If you read papers on babies, you get the sense that mostly they just stare at stuff. The vast majority of research on babies uses visual attention – usually time looking at a screen or puppet show – as its dependent measure. Some experiments use more exotic dependent variables, like operant conditioning of kicks, pacifier sucking rate, or even smiles. But since Fantz's work in the 1960s, habituation and related looking time paradigms have dominated the field. Although we're reminded occasionally that babies cry, fuss, poop, and sleep, developmentalists appear far and away most interested in looking (very nice review and critique of this idea by Dick Aslin here).
As a reader of that literature, it's been consistently amazing to me to see what M can do, even as a little baby. She is about to turn four months old next week, and the the range of her behaviors is astonishing. Even more interesting is that some of this behavioral repertoire gives clear signals to underlying cognitive processes. Here's a quick list of some things I've noticed:
Eating. M takes a lot of her meals from a bottle. Early on, she showed no recognition of the bottle itself until it touched her cheek or lip, activating the rooting reflex. But around a month or six weeks ago ago she started showing signs of recognizing the bottle as an object, and responding to it before it reached her mouth. At first, the evidence seemed inconclusive to me – she was reaching (at that point mostly unsuccessfully) for many objects, so a reach for the bottle didn't seem diagnostic. But now she shows clear signs of recognition: When she is hungry and sees the bottle, she vocalizes and opens her mouth. Although I haven't tested this systematically, her recognition seems fairly viewpoint-invariant: she can recognize the bottle in many different orientations. This provides converging evidence for object categorization in 3 - 4 month-olds. It also seems like it could be a neat method for studying vision – think specially engineered bottles with different shapes and colors...
Smiling. Ever since around six weeks, M has been a very smiley baby. She greets people with a big smile, sometimes even smiling when she is otherwise quite fussy. It's kind of fun (in a slightly sad way) to watch smiles war with crying. If she is starting to fuss you can smile at her and see a reciprocal smile fight its way through her pouty face. But so far I have seen no evidence that her smiles reflect recognition of me or her mom: she gives them quite indiscriminately right now. (I know there is other evidence for recognizing and preferring mom, via her face or even her smell, very early on; I just find it surprising that she smiles roughly as much for others as she does for us).
Also, even if I hadn't tested M's face preference to schematic stimuli early on, her smiles would be a good indicator of her recognition of pictures. M will give a big smile at a picture of a baby's face. (Before I saw this, I never understood why people gave us board books filled with baby faces.) It doesn't seem surprising now that babies recognize pictures, but people used to argue that there were "primitive peoples" (presumably tribes somewhere) who didn't recognize photos. Hence picture perception – the ability to recognize the content of pictures – would be a learned cultural skill, and so babies wouldn't recognize pictures. A beautiful study by Hochbert and Brooks (1962), in which they denied their own child access to pictures and then tested his recognition, nicely disproved this idea.
Rolling over. M has rolled over a few times, from prone (tummy) position to her back. Each time, she was interested in an object on one side of her, and she turned her head and body that way (rotating herself onto her side), then began to kick her legs. When she kicked especially hard, her center of gravity tipped over her midpoint, and she flopped onto her back. This was clearly not something she was expecting, viz. her look of complete and total surprise. The interesting thing is that she hasn't been able to reproduce this behavior in a week. This kind of motor exploration really looks like reinforcement learning, where the issue is assigning credit for the result: which of many different behaviors produced the rewarding outcome?
Vocalizations. M started cooing right around when she started smiling – a very adorable behavior. Now her vocalizations have differentiated a bit more: coos when she is in a good mood, squeals when she is starting to fuss. But the most interesting noise she makes is something we call her "cognition noise." There are several physiological measures of attention and cognition in infants, for example heart rate and pupil dilation. M presumably shows these, though we haven't measured. What we didn't bargain for is that she actually shows changes in respiration and vocalization when she is concentrating. When we give her a new toy, she stares at it, grunts, and breathes heavily. It's almost like the fan coming on for a MacBook Pro when the CPU is working hard. Adorable – and a nice external measure of attention.