Modification of depth and distance perception caused by long-term wearing of left-right reversing spectacles

Stereo and Shading Contribute Independently to Shape Convexity-Concavity Discrimination

The present study examined the joint contribution of shading and stereopsis to the perception of shape convexity–concavity. The stimuli were the images of a synthetic convex 3-D shape seen from viewpoints leading to ambiguity as to its convexity. Illumination came from either above or below, and from either the right or the left, and stimuli were presented dichoptically with normal binocular disparity, reversed disparity, or no disparity. Participants responded “convex” more often when the lighting came from above than from below. Also, participants responded that the shape was convex more often with normal than with zero disparity, and more often with zero disparity than with reversed stereopsis. The effects of lighting direction and display mode were additive—that is, they did not interact. This indicates that shading and stereopsis contribute independently to shape perception.

Rapid Learning by Walking Observers Wearing a Reversing or Inverting Prism

We measured the initial rapid learning of walking observers who wore an up–down inverting or left–right reversing prism. This prism-walking version of the ‘mirror-drawing’ experiment revealed that the learning curve as a function of the trial number was the same as that typically acquired from a traditional mirror-drawing experiment. We suggest that the initial short-term learning process involved in prism walking is similar to that in mirror drawing and is related to the high-level decision-making process involved in visuo-motor planning of actions with feedback from transformed vision.

Manual control of the visual stimulus reduces the flash-lag effect

We investigated how observers' control of the stimulus change affects temporal aspects of visual perception. We compared the flash-lag effects for motion (Experiment 1) and for luminance (Experiment 2) under several conditions that differed in the degree of the observers' control of change in a stimulus. The flash-lag effect was salient if the observers passively viewed the automatic change in the stimulus. However, if the observers controlled the stimulus change by a computer-mouse, the flash-lag effect was significantly reduced. In Experiment 3, we examined how observers' control of the stimulus movement by a mouse affects the reaction time for the shape change in the moving stimulus and flash. Results showed that the control reduced the reaction time for both moving stimulus and flash. These results suggest that observers' manual control of the stimulus reduces the flash-lag effect in terms of facilitation in visual processing.

Adaptation to left-right reversed vision rapidly activates ipsilateral visual cortex in humans

The brain mechanisms of adaptation to visual transposition are of increasing interest, not only for research on sensory-motor coordination, but also for neuropsychological rehabilitation. Sugita [Nature 380 (1996) 523] found that after adaptation to left-right reversed vision for one and a half months, monkey V1 neurons responded to stimuli presented not only in the contralateral visual field, but also in the ipsilateral visual field. To identify the underlying neuronal mechanisms of adaptation to visual transposition, we conducted fMRI and behavioral experiments for which four adult human subjects wore left-right reversing goggles for 35/39 days, and investigated: (1) whether ipsilateral V1 activation can be induced in human adult subjects; (2) if yes, when the ipsilateral activity starts, and what kind of behavioral/psychological changes occur accompanying the ipsilateral activity; (3) whether other visual cortices also show an ipsilateral activity change. The results of behavioral experiments showed that visuomotor coordinative function and internal representation of peripersonal space rapidly adapted to the left-right reversed vision within the first or second week. Accompanying these behavioral changes, we found that both primary (V1) and extrastriate (MT/MST) visual cortex in human adults responded to visual stimuli presented in the ipsilateral visual field. In addition, the ipsilateral activity started much sooner than the one and a half months, which had been expected from the monkey neurophysiological study. The results of the present study serve as physiological evidence of large-scale, cross-hemisphere, cerebral plasticity that exists even in adult human brain.