Can we regard adaptation as memory?

• ¹ Institute for Psychology, Hungarian Academy of Sciences, Hungary

Just as any other event related brain potential (ERP), the mismatch negativity (MMN) is elicited as the result of an interaction between the neural effect a stimulus and the current state of the brain. The state of the brain is partly determined by previous stimulation. What distinguishes MMN from many other ERP components is that the descriptions which have been put forward to explain within a unified framework the ERP effects categorized as MMN call upon the psychological notion of memory. The search for the neural mechanisms of MMN is, therefore, also a search for the neural implementation of a form of memory.
Stimulus specific adaptation (SSA) is a phenomenon in which previous stimulation affects the neural response to the current stimulus. Thus we can infer that the neural circuits giving rise to SSA store information about the history of stimulation. The question is: Can we regard these neural circuits (or some part of them) as a memory store in the psychological sense?

This question can be answered in a number of ways, depending on how we define memory and what we know about SSA. One possible view is to regard all stimulus after effects as memory. However, on accepting this view, we must accept that most ERP components (indeed, most neural responses) reflect some memory and thus the distinguishing feature of MMN is lost. Another view could require that it should be possible to bring the contents of “true memory” into consciousness. However, this definition is not helpful for the research of the neural mechanisms of memory and it may not even fit some of the MMN results. I will argue for a definition of memory requiring that the information stored should serve multiple processes. This criterion may be employed to distinguish internal states of a process from an information resource. The question is then whether or not the information storage inferred from SSA meets this requirement. I am not sure whether we know enough to answer this question. The reason is that whereas MMN has been observed in a variety of different stimulus configurations, SSA (for very good reasons) has been mainly studied using quite restricted types of stimulus sequences. Therefore, I will call for efforts to find SSA type phenomena with different types of stimulus paradigms.