Parafoveal Vision and Dichotic Listening

Similar sorts of findings have been obtained in neurologically intact subjects.

Earlier in these lectures, I discussed the dichotic listening paradigm developed by Cherry to study the cocktail-party phenomenon.  Subjects shadow an auditory passage presented to one ear, and ignore a competing message presented to the other ear.  And subjects can do this quite well, failing to notice much about the unattended passage -- unless, at least sometimes, when it contains highly relevant material such as the subject's own name. 

In parafoveal vision, subjects may be asked to detect the appearance of targets presented in the center of a screen.  At the same time, words can be presented on the periphery of the screen.  Typically, subjects do not notice these peripheral targets.  But they can induce semantic priming effects.  One such experiment was conducted by Bargh and PIetromonaco (1982), and discussed later in the Lecture Supplement on "Automaticity and Free Will".

Inattentional Blindness

In the classic instances of preattentive processing, the subject's attention is deliberately focused elsewhere -- on one channel of a dichotic listening task rather than the other, or to some portion of the visual field rather than somewhere else.  However, there are also other circumstances in which people do not consciously see or hear things, even when their attention is not distracted by a competing task.  Although some of these phenomena appear to reflect the fact that subjects' attention is focused on one aspect of a stimulus rather than another, in every case the subjects appear to be blind to some object or feature, despite the fact that they are staring right at it. 

The Inattention Paradigm

One of these phenomena was discovered by Mack, Rock, and their colleagues, as part of a large study of the limits of preattentive processing (e.g., Rock et al., 1992; Mack & Rock, 1998).  Their experiments employed an inattention paradigm in which subjects were presented with a seemingly straightforward but actually rather difficult task: to determine which of two crossed lines, one horizontal and one vertical, was larger.  Subjects focused on a central fixation point, the cross appeared briefly (200 milliseconds) either at fixation or in the parafoveal region, and then the subjects made their response.  Of course, this was just a cover task.  On the first two or three trials, the subjects were simply presented with lines of different length, and asked to identify which was the longer.  After establishing this attentional set, the subjects were unexpectedly presented with a new stimulus which also appeared briefly, somewhere in the region marked by the contours of the cross.  After reporting their length judgment, the subjects were also are asked whether there was another object presented, and if so where it was.  Because the subjects were not expecting the new stimulus, and were focusing their attention on the crossed lines (in order to make the comparative judgment of length), this trial is called the inattention trial.  It is important to know that this target was presented within one of the quadrants created by the cross -- so that, if subjects were scanning the lines to compare their lengths, as they were instructed to do, they should also have noticed its appearance.  Nevertheless, about 25% of the time, subjects failed to note the presence of the target.  Regardless of their response, they were then given a recognition test in which they were asked to select, from a set of alternatives, the one that most closely resembled the new stimulus. 

At this point, now that subjects had been effectively warned by the inattention trial that some other stimulus might also appear, the task shifted.  In addition to making a judgment about the cross, the subjects were asked report whether another stimulus appeared as well.  After two trials with only the cross, the new object dutifully appeared.  These are called divided attention trials: the subjects had to pay attention to the cross, in order to complete their manifest task, but they were also looking out for any target.  Finally, there were full attention trials in which subjects were instructed to ignore the cross entirely and simply detect the presence of a target. Subjects also performed the recognition task after the divided attention and full attention trials, just as they had after the inattention trial.

For technical reasons, the cross was masked after 200 milliseconds, but this did not prevent it from being clearly seen by the subjects.  The judgment task was complicated somewhat by the well-known horizontal-vertical illusion, in which a vertical line appears longer than a horizontal line of the same length.  For symmetry, it might have been interesting if the subjects had been surprised by a request to make a length judgment on a subsequent full attention trial, which would effectively would have been an inattention trial with respect to the cross.  But because the judgment task is only a cover for the real experiment, none of this really matters.     

