Endogenous versus exogenous change: Change detection, self and agency

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Abstract

The goal of this study is to characterize observers’ abilities to discriminate between endogenous (i.e., self-produced) and exogenous changes. To do so, we developed a new experimental paradigm. On each trial, participants were shown a dot pattern on the screen. Next, the pattern disappeared and participants were to reproduce it. Changes were surreptuously introduced in the stimulus, either by presenting participants anew with the dot pattern they had themselves produced on the previous trial (endogenous change) or by presenting participants with a slightly different dot pattern (exogenous changes). We analyzed awareness of the changes and behavioral adaptation to them in a dynamical manner. We observe (1) signal attenuation in the presence of endogenous change, (2) dissociation between self-attribution reports and behavioral effect of agency. We discuss the source of this sensitive attenuation as well as the relation between a minimal or core self and an extended, narrative or autobiographical self.

Introduction

Almost all of our direct knowledge of the world beyond our bodies comes from vision. A crucial characteristic of our visual system is the ability to detect change in the visual world. Change in our visual field is so important that Gibson makes it a starting point for perception:

“The central hypothesis of the theory to be presented is that the patterns and the changes of pattern of this projection are stimuli for the control of locomotion relative to the objects of the environment” (Gibson, 1958, p. 183).

“Perceivers are not aware of the dimensions of physics… They are aware of the dimensions of the information in the flowing array of stimulation that are relevant to their lives” (Gibson, 1979, p. 306).

The most commonly studied aspects of change detection concern our ability to detect exogenous change (i.e., changes introduced by others or the environment). Changes involving object location, direction, colour and velocity, for example, need to be detected and processed to enable locomotion. Despite the ecological significance of changes, the study of change detection has produced striking evidence that observers can be extremely insensitive to changes in the visual scene during an eye movement, film cut or similar masking stimulus, even when these changes are large, recurring, and expected (Rensink, 2002, Rensink et al., 1997, Rensink et al., 2000, Simons, 2000, Simons and Levin, 1998, Simons and Levin, 2003). This robust and dramatic phenomenon has been dubbed “change blindness”. It may be demonstrated in different ways (for reviews, see Rensink et al., 2000, Simons and Levin, 1997). For example, Grimes (1996) had the idea to investigate if people could notice a change in a complex scene contingent on a saccade. He asked subjects to study the details of complex images in order to perform a subsequent recognition task and told them that a change was likely to occur. Periodically, when a saccade occurred, a change occurred contingently and subjects were asked to press a button whenever they had noticed a change. Quite surprisingly, many subjects overlooked such changes. The most striking result was probably that 50% of subjects failed to notice when two cowboys’ heads were switched with each other.

At variance with these studies, the present paper deals with a rather poorly explored aspect of change detection, — the detection of endogenous change (i.e., changes originating in our own actions). This mechanism is critical for a number of reasons. First, endogenous changes are inherent to each visual stimulation (Ballard et al., 1995, Findlay and Gilchrist, 2003, Gilchrist et al., 1997, O’Regan and Noë, 2001). Second, endogenous changes could play a central role in the mechanism of change blindness (Hollingworth, 2006). In this context, the goal of the present study is to characterize observers’ ability to discriminate between endogenous changes and exogenous changes.

Several years ago, O’Regan pointed out (1992, p. 483) that “the real mystery of visual perception is: how can it be that we see so well with what an engineer would consider a very badly constructed visual system?”. Unfortunately, it is not the only mystery of visual perception. For example, another outstanding question is: how can it be that we perceive a stable world when equipped with such a noisy visual system? Indeed, ocular saccades and head movements regularly introduce changes in our visual field (Ballard et al., 1995, Findlay and Gilchrist, 2003, Gilchrist et al., 1997, O’Regan and Noë, 2001). Further, our visual re-presentations themselves are inherently noisy (Verghese, 2001, Wilken and Ma, 2004). In other words, our visual system must deal with a continuously changing and noisy visual flow. Nevertheless, observers perceive the world as a stable entity. Thus, an important issue is to determine the fate of such endogenous changes.

