The psychophysical and VEP results are consistent either with specialized elongated nonlinear RF mechanisms that pool the outputs from several smaller filters in a configuration-specific fashion or with modulatory surround effects that occur when stimuli are placed outside of the classical receptive field (CRF). Elongated receptive field have been suggested [24,25,26] as ``collector" mechanisms that pool linearly the output of a number of smaller filters from extended stimuli. According to our results the pooling should be non-linear [29,30].
Long-range spatial interactions may also be mediated by interactions between the CRF and its non-classical surround. Such effects are both spatial frequency and orientation selective [21] and may even be configuration dependent. A tendency for the response of the CRF to be facilitated by a remotely placed co-oriented, co-axial regions [27] was found.
Long-range interactions are configuration specific - with maximal facilitation occurring for collinear configurations. Facilitation is contrast dependent. It is found at or near target contrast detection threshold [29,30]. The spatial range of the facilitation extends up to a several degrees in the fovea. While long range facilitation is observed only with high contrast masks, facilitation can be found for low contrast masks with overlapping targets and masks [20,28] or with small separations [43]. For short range facilitation the effect of contrast can be described by a 'dipper function'.
Suppression may result from a combination of several mechanisms: contrast integration within the receptive field (Weber's law), the broad-band inhibition between neighboring filters (in space, orientation and spatial frequency) that has been suggested as a mechanism to normalize the cell's response, and narrow-band inhibition from local or remote locations which might affect orientation tuning curve of the receptive field. Inhibition effect are found for high contrast range and have either short-range i.e. from distance comparable to the filter size [23,30,43] or long-range i.e., from remote distances [11,17].