Abstract
Our ability to perceive visual motion is critically dependent on the human motion complex (hMT+) in the dorsal visual stream. Extensive electrophysiological research in the monkey equivalent of this region has demonstrated how neuronal populations code for properties such as speed and direction, and that neurometric functions relate to psychometric functions within the individual monkey. In humans, the physiological correlates of inter-individual perceptual differences are still largely unknown. To address this question, we used functional magnetic resonance imaging (fMRI) while participants viewed translational motion in different directions, and we measured thresholds for direction discrimination of moving stimuli in a separate psychophysics experiment. After determining hMT+ in each participant with a functional localizer, we were able to decode the different directions of visual motion from it using pattern classification (PC). We also characterized the variability of fMRI signal in hMT+ during stimulus and rest periods with a generative model. Relating perceptual performance to physiology, individual direction discrimination thresholds were significantly correlated with the variability measure in hMT+, but not with PC accuracies. Individual differences in PC accuracy were driven by non-physiological sources of noise, such as head-movement, which makes this method a poor tool to investigate inter-individual differences. In contrast, variability analysis of the fMRI signal was robust to non-physiological noise, and variability characteristics in hMT+ correlated with psychophysical thresholds in the individual participants. Higher levels of fMRI signal variability compared to rest correlated with lower discrimination thresholds. This result is in line with theories on stochastic resonance in the context of neuronal populations, which suggest that endogenous or exogenous noise can increase the sensitivity of neuronal populations to incoming signals.
Highlights
Accurate perception of visual motion is a key function of the human brain, enabling us to interpret the world around us, to predict trajectories of moving objects and to steer vehicles and control locomotion
We demonstrate in the current study that inter-individual differences in performance on a direction discrimination task of visual motion are correlated with signal variability characteristics of hMT+ but not V1
We show that pattern classification (PC), though being able to decode direction from hMT+ within subjects, is a poor tool to describe inter-individual differences
Summary
Accurate perception of visual motion is a key function of the human brain, enabling us to interpret the world around us, to predict trajectories of moving objects and to steer vehicles and control locomotion. While many psychophysical and neurophysiological studies have revealed common processing of visual motion information across participants, perceptual capabilities can differ substantially between individuals (Halpern et al, 1999). Though classical behavioral experiments average these difference to focus on the mean tendency, heterogeneity in visual motion perception can provide information on perceptual functioning. Exploring the relation between differences in performance on motion tasks and physiological signals in the visual dorsal stream can shed light on the relationships between cortical processing and perception. More evidence for a direct link between MT neuronal properties and perception comes from studies which show that microstimulation can considerably bias performance (Cohen and Newsome, 2004) and that deteriorated neuronal speed and direction selectivity accompanies aging (Yang et al, 2009; Liang et al, 2010)
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