Psst!… Pass it on! Cortical communication via the thalamus
Many neuroscientists think of the thalamus – a compact structure lying right in the centre of the brain – simply as a relay station, where sensory information from the periphery converges and is then passed on to the cortex. The cortex is thought to be the site of perception and cognition, with different cortical areas specialized to subserve different functions. Communication between cortical areas can be mediated by axonal tracts running in the white matter of the cortex. This leads readily to the view that once information reaches the cortex it is processed and integrated with other information about the external world and internal states entirely within the cortex, resulting in conscious perception or some kind of motor (or emotional) output. In most neuroimaging studies, for example, the focus is solely on activity in the cortex and the thalamus (and other lower brain areas) are explicitly ignored.
A new study by S. Murray Sherman and colleagues demonstrates unequivocally that cortical areas can also pass information indirectly via the thalamus. It has been known for some time that communication between thalamus and cortex is bidirectional. In fact, the thalamus receives far more inputs from the cortex than it does from the periphery. The circuits between thalamus and cortex can be broken down into two main types – those that drive the activity of their target neurons (whether in thalamus or cortex) and those that act more to modulate the activity of their targets, especially their temporal responsiveness. These pathways can be distinguished based on their neurochemical profiles, the types of synapses that they form and, in the case of projections from thalamus to cortex, the layers which they innervate. Driving connections from thalamus project with quite precise topography to layers 4 and 6, while modulatory connections project more diffusely within layers 1 and 5.
These modulatory connections from the thalamus are essential mediators of communication between cortical areas, due to their crucial role in the synchronization of ongoing neuronal oscillations – rhythmic patterns of activity of local ensembles of neurons. Areas where these rhythms are oscillating in phase with each other are more responsive to signals from each other – this is, in effect, a kind of frequency modulation – when one cortical area sends out a signal, it is picked up selectively by those areas that are tuned to the same frequency. This tuning can be mediated by corticothalamocortical loops (where the corticothalamic connection is driving and the thalamocortical connection is modulatory). In this scheme, however, the information itself is transferred via direct cortical connections.
The new study shows that even if these cortical connections are severed, information can still be transferred from one cortical area to another if corticothalamocortical circuits remain intact (in this case both the corticothalamic and the thalamocortical connections are driving). In a thalamocortical slice preparation from the mouse brain, the authors showed, using a new optical imaging technique, that activity in secondary somatosensory cortex (S2) could be elicited by activating primary somatosensory cortex (S1). There is nothing remarkable in that – what was remarkable was that when the direct connections between these cortical areas were severed that the activation of S2 was almost as potent. This activation depended on an intact corticothalamocortical loop – subsequent disruption of these circuits abolished activation of S2. There are thus most likely two routes of communication between these cortical areas – one direct and one via the thalamus.
These findings reinforce the important point that the function of the cortex cannot be divorced from that of the thalamus. Perception is not simply a matter of passing information along a hierarchy of processing stations – this would leave the question of who is receiving the final information. It can be envisaged instead as a process of reiterative comparison of top-down predictions with bottom-up information, much of which may be mediated by reverberating activity in corticothalamocortical circuits.