Visual space perception at different levels of depth description.

The main purpose of this study was to determine the effect of the depth description levels required in experimental tasks on visual space perception. Six observers assessed the locations of 11 posts by determining a distance ranking order, comparing the distances between posts with a reference unit, and estimating the absolute distances between the posts. The experiments were performed in an open outdoor field under normal daylight conditions with posts at distances ranging from 2 to 12 m. To directly assess and compare the observers' perceptual performance in all three phases of the experiment, the raw data were transformed to common measurement levels. A pairwise comparison analysis provided nonmetric information regarding the observers' relative distance judgments, and a multidimensional-scaling procedure provided metric information regarding the relationship between a perceived spatial layout and the layout of the actual scene. The common finding in all of the analyses was that the precision and consistency of the observers' ordinal distance judgments were greater than those of their ratio distance judgments, which were, in turn, greater than the precision and consistency of their absolute-magnitude distance judgments. Our findings raise questions regarding the ecological validity of standard experimental tasks.

Confinement has no effect on visual space perception: The results of the Mars-500 experiment.

People confined to a closed space live in a visual environment that differs from a natural open-space environment in several respects. The view is restricted to no more than a few meters, and nearby objects cannot be perceived relative to the position of a horizon. Thus, one might expect to find changes in visual space perception as a consequence of the prolonged experience of confinement. The subjects in our experimental study were participants of the Mars-500 project and spent nearly a year and a half isolated from the outside world during a simulated mission to Mars. The participants were presented with a battery of computer-based psychophysical tests examining their performance on various 3-D perception tasks, and we monitored changes in their perceptual performance throughout their confinement. Contrary to our expectations, no serious effect of the confinement on the crewmembers' 3-D perception was observed in any experiment. Several interpretations of these findings are discussed, including the possibilities that (1) the crewmembers' 3-D perception really did not change significantly, (2) changes in 3-D perception were manifested in the precision rather than the accuracy of perceptual judgments, and/or (3) the experimental conditions and the group sample were problematic.

Spared cognitive processing of visual oddballs despite delayed visual evoked potentials in patient with partial recovery of vision after 53 years of blindness.

We examined the visual and cognitive functions of a 72-year-old subject, KP, who recovered his sight after 53 years of visual deprivation. We used visual evoked potentials (VEPs) to pattern-reversal and motion-onset stimuli and cognitive responses (ERPs) during the oddball paradigm to assess the effect of long-term deprivation on a mature visual system. KP lost his sight at the age of 17 years, and light projection onto his right retina was restored at 71 years by a corneal implant. Nine months after sight recovery we recorded reproducible responses to all examined stimuli. The response to pattern reversal contained two P100-like peaks with the later peak being dominant and significantly delayed (260 ms) when compared to the P100s of two control subjects, to whom the stimuli were adjusted in size and contrast to mimic KP's vision. KP's motion-onset VEPs to full-field and peripheral stimuli had a characteristic shape with a well-defined N2 peak; however, both peaks were significantly delayed (262 and 272 ms) compared to control responses. Unlike the P100 and N2 peaks, which represent sensory detection, the P3b/P300 component of the ERP to a target event in the oddball paradigm was not further delayed. In spite of degraded vision and sensory deprivation lasting 53 years, KP displayed reproducible responses to all reported stimuli. Long-term visual deprivation and retinal detachment degraded KP's visual sensory processing, assessed by pattern-reversal and motion-onset VEPs, whereas the cognitive processing of appropriate visual stimuli was not compromised.

Perceived size and perceived direction: the interplay of the two descriptors of visual space.

The stimulus in the outdoor psychophysical experiment was formed by two rods placed on the ground plane over a range of possible distances and orientations. Observers estimated its size and direction by positioning the third rod in the neighbouring space to form an evenly spaced collinear triple of rods. The data revealed interesting similarities between the profiles of the deviations in both judgments: for size judgments, the variability of the responses was least when the targets were at a frontal orientation and gradually increased as the orientation approached the medial plane. For direction judgments, on the other hand, the variability of the responses was least when the stimuli were aligned with the observer's line of sight and gradually increased as the orientation approached the frontoparallel plane. The finding of inverse relationship between the precision of size and direction judgments is interpreted as a consequence of the unequal precision in localisation between the frontal and in-depth dimensions of visual space. The question of the best parameterisation of the observers' responses is discussed.