Abstract
The generation of a behaviorally relevant cue to the speed of objects around us is critical to our ability to navigate safely within our environment. However, our perception of speed is often distorted by prevailing conditions. For instance, as luminance is reduced, our perception of the speed of fast-moving patterns can be increased by as much as 30%. To investigate how the cortical representation of speed may vary under such conditions, we have measured the functional MRI blood oxygen level-dependent (BOLD) response of visual cortex to drifting sine gratings at two very different luminances. The average BOLD response in all areas was band-pass with respect to speed (or equivalently, temporal frequency) and thus contained no unambiguous speed information. However, a multivariate classifier was able to predict grating speed successfully in all cortical areas measured. Similarly, we find that a multivariate classifier can predict stimulus luminance. No differences in either the mean BOLD response or the multivariate classifier response with respect to speed were found as luminance changed. However, examination of the spatial distribution of speed preferences in the primary visual cortex revealed that perifoveal locations preferred slower speeds than peripheral locations at low but not high luminance. We conclude that although an explicit representation of perceived speed has yet to be demonstrated in the human brain, multiple visual regions encode both the temporal structure of moving stimuli and luminance implicitly.
Highlights
THE QUESTION OF HOW AN OBJECT ’S speed is encoded is critical to an understanding of how the visual system guides us in real-world situations
Lingnau et al (2009) have reported functional MRI (fMRI) adaptation data, suggesting that the blood oxygen level-dependent (BOLD) response is more sensitive to speed than temporal frequency, at least in middle-temporal visual area (MT) and at high contrast
Our results demonstrate that at least for narrow-band stimuli, the BOLD response to speed is band-pass, peaking between 4°/s and 10°/s in all cortical areas measured and at both luminances measured
Summary
THE QUESTION OF HOW AN OBJECT ’S speed is encoded is critical to an understanding of how the visual system guides us in real-world situations. A subsequent study that used narrow-band stimuli and a more limited speed range (Singh et al 2000) reported band-pass speed tuning in all cortical areas from V1 to MT The tuning was both quantitatively and qualitatively similar across the entire visual cortex. Lingnau et al (2009) have reported fMRI adaptation data, suggesting that the BOLD response is more sensitive to speed than temporal frequency, at least in MT and at high contrast Their results provided evidence for temporal tuning across all cortical areas at lower contrast. Both electrophysiological and imaging studies have provided mixed evidence regarding the locus and nature of speed encoding. Examining the effect of a stimulus attribute that is known to affect perceived speed on BOLD responses may render a clearer picture of how and where cortical speed encoding occurs
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