Neurobiologists have observed (see references at bottom) that neurons in the thalamus, the part of the brain that receives input connections from the body’s sensors, not only send output connections to cortical columns in the cerebral cortex, but receive reciprocal feedback inputs from the same columns. No one knows why this happens. What follows is a novel hypothesis that explains the function of the corticothalamic feedback connections as an essential part of the mechanism of sensory perception and learning.
Note: This may be my last article on Medium. I am protesting their censorship policies. Censorship is evil.
A Model of the Perceptual System of the Brain and the Cortex
In order to understand why the brain’s perceptual system uses feedback signals, it is essential to have an idea of what it is trying to do, how it is organized and the function of its subsystems.
The diagram above is the hypothesized perceptual model. It posits that the thalamus (pattern memory) is where the brain stores a huge number of elementary pattern detectors. These send their output signals directly to the cortex (sequence memory) where they connect to a myriad neuronal structures called minicolumns. These are contained inside bigger structures called cortical columns. There are approximately 100 million cortical columns in the human brain and each has 100 minicolumns on average. Each minicolum consists of 6 associate inputs and 1 primary input. The role of a cortical column is to learn as many pattern combinations as possible. Every connected minicolumn in a column is a different manifestation of the primary input of the column. The green arrow in the diagram represents the feedback signals that return to the origins of the feedforward signals, which are the pattern detectors in the thalamus. The number of feedback connections is equal to that of the feedforward connections.
Cortical columns (see Stuff I’ve Been Working on: The Cortical Column) are arranged in a feedforward hierarchy of up to 20 levels or regions. Pattern signals arrive at the bottom or entry level and percolate up the hierarchy as far as they can go. The activation of a topmost minicolumn signifies that a complex object or pattern has been detected. Normally, many top minicolumns in the hierarchy will fire simultaneously depending on the complexity of the object. Think of an object as a mountain with many peaks and plateaus. How they are clustered together to form a single object is the subject of a future article. Keep in my mind that the exact composition of an object is not learned. It is composed instantly even if the brain have never seen it before.
What is important to realize is that pattern detection does not occur until and unless a minicolumn has fired. This is how the brain handles sensory uncertainty. The problem is that pattern signals arriving from the thalamus are rarely perfect due to occlusions, noise and other accidents. The minicolumns are, likewise, rarely perfect. The brain solves the uncertainty problem by using a threshold level in its minicolumn that must be reached or surpassed in order to warrant a detection event. When this happens, a topmost minicolumn emits a feedback signal that quickly cascades down the hierarchy one level at a time, branching out as it does, all the way down to the source pattern detectors in the thalamus. The signal branches out because every one of the 7 inputs to a minicolumn is paired with a reciprocal feedback output directed down the hierarchy. In other words, when a minicolumn fires or receives a feedback signal from above, it immediately outputs 7 feedback signals down the hierarchy. This grows exponentially at each level.
Solving the Mystery of Reciprocal Corticothalamic Feedback
Two questions comes to mind. First, why does the cortex use feedback signals? Second, why must the feedback signals travel all the way down into the thalamus? Why type questions are always the best. The answers we are looking for in this case depend on gaining a good understanding of the cortical learning process:
- The most important reason for having feedback signals, as explained earlier, is that this is the fastest and most energy-efficient way to solve the uncertainty problem. The solution is to enlist the contribution of many parallel inputs during the detection process. A high enough number of signals arriving at a topmost minicolumn is enough to overcome uncertainty. Contrary to common wisdom, the brain is not a probability thinker but a cause-effect thinker. The brain assumes a perfect and deterministic world. When we recognize grandma, it’s not 50% or 90% grandma. It’s either grandma or no grandma.
- The cortex is is the seat of episodic memory. When a minicolumn receives a feedback signal, it immediately records a memory trace and the time of the activation. This is crucial because this recording affords us not only a way to recall past events but also makes it possible to predict the future. Of course, the memory trace dissipates quickly unless it is rehearsed repeatedly.
- Learning in the cortex consists of forming pattern combinations one minicolumn at a time. It is important that learning be as fast as possible. Random inputs are connected to a minicolumn and tested to see if they arrive concurrently. If an input passes the test only once, it immediately becomes a permanent connection. While this learning method is very fast, it can result in erroneous connections because of chance occurrences. There must be a way to correct the errors.
- The error correction method is straightforward. Every time a minicolumn receives a feedback signal, it strengthens every input connection that just received a strong enough signal. Bad input connections that do not fire on time rarely get strengthened and so remain weak. However, these bad connections are not severed immediately. This happens at night during REM sleep.
- Finally, the reason that the thalamus receives feedback signals is that connections to the first level of the cortical hierarchy are learned in the thalamus. The reason for this is that learning (searching for viable connections) in the thalamus is faster and easier due to the sheer number of pattern neurons. The thalamic connections must also be strengthened by feedback signals and disconnected during REM sleep if they don’t behave as expected and are therefore weak.
To sum up, feedback signals are an integral part of the brain’s cortical learning mechanism and its ability to process imperfect sensory signals. In the cortex, they contribute to episodic memory. In the thalamus, their only function is to strengthen good connections and disconnect bad ones. In a future article, I will go over how the cortex clusters large numbers of minicolumns to form invariant objects or concepts. Clusters are also part of the brain’s attention mechanism.
Feedback Connections to the Lateral Geniculate Nucleus and Cortical Response Properties
Emerging views of corticothalamic function
Stuff I’ve Been Working on: The Cortical Column
Fast Unsupervised Pattern Learning Using Spike Timing
Source: Deep Learning on Medium