In performing these experiments, Rock et al. (1992) were initially interested in asking a question about the relation between perception and attention -- to wit, which aspects of perception require attention, and which do not?  Studies of a phenomenon known as pop out had already indicated that such elementary features as color, motion, and simple shape (e.g., distinguishing between Ts and Os -- could be perceived independent of attention (Treisman, 1988), but at the time there was a dispute over whether location was also processed preattentively.  The Rock et al. (1992) studies showed that targets were successfully detected and located about 75% of the time on the inattention trials, a rate that was not significantly different from that observed on the divided attention or full attention trials.  Accordingly, they concluded that location was also processed preattentively -- a reasonable enough conclusion, given the fact that -- as they themselves noted -- you have to know where something is before you can direct your attention to it. 

On the other hand, if 75% of subjects detected the target, then 25% missed it.  Initially, Rock and Mack dismissed this phenomenon merely as noise" in their data.  But the 25% "miss" rate continued to appear, in experiment after experiment.  Moreover, the rate of misses on inattention trials increased to as much as 80% when the new object was presented right at the point of fixation, rather than when it was presented a little off-center, in the parafoveal region.  Because a substantial number of misses occurred when clearly supraliminal stimuli were presented in that portion of the visual field where visual acuity is optimal, they realized that the misses were a substantive phenomenon in and of themselves.  And because they attributed the misses to the fact that subjects were focusing their attention on the contours of the crosses, they named the phenomenon inattentional blindness (IB).

It may seem difficult to accept that subjects will entirely miss an object that it entirely within their field of vision, just because they are paying attention to something else.  However, an experiment by Neisser (1979) obtained similar effects: when subjects viewed a videotape of people passing a basketball back and forth, counting the number of passes, only 21% reported seeing a clearly visible woman who walked through the basketball court carrying an open umbrella.  A replication experiment substituted a chest-thumping gorilla for the woman: fully half of subjects failed to report the presence of the beast (Simons & Chabris, 1999, 2009).  Inattentional blindness is theoretically important because it shows that attention can be directed to specific objects, not just to particular locations in space.  But in the present context, the phenomenon is mostly interesting as a dramatic lapse of conscious perception.

Inattentional blindness may be defined as the failure to consciously perceive an object, otherwise salient, when the subject's attention and expectation are otherwise engaged.  Mack and Rock's subjects' attention is focused on the horizontal and vertical lines, and they miss the third object presented within their contours.  Inattentional blindness has at least two implications:

Inattentional Blindness and the Attentional Bugaboo

Mack and Rock (1998) began their program of research with the view that conscious perception requires attention, and thus that inattention produces a kind of blindness -- a state of "looking without seeing" (p. 1).  From their point of view, attention is "the process that brings a stimulus into consciousness... that permits us to notice something" (p. 25).  Accordingly, there is "no conscious perception at all in the absence of attention" (p. 227).  Objects may capture attention involuntarily, or attention may be deliberately directed to an object (by virtue, for example, of expectations, intentions, or mental set); in either case they enter into phenomenal awareness.  

But are Mack and Rock really right, that attention is required for consciousness?  And did their ostensibly blind subjectws really fail to see the stimuli in question?  There are reasons to be doubtful on both points.  Writing in the tradition of Eriksen (1960) and Holender (1986), Cuylany (2001) emphatically denies both claims. 

The term "demand characteristics" was introduced into psychological discourse by Orne (1962), as part of a broad critique of methodology in social-psychological research (see also Orne, 1969, 1972, 1973, 1981).  By "demand characteristics", Orne meant the totality of cues available in the experimental situation that communicate to the subject the experimenter's design, hypotheses, and predictions {for an analysis, see Kihlstrom, 2002).  Many social psychologists employ deception to hide their real intentions, but as Mack and Rock's experiments show, social psychologists are not the only ones to do so.  By fabricating the cover task of comparing the lengths of crossed lines, when they are really interested in identification of the extraneous stimulus, Mack and Rock engaged in no less a deception than did Milgram (1963) in his famous experiments on obedience to authority.  And just as Milgram's subjects may have caught on to his deception (1968), so Mack and Rock's subjects may have caught on to theirs. 

