Nearly instantaneous brightness induction

Abstract

Brightness induction is the modulation of the perceived intensity of a region by the luminance of surrounding regions and reveals fundamental properties of neural organization in the visual system. Grating induction affords a unique opportunity to precisely measure the temporal properties of induction using a quadrature motion technique. Contrary to previous reports that induction is a sluggish process with temporal frequency cutoffs of 2-5 Hz (R. L. DeValois, M. A. Webster, K. K. DeValois, & B. Lingelbach, 1986; A. F. Rossi & M. A. Paradiso, 1996), we find that induction is nearly instantaneous. The temporal response of induced brightness differs from that of luminance gratings by a small time lag (<1 ms), or by a small temporal phase lag (<0.016 cycle), and remains relatively constant across wide variations in test field height. These data are not easily explained by an edge-dependent, homogeneous filling-in process (A. F. Rossi & M. A. Paradiso, 1996); however, they are consistent with an explanation of brightness induction based on spatial filtering by cortical simple cells (B. Blakeslee & M. E. McCourt, 1999).

Figure 1

Comparison of the SBC stimuli used by DeValois et al. (1986) (a-c) and Rossi and Paradiso (1996) (d-f) with the GI stimulus (g-i). The lower row of panels illustrates horizontal one-dimensional slices through the stimuli in the upper row (short-dashed lines) and middle row (solid lines) representing the two extreme temporal phases. Note that for the GI stimulus the homogeneous test field is represented by a separate horizontal slice (long-dashed line).

Figure 2

Space-time diagrams illustrating the quadrature-motion experimental technique. Panels (c, f, i) illustrate one temporal cycle of a counterphasing grating (G1: luminance or induced). Panels (b, e, h) illustrate a luminance grating (G2) added to the test field in quadrature spatial phase to G1, in three relative temporal phases: −.25 cycles (b); 0 cycles (e) and +.25 cycles (h). Panels (a, d, g) illustrate the compound grating G1 + G2 for each combination. Leftward motion results when the temporal phase of G2 < G1, rightward motion occurs when G2 > G1, and motion energy is balanced when G2 = G1. Our technique identifies the temporal phase of an added luminance grating which matches the temporal phase of an induced grating (experimental condition) or of a luminance grating (control condition) and results in an ambiguous motion percept.

Figure 3

Psychometric functions from one subject (MM) for the 2, 8, 16 and 24 Hz temporal frequency conditions. The upper and lower panels plot data from the experimental (luminance + induced) and control conditions (luminance + luminance), respectively. For each condition, proportion “right” motion judgments are plotted against the temporal phase offset (expressed in proportion of a temporal cycle) of the added luminance grating and are fit with cumulative normal distributions. The point of subjective equality (PSE) corresponds to the temporal phase offset that results in a “right” motion judgment by the subject on 50% of the trials. The bootstrap PSE and the 95% confidence intervals (15) are indicated in each figure by the filled black symbols and horizontal error bars. The difference in the value of the bootstrap PSE between the experimental (luminance + induced) condition (upper panel, red symbols) and the corresponding control (luminance + luminance) condition (lower panel, green symbols) was taken as a measure of the phase (time) lag of induction for that condition. This difference is illustrated by the dashed lines drawn through the bootstrap PSE for the experimental (red dashes) and control (green dashes) conditions.

Figure 4

The phase (time) lag of induction as a function of temporal frequency and test field height. Panels (a-d) plot separately for each subject the difference in the bootstrap PSEs between the experimental (luminance + induced) and control (luminance + luminance) conditions as a function of temporal frequency. Test field height is indicated by the different symbols (circles = 0.5°, squares = 3.0°, triangles = 6.0°, diamonds = 9.0°). The dotted lines depict the predicted differences in the PSEs between experimental and control conditions as a function of temporal frequency which would correspond to fixed induction time lags of 0.0, 0.5, 1.0 and 2.0 ms.