Distinct neural correlates of attending speed vs. coherence of motion

Competing Visual Cues Revealed by Electroencephalography: Sensitivity to Motion Speed and Direction

Motion speed and direction are two fundamental cues for the mammalian visual system. Neurons in various places of the neocortex show tuning properties in term of firing frequency to both speed and direction. The present study applied a 32-channel electroencephalograph (EEG) system to 13 human subjects while they were observing a single object moving with different speeds in various directions from the center of view to the periphery on a computer monitor. Depending on the experimental condition, the subjects were either required to fix their gaze at the center of the monitor while the object was moving or to track the movement with their gaze; eye-tracking glasses were used to ensure that they followed instructions. In each trial, motion speed and direction varied randomly and independently, forming two competing visual features. EEG signal classification was performed for each cue separately (e.g., 11 speed values or 11 directions), regardless of variations in the other cue. Under the eye-fixed condition, multiple subjects showed distinct preferences to motion direction over speed; however, two outliers showed superb sensitivity to speed. Under the eye-tracking condition, in which the EEG signals presumably contained ocular movement signals, all subjects showed predominantly better classification for motion direction. There was a trend that speed and direction were encoded by different electrode sites. Since EEG is a noninvasive and portable approach suitable for brain-computer interfaces (BCIs), this study provides insights on fundamental knowledge of the visual system as well as BCI applications based on visual stimulation.

Effects of Temporal Expectations on the Perception of Motion Gestalts

Gestalt psychology has traditionally ignored the role of attention in perception, leading to the view that autonomous processes create perceptual configurations that are then attended. More recent research, however, has shown that spatial attention influences a form of Gestalt perception: the coherence of random-dot kinematograms (RDKs). Using ERPs, we investigated whether temporal expectations exert analogous attentional effects on the perception of coherence level in RDKs. Participants were presented fixed-length sequences of RDKs and reported the coherence level of a target RDK. The target was indicated immediately after its appearance by a postcue. Target expectancy increased as the sequence progressed until target presentation; afterward, remaining RDKs were perceived without target expectancy. Expectancy influenced the amplitudes of ERP components P1 and N2. Crucially, expectancy interacted with coherence level at N2, but not at P1. Specifically, P1 amplitudes decreased linearly as a function of RDK coherence irrespective of expectancy, whereas N2 exhibited a quadratic dependence on coherence: larger amplitudes for RDKs with intermediate coherence levels, and only when they were expected. These results suggest that expectancy at early processing stages is an unspecific, general readiness for perception. At later stages, expectancy becomes stimulus specific and nonlinearly related to Gestalt coherence.

Default-mode network dynamics are restricted during high speed discrimination in healthy aging: Associations with neurocognitive status and simulated driving behavior

Numerous daily tasks, including car driving, require fine visuospatial tuning. One such visuospatial ability, speed discrimination, declines with aging but its neural underpinnings remain unknown. In this study, we use fMRI to explore the effect of aging during a high speed discrimination task and its neural underpinnings, along with a complete neuropsychological assessment and a simulated driving evaluation in order to examine how they interact with each other through a multivariate regression approach. Beyond confirming that high speed discrimination performance is diminished in the elderly, we found that this deficit might be partly due to a lack of modulation in the activity and connectivity of the default mode network (DMN) in this age group, as well as an over-recruitment of frontoparietal and cerebellar regions, possibly as a compensatory mechanism. In addition, younger adults tended to drive at faster speeds, a behavior that was associated to adequate DMN dynamics and executive functioning, an effect that seems to be lost in the elderly. In summary, these results reveal how age-related declines in fine visuospatial abilities, such as high speed discrimination, were distinctly mediated by DMN functioning, a mechanism also associated to speeding behavior in a driving simulator.

Neural Correlates of Speed-Tuned Motion Perception in Healthy Adults

It has been suggested that slow and medium-to-fast speeds of motion may be processed by at least partially separate mechanisms. The purpose of this study was to establish the cortical areas activated during motion-defined form and global motion tasks as a function of speed, using functional magnetic resonance imaging. Participants performed discrimination tasks with random dot stimuli at high coherence, at coherence near their own thresholds, and for random motion. Stimuli were moving at 0.1 or 5 deg/s. In the motion-defined form task, lateral occipital complex, V5/MT+ and intraparietal sulcus showed greater activation by high or near-threshold coherence than by random motion stimuli; V5/MT+ and intraparietal sulcus demonstrated greater activation for 5 than 0.1 deg/s dot motion. In the global motion task, only high coherence stimuli elicited significant activation over random motion; this activation was primarily in nonclassical motion areas. V5/MT+ was active for all motion conditions and showed similar activation for coherent and random motion. No regions demonstrated speed-tuning effects for global motion. These results suggest that similar cortical systems are activated by slow- and medium-speed stimuli during these tasks in healthy adults.

Role of inter-hemispheric transfer in generating visual evoked potentials in V1-damaged brain hemispheres

Partial cortical blindness is a visual deficit caused by unilateral damage to the primary visual cortex, a condition previously considered beyond hopes of rehabilitation. However, recent data demonstrate that patients may recover both simple and global motion discrimination following intensive training in their blind field. The present experiments characterized motion-induced neural activity of cortically blind (CB) subjects prior to the onset of visual rehabilitation. This was done to provide information about visual processing capabilities available to mediate training-induced visual improvements. Visual Evoked Potentials (VEPs) were recorded from two experimental groups consisting of 9 CB subjects and 9 age-matched, visually-intact controls. VEPs were collected following lateralized stimulus presentation to each of the 4 visual field quadrants. VEP waveforms were examined for both stimulus-onset (SO) and motion-onset (MO) related components in postero-lateral electrodes. While stimulus presentation to intact regions of the visual field elicited normal SO-P1, SO-N1, SO-P2 and MO-N2 amplitudes and latencies in contralateral brain regions of CB subjects, these components were not observed contralateral to stimulus presentation in blind quadrants of the visual field. In damaged brain hemispheres, SO-VEPs were only recorded following stimulus presentation to intact visual field quadrants, via inter-hemispheric transfer. MO-VEPs were only recorded from damaged left brain hemispheres, possibly reflecting a native left/right asymmetry in inter-hemispheric connections. The present findings suggest that damaged brain hemispheres contain areas capable of responding to visual stimulation. However, in the absence of training or rehabilitation, these areas only generate detectable VEPs in response to stimulation of the intact hemifield of vision.

What you see depends on what you saw, and what else you saw: the interactions between motion priming and object priming

Both visual object priming and motion priming have been reported independently, but the interactions between the two are still largely unexplored. Here we investigated this question using a novel type of SFM stimuli, 3-D helixes, and found that the motion direction perception of an ambiguous helix can be biased by the motion direction of a preceding SFM stimulus - a classic motion priming effect. However, the effectiveness of motion priming depends on object priming: a neutral object priming produced a weak motion priming, a congruent object priming led to a strong motion priming, and critically, an incongruent object priming abolished and overpowered the motion priming. In contrast, object priming alone (in the absence of motion overlap) had little effects biasing motion perception. Taken together, these results suggest that there exists an integrated neural representation of motion and structure of 3-D SFM stimuli, and motion priming of 3-D SFM stimuli might happen at an intermediate stage between MT/V5 (which is not shape selective) and LO (lateral occipital, which is not motion selective). This novel type of stimuli, 3-D helixes, along with the prime-target paradigm, thus might offer a unique tool to examine neural bases underlying the perception of 3-D SFM stimuli and perceptual priming.