The detection of the motion of contrast modulation: a parametric study

Interaction between motion scales: When performance in motion discrimination is worse for a compound stimulus than for its integrating components

Motion direction discrimination becomes impaired when combinations of drifting high spatial frequency (HSF) and static low spatial frequency (LSF) patterns are merged into a compound stimulus. Such impairment has been suggested to occur due to an interaction between motion sensors tuned to coarse and fine scale spatial patterns. This interaction is modulated by different stimulus parameters like temporal frequency, size, the spectral components mixed, and their relative contrast. The present research precisely aims to explore in a deeper way the interaction's dependency upon the spatial frequency and the relative contrast of the components when both move coherently. Two experiments were therefore performed measuring duration thresholds (Experiment 1) and proportion of correct responses (Experiment 2) in a motion direction discrimination task. Stimuli were vertical Gabor patches of 4 deg diameter horizontally drifting with a speed of 2 deg/sec. Simple LSF and HSF stimuli as well as complex stimuli where both components moved coherently (LSFm + HSFm) were used. These were grouped in the following LSF and HSF pairs: 0.25-0.75, 0.5-1.5, 1-3 and 2-6 c/deg. Each component had a Michelson contrast of 28% or 7%, giving rise to different relative contrast combinations. Most interestingly, the results show a decrease in performance for complex stimuli with respect to each of their simple components when the LSF component has a lower contrast than the HSF one. The decrease depends on the particular spatial frequencies mixed in a stimulus. Further knowledge about the inhibitory mechanism is thus provided, revealing its joint dependency upon contrast and spatial frequency.

Second-order visual sensitivity in the aging population

Most visual and cognitive functions are affected by aging over the lifespan. In this study, our aim was to investigate the loss in sensitivity to different classes of second-order stimuli-a class of stimuli supposed to be mainly processed in extrastriate cortex-in the aging population. These stimuli will then allow one to identify specific cortical deficit independently of visibility losses in upstream parts of the visual pathway. For this purpose, we measured the sensitivity to first-order stimuli and second-order stimuli: orientation-modulated, motion-modulated or contrast-modulated as a function of spatial frequency in 50 aged participants. Overall, we observed a sensitivity loss for all classes of stimuli, but this loss differentially affects the three classes of second-order stimuli tested. It involves all modulation spatial frequencies in the case of motion modulation, but just high modulation spatial frequencies in the case of contrast- and orientation modulations. These observations imply that aging selectively affects the sensitivity to second-order stimuli depending on their type. Since there is evidence that these different second-order stimuli are processed in different regions of extrastriate cortex, this result may suggest that some visual cortical areas are more susceptible to aging effects than others.

Temporal frequency modulates the strength of the inhibitory interaction between motion sensors tuned to coarse and fine scales

The perceived direction of motion of a brief moving fine scale pattern reverses when a static coarse scale pattern is added to it (Henning & Derrington, 1988). This impairment in motion direction discrimination has been explained by the inhibitory interaction between motion sensors tuned to fine and coarse scales. This interaction depends on the particular spatial frequencies mixed, the size of the stimulus, and the relative contrast of the components (Serrano-Pedraza, Goddard, & Derrington, 2007; Serrano-Pedraza & Derrington, 2010). In this research we wanted to study the effect of speed or temporal frequency on the interaction between motion sensors. We performed three experiments where we measured duration thresholds in a motion direction discrimination task, and we also measured the proportion of correct responses. The stimuli used in the experiments were horizontally drifting vertical Gabor patches of 4° diameter (2σxy). In the first two experiments, five stimulus configurations of moving (m) and static (s) components were used: two simple stimuli, 1m c/° and 3m c/°; and three complex stimuli, 1m + 3m, 1m + 3s, and 1s + 3m. Results show that for all conditions but 1s + 3m, duration thresholds decrease (proportion of correct responses increase) with increasing speed. However, in condition 1s + 3m, duration thresholds increase from 0.5°/s to 2°/s and then decrease with increasing speed. In the third experiment we tested whether the interaction between scales is tuned to speed or temporal frequency using different conditions: 1s + 4m, 1s + 6m, 0.5s + 1.5m, and 2s + 6m. Results from duration thresholds suggest that the strength of the inhibitory interaction between motion sensors tuned to coarse and fine scales is temporal frequency tuned with a maximum around 6 Hz and a minimum between 6 and 12 Hz in the case of the proportion of correct responses.