However, if eye movements are controlled by the task, how does one access perceptual information that is not on the current agenda? In normal ongoing behavior, it is not always possible to anticipate what information is required. How does the visual system divide attention between current task goals and unexpected stimuli that may be important and may change the task demands? This issue has been referred to as the “scheduling” problem (Hayhoe,
2000; Shinoda et al.,
2001). These authors suggest two possible answers to this problem. One is that attention is attracted exogenously by the stimulus. The other possibility is that attention is attracted endogenously according to the observer's internal agenda. Traditionally, basic visual responses have been thought to be driven from the “bottom–up” by the properties of the stimulus. Two lines of research, attentional capture and inattentional blindness, provide insight into how unexpected stimuli are noticed. Folk and Gibson (
2001) referred to attentional capture as instances in which attention is drawn to stimuli without the subject's volition. Implicit measures (e.g., response times and eye movements) are generally relied on to infer shifts of attention (Jonides & Yantis,
1988; Theeuwes, Kramer, Hahn, & Irwin,
1998). Experiments by Theeuwes (
1992,
1994), Jonides and Yantis (
1988), Yantis and Hillstrom (
1994), and others found that an abrupt onset of a salient feature singleton may capture attention in a stimulus-driven, bottom–up fashion. This conclusion has been challenged by others, who claim that the ability of even stimuli such as a unique color or abrupt onset to attract attention is modulated by the current attentional set (Folk, Remington, & Johnston,
1992; Folk, Remington, & Wright,
1994; Gibson,
1996a,
1996b; Gibson & Kelsey,
1998). The inattentional blindness studies, on the other hand, directly probe subjects' awareness of unexpected stimuli. For example, stimuli that capture attention implicitly may not capture awareness (McCormick,
1997). Recent studies of inattentional blindness have tried to elucidate the factors that lead to noticing of unexpected objects (Mack & Rock,
1998; Newby & Rock,
1998). In these studies, the authors used brief presentations of simple shapes and found that about 25% of subjects reported no awareness of the unexpected item (Mack & Rock,
1998). More recently, Most, Simons, Scholl, and Chabris (
2000), Most et al. (
2001), and Scholl, Noles, Pasheva, and Sussman (
2003) used sustained and dynamic computerized task to assess the factors that might lead to noticing unexpected events. The authors found that almost 30% of the subjects failed to notice a unique item that traveled across the display for 5 s. Thus, the extent to which unexpected stimuli are processed is unclear. The extent to which these findings generalize to natural vision is also unclear. In ordinary life, the visual system deals with the entire visual field and not with displays that usually subtend only a small portion of the visual field by the use of simple, easily segmented geometric forms as stimuli. Unrestrained subjects generate head and body movements that can create a continuous stream of image motion. Also, the temporal evolution of behavior occurs over several minutes, which is not easily addressed in standard experimental paradigms. In a recent video-based study on selective looking and inattentional blindness, Simons and Chabris (
1999) explored whether the unusualness of an unexpected object influences the likelihood of detection. They found that subjects frequently (73%) failed to report a salient visual object in the context of a competing task set, thus suggesting that top–down signals may underlie the phenomenon of “inattentional blindness.” However, the relative role of top–down control versus bottom–up salience in natural vision is not clear.