'Coarse-to-fine' cyclopean processing

Previously (Popple et al, 1998 Vision Research 38 319-326) we found, using random-dot stereograms, that initial vergence increases with the size of a cyclopean disc. A corresponding improvement in stereoacuity within the disc was predicted, because disparities in the disc would be brought closer to the plane of current fixation. In the present experiment, we looked at the effect of the spatial extent of a briefly presented (< or = 500 ms) cyclopean depth pedestal on stereoacuity thresholds. Observers were required to judge the depth of a small, 1.7 deg, central disc relative to a larger surrounding disc in a random-pattern stereogram. The larger disc was set, initially, at a pedestal disparity of +/- 24 min of arc against a fixation-plane surround. The size of the larger disc was varied from 2.6 to 8.0 deg. As predicted, stereoacuity thresholds fell significantly with increasing pedestal disc size. Next, the disparity of the pedestal disc was varied. When pedestal disparity was reduced to +/- 2.4 min of arc, a disparity too small to demand vergence, the size effect disappeared except when the pedestal boundary was within 30 min of arc of the test disc boundary. We argue from this result that the effect was largely due to vergence and not cyclopean integration alone. However, the effect of pedestal size was found to persist with stimuli too brief to permit vergence (< or = 100 ms) suggesting that factors other than vergence may also play a role.     

Effects of implied action speed on estimation of event duration

Two experiments investigated the effect of manipulating the implied speed of an individual's actions on estimation of event duration. Experiment 1 requested duration estimates from eyewitnesses to a staged event. Three groups of subjects participated, and each was provided with different postevent information, which varied the implied speed of the individual that acted out the event. Results indicated that subjects who were led to believe the actor engaged in fast actions (e.g. running) generated significantly shorter duration estimates than those led to believe the speed of the actor was slow (e.g. the actor walked). Experiment 2 manipulated implied action speed by varying the commands given by a robber during a bank robbery. The marked sound track contained phrases such as ‘hurry up’, ‘come on’, ‘now!’ which imply that the bank tellers, to whom the commands were directed, had a slow action speed. The unmarked sound track contained the same number of words as the marked tape, but they did not emphasize action speed. Both versions of the video had the same actual duration. Results indicated that subjects shown the marked video gave significantly longer duration estimates, than those shown the unmarked video. The results are discussed in terms of the reconstruction of event duration, and implications for assessing the accuracy of eyewitness testimony.     

Location coding by the human visual system: multiple topological adaptations in a case of strabismic amblyopia

Amblyopia, a major cause of vision loss, is a developmental disorder of visual perception commonly associated with strabismus (squint). Although defined by a reduction in visual acuity, severe distortions of perceived visual location are common in strabismic amblyopia. These distortions can help us understand the cortical coding of visual location and its development in normal vision, as well as in amblyopia. The history of retinotopic mapping in the visual cortex highlights the potential impact of amblyopia. Theories of amblyopia include topological disarray of receptors in primary visual cortex, undersampling from the amblyopic eye compared with normal eyes, and the presence of anomalous retinal correspondence or multiple cortical representations of the strabismic fovea. We examined the distortions in a strabismic amblyope, using a pop-out localization task, in which normal observers made errors dependent on the visual context of the stimulus. The localization errors of the strabismic amblyope were abnormal. We found that none of the available theories could fully explain this one patient's localization performance. Instead, the observed behavior suggests that multiple adaptations of the underlying cortical topology are possible simultaneously in different parts of the visual field.