared on the side where the gaze cue pointed to than on the opposite side. Cole and Kuhn (2010) found that participants were faster to identify peripheral targets when targets were preceded by a centrally presented masked arrow that pointed to the target location than when the arrow pointed to the opposite location, although arrows were non-predictive for target locations. On the one hand, findings of these two studies with centrally presented cues could be conceived as demonstrations of endogenously governed shifts of attention that are induced by unconscious cues. However, the cueing effects can also be explained without the assumption that endogenous mechanisms are affected by unconscious cues, if one takes into account that these specific stimuli (arrows and eye gaze)
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can affect attention involuntarily (Ristic & Kingstone, 2006). Therefore, findings could be conceived as instances of involuntary, automatic shifts of attention which can be induced only by spatially compatible cues (Lambert et al., 2006).
Reuss, Pohl and colleagues (2011) proposed that the effect of centrally presented masked cues depends on intentionally set top down influences. Again, arrow stimuli were used as spatial cues for a letter discrimination task. Cues were either valid or invalid indicators of target locations and they were presented masked or unmasked. In contrast to Sato and colleagues (2007) and Cole and Kuhn (2010), Reuss and colleagues reported that masked cues were only effective when they validly predicted the target location whereas visible arrow cues produced validity effects even when they were not predictive for target locations.
1.3.3 Implicit learning
In addition, later studies suggest that unseen peripheral cues can direct attention to the opposite side of the cues’ location by implicit learning of a predictive relationship between the location of subliminal peripheral cues and target locations (Lambert et al., 1999;
Kentridge et al., 1999). Lambert et al. found that participants could learn to use peripheral cues when the cues were predictive for a target on the opposite side of the screen, even though participants were unable to articulate this predictive relationship. Corresponding to authors’
implicit peripheral cueing hypothesis, participants responded to the implicitly informative cues only when they were unaware of the relationship and when they were unaware of the cues. These findings contribute to the view that unconscious peripheral cues can modulate visual orientation in a way which goes beyond automatic, exogenously governed shifts of attention to the location of the cue (Lambert et al., 1999). Kentridge and colleagues (1999) found a similar effect in blindsight patient G.Y.. Peripheral cues were presented in G.Y.’s
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blind field which predicted that a target would appear at another location. After some training, G.Y. was able to shift attention away from the cue towards the probable target location.
Shifting attention voluntarily away from a cue to the predicted target location cannot be explained by simple mechanisms of automatic shifts of attention to the location of the cue but can be conceived an instance of controlled processing because it requires an interpretation of the meaning and the predictiveness of the cue.