After integrating the lab in 2003 for a short training in tract tracing, I have been involved in the exploration of the anatomical segregation between feedforward and feedback pathways in the brain and the description of the structure of the network of cortico-cortical projections. More than 350 years after Descartes it is admitted as a fact that the function of the brain can not be understood without understanding the structural constraints that underlay it’s network organization. The hierarchy of interareal projections is in our days the only model, of brain organization, that can forestand questions coming from both physiologists and anatomists. Already in the 1990’s computational models (Ulllman’95) were predicting that brain operation of conscious vision is supported by feedforward and feedback pathways converging at each level of the brain hierarchy.
The different projects are integrated milestones in the Daisy Project.
Project 1 : Feedforward and Feedback pathways in the primate cortex show sharp anatomical segregation and some morphological distinction
Information flow in the cortical hierarchy is via feedforward and feedback pathways. It is thought that feedforward projections are involved in the generation of the neuronal receptive fields while the feedback projections are modulatory processes. Both pathways are formed by pyramidal neurons for which there is no known reliable distinction. Showing that feedforward and feedback pathways constitute distinct networks will be a strong indication that they are characterised by different physiologies.
Using retrograde tracers and cell filling we explore the topology and morphological features of neurons involved in these pathways.
The main questions we are answering in this project are:
- Is there a laminar segregation of feedforward and feedback projecting neurons? Are there signature gradients?
- Do feedforward and feedback neurons have distinct morphologies?
- Do feedforward and feedback projections show important differences in topology?
- What are the relative importance of feedforward and feedback pathways?
Project 2 : Quantitative analysis of cortical connectivity reveals the connectivity profiles across cortical areas
Although the hierarchical model of cortical connectivity may appear as very instructive about the information flow in the brain, a major element of the network description is the magnitude (strength) of interconnection between the sources and a target area. Currently little is known about the influence of numbers of connections between the hierarchical stations. A connectivity profile for a given area exists if there is no other predictor (hemisphere, animal, sex, tracer, case, hierarchical type etc.) for the strength of connection than the area itself i.e. the amount of neurons in given area projecting to a target area is determined only by the target area. We analyse the connectivity of visual areas V1, V2, V4 in order to detect the existence of characteristic connectivity profiles. The existence of the connectivity profile proves that the magnitude of interconnection between two areas is determined and reflects their physiological interaction.
Project 3 Cortical networks are massively influenced by eccentricity and suggest principals of contextual processing
Single unit recordings show that eccentricity in the visual field has a marked effect on the receptive field properties of neurons in the visual cortex of both cats and monkeys. Previously we have shown that the connectivity of visual cortical areas is influenced by eccentricity. These studies showed that monkey visual areas V1, V2 and V4 have much more widespread connections in cortex subserving the periphery, compared to cortex subserving the central visual field, when tracing is done in parts of these areas processing peripheral information and vice versa.
We are constructing topological graphs of the connectivity at different eccentricities. This shows that the central visual areas were much more integrated with the ventral visual pathways and peripheral representations with the dorsal pathways. These results, suggesting that there are important differences in the cortical networks accompanying the physiological differences in central and peripheral representations, plead for a principal of contextual processing in the cortex, and ultimately question the validity of the concept of the cortical area.
