Inattentional blindness is the failure to notice a fully-visible, but unexpected object because attention was engaged on another task, event, or object.
This phenomenon is related to but distinct from other failures of visual awareness such as change blindness, repetition blindness, visual masking, and the attentional blink. In most cases, studies of inattentional blindness involve a single critical trial in which an object appears unexpectedly while observers are performing their task. At the end of the trial, observers are asked a series of questions to determine whether or not they saw the unexpected object.
Overview of inattentional blindness research
The term “inattentional blindness” was coined by Arien Mack and Irvin Rock to describe the results of their extensive studies of the visual perception of unexpected objects. Many of their studies from the early 1990s culminated in their 1998 Book entitled “Inattentional Blindness” (Mack & Rock, 1998). In their canonical task, observers view a briefly-presented cross on a computer display and attempt to judge whether the horizontal or vertical arm of the cross is longer. On a critical trial, an additional shape appears in the display, and after the trial, observers are asked whether they noticed anything other than the cross on that trial. Subsequent trials examine whether observers notice the shape now that it is expected (they know it can appear). Such trials were described as divided attention trials. Finally, observers often complete one trial in which they are told to ignore the cross and to report anything they see (see Figure 1). This “full attention” trial serves as a control condition to demonstrate that the unexpected object was perceptible even if it was not perceived on the critical trial. Using this approach, Mack, Rock, and their students and colleagues showed that people often miss the unexpected shape on the critical trial, even when it was a unique color and appeared for 200ms. Noticing rates typically ranged from 25-75% depending on the condition.
Although Mack and Rock coined the term inattentional blindness, earlier work had explored similar failures of awareness under conditions of selective attention. In perhaps the most prominent early demonstrations of this phenomenon, Ulric Neisser and his colleagues (Neisser, 1979; Neisser & Becklen, 1975) used a selective looking task to explore the role of attention in the detection of unexpected events. Their task was a visual analogue of earlier dichotic listening methods in which people often failed to notice the content of speech presented to one ear when they were actively focusing attention on speech presented to the other ear (e.g., Moray, 1959). In the studies by Neisser and colleagues, observers viewed two distinct, superimposed videos of people performing simple actions such as passing a basketball or playing a hand-slapping game. When observers focused attention on one of the events, they often failed to notice an unexpected event occurring in the other. For example, when counting the number of times several people passed a basketball while ignoring a hand-slapping game, they often failed to notice when the people in the hand-slapping stopped and shook hands (Neisser & Becklen, 1975). Recent replications and extensions of this approach by Simons and Chabris (1999) showed that such sustained inattentional blindness occurs even when the unexpected object is fully visible and the displays are not superimposed. In their study, participants counted basketball passes by players wearing white shirts and ignored passes made by players wearing black. Under these conditions, approximately 50% of observers failed to notice when a person in a gorilla suit entered the display, stopped and faced the camera, thumped its chest, and exited on the far side of the display (see movies at http://www.dansimons.com/videos.html).
More recent studies of inattentional blindness have explored how aspects of the task and stimuli contribute to inattentional blindness and the detection of unexpected objects: the role of expectations in the detection of unexpected objects (Most et al, 2005), the role of visual similarity of the unexpected objects to the attended and ignored items in the display (Most et al, 2001), the role of visual distinctiveness of the unexpected object, and the role of spatial proximity of the unexpected object to the focus of attention (Newby & Rock, 1998; Most et al, 2000). Other recent studies have examined how differences in the observers affect detection, including the effects of alcohol consumption (Clifasefi et al, 2006) or expertise in the primary task (Memmert, 2006).
Classifying a failure of awareness as inattentional blindness
All of the following criteria must hold to classify a failure of awareness as inattentional blindness as opposed to a different type of failure of awareness. Note that not all failures of awareness that result from distraction or inattention to a stimulus constitute inattentional blindness.
Observers fail to notice a visual object or event
The object or event is fully-visible and observers readily see it if they are looking for it
The failure to notice results from engagement of attention on other aspects of the display and not from aspects of the visual stimulus itself
The object or event is unexpected
Why it matters that the critical stimulus is unexpected
Traditionally, inattentional blindness refers specifically to the failure to notice unexpected objects. Some recent studies have demonstrated failures to notice objects that occur on many trials due to attentional engagement on a primary task. In such cases, the critical objects are expected, but observers fail to report them because they are engaged in another task. Although such failures of awareness can be attributed to attentional engagement, they do not precisely constitute examples of inattentional blindness. When a critical stimulus appears repeatedly during an experiment, observers do have a reason to look for it (they will be asked about it). Consequently, it might be attended, just not sufficiently to produce awareness of it. Such failures of awareness might be due to insufficient attention rather than inattention. The unexpected nature of the critical stimulus is what differentiates inattentional blindness from other failures of awareness due to distraction or attentional failures (e.g., the attentional blink).
Inattentional blindness or inattentional amnesia
Conclusions from studies of inattentional blindness are premised on the idea that a failure to report an unexpected stimulus results from a failure to see that stimulus. In principle, though, people might fail to report the unexpected stimulus even if they did see it – they could simply forget that they saw it by the time they are asked about it. That is, they have inattentional amnesia rather than inattentional blindness (Wolfe, 1999). Differentiating these alternatives might be impossible because questioning inherently occurs after the event, leaving open the possibility of forgetting. Whether or not the inattentional amnesia explanation is more plausible or palatable is a matter of debate. For the amnesia account to hold, observers would have to consciously perceive the unexpected object and then forget that they saw it, something that might be less plausible when the unexpected object is particularly distinctive or unusual (e.g., a person in a gorilla suit).
