Apparent motion of monocular stimuli in different depth planes with lateral head movements

Individual objective versus subjective fixation disparity as a function of forced vergence

Inaccuracy in the vergence eye position ("fixation disparity") can occur despite a fusion stimulus. When measured with eye trackers, this inaccuracy is referred to as "objective fixation disparity". It is a matter of debate whether objective fixation disparity can be estimated with a technically simple psycho-physical procedure, i.e. the perceived offset of aligned dichoptic nonius targets, referred to as "subjective fixation disparity". To investigate the relation between these two measures, simultaneous tests were made in far vision when placing prisms in front of the eyes (for a few seconds) in order to induce forced vergence, i.e. to vary the absolute disparity (from 1 deg divergent to 3.4 deg convergent). Frequent repeated measurements in 12 observers allowed for individual analyses. Generally, fixation disparity values and the effects of prisms were much smaller in the subjective than in the objective measures. Some observers differed systematically in the characteristics of the two types of prism-induced curves. Individual regressions showed that the subjective vs. objective slope was 8% on the average (with largest individual values of 18%). This suggests that sensory fusion shifts the visual direction of the (peripheral) binocular targets by the full amount of objective fixation disparity (since single vision was achieved); however, for the (central) monocular nonius lines this shift was more or less incomplete so that the dichoptic nonius targets indicated an individual percentage of objective fixation disparity. The subjective-to-objective ratio seems to be an individual characteristic of fixation disparity in terms of the amount and in terms of the effect of prism-induced forced vergence. Therefore, on the group level the subjective measures do not allow for a precise prediction of the objective measures.

Rehabilitation regimes based upon psychophysical studies of prosthetic vision

Human trials of prototype visual prostheses have successfully elicited visual percepts (phosphenes) in the visual field of implant recipients blinded through retinitis pigmentosa and age-related macular degeneration. Researchers are progressing rapidly towards a device that utilizes individual phosphenes as the elementary building blocks to compose a visual scene. This form of prosthetic vision is expected, in the near term, to have low resolution, large inter-phosphene gaps, distorted spatial distribution of phosphenes, restricted field of view, an eccentrically located phosphene field and limited number of expressible luminance levels. In order to fully realize the potential of these devices, there needs to be a training and rehabilitation program which aims to assist the prosthesis recipients to understand what they are seeing, and also to adapt their viewing habits to optimize the performance of the device. Based on the literature of psychophysical studies in simulated and real prosthetic vision, this paper proposes a comprehensive, theoretical training regime for a prosthesis recipient: visual search, visual acuity, reading, face/object recognition, hand-eye coordination and navigation. The aim of these tasks is to train the recipients to conduct visual scanning, eccentric viewing and reading, discerning low-contrast visual information, and coordinating bodily actions for visual-guided tasks under prosthetic vision. These skills have been identified as playing an important role in making prosthetic vision functional for the daily activities of their recipients.

Binocular Vision: Defining the Historical Directions

Ever since Kepler described the image-forming properties of the eye (400 years ago) there has been a widespread belief, which remains to this day, that an object seen with one eye is always seen where it is. Predictions made by Ptolemy in the first century, Alhazen in the eleventh, and Wells in the eighteenth, and supported by Towne, Hering, and LeConte in the nineteenth century, however, are contrary to this claimed veridicality. We discuss how among eighteenth-and nineteenth-century British researchers, particularly Porterfield, Brewster, and Wheatstone, the erroneous idea continued and also why observations made by Wells were neither understood nor appreciated. Finally, we discuss recent data, obtained with a new method, that further support Wells's predictions and which show that a distinction between headcentric and relative direction tasks is needed to appreciate the predictions.