R. van Lier, M. Vergeer, and S. Anstis (2009) reported an afterimage that produced different percepts from an inducing stimulus depending on the shape of a subsequent contour. G. Francis (2010) explained this phenomenon with a model where the contour forms a boundary that traps the afterimage color as it spreads across a surface. We conducted a series of additional model simulations and experiments to test the explanation. We first tested the hypothesis that the contour traps the afterimage color by adding additional contours. Model simulations suggest that additional contours could block color from spreading to the middle of the surface. In two experiments, additional contours blocked color spreading when they were at the inducer edges but not when they were drawn away from the inducer edges. In a second set of experiments, we investigated the model prediction that the drawn contour defines the perceived shape of the afterimage. New model simulations predict that variations in the size of the drawn contour force the perceived afterimage to vary accordingly. However, an experimental study revealed that the perceived afterimage size remains the same as the inducing stimulus. The simulation and experimental results both highlight and challenge important characteristics of the model.

Using electrophysiology, we have examined two questions in relation to musical training – namely, whether it enhances sensory encoding of the human voice and whether it improves the ability to ignore irrelevant auditory change. Participants performed an auditory distraction task, in which they identified each sound as either short (350 ms) or long (550 ms) and ignored a change in sounds’ timbre. Sounds consisted of a male and a female voice saying a neutral sound [a], and of a cello and a French Horn playing an F3 note. In some blocks, musical sounds occurred on 80% of trials, while voice sounds on 20% of trials. In other blocks, the reverse was true. Participants heard naturally recorded sounds in half of experimental blocks and their spectrally-rotated versions in the other half. Regarding voice perception, we found that musicians had a larger N1 ERP component not only to vocal sounds but also to their never before heard spectrally-rotated versions. We, therefore, conclude that musical training is associated with a general improvement in the early neural encoding of complex sounds. Regarding the ability to ignore irrelevant auditory change, musicians’ accuracy tended to suffer less from the change in sounds’ timbre, especially when deviants were musical notes. This behavioral finding was accompanied by a marginally larger re-orienting negativity in musicians, suggesting that their advantage may lie in a more efficient disengagement of attention from the distracting auditory dimension.

Retinal lateral inhibition is one of the conventional efficient coding mechanisms in the visual system that is produced by interneurons that pool signals over a neighborhood of presynaptic feedforward cells and send inhibitory signals back to them. Thus, the receptive-field (RF) of a retinal ganglion cell has a center-surround receptive-field (RF) profile that is classically represented as a difference-of-Gaussian (DOG) adequate for efficient spatial contrast coding. The DOG RF profile has been attributed to produce the psychophysical phenomena of brightness induction, in which the perceived brightness of an object is affected by that of its vicinity, either shifting away from it (brightness contrast) or becoming more similar to it (brightness assimilation) depending on the size of the surfaces surrounding the object. While brightness contrast can be modeled using a DOG with a narrow surround, brightness assimilation requires a wide suppressive surround. Early retinal studies determined that the suppressive surround of a retinal ganglion cell is narrow (< 100–300 μm; ‘classic RF’), which led researchers to postulate that brightness assimilation must originate at some post-retinal, possibly cortical, stage where long-range interactions are feasible. However, more recent studies have reported that the retinal interneurons also exhibit a spatially wide component (> 500–1000 μm). In the current study, we examine the effect of this wide interneuron RF component in two biophysical retinal models and show that for both of the retinal models it explains the long-range effect evidenced in simultaneous brightness induction phenomena and that the spatial extent of this long-range effect of the retinal model responses matches that of perceptual data. These results suggest that the retinal lateral inhibition mechanism alone can regulate local as well as long-range spatial induction through the narrow and wide RF components of retinal interneurons, arguing against the existing view that spatial induction is operated by two separate local vs. long-range mechanisms.

The perceived duration of a moving stimulus correlates positively with its speed. It is not known whether such duration dilation depends on the physical or apparent speed. Here we show the latter to be true. The perceived duration of a shortly presented (500, 900, 1300 ms) Gabor patch whose carrier moved at 1°/s in a direction opposite to a background of random black dots rigidly moving at 3°/s appeared to last 20% longer and to drift 240% faster than the same Gabor carrier moving in the same direction as the random-dot background. Assessment of the perceived speed of each of the two configurations relative to a moving Gabor patch in the absence of the moving background allowed the comparison of the observed duration dilation with that obtained as a function of the corresponding physical speeds, which should have yielded a dilation of only 7%, i.e., three times less. In line with the proposal that perceived duration correlates with the strength of the neural response evoked by the stimuli to be timed, the present data can be accounted for by the increased responsiveness of antagonistic center-surround motion-receptive fields when stimulated with center-surround antagonist motions.

The spatial and spectral topography of the cone mosaic set the limits for detection and discrimination of chromatic sinewave gratings. Here, we sought to compare the spatial characteristics of mechanisms mediating hue perception against those mediating chromatic detection in individuals with known spectral topography and with optical aberrations removed with adaptive optics. Chromatic detection sensitivity in general exceeded previous measurements and decreased monotonically for increasingly skewed cone spectral compositions. The spatial grain of hue perception was significantly coarser than chromatic detection, consistent with separate neural mechanisms for color vision operating at different spatial scales.