Introduction

It has been experimentally observed that the receptive fields (RF) of cortical cells have a dynamic nature. For instance, it was found that some time (of the order of minutes) after the occurrence of a retinal lesion the area of the RF increased by a factor of order 5 (Gilbert and Wiesel, 1992), and that cortical cells with their classical RF inside the damaged region recovered their activity. A similar effect can be obtained without the existence of real lesions. Stimuli can emulate the lesion if they are localized; that is, if there is some small part of input space that receives stimulation strongly different from their surround. Lack of stimulation in a small region of the visual space produces an effect similar to a scotoma. Experiments with localized stimuli have been done in both the visual (Pettet and Gilbert, 1992) and the somatosensory systems (Jenkins et al., 1990).

These changes in the RFs of cortical neurons can be quantitatively studied with psychophysical experiments. For instance, changes in RF sizes are reflected in a systematic bias in feature localization tasks. It has been found (Kapadia et al., 1994) that the ability to determine the relative position of a short line segment in the middle of another two, presented close to the border of the artificial scotoma, was strongly biased in a way that is consistent with the expansion of RFs of neurons in the cortical scotoma.

It has been speculated (Gilbert, 1992; Pettet and Gilbert, 1992) that the expansion of RF sizes is responsible for the perceptual filling-in effect (Ramachandran and Gregory, 1991) and other visual illusions.