L5tt cells are attentional (paper summaries)

[1] High-frequency burst spiking in layer 5 thick-tufted pyramids of rat primary somatosensory cortex encodes exploratory touch

[2] Active dendritic currents gate descending cortical outputs in perception

Main Point
L5tt cells cause the brain to notice a stimulus. To do so, they burst, triggering detection via thalamus, SC, and striatum.

L5tt cells can burst upon touch.

This study just measured spiking while the rat whisked and touched an object.

Many L5tt cells were capable of responding to touch by bursting. Most of this paper addresses possible complications, so it’s fairly safe to take that result as-is. For example, they can burst upon touch regardless of their long-range projections.

Bursting might overcome the generally depressed corticothalamic synapses, causing thalamocortical cells to fire.

I’m mentioning this study because the other study is a bit ambiguous. It uses calcium imaging, for example.

Bursts are required to notice stimuli, at least very weak ones. (Or at least apical calcium events are required.)

This study measured the perceptual threshold, meaning the stimulus strength which the mouse noticed half the time. You can imagine trying to notice a very weak stimulus.

If the mouse notices the stimulus, the L5tt cells burst (or at least have an apical calcium signal closely associated with bursting). Otherwise, they don’t. Activating their apical dendrites reduces the perceptual threshold.

The L5tt cells in this study project to thalamus, superior colliculus, striatum, pons, and medulla. Silencing their synapses in thalamus or superior colliculus strongly impaired the mouse’s detection performance. The same goes for the striatum, but the effect was smaller. There was no effect for pons or medulla.

They repeated the experiment without the behavioral task or reward. This reduced the average calcium signal a lot.

That contradicts the other study, where they burst without a behavioral task or reward. However, the stimuli in this study were very weak, so I think the mice just didn’t notice them. There are other possible explanations, though.


Related to your reference #2 is this previous publication by some of the same authors:

Active cortical dendrites modulate perception

Naoya Takahashi, Thomas G. Oertner, Peter Hegemann, and Matthew E. Larkum, 2016
Link: Science | AAAS


There is as yet no consensus concerning the neural basis of perception and how it operates at a mechanistic level. We found that Ca2+ activity in the apical dendrites of a subset of layer 5 (L5) pyramidal neurons in primary somatosensory cortex (S1) in mice is correlated with the threshold for perceptual detection of whisker deflections. Manipulating the activity of apical dendrites shifted the perceptual threshold, demonstrating that an active dendritic mechanism is causally linked to perceptual detection.

And before that, Larkum wrote this review about apical dendrites, which describes how apical dendrites work at a mechanistic level (but does not explore their purpose).


Thalamocortical interactions

S Murray Sherman, 2012
Free version: Thalamocortical Interactions - PMC

Glutamatergic pathways dominate information processing in the brain, but these are not homogeneous. They include two distinct types: Class 1, which carries the main information for processing, and Class 2, which serves a modulatory role.

[…] the Classes are the same for cortical and thalamic circuitry. These are also the only two classes of glutamatergic input so far seen.

Figure 1 shows the phenomenon of paired-pulse depression & facilitation.
Class 1 synapses (depressing) are tuned to respond to individual spikes and to ignore burst-firing.
Class 2 synapses (facilitating) are tuned to respond to burst-firing and to ignore individual spikes.

At the time of publication of this review, it wasn’t possible to directly prove that apical dendrites have synapses with only class 2 presynaptic axons. However they did find that class 1 and class 2 synapses are predominantly proximal and distal (respectively), and apical dendrites are by definition also distal dendrites.

Therefore it appears that: apical dendrites are tuned to respond to burst-firing, and they can respond by causing their cell to burst-fire.


Cortical control of behavior and attention from an evolutionary perspective

S. Murray Sherman, and W. Martin Usrey, 2021


For animals to survive, they must interact with their environment, taking in sensory information and making appropriate motor responses. Early on during vertebrate evolution, this was accomplished with neural circuits located mostly within the spinal cord and brainstem. As the cerebral cortex evolved, it provided additional and powerful advantages for assessing environmental cues and guiding appropriate responses. Importantly, the cerebral cortex was added onto an already functional nervous system. Moreover, every cortical area, including areas traditionally considered sensory, provides input to the subcortical motor structures that are bottlenecks for driving action. These facts have important ramifications for cognitive aspects of motor control. Here we consider the evolution of cortical mechanisms for attention from the perspective of having to work through these subcortical bottlenecks. From this perspective, many features of attention can be explained, including the preferential engagement of some cortical areas at the cost of disengagement from others to improve appropriate behavioral responses

I highly recommend reading the whole article. It is short, informative, and novel.
Here are some interesting sections which indirectly support the claim that L5tt cells are attentional:

These links between cortex and behavior involve layer 5 corticofugal projections. The only other source of subcortical projection arises from a population of layer 6 cells: some innervate thalamus and others the claustrum, but of great importance in the context of this perspective, in no case do these innervate obvious subcortical motor centers (Sherman and Guillery, 2013; Sherman, 2016; Usrey and Sherman, 2019). It is also worth noting that cells in layers 5 and 6 that project subcortically rarely project to other cortical areas, and vice versa (Petrof et al., 2012). Some further details on differences between layer 5 and 6 corticofugal projections are provided below. It is the layer 5 cells that innervate numerous subcortical motor centers (Deschenes et al., 1994; Bourassa and Deschenes, 1995; Bourassa et al., 1995; Kita and Kita, 2012; Prasad et al., 2020) and thus appear to be the effective route whereby cortex influences behavior.

An alternative way to consider attention is from the perspective of the selection process whereby the appropriate […] circuits are brought into play to provide the best response to any environmental challenge at any given time. All of the other results described for attention can be seen as a logical and predictable consequence of this process.

And I just found this part interesting:

many aspects of sensory perception also appear to function without cortex. A classic example is the Sprague effect, in which cats without visual cortex can detect and locate novel visual stimuli on the basis of functioning of the superior colliculus (Sprague, 1966). This ability to detect stimuli strictly with subcortical circuits has also been extended to the auditory and somatosensory systems (Lomber et al., 2007; Hong et al., 2018). […] The main point here is that during pre-mammalian vertebrate evolution, brainstem centers formed the capacity for quite effective sensorimotor processing, typically reaching maximum effectiveness in evolution of circuits involving midbrain structures such as the optic tectum, which is the homolog of the mammalian superior colliculus, although older and simpler telencephalic structures also exist in primitive vertebrates such as the lamprey (Herman et al., 2018; Krauzlis et al., 2018; Gharaei et al., 2020; Basso et al., 2021). Many and perhaps all of these older circuits remain viable and in use in mammalian brains.