Kouider et al have recently reported that infants’ cortical activity (when viewing faces) is isomorphic to that of adults who consciously perceive faces. They take their data to suggest that conscious perception develops between 5 months of age and 12-15 months of age. After reading their paper, I wonder whether we should take the data to be indicating something else.
In what follows, I will explain my understanding of the experiment and its results. Then I will point out some strange features of their data. Finally, I will argue that what they have found is not a neural marker of conscious perception, but a marker of unconscious perception.
In this study experimenters were looking to see if the event-readiness-potential (ERP) components that manifest when adults “consciously perceive” faces ( see Figure 3 below: [a] early posterior negativity, or EPN [b] N290, [c] P400, and [d] late slow wave, or LSW) would manifest among three groups of infants—5-months old, 12-months old, and 15-months old. If the same (or similar) ERP components manifest, then we could assume that the infants are doing something similar to adults; that is, they the infants would be “consciously perceiving”—or so the authors assume. With this assumption in hand, experimenters set out to see how babies respond to various durations of face stumuli (i.e. 17ms, 33ms, 50ms, 100ms, 150ms, 200ms, 250ms, 300ms). It turns out that the 12-month old and 15-month old infants (and not the 5-month old infants) show ERP components that are similar to adult ERP components. See figure 3 below.
The aforementioned adult ERP components are taken to indicate the cortical threshold of conscious perception. And since the authors take the ERP components of their infant subjects to be similar to certain, more rapidly onsetting, adult ERP components (e.g. the infant’s P400 to be isomorphic to the adults P300), they conclude that, “the brain mechanisms underlying the threshold for conscious perception are already present in infancy, but undergo a slow acceleration during development” (Kouider 2013, 376). In other words, they take infants to show the neural marker of conscious perception.
This interpretation is rather theory-laden in that it assumes that the interpretation of one set of ERP components from one population can be generalized to another similar—not identical—set of ERP components from another similar—not identical—population. Indeed, this move seems to require a good deal of faith in generalization. But I do not wish to quibble about this. Rather, I wish to point out that the authors might have missed something. Before I point this out, let me note one more interesting feature of this data.
For reasons unknown to me, the authors leave out the 5-month olds ERP data for trials where faces were presented for less than 50ms. If the ERP graphs for these presentations were at all like the 12-month and 15-month infants, then there would be no significant difference between 12-month olds and 5-month olds (Figure 3 attached as PNG with arrows pointing to the areas I mention), and part of the authors’ interpretation would be undermined—namely, the part about the mechanisms of “conscious perception” developing between 5 months of age and 12-15 months of age. And if the ERP graphs for the sub-50ms presentations among 5-month olds are different than the ERP graphs for sub-50ms presentations among 12-15-month olds, then Kouider et al might have missed something.
Now I can mention what I think they might have missed. Notice in figure 3, that 12-month olds and 15-month olds do not show the characteristic ERP components of “conscious perception” when faces are presented for 50ms or less. The N290 is missing and the P400 becomes a positive curve that is too gradual to be characterized by a singe time-index. After all, the positive curve begins around 200ms and ends around 650ms. I presume the authors would interpret this ERP curve as either a “lack of conscious perception” or “unconscious perception.” But surely it is the latter, if it is either. To say it is the former is to say (according to the authors’ assumptions) that somewhere between 5 months of age and 12-15 months of age infants ‘develop a lack of conscious perception’—a statement which is clearly confused. So we have to assume to latter interpretation. On this interpretation the graph suggests that if something is developing between 5-month old infants and 12-month old infants, it is not conscious perception, but unconscious perception.
For those who are interested in the philosophy and science of conscious perception, I wonder what you think. Does my interpretation of Kouider et al seem fair?
Kouider et al (2013). “A Neural Marker of Perceptual Consciousness in Infants” Science 340: 376-380.