How attention works in the brain is becoming clearer through empirical and computer modeling work.
The current World Series is a tour de force of mental concentration and attention. Batters intently watch a pitch, and have milliseconds to decide that it isn't any good. Pitchers study the opposing batters for any signs of gullibility. Managers face excruciating decisions on when to pull a pitcher in danger. Spectators decide whether to get drawn into the pitcher's duel, or chat with their neighbors. Advertisers measure attention in dollars and cents.
The economics of attention may have reached a fever pitch, but the physiology of attention is only slowly being revealed. Attention is obviously closely aligned with consciousness, so progress on one implies progress on the other as well. The current paper is a computer modeling project, trying to simulate the core connections and behavior between the thalamus and cortex that are involved in sensory perception. For instance, mice are given a slight push on a whisker. If they respond to that, it shows they perceived it. At a very light threshold level, the chances of perception can tuned to 50%, and perceptual events can have more to do with the mental status of the mouse and the history of whisker stimulation than it does with the (consistent) strength of the stimulus. A similar threshold phenomenon holds in other forms of sensing and perception, such as binocular rivalry. Indeed, in binocular rivalry of vision, there is a slow switching back and forth between each image, based on neural accommodation after a few seconds attending to one of the images. Such threshold levels of perception are the bread and butter of research on attention.
"Given the ubiquity of the thalamocortical circuit architecture across sensory modalities, we, along with others, have proposed that reverberant bursting activity in L5PT [thick-tufted layer 5 pyramidal-tract neurons]– matrix thalamus loops may be a necessary component part in a domain general mechanism of perceptual awareness."
"Optogenetic excitation of the apical dendrites reduced the animal’s threshold for awareness, increasing both hits and false-alarms. In turn, pharmocological inhibition of the apical dendrites and POm [posteromedial thalamic nucleus] (a matrix-rich higher-order thalamic nucleus with closed loop connections to barrel cortex) increased the animal's perceptual threshold."
The model was based on the physiology of thalamo-cortical loops, which are very common in sensory, motor, and other circuits. Attention is not something that "happens" in one place, but rather appears to be a state of the network, after negotiation between upper and lower levels. Strong stimuli, such as the roar of the crowd after a home run, push their way to the top of the attention chain. Conversely, a quest for a hot dog can lead to highly focused top-down attention on planning and making a trip to the concession stand, while ignoring everything else going on in the game.
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| A bit of physiology, showing how mouse neurons connect between cortical and thalamus levels, and how they look at various levels within the cortical sheet. |
These researchers found that by making a faithful model of the neurons and connections found by physiology, they could then functionally model quite faithfully the actions of this loop, including its perceptual thresholds, stochastic activation, and tendency to accommodate (dampen) repeated stimuli. A key aspect of the physiology is the layering of the cortex. Evolution has left clear marks in brain areas of various vintages, in the form of layer organization. The most primitive areas like the brain stem and cerebellum have no layering at all, but rather have anatomical structures, bulbs, and sub-nuclei. Less primitive subcortical (limbic) areas have roughly three layers of cells, whereas the neocortex has a structure that is a uniform sheet, modularized not so much into structural bulbs, knobs, etc, but in a more regularized arrangement of six distinct layers, with columns of activity for parcellation of function. This regular arrangement is replicated all over the cortex and (re-)used for innumerable functions, from sensation and motor control to decision making and emotional control. In general, the middle layer (4) contains the most cell bodies, (called the granular layer), and connects extensively to the other layers. The upper layers (numbered 1,2,3, and closest to the outside of the brain) receive inputs from the thalamus and other areas, while the lower layers (5 and 6 ) send outputs to lower areas of the brain, for motor control, attention control, etc.
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| The authors simulate cells and connections from the known physiology. Cortical levels shown on left, and an example of one set of connected, active, firing cells on right. |
The thalamocortical loop, therefore, as illustrated by the authors, is a cycle of connections from the thalamus into cortical layer 1, which connects to cells stationed in layer 5, which then send axons out back to the thalamus. The thalamus is a large structure nested within/under the hippocampus, basal ganglia, and corpus collosum, that mediates cortical signals to and from the rest of the nervous system, including senses. The authors basically find that they can reproduce the signature bursting behavior of this circuit that others have argued is a sign of attention. That is, the brief set of six pulses between 0.1 and 0.2 seconds above, which is quickly shut down by continuing activity from the inhibitory basket cells (orange, BC). I can't speak to the details either of the modeling or the physiology it is based on, but the authors try their best to hew to realistic cells and physiological circuits, making the case that the neural behavior they get out is a realistic simulation, which can then be used for other perturbations and studies of this system.
A question that arises is the relationship of consciousness with attention. Are the neural patterns characteristic of attention all that is required to also be conscious? A review of the field says no, they are different, or at least that consciousness is a broader concept that contains attention, but also can contain bare awareness without specific focus. I am not so sure, since attention can presumably be directed inside to our own thoughts and memories, that constitutes the floating kind of awareness of bare consciousness. Without attention to anything, we lack conscious content, and thus, perhaps consciousness itself. The idea that we can meditate our way to a content-less consciousness is time and again disproven by the practice of meditation. It finds that a focus is essential, not to empty the mind of all contents, which I believe is impossible, but to control those mental contents and attain a controlled level of lucid dreaming or mantra-driven reduced consciousness that seems to be the goal of meditation.
The authors of this paper mention that their view of attention is very compatible with each of several reigning theories of consciousness, inchoate as those are, so we do seem to be heading, ever so slowly, towards a solution of this long-standing problem.
- The last time the President and military were at such loggerheads.
- China knows what it is doing.
- Make more market housing.
- The NewsHour asks scientists how they are doing.
- We are going there.
