Visual Evoked Potentials to Geometric Forms: Effects of Spatial Orientation

Abstract

Five outlined geometric figures of equal contour length were presented monocularly below the fixation point (FP); an upright equilateral triangle (upright triangle), an inverted equilateral triangle (inverted triangle), an upright square (square), a 45°‐tilted square (diamond) and a circle. The angular separation between FP and the top of the figures was held constant. Transient visual evoked potentials (VEPs) were recorded monopolarly from inion, 4, 7, 10 cm above it (l, l4, l7, l10) and Cz for ten subjects. The grand‐averaged subtracted waves were obtained between the figure and blank (control) conditions. N1 (mean peak latency: 155 ms) and P2 (240 ms) waves were identified. ANOVAs were conducted for the latency and amplitude values. Main results were as follows: On the N1 amplitude at l4 and l, the upright triangle, inverted triangle and diamond figures evoked significantly larger responses than did the square and the circle. However, no significant difference was found among the former three figures. These findings on the N1 amplitude suggest that the effect of a triangle is orientation‐free and that of a square is orientation‐bound.

Effects of angularity of the figures with sharp and round corners on visual evoked potentials

“…However, the orientation could be a crucial signal for shape discrimination (Ishii & Tsuji, 1979). Its effect must therefore be examined in order to identify the formspecific response (Ito et al, 1997). Experiment 2 therefore investigated the VEPs when the figures were inverted (see Figure 1b).…”

“…Because of the importance of form to visual perception, we have conducted psychophysiological studies of its effect upon transient VEPs by using the fundamental geometric forms of the equilateral triangle, the square and the circle (Ito, Kuwabara, Sugata, Suzuki, & Kawai, 1996, 1998Ito & Sugata, 1995;Ito, Sugata, & Kuwabara, 1997). The main aims of these studies were to accumulate meaningful psychophysiological data on form perception by means of the VEP as an objective measure, to identify form-related VEP components, and to understand how information regarding form is processed in the underlying cortical areas.…”

Visual evoked potentials to the geometric forms in the randomized presentation

“…Both waves showed the monopolar potential distribution on the scalp, with the peaks at I5. The N wave is what we called N1 in our previous studies (Ito & Sugata, 1995;Ito et al, 1997). Figure 3 shows the averaged peak latencies of the N and P waves at four locations except Fz.…”

“…The triangle elicited a significantly larger negative (N1) response than the square or the circle. Ito et al (1997) examined the effects of geometric form and orientation, and found that the upright triangle, the inverted triangle and the diamond evoked significantly larger N1 responses than the square and the circle.…”

“…The subjects of our previous studies (Ito & Sugata, 1995;Ito et al, 1997) had prior knowledge of the differential psychological effects of geometric figures, for example that the triangle is the most visible of them (Torii, 1969;Yokose, 1968). When a figure is repeatedly presented at a fixed position on the visual field, such related knowledge may lead subjects to have different preparatory states, which in turn result in different levels of arousal.…”