If you quickly glance at Claude Monet’s “The Rue Montorgueil in Paris. Celebration of June 30, 1878” as you visit the Musée d’Orsay, you can instantly recognize individual objects in this scene depicting a french celebration with crowds of people walking in the street. Look closer. The blocks of color on the flags are so blurred you may have trouble distinguishing one flag from the next. The crowds of people in the street are almost inseparable into individual persons. The typical Parisian gates on the bottom of the windows are nothing but dark blotches. Still, you can recognize these objects and people for what they are, even in a painting you have probably never seen before.
This ability has to do with a kind of memory your brain uses to identify something that you see, and it is called “object-recognition” memory (ORM) (Winters et al., 2008). Generally, the hippocampus is a brain part known to be very important in this kind of memory (Winters et al., 2008). This structure mainly houses declarative memories, such as the abilities to remember the name of the dessert that you picked up at a boulangerie or to tell taxi drivers the address you’re staying at while in Paris (Winters et al., 2008). Despite the importance of the hippocampus, it is not the only structure in the brain that houses memory. Brain structures like the orbitofrontal cortex, amygdala, and cerebellum also house components of memory. Even recently, parts of the cortex that decode visual information have been found to be important in certain types of memory, like ORM (Winters et al., 2008).
The brain processes visual information in a hierarchical manner. The photoreceptors in the eye sense spots of light and send signals to other neurons in the brain that activate to increasingly specific images. An important part of this system is the area known as V2, which splits into two parts. The lower area, known as the ventral part, allows for the abilities to tell the shape of a baguette, the size of the Eiffel Tower, and the color of the bright red strawberries in the markets (López-Aranda et al., 2009). In an attempt to try to decode this part of the brain, researchers have been looking at this part of the cortex to determine which neurons do what. Certain neurons in layers of this cortex have been tested, such as the layer 3 neurons playing a part in visual processing; however, the function of layer 6 neurons in this area remained a mystery (López-Aranda et al., 2009). In a recent study, López-Aranda and associates took on the challenge of determining the function of these neurons in the 6th layer of this part of the visual cortex (López-Aranda et al., 2009). The researchers hypothesized that these neurons play a role in ORM (López-Aranda et al., 2009).
To determine the function of neurons in layer 6, López-Aranda and associates set up an experiment where rats would have the opportunity to recognize objects they had seen before (López-Aranda et al., 2009). The experimenters placed two identical objects with the rats for three minutes, then they removed the items (López-Aranda et al., 2009). Later, during testing sessions at different timepoints, the rats had the opportunity to explore one of the old items and a new item that they had never seen (López-Aranda et al., 2009). The amount of time that the rats spent around each of the objects, old and new, was measured (López-Aranda et al., 2009). If the rats spent a great deal of time around the new item and ignored the old item, the researchers concluded that the rats remembered the old item using ORM, and they would rather use their time to explore the novel item (López-Aranda et al., 2009). If the rats spent an equal amount of time around the old and the new item, then the rats did not remember the old item and spent time examining both the new and the old items (López-Aranda et al., 2009).
The normal rats were able to remember the old object for 45 minutes, but not for 60 minutes (López-Aranda et al., 2009). When the researchers inserted more RGS-14 genes, which produces a regulator of G-protein signaling protein, into layer 6 neurons of the V2 cortex three weeks before the ORM testing, the rats remembered the object for at least 24 weeks (López-Aranda et al., 2009). With more of the RGS-14 protein in these neurons, the rat’s had longer ORM (López-Aranda et al., 2009). To make sure that this ORM enhancement was unique to this combination of RGS-14 protein in the layer 6 V2 neurons, this protein was overexpressed in three other brain locations and there was no significant ORM increase (López-Aranda et al., 2009).
Further proving their point, López-Aranda and associates killed off the neurons in layer 6 of V2 with the hypothesis that this would hinder ORM (López-Aranda et al., 2009). At 45 minutes, the rats with the missing neurons did not spend more time around the new object; they couldn’t remember that they had seen the old one before (López-Aranda et al., 2009). Just to make sure that the ORM ability was unique to V2 layer 6 neurons, neurons in other structures were eliminated (López-Aranda et al., 2009). These eliminations did not change the rats’ ORM ability to remember the old object (López-Aranda et al., 2009). Based on these data showing that increased RGS-14 protein expression in layer 6 neurons of V2 increases ORM memory, and elimination of the layer 6 V2 neurons decreases ORM memory, the experimenters wondered if these neurons were important for ORM memory formation or if they were the sites where the ORM memories were stored (López-Aranda et al., 2009).
In one more experiment, rats that had increased RGS-14 protein in the layer 6 neurons of V2 were exposed to new objects (López-Aranda et al., 2009). After that, the neurons in this layer were destroyed (López-Aranda et al., 2009). The rats had some ability to recall the objects that they had previously seen, but if they were exposed to objects after the neurons were destroyed, they could not remember them at all (López-Aranda et al., 2009). This experiment shows the importance of V2 layer 6 neurons in acquiring the memory, but because the rats remembered the old objects they had seen before the neurons were destroyed, this memory could not have been stored in the V2 layer 6 neurons (López-Aranda et al., 2009). While the V2 layer 6 neurons may not house the memories that allow you to recognize objects, they are important in being able to form the memories so you can recognize the French flags and the people in Monet’s painting (López-Aranda et al., 2009).
To wrap up this post, your visual system is even more amazing than just being able to sense the brushstrokes and colors when you look at the impressionist paintings in the Musée d’Orsay. Parts of it have to do with making memories of what you have seen, so that you can apply them to your future experiences. The brain is just like the deep ocean or distant space; we are figuring it out, but so much remains unknown. This study shows that the visual system is involved with more than just processing visual information. Just like in Monet’s painting where the lines that define the objects are blurred, the functions of the visual cortex seem to be unclear as it is is implicated for other things besides vision, like ORM.
López-Aranda, M. F., López-Téllez, J. F., Navarro-Lobato, I., Masmudi-Martín, M., Gutiérrez, A., Khan, Z. U. (2009). Role of layer 6 of V2 visual cortex in object recognition memory. Science 325:87-89.
Winters, B. D., Saksida, L. M., Bussey, T. J. (2008). Object recognition memory: neurobiological mechanisms of encoding, consolidation, and retrieval. Neuroscience and Biobehavioral Reviews 32:1055-1070.
~ by Emily Aidan Berthiaume