As I’ve mentioned many times in class, one of the main jobs of the parietal lobe is to integrate sensory information coming from your body and the external world.  The reason for this is so you can navigate more effectively.  The back half of the brain is generally thought to be involved in processing all the sensory information that feeds into it, then that information is sent to the front half of the brain which is responsible for acting on that information.  One piece of information the parietal lobe sends forward to the front half of the brain is a priority map of the environment (Bisley & Goldberg, 2010).

Essentially, sensory information from your eyes and ears pours into your brain through various sense organs, and from that, after some initial processing in association areas that surround the primary sensory cortices in the bran, this information is integrated into a priority map that gives precedence to those things in the environment to which you need to pay attention.  What is particularly interesting about this map is that it isn’t  driven by features in the environment that are perceptually distinct.  It’s tempting to think that the map is constructed such that bright and noisy and smelly and tasty things stand out, and that is partially the case. This is what we would expect if the mapping were produced through bottom-up processes.

In bottom-up processing, we start with the small details, the little pieces of information that make up the whole.  We begin to put the information together until we have the whole thing.  This is in contrast to top-down processing which says just the opposite, that we start with the larger picture, the bigger concept, and work our way down to the smaller details.  As with many things in psychology there has always been a debate about whether information processing is top-down or bottom-up.  As is also often the case, it is a combination of the two, and the priority map the parietal lobe produces is a very good example of the combination of both types of processing information.

As I mentioned, sensory information from the environment is sent to the cortex, processed in association areas, and then integrated in the parietal lobe and other parts of the brain.  From all that information, which is processed in a bottom-up fashion, the parietal lobe then creates a map of things in the environment that you should be behaviorally interested in and paying attention to.  This is where the top-down processing is added to the mix.  When you are in a familiar environment, there may be several perceptually salient features.  There may be a bright light shining nearby, or intermittent ambient noise coming from the weather or other organisms.  But there may also be an area that you should remember and should consider exploring because it might have something good for you to eat contained within it.  If your attention was guided strictly by the brightest, loudest, smelliest things in the environment, you wouldn’t be able to focus your attention onto areas that might be useful to you.  So we obviously need a mechanism to tune out things that might be perceptually strong, but behaviorally irrelevant.  That’s exactly what a priority map does (Bisley & Goldberg, 2010).

How is this represented in the brain?  That is a good question, because if you recall, in an earlier lecture I mentioned that one of the ways the nervous system encodes for stimulus intensity is through frequency.  A strong stimulus is encoded either by neurons having a lot of action potentials, or by a large number of neurons firing at the same time, or a combination of both of these.  It turns out, that’s exactly how priority is encoded in the parietal lobe.  Those things in the environment that should be paid attention to because they are behaviorally relevant are encoded by greater neural activity on the map than those things that are of less interest.  So while physical intensity is directly encoded at least partially by frequency, the increase in neural activity that corresponds to behaviorally significant things in the environment is an aspect the brain adds to the information once it’s been processed.

As I was reading through material about this, it got me thinking about habituation, and the neural mechanisms that mediate that.  Habituation is a type of non-associative learning in which we stop responding to stimuli that are repetitive and contain no important information.  For example, when I first moved to New York City, I lived right next to the emergency room of Bellevue Hospital.  For the first month or so, I barely was able to sleep due to the constant sound of sirens.  New York is a large city, and emergencies are always happening, so there were a lot of ambulances.  Over time, however, I gradually began tuning the sound of the sirens out, until I no longer heard them.  This was brought home to me when my family called a few months after I had been there, and as we were talking, my father asked me what in the world was going on because he could hear all the sirens.  I actually asked “What sirens?” that’s how well I was able to tune them out at that point.  In the laboratory, habituation can be tested by repeatedly presenting a stimulus, such as a particular flavor or a particular sound, and measuring physiological and behavioral responses to it.

It seems logical that habituation, which is a mechanism by which sensory thresholds are gradually increased and which biases an organism toward novelty, must factor into the priority maps of the environment that the parietal lobe constructs for us.  The same bottom-up and top-down processing is possibly at work with habituation as well, with the actual stimulus from the environment to be habituated to being part of the small details of bottom-up processing.  But then, once we have a better picture of the whole environment, and we know that that stimulus we’ve been hearing or seeing that captures our attention isn’t particularly important, we can tune it out in favor of things to which we should be paying attention.

References

Bisley, J.W. & Goldberg, M.E. (2010). Attention, intention, and priority in the parietal lobe. Annual Review of Neuroscince, 33, 1-21 doi: 10.1146/annurev-neuro-060909-15283

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