Electronic Thesis and Dissertation Repository

Thesis Format



Master of Science




Minda, John P.


It remains an open question whether modern neuroimaging clearly dissociates the Explicit system that learns by encoding rules, and the Implicit system that learns by encoding information-integration boundaries. Further, there are nearly no applications of fNIRS as a modality in studying category learning. We conduct two behavioural experiments to validate a carefully controlled categorization task intended to dissociate Explicit and Implicit systems. Then we apply fNIRS neuroimaging within-subjects to localize a neuroanatomical dissociation. We localized two effects to R DLPFC (1) a simple single-dissociation of higher activity in RB categorization, and (2) a negative relationship between overall task performance and the magnitude of neural activation. We conclude that explicit and implicit cortical activity are dissociable by neuroimaging, and that fNIRS is a feasible modality to study human categorization.

Summary for Lay Audience

It is already well known that there are at least two ways in which humans learn new categories. The first method applies to categorizing objects such as cups and mugs. How would you distinguish between cups and mugs? It turns out the mind narrows in on one main property, in this case—the handle. After seeing many cups and mugs you might notice that most mugs have a handle. You may come to formulate some kind of a rule: "mugs have handles, cups don't". With that bit of language, you have now categorized most of the cups and mugs you'll ever see. This is known as explicit categorization.

However, sometimes this explicit method may not work for certain types of objects such as cats and dogs. If you look closely, you may realize that it's difficult to narrow in on one 'property' that cleanly divides cats and dogs. Is it the size? The amount of fluff? The whisker length? There is no single clear distinction. However, all of us easily classify cats and dogs. The second way we classify is based on picking up associations between certain objects we see and a label or action that corresponds with it. This is called implicit categorization.

If we learn categories in two different ways, does that mean there are two different circuits of the brain involved? This study aims to answer that question. We do that by designing two tasks, one explicit task, one implicit. Then we use functional near infrared spectroscopy (fNIRS) to measure the blood flowing through the brain at different locations. More blood flow tells us that one part of the brain is working harder. Then we compare blood flow patterns between explicit and implicit.

We find that there was more activity in the right dorsolateral prefrontal cortex (located above your right eyebrow, where the hairline begins) when people are doing explicit categorization. Our findings align with previous work on memory, language, and reasoning. We therefore conclude that there is likely to be distinct underlying brain circuits for explicit and implicit category learning.