By: Ryan Hunsaker
I have been having some interesting conversations on twitter with John Kubie (Link to his blog) concerning pattern separation and he has me completely flummoxed by a brilliantly simple question.
I realized his question was not pedantic as I had originally thought as I argued with him but rather was quite insightful and raised a philosophical point that is critical to understand what we mean when we say pattern separation. Since I feel this is an essential point we need to define as a field, I am throwing his question out to the group.
His deceptively simple question was : “What is a stimulus”?
Now, before answering, think about all that baggage that the term stimulus carries with it. Okay, now that we hopfully have all confused ourselves you can read on a bit further and I will explain my thinking on the topic.
Since I like to focus in on methodological questions, I chose to go down this road since this is an important methodological issue that needs a resolution. Also, the answer to this question seems to me to be the fundamental assumption upon which the rest of our research depends.
I used to have an easy answer for this question, but I have lost it of late. I still believe my old answer, but I am no longer confident using it as more than a working definition at best. Part of this reason is that Ray published a paper recently in Neurobiology of Learning and Memory that demonstrated that rats with dentate gyrus lesions result in an inability to remember a stimulus if it was provided in a spatial context, but did not result in a similar deficit if the stimulus was presented in front of blank space. In this case, it was a novel object recognition test is red or clear box. He was not the first to show the hippocampus can be involved for object recognition if there is a distracting context in rats, but the dentate gyrus lesion effect has a lot of theoretical implications for my work.
How I interpret these data is that the dentate gyrus is in a unique anatomical location to process large amounts of sensory information and to generate contextual representations (particularly spatial representations in rodents). If an object is presented is way that can be combined with the extramaze cues, then the object becomes part of the context and the rat cannot process and subsequently recall this representation without a dentate gyrus (i.e., a new object in that space will share the neural representation of the first object because of the larger context). If the object is presented in front of blank space, then there is no information for the dentate gyrus to generate a complex spatial representation/context, and the rat can use the normal cortically-mediated methods to recall the objects.
In a similar vein and more to the point of this post to spur discussion, how do we define a stimulus for our experiments? Can it be something that can be readily encoded in parts (for example a coat rack with a hat on it can be either encoded as a single entity or two items with a spatial relationship)? Does it clearly have to be a single item? Is it a problem if it can be given an easy to remember name? Can the item we test them on be lost in the background easily (figure/ground problem)? Is the test item actually a stimulus in itself or is it the result of a neural computation (Ray and I use the distance between objects as the manipulation, forcing spatial location to be the remembered stimulus)? Can it be a unitized representation of multiple stimuli? … And so on and so on.
I don’t know any of these answers. But I think this is one of the methodological questions we need to grapple with so we can start working toward cementing definitions and generating increasingly sensitive paradigms that test what we think we are testing.
Now to the tl;dr of this post, anyone want to jump into the trenches with me and venture an answer to John’s question? What exactly do we mean when we say, “stimulus”?