As suggested by Frith and collaborators (Frith, Blakemore, & Wolpert, 2000), awareness of endogenous changes could be unnecessary and perhaps even confusing. For example, saccades, head and body movements or eye blinks are different endogenous sources of visual changes to which it is probably adaptive not to be sensitive. In this context, Frith (Frith et al., 2000) assumes that one of the fundamental properties of consciousness is to maintain the stability of the consciously perceived world, that is, to maintain the stability of the contents of awareness in spite of continuously changing stimulation.

“The changes in re-presentation that result from our own movements are entirely predictable on the basis of those movements and therefore do not require our attention. It seems plausible that to be aware of re-presentations which changed every time we moved our bodies, or even our eyes, would be a positive disadvantage” (Frith et al., 2000, p. 1775).

Over the past decades, endogenous change blindness has enjoyed particular interest in a different domain — action. Researchers have increasingly studied how we can distinguish between sensations that are produced by our own movements and sensations that are caused by a change in the environment (Decety, 1996, Jeannerod, 1988, Jeannerod, 1997, Wolpert, 1997, Wolpert et al., 1995). These studies have repeatedly demonstrated that the sensory consequences of self-generated movements are perceived differently than identical sensory inputs that generated externally. In particular, there is now substantial evidence that the sensory effects of self-produced movement are attenuated (Blakemore et al., 1998, Blakemore et al., 2000, Claxton, 1975, Collins et al., 1998, Milne et al., 1988, Weiskrantz et al., 1971). A recent study by Blakemore and collaborators (Blakemore, Frith, & Wolpert, 1999) is relevant in this context. Using a robotic interface, delays of 100, 200 and 300 ms and trajectory rotations of 30°, 60° and 90° were introduced between the movement of the left hand and the resultant tactile stimulation on the right palm. Increase in temporal and spatial discrepancies between the subject’s movement and the resultant tactile stimulation enable to differentiate between the perception of self-produced sensation (no delays and no trajectory rotations) and the perception of externally produced sensation. Participants were asked to rate the tactile stimulus in terms of several sensations, including tickliness (painful, intense, pleasant, irritating, and tickly). Interestingly, the authors observed a systematic increase in the sensation experienced as the discrepancy between the applied movement and the felt movement increased in time or space. In other words, conscious experience of being tickled is tightly dependent on the source of the action. Nevertheless, this phenomenon of sensitive attenuation has known few echoes in the domain of change detection. As observed for self-produced movement, we could imagine that change detection could be less sensitive to endogenous change than exogenous change, in other words, that the change detection is source dependent.

At the same time, such findings raise questions about the nature of the self, and in particular the question of how one’s actions are distinguished from the actions of others. Historically, philosophical and psychological approaches to the self have focused on the mechanism of self-attribution or, in other words, one’s ability to refer to oneself as the author of one’s own actions (for reviews, see De Vignemont and Fourneret, 2004, Gallagher, 2000). The feeling of being causally involved in an action (the sense of agency, Gallagher, 2000) is critical for a number of reasons, in particular in the formation of self-consciousness and in human social communication (Daprati et al., 1997, Georgieff and Jeannerod, 1998). However, if these different protocols using attribution judgements have proven to be useful in our understanding of the self, the self cannot be reduced to the question of self-attribution. Classically, we may distinguish two different aspects of the self — the ‘narrative’ self and the ‘minimal’ self (Gallagher, 2000). The narrative self corresponds to “a more or less coherent self (or self-image) that is constituted with a past and a future in the various stories that we and others tell about ourselves” (Gallagher, 2000, p. 15). Clearly, introspective reports deal with this first aspect of the self. The minimal self, on the other hand, corresponds to a more primitive and embodied sense of self. It is the pre-reflective feeling that a given movement is performed by me, or that a given experience is had by me. This reference to self is distinguished from the autobiographical sense of having a narrative self that persists across experiences. The minimal self is more like an instantaneous feeling of “mineness” with which experiences are labelled. As suggested by Gallagher (2000), this aspect of the self depends on an ecologically embedded body, but one does not have to know or be aware of this to have an experience that still counts as a self-experience. In other words, the minimal self cannot be reduced to self-attribution reports.

