Stereovision for action reflects our perceptual experience of distance and depth

Abstract

Binocular vision is widely recognized as the most reliable source of 3D information within the peripersonal space, where grasping takes place. Since grasping is normally successful, it is often assumed that stereovision for action is accurate. This claim contradicts psychophysical studies showing that observers cannot estimate the 3D properties of an object veridically from binocular information. In two experiments, we compared a front-to-back grasp with a perceptual depth estimation task and found that in both conditions participants consistently relied on the same distorted 3D representation. The subjects experienced (a) compression of egocentric distances: objects looked closer to each other along the z-axis than they were, and (b) underconstancy of relative depth: closer objects looked deeper than farther objects. These biases, which stem from the same mechanism, varied in magnitude across observers, but they equally affected the perceptual and grasping task of each subject. In a third experiment, we found that the visuomotor system compensates for these systematic errors, which are present at planning, through online corrections allowed by visual and haptic feedback of the hand. Furthermore, we hypothesized that the two phenomena would give rise to estimates of the same depth interval that are geometrically inconsistent. Indeed, in a fourth experiment, we show that the landing positions of the grasping digits differ systematically depending on whether they result from absolute distance estimates or relative depth estimates, even when the targeted spatial locations are identical.

Linear–nonlinear models of the red–green chromatic pathway

“…We have proposed two types of nonlinearity that might account for the hue shifts (Stockman et al, 2017b). First, a slew-rate limited mechanism that restricts the rate at which internal representations that determine hue can change.…”

“…We begin by assessing how well the models can simultaneously account for three important sets of data taken from two previous papers (Stockman et al, 2017a;Stockman et al, 2017b). First, (a) the chromatic temporal contrast sensitivity functions (TCSFs) that are often used to infer overall temporal processing in the chromatic pathway (e.g., de Lange, 1958;Kelly, 1974), second (b) the functions showing the dependence of the hue shift on the second harmonic phase delay in two-component waveforms, and third (c) the TCSFs for discriminating between the mean hue shift produced by a sawtooth flickering waveform and the same waveform with its polarity inverted.…”

“…The experimental details can be found in our previous publications (Stockman et al, 2017a;Stockman et al, 2017b). Briefly, the experiments were programmed in MATLAB with the Cambridge Research Systems toolbox and presented via a VSG2/5 system (Cambridge Research Systems Ltd., Rochester, Kent, UK) on a carefully linearized and calibrated 21 in.…”

“…The overall attenuation characteristics of the model should be consistent with the chromatic TCSFs shown in both panels of Figure 2 as orange diamonds: for observers KR in the left-hand panel and for JA in the right-hand panel. These data are replotted from figure 2 of Stockman et al (2017b) and show log 10 chromatic sensitivity as a function of frequency (Hz, logarithmic axis). To obtain these data, the modulation of equiluminant red/green sinusoidal flicker was varied in a two-alternative forced-choice experiment to find the relation between modulation and detection performance.…”

“…In particular, L-and M-cone-isolating sawtooth waveforms, varying around a mean yellow-appearing chromaticity, had different hues depending on whether the sawtooth was slowly-rising-rapidly-falling or rapidly-rising-slowly-falling. In general, the hue shifted in the direction of the slowly changing slope, so that the mean hue of Lor M-cone isolating sawtooth flicker that went slowly-redder-rapidly-greener appeared redder, whereas the hue of sawtooth flicker that went slowly-greener-rapidly-redder appeared greener (Stockman et al, 2017b). Measurements of the effects of varying either the modulation of the waveforms or the rates of change of their rising and falling slopes suggested that the shifts in the mean hue depended mainly on the first and second harmonics of the flickering waveforms (Stockman et al, 2017b).…”