Kouider et al have recently reported that infants’ cortical activity (when viewing faces) is isomorphic to that of adults who consciously perceive faces. They conclude that conscious perception develops between 5 and 15 months of age. After reading their paper, I want to consider a different conclusion. Perhaps Kouider et al didn’t find a marker of conscious perception. Maybe they found 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. Infants’ ERPs seem to be similar to certain, more rapidly onsetting, adult ERPs. For example, the infant’s P400 is supposed to be isomorphic to the adults P300. So Kouider et al 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 a similar neural marker of conscious perception as adults.
Comparing Infants’ ERP Data
For reasons unknown to me, Figure 3 does not show data from ≤ 50 ms trials for 5-month olds. It does for 12- to 15-month-olds. So it is hard to tell if we should assume that these trials were the same or different between these age groups. If the ERP graphs for these presentations were alike between the older and younger infants, then this would be uninteresting. But if the ERP graphs for the sub-50ms presentations among 5-month olds were as different as they look in Figure 3, then that is noteworthy.
In Figure 3, 12-month olds and 15-month olds do not show the characteristic ERP components of “conscious perception”. 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 new 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 15 months of age infants develop a lack of conscious perception—and that statement is, at best, confused. So we have to assume to latter interpretation. On that interpretation, the graph suggests not that conscious perception is developing, but unconscious perception is developing.
Kouider et al (2013). “A Neural Marker of Perceptual Consciousness in Infants” Science 340: 376-380.