Inattentional blindness or inattentional agnosia
Another alternative to the inattentional blindness account is that observers see the critical object in the display but do not process it extensively and consequently do not retain it. In essence, they experience inattentional agnosia (see Simons, 2000). They might see that there is something in the display, but not identify it as a gorilla. In fact, they might not identify it as a coherent object at all. Under this explanation, something is perceived, but it is not perceived as some “thing.” Because it is not encoded as a thing, it is not remembered and reported after the display is removed. However, evidence that the critical object can prime a subsequent response suggests that it is processed to some extent, even when it is not reported.
Change blindness or inattentional blindness
Change blindness refers to the failure to notice something different about a display whereas inattentional blindness refers to a failure to see something present in a display. Although these two phenomena are related, they are also distinct. Change blindness inherently involves memory — people fail to notice something different about the display from one moment to the next; that is, they must compare two displays to spot the change (see Simons & Rensink, 2005). The signal for change detection is the difference between two displays, and neither display on its own can provide evidence that a change occurred. In contrast, inattentional blindness refers to a failure to notice something about an individual display. The missed element does not require memory – people fail to notice that something is present in a display. In a sense, most inattentional blindness tasks could be construed as change blindness tasks by noting that people fail to see the introduction of the unexpected object (a change – it was not present before and now it is). However, inattentional blindness specifically refers to a failure to see the object altogether, not to a failure to compare the current state of a display to an earlier state stored in memory.
What is perceived in spite of inattentional blindness?
Studies of inattentional blindness demonstrate that people fail to notice unexpected objects in a display. Or, more precisely, that they fail to report having noticed an unexpected object. The information from the unexpected object is filtered from awareness by the time people are asked about it. However, it is unclear how much processing of the unexpected object occurs before this filtering. In its strongest form, the word “blindness” implies that the information is processed minimally if at all. However, other evidence suggests that the unexpected object is processed and that it can influence perception. For example, when the unexpected object involves the grouping of background dots in a display, the unreported grouping can affect judgments of line length in the Mueller-Lyer illusion (Moore & Egeth, 1997). And, as for early studies of dichotic listening (Treisman, 1964), some stimuli apparently are less subject to inattentional blindness. For example, observers typically fail to see common words in the Mack and Rock task, but they do see their own name when it appears unexpectedly (Mack & Rock, 1998). Observers also show some priming from unreported words as evidenced by a tendency to complete word fragments with the unreported word rather than other more common words (Mack & Rock, 1998). These findings suggest that the unexpected object is processed, possibly to a semantic level, even when it is unreported. If so, they also suggest that the inattentional agnosia explanation is wrong, at least at some level — semantic processing implies that the object was identified at some level of the visual system.
Evidence that the unreported stimulus is processed to some extent is reminiscent of other research on subliminal perception, the idea that unseen stimuli exert an influence on perception or possibly behavior. However, care must be taken to draw strong inferences about implicit or subliminal perception from the inattentional blindness tasks. Inattentional blindness tasks rely on a single critical trial to determine whether or not an unexpected object was consciously perceived. However, the inference that it was not perceived depends on a report after the trial, and such reports are subject to many influences other than just whether or not the object was seen. For example, some people might be more hesitant to report an incredible object when they lack certainty that they saw it (in signal detection terms, they respond conservatively). If so, they might not report the unexpected object even if they have some inkling that something might have been present in the display. In other words, they might have consciously perceived the unexpected object, but been hesitant to say so definitively. With only one critical trial, the inattentional blindness task is poorly designed to discriminate between the ability to detect the stimulus and biases in the tendency to report the stimulus. That said, the unexpected object does fall below a subjective threshold for awareness in that people do not report it. And, that subjective threshold for awareness may be practically important, even if observers objectively saw something.
Evidence for inattentional blindness comes mostly from relatively simple laboratory tasks, but the phenomenon likely has many daily analogues. For example, automobile accident reports frequently report driver claims that they “looked but failed to see” the other vehicle. Many collisions between cars and motorcycles involve cars turning in front of an oncoming motorcycle, with the car driver not seeing the motorcyclist. Given that in many contexts, motorcycles are less common that cars, inattentional blindness is more likely. Critically, the difficulty of the primary task in an inattentional blindness task increases the probability that people will miss the unexpected object. In practical terms, the more people focus on aspects of their visual world other than the detection of unexpected objects, the less likely they are to detect such objects. Recent evidence suggests that talking on a cell phone, for example, dramatically increases the probability of missing an unexpected object (Scholl et al, 2003).
Although inattentional blindness constitutes a limit of the visual system, it also illustrates a critical aspect of visual processing. Specifically, it reveals the role of selective attention in perception. Inattentional blindness represents a consequence of this critical process that allows us to remain focused on important aspects of our world without distraction from irrelevant objects and events. Only when those unselected aspects of our world are both unexpected and important does inattentional blindness have practical consequences. And, inattentional blindness itself may be useful in some contexts. For example, by guiding selective attention to one part of a display, it is possible to reduce the visual quality of a different part of the display with minimal consequence, possibly allowing greater visual compression in motion sequences (Cater et al, 2002).