About a month ago, I had the chance to attend the CCN conference in Philadelphia. This post is not about all the great talks and posters I saw, the new friends I made, nor the fascinating and thought-provoking discussions I had. It’s a great conference, but this is a post about a troubling and ironic theme that I heard more than a few times from multiple speakers. The troubling part is that behaviorism is making a comeback. The ironic part is that it is driven by a methodology that is intended to replicate and elucidate the details of mental representations: deep neural networks.
In 2014, a landmark paper by Dan Yamins and others in Jim DiCarlo’s lab set the stage: they essentially showed that each layer in a 4-layer “deep” neural network trained to do object recognition could be mapped to representations found along the primate ventral stream, which is known to be involved in visual object recognition in the brain. Importantly, they went a step further and showed that the better a neural network was at classifying objects, the better it was at explaining representations in the ventral stream. This was (and is) a big deal. It was proof of a theory that had been floating in many researcher’s minds for a long time: the ventral stream analyzes the visual world by hierarchical processing that culminates in disentangled representations of objects.
So where do we go from there? Since 2014, the deep learning community has progressed in leaps and bounds to bigger, faster, better performing models. Should we expect Yamins et al’s trend to continue – that better object recognition gives us better models of the brain for free? The evidence says no: sometime around 2015, “better deep learning” ceased to correlate with “more brain-like representations.”
This is why I was surprised to hear so many speakers at CCN suggest that, to paraphrase, “to make neural networks better models of the brain, we simply need bigger data and more complex behaviors.” It all reduces to inputs and outputs, and as long as we call the stuff in between “neural,” we’ll get brain-like representations for free!
I’m not alone in questioning the logic behind this approach. A similar point to mine was articulated well by Jessica Thompson on Twitter during the conference:
where does the notion that “the more complex the problem, the fewer the number of solutions that exist to it” come from? It’s come up a couple times today at #CCN2018 and I don’t get the intuition
— Jessica Thompson🧠🤖🤔👩💻✊ (@tsonj) September 7, 2018
Of course, a neural network that solves both problem A and problem B will be more constrained than one that solves either A or B alone. Bigger data makes for more constrained models, as long as the model’s outputs – its behaviors – are limited. Is it obvious, though, that adding complexity to the behaviors we ask of our models will likewise push them towards more human-like representations? Is it clear that this is the most direct path towards AI with human-like cognitive properties? My concern is for a research program built around neural networks that nonetheless fixates on the inputs and outputs, stimulus and behavior. This is simply behaviorism meets deep learning.
Now, nearly every cognitive scientist I’ve ever met is happy to denounce the old, misguided doctrines of “behaviorism.” Calling someone a behaviorist could be taken as a deep insult, so allow me to clarify a few things.
An extremely brief history of behaviorism
The behaviorists were not as crazy as folk-history sometimes remembers them to be. To the more extreme behaviorists, led by B. F. Skinner, there was no explanatory power in internal representations in the mind, since they were assumed to be either unobservable (at best based on introspection) or reducible to inputs and outputs (Skinner, 1953, p.34). It should be noted, however, that even Skinner did not reject the existence of mental representations themselves, nor that they were interesting objects of scientific study. He simply rejected introspection, and hoped everything else would have a satisfying explanation in terms of a subject’s lifetime of stimuli and behaviors. This is not unlike the suggestion that the representations used by neural networks should be understood in terms of the dataset and the learning objective. So, why did behaviorism fall out of favor?
Behaviorism’s decline began with the realization that there are many aspects of the mind that are best understood as mental representations and are not easily “reducible” to stimuli or behavior – perhaps not a surprising claim to a modern reader. The classic example is Tolman’s discovery of cognitive maps in rats. Tolman demonstrated that mental representations are not only useful and parsimonious explanations, but are also measurable in the lab. Historically, his results spurred a shift in the emphasis of psychologists from measurement and control of behavior to understanding of the mental representations and processes that support it.
As in Yamins et al (2014), this has always been the goal of using deep neural networks as models of the brain: starting with the right architecture, optimizing for a certain behavior gives us brain-like representations “for free.” Wouldn’t it be ironic then if deep neural nets led cognitive scientists back to behaviorism?
What are the alternatives?
The alternative to the behaviorist approach is that our models in cognition and neuroscience should be guided by more than just matching the inputs and outputs of the brain.1 The difficult but incredibly important problem here is characterizing what are the right constraints on the stuff in between. Training their model to do object recognition was interesting, but I think the success of Yamins et al (2014) came from their 4-layer model architecture which was designed to match known architectural properties of the ventral stream.2 It’s perhaps no surprise that neural networks pushing hundreds of layers have ceased to be good models of the ventral stream.
So, what kinds of constraints should we put on our model architectures? This problem needs to be approached from many directions at once: anatomical constraints of what connects to what, functional constraints on the class of computations done by each part of the model, and normative constraints like Bayesian learning and inference of latent variables. We need to look to ideas from the unsupervised learning literature on what makes good “task-independent” representations. In other words, our models need the right inductive biases. They should mimic human learning not just in the “big data” regime with millions of input/output examples, but in the limited-data regime as well.
This is not an exhaustive set of criteria and I don’t claim to have the right answer. However, I do believe that anyone interested in understanding how the brain works needs to invest more in understanding anatomical, functional, and normative constraints on representations than simply pushing in the direction of task-optimized black-boxes.
- Yamins, D. L. K., Hong, H., Cadieu, C. F., Solomon, E. A., Seibert, D., & DiCarlo, J. J. (2014). Performance-optimized hierarchical models predict neural responses in higher visual cortex
- Skinner, B. F. (1953). Science and human behavior
- Tolman, E. C. (1948). Cognitive maps in rats and men. Psychological review
- For a philosophical perspective, consider John Searle’s Chinese Room thought experiment. Searle imagines a room with a mail slot, where queries written in Chinese characters are the “input” and responses are passed back out through a different slot, also written in Chinese (let’s say Mandarin). From an input/output perspective, the system appears to be an intelligent agent who is fluent in Mandarin and providing thoughtful answers to the input queries. The catch in Searle’s thought experiment is that inside the room is someone who only speaks English, but who has a very large book of “rules” for handling different types of inputs. This person, with the help of the book, is implementing an AI program that can intelligently respond to questions in Mandarin. This thought experiment can be taken in a few different directions. Here, I would argue that it’s an example of how simply matching the inputs and outputs of some desired behavior doesn’t necessarily give what you would expect or want in between.
- Further work led by Yamins & DiCarlo has continued in the direction of testing various architectural constraints like using recurrence.