Attention and consciousness/how to manipulate attention
Psychophysical tools to manipulate top-down attention
Top-down attention and consciousness are usually tightly coupled. To dissociate these two, experimental tools that manipulate either one independently in a specific manner with few side effects are called for.
There exist at least two forms of selective attention: stimulus-driven, bottom-up, saliency-mediated attention as well as task- and goal-dependent top-down attention. Previously neutral stimuli (such as text, or abstract images) can be associated with reward or punishment to acquire additional saliency. Biologically relevant stimuli may be preferred or disliked based on individual differences (e.g., snakes, spiders, and nude pictures).
A variety of techniques to manipulate these components of attention has been invented. It is not always easy to compare them, as each method interferes with attention at a different level of processing (Sperling and Dosher, 1986; VanRullen et al., 2004).
In Posner’s cueing paradigm, popular in the study of orienting (Posner et al., 1980), a target is preceded by an informative or a non-informative cue that appears at the target location or at fixation. Attentional effects are inferred in terms of reaction time and/or accuracy of target detection. Variants of the method demonstrated that an invisible cue can direct exogenous attention to a particular spatial location (McCormick, 1997; Kentridge et al., 2004; Rajimehr, 2004; Feng et al., 2006; Jiang et al., 2006; Sumner et al., 2006), clear support for the orienting of exogenous attention without the intervention of consciousness.
In visual search, subjects need to find a target among distractors; reaction time is related to the number of distractors. When the search slope is steep, the search process is said to be serial, and when flat, parallel. The former is usually taken as the evidence of serial processing by top-down attention. However, the steep serial search may arise due to completely bottom-up factors (VanRullen et al., 2004). This exemplifies a case where dual-tasks and visual search methods may yield inconsistent results.
The dual-tasks paradigm (Sperling and Dosher, 1986; Braun and Sagi, 1990; Braun and Julesz, 1998) manipulates top-down, focal attention without affecting bottom-up saliency: a central, attentionally-demanding discrimination task is present at the center of gaze, while a secondary stimulus is projected somewhere into the periphery (Figure 1). Subjects carry out either the central, the peripheral, or both tasks simultaneously while the scene and its layout remain the same.
Surprisingly, seemingly complex peripheral tasks can be done equally well under either single or dual-task condition (Li et al., 2002; Reddy et al., 2004; Reddy et al., 2006) (Figure 2, left), while other, computationally simpler tasks deteriorate when performed simultaneously with the central task (Figure 3, right). The dual-task paradigm quantifies what type of stimulus attributes can be signaled and possibly consciously perceived in the near absence of spatial attention (VanRullen et al., 2004).
Independent manipulation of top-down attention and visibility
Most importantly, the dual-task paradigm can be combined with a multitude of visual illusions that render stimuli invisible, allowing the independent manipulation of top-down attention and consciousness (Figure 3), although such a full factorial analysis for many popular experiments awaits future work Attention and Consciousness/Opposite Effects.
Caveat of dual-task paradigm
The inference of attentional requirements from dual-task performance demands caution. High proficiency in such tasks is only achieved after extensive training of many hours. Such an extended training phase renders the experience of the task quite different for trained subjects from what naïve subjects experience (Joseph et al., 1997; Braun, 1998).
Finally, there is a class of neurological conditions as well as visual illusions in normal subjects where stimuli become invisible because of impairments in the mechanisms of top-down or bottom-up attention. Hemineglect and extinction (Driver and Mattingley, 1998), attentional blink (Raymond et al., 1992; Chun and Potter, 1995), inattentional blindness (Mack and Rock, 1998), and change blindness (Simons and Rensink, 2005) are sometimes used as positive evidence for “without attention, no consciousness” (O'Regan and Noe, 2001). Although some attributes of the visual input need attentional amplification to rise to the level of consciousness, other aspects, such as the gist of the scene and its emotional content, are quite resistant to such attentional manipulations (Mack and Rock, 1998; Anderson and Phelps, 2001).
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