Direct evidence for encoding of motion streaks in human visual cortex

Deborah Apthorp,David Alais,Bahador Bahrami,Christian Kaul,Geraint Rees,D Samuel Schwarzkopf

doi:10.1098/rspb.2012.2339

Abstract

Temporal integration in the visual system causes fast-moving objects to generate static, oriented traces (‘motion streaks’), which could be used to help judge direction of motion. While human psychophysics and single-unit studies in non-human primates are consistent with this hypothesis, direct neural evidence from the human cortex is still lacking. First, we provide psychophysical evidence that faster and slower motions are processed by distinct neural mechanisms: faster motion raised human perceptual thresholds for static orientations parallel to the direction of motion, whereas slower motion raised thresholds for orthogonal orientations. We then used functional magnetic resonance imaging to measure brain activity while human observers viewed either fast (‘streaky’) or slow random dot stimuli moving in different directions, or corresponding static-oriented stimuli. We found that local spatial patterns of brain activity in early retinotopic visual cortex reliably distinguished between static orientations. Critically, a multivariate pattern classifier trained on brain activity evoked by these static stimuli could then successfully distinguish the direction of fast (‘streaky’) but not slow motion. Thus, signals encoding static-oriented streak information are present in human early visual cortex when viewing fast motion. These experiments show that motion streaks are present in the human visual system for faster motion.

Highlights

Blurred lines or ‘motion streaks’ along the trajectory of a moving object have long been used in art and photography to illustrate fast motion
If slower motion is encoded by neurons whose preferred orientation is orthogonal to their preferred direction [5,26,27,28], we expected instead to see the opposite pattern after adaptation to slow motion
Our fMRI results, using a conservative correction for multiple comparisons, showed successful generalization from training the decoding of static-oriented stimuli to testing the decoding of direction of faster motion in area V2, whereas our psychophysical results suggest that faster and slower motions 7 may be processed by distinct neural substrates

Summary

Introduction

Blurred lines or ‘motion streaks’ along the trajectory of a moving object have long been used in art and photography to illustrate fast motion (figure 1a). Multi-voxel pattern analysis (MVPA) can reveal selectivities for orientation and direction of motion of visual stimuli from population fMRI responses by exploiting information contained by the spatial pattern of signals in a brain region [14,15,16] To test whether these distinct neural effects of faster and slower motion existed in the human brain, we first performed a psychophysical adaptation study testing contrast threshold elevation for static-oriented patterns after viewing fast or slow motion. In a second experiment, using fMRI MVPA, we investigated whether ‘motion streaks’ contributed to motion processing in the human brain by testing whether activity patterns in neuronal populations selective for orthogonal static orientations might be sufficient to determine the direction of motion of faster (but not slower) moving stimuli that produced ‘motion streaks’ with the same orientations (and vice versa). We found that a classifier trained on patterns of brain activity while viewing static-oriented stimuli could successfully decode the direction of dot stimuli moving fast enough to form streaks, but not those moving at speeds below the streak threshold

Methods

Results

Discussion