Even distorted felt size engenders visual-haptic integration.

Intersensory interactions predicted by the sensory precision hypothesis have been infrequently examined by distorting the reliability of size perception by touch. Consequently, participants were asked to see one size and manually feel another unseen size either with bare fingers or with fingers sleeved in rigid tubes to decrease the precision of touch. Their subsequent visual estimates of the perceived size favored the more precise modality. Experiment 1 (N = 46) varied the intersensory discordance to examine whether the estimate arose from trivial response biases or from perceptual binding effects. Experiment 2 (N = 32) examined the presence of the perceptual effect in the absence of discordant sensory cues. Results favored a perceptual interpretation because the haptic and visual cues merged regardless of the discordance amount only when the stimulation arose from separate sources. The observed interaction between touch imprecision and visual bias is consistent with computational models of optimal perception.

Touch Precision Modulates Visual Bias.

The sensory precision hypothesis holds that different seen and felt cues about the size of an object resolve themselves in favor of the more reliable modality. To examine this precision hypothesis, 60 college students were asked to look at one size while manually exploring another unseen size either with their bare fingers or, to lessen the reliability of touch, with their fingers sleeved in rigid tubes. Afterwards, the participants estimated either the seen size or the felt size by finding a match from a visual display of various sizes. Results showed that the seen size biased the estimates of the felt size when the reliability of touch decreased. This finding supports the interaction between touch reliability and visual bias predicted by statistically optimal models of sensory integration.

Again, knowledge of common source fails to promote visual-haptic integration.

The present study tested whether knowledge of a common source of conflicting visual-haptic stimulation promotes intersensory integration. 40 undergraduates manually felt the size of a square while viewing it through a lens that minified its visual size by half. Participants, however, could experience the haptic and the visual stimulation as emanating from either a common source or different sources. Their subsequent matches of the perceived size were biased by the felt size of the square, irrespective of whether they experienced the intersensory stimulation as coming from one or two sources. These results strengthen previous findings suggesting that the integration of sensory discordant information depends on bottom-up rather than on top-down processes.

A "unity assumption" does not promote intersensory integration.

An account of intersensory integration is premised on knowing that different sensory inputs arise from the same object. Could, however, the combination of the inputs be impaired although the "unity assumption" holds? Forty observers viewed a square through a minifying (50%) lens while they simultaneously touched the square. Half could see and half could not see their haptic explorations of the square. Both groups, however, had reason to believe that they were touching and viewing the same square. Subsequent matches of the inspected square were mutually biased by touch and vision when the exploratory movements were visible. However, the matches were biased in the direction of the square's haptic size when observers could not see their exploratory movements. This impaired integration without the visible haptic explorations suggests that the unity assumption alone is not enough to promote intersensory integration.

Practise strengthens haptic capture.

When do haptic estimates of discordant visual-haptic size capture vision? Observers looked at a square through a minifying lens (50%) whilst they simultaneously touched the square from below through a hand-concealing cloth. Their subsequent match of the square's size, rendered by touching a set of comparison squares, was haptically biased when they practised estimating the square's size (Experiment 1, N = 72), when they actively explored rather than passively touched the square (Experiment 2, N = 24), but not when they were uninformed before inspecting the square that they would estimate its size (Experiment 3, N = 36). Evidently, the haptic exploratory strategies occasioned by the practise influenced the integration of the felt size and the seen size by weighing the haptic input more than the visual input, and this weight shifting manifested itself by strengthening haptic capture.