Certain details of Mack and Rock's procedures and results lend weight to Dulany's critique. 

As attractive as the notion of inattentional blindness might be, it has to be said that there are as yet no good answers to many of Dulany's criticisms.  To a great extent, the intransigence of Dulany's arguments stems from the fact that, like Eriksen before him, he resists the identification of conscious awareness with introspective self-report. (In fact, Dulany was a graduate student of Eriksen's at the University of Illinois before joining him on the faculty there.  In their one joint publication, Dulany and Eriksen (1959) argued that verbal self-reports were no more than accurate than covert psychophysiological responses as indices of stimulus discrimination.)

Dulany expresses a positivist's reserve about introspection, but he is no radical behaviorist.  Far from denying the existence or importance of consciousness, Dulany has asserted a thoroughgoing mentalism that identifies consciousness with symbolic representation, and he has defined symbolic processing so broadly -- encompassing everything that happens after sensory transduction and before motor transduction (Dulany, 1991, 1997).  Some symbolic codes are literal, close to a perceptual representation, while others are symbolic, more abstract and full of meaning, but none of them are any more or less conscious than the others because from his point of view all symbolic codes are represented in consciousness. 

In the end, Dulany adheres to the "grand principle" {Dulany, 1997, p. 184} that "mental episodes consist of nonconscious operation upon conscious intentional states, yielding other conscious intentional states".  In other words, mental processes may be unconscious, but mental contents are conscious.  This position comports well with the traditional view, in cognitive psychology, that procedural knowledge is unconscious, inaccessible to introspective conscious access in principle, but that declarative knowledge is accessible to conscious awareness.  But a major thrust of this course is that mental contents -- percepts, memories, and thoughts -- can also be unconscious, in the sense that they can interact with ongoing experience, thought, and action in the absence of, or independent of, conscious awareness.  That is what implicit memory, implicit perception, and other, cognate phenomena (yet to be discussed) are all about. 

It is possible that Dulany is right, that only mental processes are unconscious, and that mental states, as symbolic representations, are always conscious.  But there is a danger here, and it is of the same kind that Eriksen's arguments posed for studies of subliminal perception.  Eriksen identified consciousness with discriminative behavior; but if the evidence for unconscious percepts, memories, and the like consists in observations of discriminative behavior in the absence of self-reported awareness, he has ruled the psychological unconscious out of existence by definitional fiat.  Similarly, Dulany identifies consciousness with symbolic representations; but if the evidence for unconscious percepts, memories, and the like consists in observations like priming outside of awareness, then he too has ruled the psychological unconscious out of existence.  We are left with the traditional view in cognitive psychology, which identifies the unconscious with automaticity and procedural knowledge, and consciousness with all mental states and contents and states. 

Varieties of Attentional Blindness

Like Eriksen's (1960) before them, Holender's (1986) critique of preattentive semantic processing and Dulany's (1997) criticisms of inattentional blindness are not trivial, and should have the effect of forcing researchers to clean up their methodologies, putting evidence for preattentive, preconscious processing on ever-firmer empirical grounds.  This exactly what happened with research on subliminal perception (Greenwald et al.,1996; Draine & Greenwald, 1998).  In the meantime, other investigators have opened several new lines of research on the general questions of attention, conscious awareness, and preattentive, preconscious processing.  The studies described previously in this chapter examined processing when subjects' attention was directed elsewhere -- to one channel in dichotic listening, to foveal rather than parafoveal regions, to some objects but not others.  By contrast, these studies document anomalies of awareness even when subjects are directing their attention right at the target.  For this reason, I classify these studies as concerned with attentional blindness -- deficits in conscious visual processing that occur despite the subject's appropriate deployment of attention, and accurate expectations (see also Kanai et al., 2010). 

Paralleling the conclusions of Mack and Rock (1997) concerning inattentional blindness, we can conclude from the phenomena of attentional blindness that Attention does not guarantee conscious perception.  But again, in the present context, we are interested not so much in the blindness itself -- though that's interesting.  What we're interested in is whether there is unconscious, implicit perception of the stimuli in question.  Can you get priming from a stimulus for which a subject is attentionally blind?