In this context, we have to make a distinction between the fact that I own a certain mental or bodily state, and the fact that I recognize this state as mine (see also, Bulot, Thomas, & Delevoye-Turrell, 2007). From a conceptual, a phenomenological and an empirical point of view, the relations between a minimal or core self and an extended, narrative or autobiographical self remain controversial. They may be seen to be complementary notions. But is the core self a (logical and temporal) precondition for the extended (narrative or autobiographical) self? Or is the core self, on the contrary, a subsequent abstraction; is it simply a stripped-down version of what must count as the genuine and original self (Zahavi, 2005)? To resolve this question, the study of the self needs to go further than the simple use of attribution judgements and to explore the possible dissociations between the minimal and the narrative self in change detection. In particular, if the minimal self is a precondition for the narrative self and could exist in absence of self-attribution reports, explicit judgement tasks are no longer sufficient, and it becomes a key concern to find an implicit measure of agency, — one that is sensitive to the minimal self.

In this light, the main goal of this study is to explore how the visual system determines whether the perceived changes find their source from endogenous factors (internal error) or from real environmental changes (external change), and to better define the concept of self. From this perspective, we address two interrelated questions. First, how does detection of endogenous change differ from detection of exogenous change? As discussed by Bulot and collaborators (Bulot et al., 2007), a crucial point in our understanding of agency is to compare the experiences of the participant in case of one’s actions and in case of others’ actions. Given the results obtained by Blakemore and collaborators on tickling (Blakemore et al., 1999), one would expect that the fact to be the agent of change may decrease our sensitivity to this change. Second, what relationship exists between the minimal self and the narrative self in change detection. The current issue is the possible dissociation between minimal and narrative self and the presence of a non-conceptual access to the self – a more primitive self-consciousness that does not depend on the use of a first-person pronoun. The fact that neonates are able of imitating the facial gestures of others in a way that rules out reflex or release mechanisms (Meltzoff and Moore, 1977, Meltzoff and Moore, 1983) have been interpreted as a proof that the human infant is already equipped with a minimal self that is embodied, enactive and ecologically tuned, even in a pre-linguistic period (Bermúdez, 1996, Gallagher, 1996, Gallagher, 2000, Rochat, 1995). If so, we could imagine that narrative self and minimal self could operate independently of each other in certain conditions in adult participants. Given that awareness of internal change is unnecessary or even confusing (Frith et al., 2000), we may expect that change detection is one of those conditions.

To explore these questions, we used an adaptation of the method of serial reproduction first introduced by Bartlett (1932). On each trial, participants were first shown a dot pattern on the screen. Next, the pattern disappeared and participants had to reproduce it. In a first condition, the target pattern remained the same over 20 trials. In the other two conditions, the target pattern changed over successive trials, and the source of the change was manipulated (exogenous or endogenous). In the latter two conditions, participants were not informed of the occurrence of these changes. We measured both awareness of the change (through verbal reports) and behavioral adaptation to this change (through a reproduction task). By comparison with protocols that only use attribution judgements, this paradigm presents several advantages. First, by using two different sources of change, we can compare the experiences of the participant when she is the author of the change and when another agent produces the change. In this way, we hope to bring to light the role of agency in the process of change detection. Second, this paradigm combines behavioral measures and verbal reports. Through this combination, we expect to clarify the dissociation between experiencing oneself as the author of an action (minimal self) and judging oneself as the author of an action (narrative self). Indeed, verbal reports about where and what is the change remain the main measure, and often the only, to identify change detection. However, in recent studies it was suggested that our brain knows more than we can tell (Hollingworth and Henderson, 2002, Hollingworth, 2003, Fernandez-Duque and Thornton, 2003, Laloyaux et al., 2006, Mitroff et al., 2004).

