Overflow, first-sight, and vanishing point distances in visual imagery

Reduced visual acuity is mirrored in low vision imagery

Research has examined the nature of visual imagery in normally sighted and blind subjects, but not in those with low vision. Findings with normally sighted subjects suggest that imagery involves primary visual areas of the brain. Since the plasticity of visual cortex appears to be limited in adulthood, we might expect imagery of those with adult-onset low vision to be relatively unaffected by these losses. But if visual imagery is based on recent and current experience, we would expect images of those with low vision to share some properties of impaired visual perception. We examined key parameters of mental images reported by normally sighted subjects, compared to those with early- and late-onset low vision, and with a group of subjects with restricted visual fields using an imagery questionnaire. We found evidence that those with reduced visual acuity report the imagery distances of objects to be closer than those with normal acuity and also depict objects in imagery with lower resolution than those with normal visual acuity. We also found that all low vision groups, like the normally sighted, image objects at a substantially greater distance than when asked to place them at a distance that 'just fits' their imagery field (overflow distance). All low vision groups, like the normally sighted, showed evidence of a limited visual field for imagery, but our group with restricted visual fields did not differ from the other groups in this respect. We conclude that imagery of those with low vision is similar to that of those with normal vision in being dependent on the size of objects or features being imaged, but that it also reflects their reduced visual acuity. We found no evidence for a dependence on imagery of age of onset or number of years of vision impairment.

Effects of content and viewing distance on the preferred size of moving images

While visual size preferences regarding still objects have been investigated and linked to the “canonical size” effect—where preferred on-screen size was significantly related to objects’ real-world size—the visual size preferences related to moving images of natural scenes has not been researched. In this study, we measured the preferred size of moving images of natural scenes and short duration and investigated the effect of viewing distance on size preferences. Our results showed that the preferred size varied strongly depending on content, and we found moving images’ canonical size effect. The preferred size in images of scenery was significantly larger than in images of persons, and there was a positive correlation between the preferred size and the real-world physical size of the main subjects in the images. When the viewing distance was doubled, the preferred size increased about 10% as a ratio to screen size—in contrast to the findings of a previous study. While the rationale for these findings is not yet clear, our analysis suggests that neither the motion component in the images nor the nature of their background area are contributing factors. We suggest that environment, viewing distance, and screen size may contribute to this effect.

Tests of the abilities to judge ratios of extensive and intensive sensory magnitudes

The assumption that people are able to numerically judge ratios of sensory magnitude is often used to scale sensations. In this study, we tested this assumption for the extensive sensation of length and the intensive sensations of heaviness and brightness. A ratio model fit judged length ratios accurately and fit judged heaviness and brightness ratios inaccurately. Also, most participants could not judge brightness ratios, even if they seemed able to respond nonverbally to the brightness ratios. These results suggest that people may generally be able to judge extensity ratios but not intensity ratios. Participants used auxiliary judgment operations to compensate for their lack of ability to judge intensity ratios. These auxiliary, nonratio operations were found to yield linear scales of intensive sensory magnitude.

Canonical visual size for real-world objects

Real-world objects can be viewed at a range of distances and thus can be experienced at a range of visual angles within the visual field. Given the large amount of visual size variation possible when observing objects, we examined how internal object representations represent visual size information. In a series of experiments which required observers to access existing object knowledge, we observed that real-world objects have a consistent visual size at which they are drawn, imagined, and preferentially viewed. Importantly, this visual size is proportional to the logarithm of the assumed size of the object in the world, and is best characterized not as a fixed visual angle, but by the ratio of the object and the frame of space around it. Akin to the previous literature on canonical perspective, we term this consistent visual size information the canonical visual size.

The psychophysics of imagery

A series of experiments considers the extent to which the interrelations among subjective magnitudes aroused by images corresponds to those for subjective magnitudes aroused by physical stimuli. In Experiment 1, 68 undergraduates typed phrases in response to graded categories regarding the imagined magnitude of lights, sounds, and smells. In Experiment 2, 5 undergraduates and, in Experiment 3, 3 graduate students then magnitude estimated the image intensity aroused by each of these stimulus phrases. In Experiments 4 and 5, the same subjects performed cross-modality matches between phrases arousing images for different attributes (light, sound, and smell). Statistical analysis indicates that estimates based on images display many of the same patterns as those based on physical stimuli. The major exception involves sequence effects, present for actual stimuli but not for images.

Reproduction of object shape is more accurate without the continued availability of visual information

An unfamiliar configuration lying in depth and viewed from a distance is typically seen as foreshortened. The hypothesis motivating this research was that a change in an observer's viewpoint even when the configuration is no longer visible induces an imaginal updating of the internal representation and thus reduces the degree of foreshortening. In experiment 1, observers attempted to reproduce configurations defined by three small glowing balls on a table 2 m distant under conditions of darkness following 'viewpoint change' instructions. In one condition, observers reproduced the continuously visible configuration using three other glowing balls on a nearer table while imagining standing at the distant table. In the other condition, observers viewed the configuration, it was then removed, and they walked in darkness to the far table and reproduced the configuration. Even though the observers received no additional information about the stimulus configuration in walking to the table, they were more accurate (less foreshortening) than in the other condition. In experiment 2, observers reproduced distant configurations on a nearer table more accurately when doing so from memory than when doing so while viewing the distant stimulus configuration. In experiment 3, observers performed both the real and imagined perspective change after memorizing the remote configuration. The results of the three experiments indicate that the continued visual presence of the target configuration impedes imaginary perspective-change performance and that an actual change in viewpoint does not increase reproduction accuracy substantially over that obtained with an imagined change in viewpoint.

Separating the effects of alertness from the effects of encoding in a pitch-imagery task

The facilitatory effects of auditory imagery have often been attributed to priming. However, the facilitatory effects of a prime on subsequent processing can be separated into effects related to specific perceptual processes, such as encoding, and effects related to more general processes, such as alertness. Whereas research on auditory imagery has focused on code-activated facilitation, the role of alertness in image-induced facilitation has been largely presumed rather than empirically investigated. Therefore, a study was designed to isolate the facilitatory effects of alertness from those of encoding. The results show that the nonspecific alerting effects of auditory imaging were quite extensive--a nearly 78-ms decrease in reaction time--compared with the effects of encoding.

Imagery, memory, and size-distance invariance

The size-distance invariance hypothesis (SDIH) was examined for remembered and imaged stimuli. In Experiment 1, subjects gave remembered and imaged distances of familiar objects and imaged distance of nondescript rods. The relationship between stated size and distance is more adequately described by power functions with exponents less than 1 than by the more restricted SDIH (exponent of 1). In Experiment 2, subjects gave distance estimates to recalled and imaged familiar objects and described the visual context in which each object was situated. A different group then sorted the contexts into categories based on general similarity. There were no significant differences between distance estimates based on memory and those based on imagery, and the visual contexts were not sorted according to whether they were generated in the memory or in the imagery conditions. In Experiment 3, subjects estimated the distances to objects in an outdoor setting. A linear relationship was found between estimated and physical distance, suggesting that the lower exponents obtained in Experiments 1 and 2 were not artifacts of the distance judgment procedure.