Repetition Blindness

The phenomena of attentional blindness are most commonly observed in laboratory conditions employing a technique called rapid serial visual presentation, or RSVP (Potter, 1969; Forster, 1970), in which subjects view a series of visual stimuli presented one after another in very quick succession -- about 100 milliseconds each.  In an early experiment by Kanwisher and her colleagues, subjects viewed sentences such as It was work time so work had to get done, presented one word at a time, and then were asked to report verbatim what they had seen.  Note that the sentence contains a repetition of the word work.  We can call the first instance of the repeated word R1, and the second instance R2.  Under RSVP conditions, streaming about 10 words per second, a large number of subjects -- about 60% -- omit R2: they report "It was work time so had to get done", despite the fact that the sentence is obviously incomplete (Kanwisher, 1987).  When R2 is not the same as R1, it is missed only about 10% of the time.

The procedure here is a little tricky: because most skilled readers do not read every word in a sentence, there is a tendency for them to fill in the gaps with meaningful words -- just as we generally ignore misspellings, missed words, and other typographical errors.  To counteract this tendency, Kanwisher carefully instructed subjects to report what they saw verbatim.  And she also included a number of sentences that actually had words missing, so that subjects would not be surprised when they reported an incomplete or anomalous sentence.

A later experiment by Kanwisher and Potter(1990, Experiment 1) makes the effect even clearer.  In this study, subjects saw three different kinds of sentences. 

• In the repeated condition, the sentence was a standard RB stimulus, such as We were anxious for autumn well before autumn arrived.
• In the synonym condition, R1 was replaced by a synonym, as in We were anxious for fall well before autumn arrived.
• In the unrepeated condition, R1 was replaced by a semantically unrelated word that made sense in the context of the sentence, such as
  We were anxious for apples well before autumn arrived.

Across the three conditions, subjects correctly reported R1 approximately 94% of the time; however, they reported R2 only 40% of the time in the repeated condition, compared to an average of about 88% for synonyms and unrepeated condition.  The fact that RB occurs for a repetition, but not a synonym, indicates that RB occurs at the level of a perceptual (or possibly lexical) representation, but not at the level of meaning. 

RB occurs when one instance of a word -- a token of the type -- appears fairly quickly after the first -- within one or two words, in fact, corresponding to a lag of about 200-300 milliseconds at a presentation rate of eight words per second.  If the repeated word is delayed by about 500 milliseconds, either by positioning the word later in the sentence or by slowing the rate of presentation, RB does not occur. 

Such findings led Kanwisher (1987, 2001) to propose a type-token individuation hypothesis of RB.  According to this hypothesis, conscious awareness requires both the activation of a memory representation of an event (the type) and the individuation of that activated structure as the representation of a discrete event (a token of the type).  The first time the subject sees the word autumn, the perceptual representation of that word is activated and a token individuated; but when autumn is repeated so quickly, individuation does not occur, and so the subject does not see it as a different, discrete event -- as another token of the type.  The event need not be a word: RB has also been observed for digits and letters, pictures, and simple and complex shapes -- including impossible figures  According to Kanwisher, the lack of individuation creates a failure of conscious perception: the subject is not consciously aware of the token's repetition.  

The Attentional Blink

Another phenomenon of attentional blindness is known as the attentional blink (AB), -- a phenomenon initially noticed by Broadbent and Broadbent (1987), but given its name by Raymond, Shapiro, and their associates (Raymond et al., 1992; Shapiro et al., 1994; for a review, see Shapiro, 1994, 1997}.  In AB experiments, subjects view a series of stimuli (such as letters) presented in rapid succession -- say, 30 items over a period of a three seconds, or a rate of about 100 milliseconds per item (this is roughly the same rate as in RB experiments). On each trial, the subject's task is to report the identity of a distinctive target letter -- for example, the one printed in red as opposed to black.  However, the subjects are also asked to detect whether a second probe -- for example, the letter "R" -- also appeared in the letter series.  Of course, the presence of the primary target (T1) and secondary probe (T2), and their locations within the string of letters, varied from trial to trial.  Control subjects performed the probe-detection task only, without the target-identification task.  In a typical experiment (Shapiro et al., 1994), control subjects showed a high level of accuracy, approximately 85%, regardless of where in the sequence T2 was placed.  However, the experimental subjects missed T2 about half the time when it was presented within about 500 milliseconds (i.e., within five items) after T1.  If T2 was presented six to eight items later, outside the 500-millisecond interval, detection rates for experimental subjects were essentially the same as those of the controls. 

