Temporal response properties to second-order visual stimuli in the LGN of cats

Specificity of brain reactions to second-order visual stimuli

The second-order visual mechanisms perform the operation of integrating the spatially distributed local visual information. Their organization is traditionally considered within the framework of the filter-rectify-filter model. These are the second-order filters that provide the ability to detect texture gradients. However, the question of the mechanisms' selectivity to the modulation dimension remains open. The aim of this investigation is to answer the above question by using visual evoked potentials (VEPs). Stimuli were textures consisting of staggered Gabor patches. The base texture was nonmodulated (NM). Three other textures represented the base texture which was sinusoidally modulated in different dimensions: contrast, orientation, or spatial frequency. EEG was recorded with 20 electrodes. VEPs of 500 ms duration were obtained for each of the four textures. After that, VEP to the NM texture was subtracted from VEP to each modulated texture. As a result, three different waves (d-waves) were obtained for each electrode site. Each d-wave was then averaged across all the 48 observers. The revealed d-waves have a latency of about 200 ms and, in our opinion, reflect the second-order filters reactivation through the feedback connection. The d-waves for different modulation dimensions were compared with each other in time, amplitude, topography, and localization of the sources of activity that causes the d-wave (with sLORETA). We proceeded from the assumption that the d-wave (its first component) represents functioning of the second-order visual mechanisms and activity changes at the following processing stages. It was found that the d-waves for different modulation dimensions significantly differ in all parameters. The obtained results indicate that the spatial modulations of different texture parameters caused specific changes in the brain activity, which could be evidence supporting the specificity of the second-order visual mechanisms to modulation dimension.

Interaction of first- and second-order signals in the extraction of global-motion and optic-flow

The intention of this series of experiments was to determine the extent to which the pathways sensitive to first-order and second-order motion are independent of one another at, and above, the level of global motion integration. We used translational, radial and rotational motion stimuli containing luminance-modulated dots, contrast-modulated dots, or a mixture of both. Our results show that the two classes of motion stimuli interact perceptually in a global motion coherence task, and the extent of this interaction is governed by whether the two varieties of local motion signal produce an equivalent response in the pathways that encode each type of motion. This provides strong psychophysical evidence that global motion and optic flow processing are cue-invariant. The fidelity of the first-order motion signal was moderated by either reducing the luminance of the dots or by increasing the displacement of the dots on each positional update. The experiments were carried out with two different types of second-order elements (contrast-modulated dots and flicker-modulated dots) and the results were comparable, suggesting that these findings are generalisable to a variety of second-order stimuli. In addition, the interaction between the two different types of second-order stimuli was investigated and we found that the relative modulation depth was also crucial to whether the two populations interacted. We conclude that the relative output of local motion sensors sensitive to either first-order or second-order motion dictates their weight in subsequent cue-invariant global motion computations.

Visual responses to contrast-defined contours with equally spatial-scaled carrier in cat area 18

Contrast-defined contours are one type of second-order contours, across which there are no differences in luminance. Although they can be always perceived, their responses have been only investigated when the spatial frequency of carrier, the background texture whose contrast is modulated to form contours, is much higher than that of contrast-defined contours, due to the interference of responses to luminance contours in other cases. In the present study, we examined visual responses in cat area 18 to the contrast-defined contours with carrier at same spatial frequency equal to neuron's preferred value for luminance contours, by establishing a control stimulus including all the luminance components but lack of the contrast contour information. Using single unit recording and intrinsic optical imaging, we demonstrated that contrast gratings with equally spatial-scaled carrier induced responses in a proportion of cat area 18 neurons with the preferred orientation similar to that for luminance contours, and the responses generated orientation maps similar to those for luminance contours. Our finding suggests that early visual cortex can process second-order contours regardless of the spatial frequency of carriers, in a way similar to the processing of luminance contours. This uniform manner of early visual processing might underlie the visual detection of both luminance contours and non-luminance second-order contours.