The P300 event-related potential (ERP) can be used to infer whether an observer is looking at a target or not. Common practice in P300 experiments and applications is that observers are asked to fixate their eyes while stimuli are presented. We investigated the possibility to differentiate between single target and nontarget fixations in a target search task involving eye movements by using EEG epochs synchronized to fixation onset (fixation-related potentials: FRPs). Participants systematically scanned search displays consisting of six small Landolt Cs in search of Cs with a particular orientation. After each search display, they indicated whether and where target Cs had been presented. As expected, an FRP component consistent with the P300 reliably distinguished between target and nontarget fixations. It was possible to classify single FRPs into target and nontarget FRPs above chance (on average 62% correct, where 50% would be chance). These results are the first step to practical applications such as covertly monitoring observers' interests and supporting search tasks.
In addition to the rod and cone photoreceptors the retina contains intrinsically photosensitive retinal ganglion cells (ipRGCs). These cells express the photopigment melanopsin and are known to be involved in reflexive visual functions such as pupil response and photo-entrainment of the circadian rhythm. It is possible that the ipRGCs contribute to conscious visual perception, either by providing an independent signal to the geniculo-striate pathway, or by interacting with and thus modifying signals arising from “classical” retinal ganglion cells that combine and contrast cone input. Here, we tested for the existence of an interaction by asking if a 350% change in melanopsin stimulation alters psychophysical sensitivity for the detection of luminance flicker. In Experiment 1, we tested for a change in the threshold for detecting luminance flicker in three participants after they adapted to backgrounds with different degrees of tonic melanopsin stimulation. In Experiments 2 and 3, this test was repeated, but now for luminance flicker presented on a transient pedestal of melanopsin stimulation. Across the three experiments, no effect of melanopsin stimulation upon threshold flicker sensitivity was found. Our results suggest that even large changes in melanopsin stimulation do not affect near-threshold, cone-mediated visual perception.
Quantitative data of Muller-Lyer illusions from the literature were analyzed according to three different models. All three models predict the illusion effect, although with different magnitude and different parameter dependency. First, a filter model describing a certain amount of blurring of the retinal picture seems partly responsible for the observed illusion. With reasonable estimation of the filter constants, however, a sufficient magnitude of illusion cannot be obtained. A second model of oriented line or bar receptors is even less effective in explaining the observed length illusions. A third model, consisting of a size-constancy operator triggered by depth cues, may predict effects larger than actually observed. It is concluded that figural aspects such as depth-inducing cues are mainly responsible for the illusion effects observed in Muller-Lyer figures.
There is theoretical and empirical support for long-term adaptation of human vision to chromatic regularities in the environment. The current study investigates whether relationships of luminance and chromaticity in the natural environment could drive chromatic adaptation independently and differently for bright and dark colors. This is motivated by psychophysical evidence of systematic difference shifts in red-green chromatic sensitivities between contextually bright- versus dark-colored stimuli. For some broad classes of scene content, consistent shifts in chromaticity are found between high and low light levels within images. Especially in those images in which sky and terrain are juxtaposed, this shift has direction and magnitude consistent with the observed psychophysical shifts in the red-green balance between bright and dark colors. Taken together, these findings suggest that relative weighting of M- and L-cone signals could be adapted, in a luminance-dependent fashion, to regularities in the natural environment.
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