Of course, the attentional blink is not an actual eyeblink: subjects rarely blink their eyes during an RSVP trial.  Nor is the attentional blink a phenomenon of visual masking, because it does not occur if the subject is not instructed to identify (i.e., pay attention to) the preceding T1.  Instead, it is as if the subject has blinked, by analogy to the actual eye-blinks that occur after subjects shift their gaze from one location to another.  Shapiro and Raymond suggested that T1 is identified preattentively, on the basis of its physical characteristics -- e.g., that it is red as opposed to black.  Attention is then directed toward T1 in support of the identification process (e.g., that it is a T rather than an L), but it takes time for attention to shift back to the stream of RSVP stimuli, giving T2 time to be lost through decay or interference.  If T2 occurs soon enough after the target, then, it is likely to be missed.  However, T2 probe is not missed if the subject is instructed to ignore T1 -- thus supporting an attentional interpretation of the effect. 

Although AB is not an actual eyeblink, it is nonetheless a real blink.  Sergeant and Dehaene (2004) asked subjects to rate the visibility of the T2 at various lags after T1.  After presentation of T1, the visibility of T2 decreased immediately at a lag of 2 (that is, with only a single stimulus between T1 and T2, and returned to baseline immediately at a a lag of six (that is, with five stimuli between the two targets).  The disappearance and reappearance of T2 occurred in a discontinuous, all-or-none fashion, with not even a hint of continuous degradation and recovery.  This pattern was quite different from that observed with backward masking, which produced an immediate degradation of the preceding stimulus (that is, at a lag of 1), and a gradual increase as the SOA lengthened.  Sergent and Dehaene suggest that their results are most compatible with a two-stage model of the AB in which each stimulus in the RSVP display is subject to automatic, preconscious pickup, but conscious processing of T1 consumes cognitive resources that are necessary for conscious required for conscious processing of T2 (Chun & Potter, 1995).

Change Blindness

Another variant on attentional blindness is change blindness (CB) , which occurs when subjects fail to notice rather large alterations in familiar scenes (Rensink, 1997; for comprehensive reviews, see Rensink, 2004; Simons, 2000; Simons, 2005; Simons, 1997).  In the flicker paradigm, subjects are presented with a picture of a natural scene rapidly alternating with a slightly modified picture in which there has been some change in the presence or absence of some object or feature, its color, orientation, or location -- about 240 msec per stimulus.  In this case, subjects will readily detect the change, apparently because the alteration creates a "luminance transient" that draws attention to the spatial location in which the change has occurred.  But if the two images are separated by a blank gray field, presented for only 80 msecs, images the situation is a little different.  In the laboratory, the difference between the two stimuli is typically restricted to only a single modification in appearance (i.e., the presence or absence of some object or feature), color, or location. The two versions alternate  back and forth on a random schedule, each version appearing for only a very short time (typically 240 msec), and separated by a gray field that appears for an even shorter period of time (typically 80 msec).  On a single trial, presentation continues in this manner for 60 seconds, or until the subject detects the change.  The purpose of this procedure is to simulate the situation in real-world vision, in which the observer must integrate information across saccadic eye movements -- in this case, simulated by the gray field interposed between the two versions of the stimulus.  In the absence of the gray field, subjects will immediately notice the change

Change blindness falls under the rubric of attentional blindness because the subject is not aware that the second image is different from the first. That is a lapse of consciousness.