Perceptual interaction of local motion signals

Abstract

Motion signals are a rich source of information used in many everyday tasks, such as segregation of objects from background and navigation. Motion analysis by biological systems is generally considered to consist of two stages: extraction of local motion signals followed by spatial integration. Studies using synthetic stimuli show that there are many kinds and subtypes of local motion signals. When presented in isolation, these stimuli elicit behavioral and neurophysiological responses in a wide range of species, from insects to mammals. However, these mathematically-distinct varieties of local motion signals typically co-exist in natural scenes. This study focuses on interactions between two kinds of local motion signals: Fourier and glider. Fourier signals are typically associated with translation, while glider signals occur when an object approaches or recedes. Here, using a novel class of synthetic stimuli, we ask how distinct kinds of local motion signals interact and whether context influences sensitivity to Fourier motion. We report that local motion signals of different types interact at the perceptual level, and that this interaction can include subthreshold summation and, in some subjects, subtle context-dependent changes in sensitivity. We discuss the implications of these observations, and the factors that may underlie them.

Figure 1

Stimulus examples. Panel A: Space–time slices of stimuli containing F (left column), G contraction (middle column), and G expansion (right column) signals at maximum correlation strength. Top row: Even parity, corresponding to standard F motion, white G contraction, and white G expansion (see Table 1). Bottom row: Odd parity, corresponding to reverse-phi F motion (not used here), black G contraction, and black G expansion. Panel B: Space–time slices of stimuli containing mixtures of F and white G contraction, in the proportions used in these experiments. Top row: Five example stimuli along a ray ending with C_F = 0.1 and C_G = 0.5. Bottom row: Five example stimuli along a ray ending with C_F = 0.05 and C_G = 0.95.

Figure 2

Psychophysical performance for direction judgments for stimuli containing mixtures of F and G signals. Contour maps show fraction correct as a function of their signal strengths, C_F (abscissa) and C_G (ordinate). Upper quadrant shows responses for stimuli containing white Gs; lower quadrant shows responses for stimuli containing black Gs. The abscissa corresponds to pure F stimuli. First and third rows: Mixtures of F and G contraction. Second and fourth rows: Mixtures of F and G expansion. The lines indicate the rays that were studied, and the points on the rays the specific signal combinations. When two motion cues were present, they were always in consistent directions.

Figure 3

Fits of Weibull functions to performance along each ray. Panel A: F and G contraction; Panel B: F and G expansion. In each panel, the seven individual plots show the measured fraction correct along a single ray and the fitted Weibull function. The abscissa extends to the maximal total motion strength on that ray error bars indicate 95% CIs determined by bootstrap. Data are shown with a line color indicating glider parity (red: white G; blue: black G; green: pure F [no G]) and line style indicating the maximal motion strength at the end of the ray (solid: C_F = 0, C_G = 1; dashed: C_F = 0.05, C_G = 0.95; dot-dash: C_F = 0.1, C_G = 0.5; dotted: C_F = 0.1, C_G = 0). Black solid lines show the Weibull fit. The central plot shows all of the fits superimposed. In each panel, Weibull functions had the same shape parameter: b = 1.65 in Panel A, b = 1.70 in Panel B. Subject EIN, dataset 1.

Figure 4

Isodiscrimination curves for combinations of F and G contraction. Blue curves connect the distances a_r along each ray at which a fraction correct of 0.75 is reached (see Methods). Purple dashed lines: 95% confidence limits. Ordinate shows G contraction strength, upper quadrant for white Gs and lower quadrant for black Gs.

Figure 5

Comparison of sensitivity (1/threshold; i.e., 1/a_r in Equation 1) to F motion in the context of G contraction (red) and expansion (green). Error bars indicate 95% CIs. Braces connect data from subjects run in both kinds of experiments. Note that for subject TS, there was no measurable sensitivity to F motion in the context of G expansion.

Figure 6

Comparison of fraction correct (FC) for F motion, with strength C_F = 0.1, in three contexts: G contraction (red), G expansion (green), and random movies (blue). Error bars indicate 1 SEM. Brackets indicate significant (p < 0.05) differences in FC for F (ordinate) motion via a one-tailed (G contraction > G expansion > random) t test, paired across blocks. FC for the context trials (abscissa) is significantly different at p < 0.05 for every within-subject comparison.