Larkum 2012, http://dx.doi.org/10.1016/j.tins.2012.11.006
A basic feature of intelligent systems such as the cerebral cortex is the ability to freely associate aspects of perceived experience with an internal representation of the world and make predictions about the future. Here, a hypothesis is presented that the extraordinary performance of the cortex derives from an associative mechanism built in at the cellular level to the basic cortical neuronal unit: the pyramidal cell. The mechanism is robustly triggered by coincident input to opposite poles of the neuron, is exquisitely matched to the large- and fine-scale architecture of the cortex, and is tightly controlled by local microcircuits of inhibitory neurons targeting subcellular compartments. This article explores the experimental evidence and the implications for how the cortex operates.
A State of Attention
This is my response to the article (Larkum 2012) which discusses backpropagation activated Ca 2+ (BAC) spike firing in Pyramidal neurons in the neocortex. Larkum argues that BAC firing is a critical component of cortical feedback. I disagree with this assessment; I argue that BAC firing is a critical component of attention. I hypothesize that BAC firing is a distinct computational state of neurons which represents a state of attention. This additional state fits into the HTM paradigm and explains how to control an HTM system.
Here I summarize key evidence from (Larkum 2012). Pyramidal neurons have a calcium-spike initiation area in their apical dendrite, which is isolated from lower areas of the neuron. The apical dendrite is a separate compartment from the basal dendrites which is usually unable to initiate APs. If the basal dendrites first initiates an AP then the apical dendrite becomes suddenly very sensitive and if activated then the neuron will emit 2-4 APs at approximately 200 Hz, an event known as backpropagation activated Ca 2+ (BAC) spike firing. Apical dendrites, like basal dendrites, utilize NMDA receptors and dendritic spines. Some inhibitory neurons target apical dendrites.
A State of Attention
BAC-firing in pyramidal neurons represents a state of attention. This state is in addition the HTM states of: inactive, unpredicted-active, and predicted-active; the lower half of the neuron is a state of the art HTM neuron. Pyramidal neurons are either at attention or at ease. Neurons enter the state of attention if they are activated by both their basal and apical dendrites. This rule may have exceptions, such as if there is no feed forward proximal input and the whole cortical area becomes inactive and unused, or if there is overwhelming apical input. Apical dendrites utilize NMDA receptors which indicates that they operate in a similar manner to basal dendrites, including that they constantly learn in an unsupervised manner. This means even if you aren’t paying attention that the apical dendrites are getting ready to pay attention.
BAC firing could be reliably detected by its signature short high frequency bursts. It’s possible that apical dendrites are tuned to respond primarily to BAC-firing (at attention) inputs, with other inputs ignored or having a lesser effect. This would have an important effect: it would allow assemblies of neurons at attention to persist over time by forming stable local recurrent networks through apical dendrites. It would also block assemblies of neurons which are at ease from interfering with or spreading through apical dendrites. Attention would persist across cortical areas assuming that topologically related areas were connected through apical dendrites, which is important because the “thousand brains” theory states that objects are represented by all neurons which contain the object in their proximal receptive field. This means that as an object moves across an animals field of vision its representation in the brain moves through many cortical areas, and attention follows with it. Attention would persist without any intervention.
Attention would make all of the neocortex into a single global workspace where ideas come together in a controlled fashion to produce a desired result. All neurons which are at attention would participate by broadcasting pertinent information. This would allow arbitrary areas of the cortex to cooperate, and areas which aren’t currently participating quietly watch and wait.
Methods of Cortical Control
The basal ganglia performs reinforcement learning which concerns what good or bad things may befall the animal. The basal ganglia projects to the thalamus, which in turn projects to the cortex, and then finally to the muscle neurons. I hypothesize that the thalamus uses reinforcement learning to associate conditions in the basal ganglia and cortex with ways to manipulate the cortex. Cortical pathways which connect to muscles or other brain outputs will be especially controlled. Here are three ways that the thalamus could manipulate the cortex:
Turn off all of the feed forward proximal input, which will shut down a whole cortical area. This could be used either to restart the network or to allow attention to inject an idea into an unused cortical area. This mechanism could allow attention to recruit unused cortical areas to help with problems which the unused areas have experience solving. This could also prevent the motor cortex from performing an undesired action.
Turn off attention. There are inhibitory interneurons which target the apical dendrites, both in layer 1 and Martinotti cells throughout the neocortex. This inhibition would prevent activity from participating on the global workspace. Activity suppressed in this way is still present in the cortex and can be brought back to attention if needed.
Promote attention in a cortical area by increasing the overall magnitude of the feed forward proximal input so that neurons activate as strongly as BAC firing neurons. This could allow an idea to gain attention where before there was no attention.