In this context, we used a new behavioral index — the variability of participants’ responses, as a possible implicit measure of change detection. Variability here specifically refers to the difference, as measured over time, between successive responses produced by a participant to a stimulus. The use of such a measure is based on two assumptions. First, as shown by previous results (Berberian, 2007, Berberian et al., 2007, Giraudo and Pailhous, 1994, Giraudo and Pailhous, 1999, Sarrazin et al., 2004), variability and its dynamics is a good behavioral index of memory stability. Second, the results obtained by Giraudo and Pailhous (1999) seemed to indicate that the level of variability can be used as an indicator for the presence of visual change detection. Indeed, using a memorization and reproduction task and presenting targets that varied randomly in time, these authors showed an increase in reproduction variability during target presentation. Different explanations could be proposed to explain this increase in variability. A first possibility is that the increase in variability could be caused by interference between multiple re-presentations. However, Giraudo and Pailhous observed that the distance to the target decrease during the first trials. In other words, learning takes place whereas the stimulus changes for each trial. Such learning process is not coherent with the possible existence of multiple re-presentations. Second, this increase in variability could be the consequence of an inability to stabilize a reference in memory in result to the change presenting in the stimulus. However, as soon as the disruption ended (the end of unstable targets presentation), an abrupt decrease in variability reproduction appeared, and a level of image consistency (i.e., image variability in steady state) equivalent to that observed in the previous experiments was instantaneously reached at that point. In other words, the level of image consistency is not modified in a durable way and, even in presence of stimulus instability, participants are able to create a reference in memory. Finally, the third alternative is that the increase of the variability level is the consequence of an increase in variation around a stabilized reference. This third alternative, congruent with the different results obtained by Giraudo and Pailhous, indicate that the increase of variability level expressed a perceptual sensitivity to change. In this context, response variability in a visuospatial memorizing task seems a good index of change detection and thus a relevant paradigm to study the mechanisms underlying implicit sensitivity to change.

Section snippets

Experiment 1

There is a now a huge number of studies dedicated to change detection. Yet little is known about how precisely internal change is detected. Our first experiment deals with this fundamental problem. To address this issue, we asked participants to memorize and reproduce dot patterns several times in succession. Specifically, on each trial, a pattern of dots (see Fig. 1) appeared on the screen for 5 s and then disappeared from view. Participants were then to attempt to reproduce the dot pattern by

Experiment 2

In this second experiment, we tried to clarify the origin of the decrease in response variability observed in presence of endogenous change. In this context, we conducted a modified replication of the first experiment. The question is the following: Is decrease in variability for endogenous change present even in the absence of a direct correspondence between target configuration and participant responses? To address this issue, we delayed the re-presentation of participants’ own previous

General discussion

The goal of these experiments was to characterize observers’ ability to discriminate between endogenous changes and exogenous changes and to explore the role of agency and self in change blindness situations. In this perspective, we used a novel paradigm based on a memorization task in which participants are to memorize and reproduce dot patterns several times in succession. Changes were introduced in the stimulus pattern and the source of these changes was manipulated (endogenous or

Conclusion

Using a novel methodological approach based on mnesic reproduction and on the analysis of response variability, we explored change blindness in the presence of endogenous changes. Our method, through which people provide both behavioral and change detection measures, makes it possible to track the dynamics of sensitivity to change over the entire experiment. Our main results replicated the results of previous studies suggesting that our visual system can continuously specify if the changes that

Acknowledgments

Bruno Berberian is supported by a post-doctoral grant from the Fyssen Foundation. This research was also supported by an institutional grant from the Université Libre de Bruxelles to A.C., by Concerted Research Action 06/11-342 titled “Culturally modified organisms: What it means to be human in the age of culture”, financed by the Ministère de la Communauté Française – Direction Générale l’Enseignement non obligatoire et de la Recherche scientifique (Belgium), and by European Commission Grant

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