Early processing in the human lateral occipital complex is highly responsive to illusory contours but not to salient regions

Abstract

Human electrophysiological studies support a model whereby sensitivity to so-called illusory contour stimuli is first seen within the lateral occipital complex. A challenge to this model posits that the lateral occipital complex is a general site for crude region-based segmentation, based on findings of equivalent hemodynamic activations in the lateral occipital complex to illusory contour and so-called salient region stimuli, a stimulus class that lacks the classic bounding contours of illusory contours. Using high-density electrical mapping of visual evoked potentials, we show that early lateral occipital cortex activity is substantially stronger to illusory contour than to salient region stimuli, whereas later lateral occipital complex activity is stronger to salient region than to illusory contour stimuli. Our results suggest that equivalent hemodynamic activity to illusory contour and salient region stimuli probably reflects temporally integrated responses, a result of the poor temporal resolution of hemodynamic imaging. The temporal precision of visual evoked potentials is critical for establishing viable models of completion processes and visual scene analysis. We propose that crude spatial segmentation analyses, which are insensitive to illusory contours, occur first within dorsal visual regions, not the lateral occipital complex, and that initial illusory contour sensitivity is a function of the lateral occipital complex.

Figure 1

Illustration of the stimuli. (a) IC-forming stimulus; to be viewed at 30 cm distance to simulate experimental conditions (i.e., 7° illusory contour), IN configuration. (b) SR-forming stimulus; to be viewed at 30 cm distance, IN configuration. (c) Control stimulus for the IC condition, OUT configuration. (d) Control stimulus for the SR condition, OUT configuration.

Figure 2

Electrophysiological response to IC and SR stimuli over the entire epoch (-100 to 500ms). (a) Group-averaged VEPs for each stimulus type over four posterior scalp sites (see inset for locations and color scheme). (b) Point-wise paired t-tests (“statistical cluster plots”) comparing IN (pacmen facing in) vs. OUT (pacmen facing out) configurations of the three inducer types (p=0.01 threshold, over -100 to 500 ms time period). Color represents the result of the t-test. Electrodes are arranged left to right for seven scalp regions, demarcated by solid white lines: occipital, parieto-occipital, parietal, central, fronto-central, frontal, and fronto-polar.

Figure 3

VEP difference waveforms (IN-OUT) to IC and SR stimuli over the entire epoch (-100 to 500ms). (a) Group-averaged VEP difference waveforms over four posterior electrodes (see inset for locations and color scheme). Vertical lines indicate time periods of interest (154-203ms and 404-469ms). (b) Point-wise paired t-tests (“statistical cluster plots”) comparing IC difference vs. SR difference responses (p=0.01 threshold, over -100 to 500 ms time period). Electrodes are arranged into five scalp regions along the vertical axis; regions are displayed to the left of the plot.

Figure 4

Electrical imaging results. (a) Group-average global field power (GFP) waveforms (see inset for the color scheme). Time-periods of significant GFP t-test and TANOVA analyses are demarcated on the x-axis. The TANOVA analysis compared response topography between difference waveforms (IN-OUT) for the two stimulus types as a function of time. Time intervals of significant (p<0.01) topographic difference using a non-parametric Monte Carlo bootstrapping procedure are displayed. (b) VEP difference waveform topographies for the two conditions.

Figure 5

Source estimations. Group-averaged LAURA distributed linear source estimations were calculated for the VEP difference waveforms over the 154-203ms time period (panel a) and the 404-469ms time period (panel b). The mean differences between these source estimations are shown in the right column. Significantly stronger contributions from LOC sources were found in the early time period for the IC condition (left Brodmann’s area 19/37, cluster 1) and in the later time period for the SR condition (right Brodmann’s area 37, cluster 4). (c) Mean scalar values of activations within clusters and t-test results between conditions.