Reaching beyond the classical receptive field of V1 neurons: horizontal or feedback axons?
Introduction
The traditional, feedforward, model of the visual system invokes a cascade of processing stages, beginning with relay of retinal input to the neurons of area V1, via the lateral geniculate nucleus (LGN), and subsequent processing through a hierarchy of cortical areas [102]. According to this model, cells at each successive stage process inputs from increasingly larger regions of space, and code for increasingly more complex aspects of visual stimuli, a complex abstraction of the visual world achieved by cells at the highest visual centers. The selectivity of a neuron to a given parameter1 is supposed to result from the ordered convergence of afferents from the lower stages. One example is the emergence of orientation selectivity in area V1 neurons. Following the initial model of Hubel and Wiesel [39], several authors have claimed that orientation selectivity can be entirely generated by the ordered arrangement of afferents from the LGN. This view has been subject to debate for several decades (for reviews see [26], [94]). In particular, there is evidence that local intracortical excitatory and inhibitory mechanisms play a role in amplifying and sharpening the orientation tuning of V1 neurons.
Although feedforward models can perform a surprising number of object recognition tasks in a simple environment [98], they fail to identify an object in a cluttered environment, where the object can be partially masked or occluded by other objects. Proper interpretation of partially occluded figures requires global information about the 3D interpretation of the scene to guide the alignment of edges. The computations related to such global-to-local interactions should be flexible, i.e. different sets of global cues should lead to different types of local processing of edges. Furthermore, to mediate perceptual completion across several degrees of visual angle, these computations should involve exchange of information across distant regions of the visual field (often >10°). Finally, to allow for interpretation of an image within the timeframe of inter-saccadic times (200 ms), they should be fast.
There is compounding evidence that flexible and rapid long-distance computations do occur in the visual system, even at the lower levels of cortical processing. Thus, for example V2 and V1 cells respond to illusory contours2 [34], [87], [103], and to occluded contours defined by contextual depth cues [7], [96]. More generally, the response of cells in V1 (and extrastriate cortex) can be modified by contextual stimuli lying far outside the neurons’ receptive field [1], [10], [30], [60], [67]. What are the neural circuits underlying these fast, long-distance, global-to-local interactions? In this chapter we present a set of anatomical and physiological data exploring this issue.
Section snippets
Spatial properties of center–surround interactions in V1: where is the center, where is the surround?
The typical neural signature of long-range spatial computations at the single cell level is represented by center–surround interactions. These have been described for the first time in the retina 50 years ago [52], and are thought to endow retinal ganglion cells with the ability of signaling relative contrast rather than local luminance. Center–surround interactions are also observed in the responses of cortical cells, for which it is possible to define a discharge center or receptive field,
Spatial properties of V1 horizontal connections
V1 horizontal connections are long-range, reciprocal, intra-laminar projections prominent in the upper layers of V1 [28], [75] (but also present in layers 4B/upper 4Cα of primates and 5 and 6 of primates and carnivores). These connections arise from excitatory neurons, show a periodic, patchy pattern of termination, and contact predominantly (about 80%) excitatory neurons but also (about 20%) inhibitory neurons [48], [62], [64]. Horizontal axons have been found to link preferentially cortical
Spatial properties of feedback connections to V1
On the basis of laminar origin and termination, connections between visual cortical areas have been classified as feedforward or feedback, and a hierarchical organization of cortical areas has been proposed [22], [25], [76]. Area V1, at the bottom of the hierarchy, receives its main feedforward inputs from the thalamus, and sends feedforward projections to several extrastriate cortical areas which, in turn, send projections back to V1. In both cat and monkey, feedback connections to V1 arise
Conclusions
There is thus evidence for involvement of feedback connections as well as horizontal connections in center–surround interactions in area V1 neurons. The different extents of the divergences of horizontal and feedback connections from different areas (Fig. 7) provide a progressive tiling of the surround by horizontal and feedback connections from areas located further and further away from V1. This extensive overlap between the different feedback connections may explain why effects of V2
Acknowledgments
This work was supported by: European Community Grant Viprom Biomed 2, Wellcome Trust Grant 061113, and Research to Prevent Blindness, Inc., New York, INSERM, CNRS, GIS Cognisciences.
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1.23 - Network Properties of Visual Cortex
2020, The Senses: A Comprehensive Reference: Volume 1-7, Second Edition