Asymmetries in the discrimination of motion direction around the visual field

Abstract

The discriminability of motion direction is asymmetric, with some motion directions that are better discriminated than others. For example, discrimination of directions near the cardinal axes (upward/downward/leftward/rightward) tends to be better than oblique directions. Here, we tested discriminability for multiple motion directions at multiple polar angle locations. We found three systematic asymmetries. First, we found a large cardinal advantage in a cartesian reference frame - better discriminability for motion near cardinal reference directions than oblique directions. Second, we found a moderate cardinal advantage in a polar reference frame - better discriminability for motion near radial (inward/outward) and tangential (clockwise/counterclockwise) reference directions than other directions. Third, we found a small advantage for discriminating motion near radial compared to tangential reference directions. The three advantages combine in an approximately linear manner, and together predict variation in motion discrimination as a function of both motion direction and location around the visual field. For example, best performance is found for radial motion on the horizontal and vertical meridians, as these directions encompass all three advantages, whereas poorest performance is found for oblique motion stimuli located on the horizontal and vertical meridians, as these directions encompass all three disadvantages. Our results constrain models of motion perception and suggest that reference frames at multiple stages of the visual processing hierarchy limit performance.

Figure 1.

Hypothetical asymmetries in sensitivity to motion direction in a cartesian and polar reference frame. Arrow length indicates predicted sensitivity. Left to right: Cardinal (green) and oblique (purple) directions in a cartesian reference frame. The cardinal arrows are longer to depict a predicted cartesian cardinal advantage. Cardinal (brown) and oblique (teal) directions in a polar reference frame. The longer arrows depict a predicted polar cardinal advantage. Radial (blue) and tangential (red) directions in a polar reference frame. The longer blue arrows depict a predicted radial advantage. Combined advantages (gray) in both reference frames assuming an equal weight of cartesian cardinal, polar cardinal and radial advantages. The largest asymmetry effects are predicted along the primary meridians of the visual field (shown as difference in grayscale value). In principle, an unequal combination of these three factors might be observed.

Figure 2.

Experimental design. (Left) Observers performed a 2AFC motion direction discrimination task. After a fixation interval, the observer viewed a drifting Gabor pattern, and indicated whether the drift direction was clockwise or counterclockwise relative to the standard direction (here, upward), and then received auditory feedback. (Right) There were 64 conditions, crossing eight standard directions and eight polar angle locations: four locations were on the primary meridians (solid red lines) and four locations were off the primary meridians (dashed red lines) of the visual field. The standard direction was constant within an experimental session.

Figure 3.

Sensitivity estimates demonstrate a cartesian cardinal advantage. (A) Example psychometric fits for S01 at location 90 degrees demonstrates high sensitivity and small bias magnitude (red curve/text) for upward compared to low sensitivity and high bias magnitude (blue curve/text) for upper leftward motion directions at this location. Each curve was estimated from 200 trials (20 trials/tilt angle). At this location, sensitivity to upwards directions = 0.97 (units: 1/degrees) and bias magnitude = 0.07 (units: degrees); sensitivity to lower rightwards directions = 0.53 and bias magnitude = 2.10. (B) Mean sensitivity (represented by arrow length) for each motion direction collapsed across all eight locations. Sensitivity was greater for cartesian cardinal than cartesian oblique directions. (C) Mean sensitivity to cartesian cardinal and cartesian oblique directions grouped based on whether the stimulus was located ON or OFF the primary meridians of the visual field. Grouping the data this way shows that the cartesian cardinal advantage was greater on than off the primary meridians.

Figure 4.

Sensitivity to motion direction varies systematically around the visual field. (A) Mean sensitivity for cardinal compared to oblique motion directions in cartesian reference frame. Each plotted point represents the mean performance for each condition across observers at eight polar angle locations. Each value prior to averaging was derived from four psychometric fits per condition for a given observer. Lines connect the dots. Error bars not visible were smaller than the plotting symbols. (B) Same data points as in A but re-grouped to compare mean sensitivity for polar cardinal compared to polar oblique directions at eight polar angle locations. (C) Mean sensitivity for radial compared to tangential directions at eight polar angle locations.

Figure 5.

Sensitivity negatively correlates with bias magnitude. Opaque dots represent average sensitivity/bias magnitude estimates per observer. Translucent dots represent estimates derived from each psychometric fit. As sensitivity increased, bias magnitude decreased, demonstrating a negative correlation.

Figure 6.

Model comparison. Several linear mixed effects models were tested. Observer identity was included as a random effect in all models. Directional asymmetries were fixed effects, with one additional effect added for each model. The Δ BIC scores were computed for each model for the sensitivity and bias magnitude measures.

Figure 7.

Parameter estimates and model fits demonstrate how directional asymmetries can explain behavioral variation in motion discrimination task. (A) Model 4 estimated weights for each asymmetry, relative to the global mean. Error bars represent 68% confidence intervals. (B) Directional asymmetries were greatest for locations at the primary meridians, the locations which have shared cardinal directions in cartesian/polar reference frames. Each polar plot shows mean performance for eight directions at a unique polar angle location. Lines represent the model fits reconstructed as β₀ + β₁ (cartesian_cardinal_vs_oblique_im) + β₂ (polar_cardinal_vs_oblique_im) + β₃ (radial_vs_tangential_im).