Neurons in the mammalian visual cortex have been
found to respond to second-order features which are not
defined by changes in luminance over the retina (Albright,
1992; Zhou & Baker, 1993, 1994, 1996; Mareschal &
Baker, 1998a,b). The detection of these
stimuli is most often accounted for by a separate nonlinear
processing stream, acting in parallel to the linear stream
in the visual system. Here we examine the two-dimensional
spatial properties of these nonlinear neurons in area 18
using envelope stimuli, which consist of a high spatial-frequency
carrier whose contrast is modulated by a low spatial-frequency
envelope. These stimuli would fail to elicit a response
in a conventional linear neuron because they are designed
to contain no spatial-frequency components overlapping
the neuron's luminance defined passband. We measured
neurons' responses to these stimuli as a function
of both the relative spatial frequencies and relative orientations
of the carrier and envelope. Neurons' responses to
envelope stimuli were narrowband to the carrier spatial
frequency, with optimal values ranging from 8- to 30-fold
higher than the envelope spatial frequencies. Neurons'
responses to the envelope stimuli were strongly dependent
on the orientation of the envelope and less so on the orientation
of the carrier. Although the selectivity to the carrier
orientation was broader, neurons' responses were clearly
tuned, suggesting that the source of nonlinear input is
cortical. There was no fixed relationship between the optimal
carrier and envelope spatial frequencies or orientations,
such that nonlinear neurons responding to these stimuli
could perhaps respond to a variety of stimuli defined by
changes in scale or orientation.
Image Modification Based on Spatial Frequency Components for Visual Attention Retargeting
