Methodological Pitfalls in the “Objective” Approach to Consciousness: Comments on Busch et al. (2009)

Abstract

One major problem in the empirical investigation of consciousness is to identify a so-called objective measure of the presence or absence of a specific conscious experience. An objective measure, in this context, refers to a measure of how well a subject is able to solve a task or to a report, given by the subject, which does not explicity refer to his or her own conscious experience. Such task performance or report may be influenced by conscious as well as unconscious processes. Subjective measures, on the other hand, are defined as reports (verbal or other kinds) made by a subject directly about his or her conscious experience. The paper by Busch, Frund, and Herrmann (2009) is an important and interesting suggestion of how to find neural correlates involved in change detection and change blindness, but it also claims to infer knowledge about conscious experiences from its data. This commentary will focus on this last claim. In their study, the authors present a change blindness experiment in which they investigated whether change detection (sensing) and change identification (seeing) rely on different or similar neural processes. The authors successfully identified some ERP components that were similar for both conditions (the VAN and the P3) and some other components that specifically occurred for changes that were identified (the change-related positivity and the N2pc). In a second experiment (visual search), the authors showed that the N2pc reflected selective attention, whereas the change-related positivity was specific for change identification. They conclude that sensing and seeing a change rely on different neural processes. These results are based on signal detection theory (SDT) according to which data are analyzed and interpreted. SDT is a model of how systems detect signals among noise, and it has repeatedly proven applicable to the human perceptual system (Green & Swets, 1966). SDT provides an objective measure of a subjectʼs capacity to detect stimuli (d0) and a criterion for detection (C). Thus, the theory suggests a way to obtain data about a subjectʼs perceptual capacity that is not based on subjective verbal reports but on “objective reports,” that is, their task performance. There is, however, an important conceptual and empirical distinction between signals and reports (Overgaard, 2009). In this terminology, different from the SDT terminology, signals refer to the “uncontrolled behaviors such as reflexes” of a subject (Overgaard, 2009, p. 16)—that is, the observation of behavior that is not as such intended to inform an observer yet may be of use as data to analyze some cognitive process. Reports, in contrast, are communications from the subject; this may be a verbal statement describing a complex scenery or something as simple as a button press when a target is present—the important part being that the subject is intending a communication using a report. SDT makes no such demands that reports are based on intended communications or on reports directly about consciousness. Accordingly, a signal in SDT is not necessarily an expression of what a subject has perceived consciously. In order to measure d0 and C, subjects are performing a task, for example, a visual detection task. One problem that has previously been identified is that task performance is not a good guide to conscious experiences because unconscious factors might also be involved (Lau, 2008). To illustrate this point, Lau (2007) describes an experiment where the same d0 is constant while reports of experience differed in subjects over time. Therefore, d0 as well as C seem blind to the conscious experience the subjects has while performing the task. They seem to be of use when describing those cognitive events that make an overt behavior possible (such as responding to the presence of a target), that is, the so-called objective aspects of perception. However, Busch et al. (2009) are more ambitious than that. They claim explicitly to be investigating the subjective aspects as well using SDT—an approach that we, as argued above, find problematic as a matter of principle. The experiments by Busch et al. (2009) do however contain two important deviations from what may be considered a “standard” SDT design. Those deviations, it Aarhus University Hospital, Hammel, Denmark, University of Aarhus, Denmark, Aarhus University Hospital, Aarhus, Denmark