We are interested in understanding how the thalamocortical microcircuit(s) generate(s) all the different receptive field structures, and their associated functional responses properties, that can be found in primary visual cortex. For instance, the orientation of a line at a given position in visual space is represented by a small volume of primary visual cortex, known as an orientation column, which spans from the surface to the white matter. Although cells within such a column share some response properties (like orientation selectivity and ocular dominance), they are functionally diverse. Two main functional cell types have been described, simple cells and complex cells. When mapped with simple stimuli (like a sparse-noise consisting of individually flashed bright and dark spots), simple receptive fields comprise separate and elongated subregions that evoke responses when, against a neutral grey background, we turn the bright spot on (on-subregion) or the dark spot on (off-subregion). In contrast, complex cells make up a very heterogeneous population and they tend to produce similar responses throughout the entire receptive field (‘on’ or ‘off ’ responses, ‘on–off ’ responses or no responses al all to the sparse-noise protocol, requiring richer stimuli to be visually driven).

To correlate these different functional response properties with position within the cortical microcircuit, we use a variety of experimental approaches: mainly whole-cell intracellular recordings in vivo, combined with 3-dimensional anatomical reconstruction of the recorded cells; and multiple, simultaneous, extracellular recordings using matrices of seven electrodes coated with fluorescent dyes.

Our results support the idea that simple cells and complex cells may represent two different stages of receptive field construction. In a first stage, simple cells would be constructed from thalamic inputs and, in a second step, simple cell inputs would be pooled to generate complex receptive fields. Furthermore, we have shown systematic changes in synaptic receptive-field structure, synaptic physiology and relative orientation tuning of excitatory and inhibitory inputs at the different stages of cortical processing. Such progressive transformations in the way information about the visual scene is represented along the early visual pathway will help us shed light on the specific aspects of the cortical microcircuit.

Main collaborators

Judith A. Hirsch          http://jah.usc.edu

Jose-Manuel Alonso    http://www.sunyopt.edu/research/alonso.shtml

Progression of RF-structures along the early visual pathway.

Stereogram made by Jose-Manuel Alonso (based on Martinez and Alonso, 2001, Neuron 32, 515-525). The figure shows a complex RF recorded in layer 3, in green representing overlapped on an off responses; a simple receptive field with two subregions recorded in layer 4, an off-subregion in blue and a an on-subregion in orange; and a thalamic on-center receptive field recorded in lamina-A of the LGN, only the receptive-field center is plotted in red. The three cells were simultaneously recorded and their receptive fields were well overlapped in visual space. Our results show that a hierarchical model of connectivity could explain the emergence of all three different receptive field types along the early visual pathway.