ELECTROPHYSIOLOGICAL CORRELATES OF TEXTURE SEGREGATION IN THE HUMAN VISUAL EVOKED-POTENTIAL

We investigated whether the visual evoked potential (VEP) reflects cortical processing associated with preattentive texture segregation. On a visual display unit we presented stimuli with various arrangements of oriented line segments that either led to the appearance of a "preattentive" checkerboard or did not. Two presentation modes were used (pattern onset at 1 Hz and rapid pattern change at 4.3 Hz), while luminance (57 cd/m2) and contrast (92%) of the line segments remained constant. VEPs were recorded in 7 human subjects. The VEP was analyzed as a linear combination of putative components, which are evoked by either local pattern, quasi-local orientation contrast or global preattentive structure. In the transient VEP, we found a negativity over the posterior pole at a latency between 161 and 225 msec (FWHM) in the linear combination designed to extract segregation-specific components. Peak amplitude reached 3.1 +/- 0.8-mu-V (mean +/- SEM) at 199 msec. This negative peak appeared only for textures containing orientation contrast. Steady-state analysis of the rapid presentation also revealed a significant component (P = 0.002) associated with texture segregation. These potentials either represent processing of orientation contrast or global processing of texture segregation. The results suggest that specific surface potentials, differing from cognitive potentials, can be derived which are associated with preattentive processing.