This increase in effectiveness may be achieved in various ways at the synaptic level, including a strengthening of excitatory connections and an adaptation of inhibitory connections (65). Since the horizontal connections, as described above, involve push-pull interactions between excitation and inhibition, a reduction in the inhibition would unmask the connections, boosting their strength from a subthreshold influence to a suprathreshold, driving influence. Moreover, since the ability to strengthen these connections is itself under inhibitory control, where with less inhibition there is an increased probability of producing a use-dependent change in the excitatory connection (66), one might produce an increase in the strength of the horizontal connections by a cascade of mechanisms. The precise synaptic mechanisms governing plasticity in this system, however, remain to be worked out.
The response properties of cells in primary visual cortex are considerably more complex than was previously believed. The complexity is manifest as both a context dependency and a dependency on the prior history of stimulation. As a result of these findings it is clear that the primary visual cortex carries information about higher-order characteristics of the visual stimulus rather than a mere representation of the line segments of which it is composed. Instead, it provides information about the character of the conjunctions between contours and surfaces in the visual image. The perceptual consequences of the dynamic changes in RF structure and cortical functional architecture depend on the time scale of the plasticity. Changes occurring over the longest time periods may play a role in recovery of function after lesions of the central nervous system, but under normal circumstances may be involved in perceptual learning. Over shorter time scales, the effect may represent a continuing process of normalization and calibration of the visual system, as well as the linkage of contours and fill-in of surfaces common to a single object. Several characteristics of the phenomena described above bear emphasizing: cells in area V1 are increasingly being seen as being involved in complex perceptual tasks, mediating the process of linkage of contours and integrating visual information over visual space. These processes are likely to involve a differential strengthening and weakening of subsets of connections within extensive axonal fields, the long-range horizontal connections representing a likely substrate for many of the observed effects. Because of these connections any cortical cell has a wider range of potential properties it can potentially express than is manifest at any given time. An important question to be addressed is to differentiate those contextual effects and dynamic changes in RFs that are due to the intrinsic horizontal connections, hence reflecting bottom-up processes, from those that arise from feedback connections, reflecting top-down influences. Though the precise synaptic mechanisms remain to be worked out, the fact that the effects have been observed in primary visual cortex, where much of the detailed functional architecture, connectivity, and RF properties have been worked out in considerable detail, makes accessible an understanding of the mechanisms of higher-order perceptual phenomena.
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