There are articles asking, “Are we ever going to understand the brain?” They imply that we have been studying the brain for long enough to be able to say how it works, if we are ever going to, and therefore hinting that it is a permanent mystery. But every week or so some new wrinkle on the brain’s nature comes to light. The brain is far more complicated and far less understood than many think.
Recently a paper appeared that pointed to a wholly new feature of neurons. (citation below) Johansson and his colleagues demonstrate a surprising feature of at least some neurons. They looked at a well known response. When a puff of air is directed at the eye, there is a blink. If this is done over and over with the same time interval between a signal and the puff, a reflex is formed so that the blink happens at just the right time to protect the eye from the puff. This is a standard conditioned reflex and we thought we understood conditioned reflexes. The researchers found that the learning of the time between signal and puff was not a function of a network of cells but an internal function of one type of cell. “The data strongly suggest that the main timing mechanism is within the Purkinje cell and that its nature is cellular rather than a network property. Parallel fiber input lacking any temporal pattern can elicit Purkinje cell responses timed to intervals at least as long as 300 ms. … In addition, the data show that a main part of the timing of the conditioned response relies on intrinsic cellular mechanisms rather than on a temporal pattern in the input signal. ” We have been modeling neurons as firing, or not, as a result of the strength of the signals at their synapses; and firing, if they do, immediately. Any timing effects were assumed to be produced by network structures. Neurons were modeled as very fancy switches but with no timing capabilities. Now understanding has changed. Large changes in understanding, like this one, happen regularly. We are a long way from understanding the mechanisms in the brain.
Here is the Significance and Abstract:
The standard view of neural signaling is that a neuron can influence its target cell by exciting or inhibiting it. An important aspect of the standard view is that learning consists of changing the efficacy of synapses, either strengthening (long-term potentiation) or weakening (long-term depression) them. In studying how cerebellar Purkinje cells change their responsiveness to a stimulus during learning of conditioned responses, we have found that these cells can learn the temporal relationship between two paired stimuli. The cells learn to respond at a particular time that reflects the time between the stimuli. This finding radically changes current views of both neural signaling and learning.
The standard view of the mechanisms underlying learning is that they involve strengthening or weakening synaptic connections. Learned response timing is thought to combine such plasticity with temporally patterned inputs to the neuron. We show here that a cerebellar Purkinje cell in a ferret can learn to respond to a specific input with a temporal pattern of activity consisting of temporally specific increases and decreases in firing over hundreds of milliseconds without a temporally patterned input. Training Purkinje cells with direct stimulation of immediate afferents, the parallel fibers, and pharmacological blocking of interneurons shows that the timing mechanism is intrinsic to the cell itself. Purkinje cells can learn to respond not only with increased or decreased firing but also with an adaptively timed activity pattern.
Johansson, F., Jirenhed, D., Rasmussen, A., Zucca, R., & Hesslow, G. (2014). Memory trace and timing mechanism localized to cerebellar Purkinje cells Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1415371111