How specific — or sparse — is the neural representation of a memory trace? Quian Quiroga and colleagues now have an article in Neuron (PDF), where they describe their well-known studies using single-cell recordings to well-known faces. As you most likely know, this has given rise to the debate about the “Jennifer Aniston neuron”. Their findings, briefly put, have demonstrated that single cells show quite specific responses to very specific visual stimuli. While one cell may have a preferential response to the Sidney Opera House, another responds dramatically more to Hale Berry, while yet another cell responds to, well, Jennifer Aniston.
The Quiroga studies have re-iterated the debate (if it ever went dead) about how specific the neural coding is in the brain. Is it really so that the brain has such a specific code that one cell can represent one percept? Do we have a grandmother cell, a President Nixon cell and Marilyn Monroe cell?
Today, there is wide agreement that the one-cell-one-percept idea is untenable and unsupported by the literature. Rather than cingle cells, we see that networks represent a percept, rather than single cells. However, the findings by Quiroga et al. have nevertheless stunned the scientific (and global) community with regard to just how specific the neural code can be, and that it can be detected in a single neuron. The findings that we can record how one single neuron responds to one, and only one, percept, is quite surprising.
So forget about the grandmother cell, right? Or maybe not. After all, following the idea from these findings, we should not be surprised that there would in fact be one neuron that responded preferentially to our grandmother. Yes, it would be an expression of a “network code” representing our grandmothers as such, but nevertheless, you may in fact have that one neuron that responds to ol’ granny.
While we leave it at that, it is still surprising that this team of researchers use the term “medial temporal lobe” (or MTL for short). Why doe they say that there is sparse coding in the MTL? It’s a rather big region, and a region packed with qualitatively different regions. Not only are these regions different anatomically, but also functionally, they are thought to be involved in different functions. The perirhinal cortex is involved in processing (and encoding) of complex visual objects as well as novelty processing and working memory, the (posterior) parahippocampal cortex is involved in spatial processing (remember the parahippocampal place area). The entorhinal cortex has a medial and lateral part that deal with spatial and object information, respectively. And in addition to the hippocampus and amygdala, with their quite different functions, we may extend the MTL concept to include the temporopolar cortex, and maybe even the inferotemporal cortex.
Where in this complex system do Quiroga et al. find their sparse coding? Everywhere? My bet is on the perirhinal cortex due to its involvement in complex visual object processing. I’ve come to know that the researchers have not had structural scans available to determine the exact location of their electrodes. The scans have obviously been made, but they have not been able to use that information (hush hush, don’t tell anyone…).
So, while these studies are indeed important to our understanding of the coding of specific information, we’re left with a huge gap in terms of their anatomical properties. While most of the research community focuses on MTL subdivisions to an increasing extent, it is a bit puzzling to me that nobody have ever criticised these studies for their sparsity of anatomical information. Maybe I’ll be the first?