Horizontal-vertical anisotropy with respect to bias and sensitivity.

Previous research suggests that participants are biased to assess horizontal distances as smaller than vertical distances. We explored the effect of horizontal versus vertical orientation on the sensitivity, as well as the bias, of distance judgments. In contrast to participants of most past studies, our participants never directly compared horizontal and vertical distances. On each trial, participants judged the horizontal or vertical distance between a pair of horizontally or vertically oriented dots as being greater than or less than the average distance between previously presented pairs of dots of the same orientation. When the same participants judged horizontal and vertical distance, the distance judged greater than average 50% of the time (P50) was larger for horizontal than for vertical judgments-a pattern consistent with the horizontal-vertical bias of past work. When different participants judged horizontal and vertical distance, the horizontal and vertical P50s did not differ. When participants did not know whether they would judge horizontal or vertical distance on the coming trial, sensitivity was lower for horizontal than vertical judgments. When participants knew what kind of judgment they would make on the coming trial, sensitivity did not differ for horizontal and vertical judgments. We consider the implications of these findings for accounts that attribute the horizontal-vertical bias (1) to real-world-retinal mapping, (2) to the elliptical shape of the visual field, or (3) to the asymmetry of the receptive fields used to assess frontal distance. We suggest that distance field asymmetry must be invoked to explain the present sensitivity effect.