Visual evoked potentials elicited by chromatic motion onset

Visual evoked potentials to change in coloration of a moving bar

In our previous study we found that it takes less time to detect coloration change in a moving object compared to coloration change in a stationary one (Kreegipuu etal., 2006). Here, we replicated the experiment, but in addition to reaction times (RTs) we measured visual evoked potentials (VEPs), to see whether this effect of motion is revealed at the cortical level of information processing. We asked our subjects to detect changes in coloration of stationary (0(°)/s) and moving bars (4.4 and 17.6(°)/s). Psychophysical results replicate the findings from the previous study showing decreased RTs to coloration changes with increase of velocity of the color changing stimulus. The effect of velocity on VEPs was opposite to the one found on RTs. Except for component N1, the amplitudes of VEPs elicited by the coloration change of faster moving objects were reduced than those elicited by the coloration change of slower moving or stationary objects. The only significant effect of velocity on latency of peaks was found for P2 in frontal region. The results are discussed in the light of change-to-change interval and the two methods reflecting different processing mechanisms.

Misperceptions of speed for chromatic and luminance grating stimuli

Errors in the perception of speed of moving visual stimuli can occur when presented stimuli are of unequal contrast and when they appear alongside additional modifier stimuli that move at different speeds. We have examined these misperceptions for chromatic and luminance grating stimuli in order to assess to what extent these different kinds of motion cue might be utilised in the analysis of speed of moving objects. We show that the dependence on contrast of speed matching for luminance and chromatic stimuli is similar over a range of stimulus speeds greater than 4 deg/s. Differences between the contrast dependencies of speed perception for chromatic and luminance stimuli are only evident at slow speeds (< 4 deg/s) and low contrasts. The presence of modifier stimuli can directly influence the perceived speed at both high and low velocities and contrasts. This influence was found to be independent of the modifiers' chromaticity and was greatest when the modifiers were adjacent to and presented simultaneously with the test and reference stimuli. However, the modifiers were still able to induce measurable changes in perceived speed for increased separations over space and time. Taken together these results indicate that whilst differences do exist in the contrast dependencies of speed perception for chromatic and luminance stimuli, they are evident only for a narrow range of stimulus parameters (i.e. low speed and low contrast). There appears to be ample scope for interactions between chromatic and luminance contrast in speed perception where there is the capacity to pool this information over a relatively broad spatio-temporal extent.

A primer on motion visual evoked potentials

Motion visual evoked potentials (motion VEPs) have been used since the late 1960s to investigate the properties of human visual motion processing, and continue to be a popular tool with a possible future in clinical diagnosis. This review first provides a synopsis of the characteristics of motion VEPs and then summarizes important methodological aspects. A subsequent overview illustrates how motion VEPs have been applied to study basic functions of human motion processing and shows perspectives for their use as a diagnostic tool.

Flicker VEPs reflecting multiple rod and cone pathways

In an attempt to determine whether the relative contributions of magno-mediated and parvo-mediated inputs to the cortex are significantly altered in the transition from cone to rod vision, VEPs were recorded at different luminance levels (photopic to scotopic) for 2Hz square-wave, isochromatic flicker. The VEP mass response appears capable of reflecting major parvo-mediated contributions even at luminance levels for which responses from individual cells in the parvocellular pathway are reported to be weak. Our findings suggest that parvo-mediated responses are the dominant source of high-contrast isochromatic flicker VEPs at all light levels.

Chromatic sensitivity of neurones in area MT of the anaesthetised macaque monkey compared to human motion perception

We recorded activity from neurones in cortical motion-processing areas, middle temporal area (MT) and middle posterior superior temporal sulcus (MST), of anaesthetised and paralysed macaque monkeys in response to moving sinewave gratings modulated in luminance and chrominance. The activity of MT and MST neurones was highly dependent on luminance contrast. In three of four animals isoluminant chromatic modulations failed to activate MT/MST neurones significantly. At low luminance contrast a systematic dependence on chromaticity was revealed, attributable mostly to residual activity of the magnocellular pathway. Additionally, we found indications for a weak S-cone input, but rod intrusion could also have made a contribution. In contrast to the activity of MT and MST neurones, speed judgments and onset amplitude of evoked optokinetic eye movements in human subjects confronted with equivalent visual stimuli were largely independent of luminance modulation. Motion of every grating (including isoluminant) was readily visible for all but one observer. Similarity with the activity of MT/MST cells was found only for motion-nulling equivalent luminance contrast judgments at isoluminance. Our results suggest that areas MT and MST may not be involved in the processing of chromatic motion, but effects of central anaesthesia and/or the existence of intra- and inter-species differences must also be considered.

Distinct contrast response functions in striate and extra-striate regions of visual cortex revealed with magnetoencephalography (MEG)

OBJECTIVE

To spatially and temporally characterise the cortical contrast response function to pattern onset stimuli in humans.

METHODS

Magnetoencephalography (MEG) was used to investigate the human cortical contrast response function to pattern onset stimuli with high temporal and spatial resolution. A beamformer source reconstruction approach was used to spatially localise and identify the time courses of activity at various visual cortical loci.

RESULTS

Consistent with the findings of previous studies, MEG beamformer analysis revealed two simultaneous generators of the pattern onset evoked response. These generators arose from anatomically discrete locations in striate and extra-striate visual cortex. Furthermore, these loci demonstrated notably distinct contrast response functions, with striate cortex increasing approximately linearly with contrast, whilst extra-striate visual cortex followed a saturating function.

CONCLUSIONS

The generators that underlie the pattern onset visual evoked response arise from two distinct regions in striate and extra-striate visual cortex.

SIGNIFICANCE

The spatially, temporally and functionally distinct mechanisms of contrast processing within the visual cortex may account for the disparate results observed across earlier studies and assist in elucidating causal mechanisms of aberrant contrast processing in neurological disorders.

Direction tuning of human motion detection determined from a population model

Mass responses that are obtained using electrophysiological or psychophysical techniques are inadequate to characterize motion detectors at the single-unit level. Therefore, we have modelled a population of motion detectors and fitted their mass response to motion-onset EEG data. By examining a single unit of the modelled population we could assess the range of directions and the level of adaptation an individual motion detector responded to. Stimuli were patterns of randomly distributed dots. After subjects had adapted to either a non-moving pattern or to motion in one fixed direction, the pattern moved in one of seven possible test directions. The population model assumed elementary motion detectors with a Gaussian angle-selectivity profile. The population response was calculated as the sum of all adaptation-weighted individual responses to the test direction. In all subjects, we observed sizeable effects of adaptation on test directions close to the adapted direction and small amounts of direction-independent adaptation for all test directions. The model fit explained 75% (Oz) and 94% (lateral derivation) of the total variance and revealed a motion detector bandwidth (full width at half maximum) of 72 degrees (Oz) and 62 degrees (lateral derivation). The maximal adaptation depth was estimated as 72% (Oz) and 66% (lateral derivation). Estimates inferred from the population model representing human motion detectors were found to be close to those obtained by single-cell data from non-human primates.

Contrast coding and magno/parvo segregation revealed in reaction time studies

Reaction times (RTs) are obtained for a wide range of contrasts of vertical sinusoidal gratings. The data are plotted as a function of the reciprocal of contrast. In some conditions, a single linear function accounts for the data. In others a clear bi-linear function is obtained. The low and high contrast regions of the function are interpreted as representing magno and parvo activity, respectively. RT-based supra-threshold sensitivity functions are obtained for different luminances, stimulus durations and eccentricities and these are compared with conventional threshold-based sensitivities to establish the extent to which RTs and contrast sensitivity are constrained by the same sensory processes.

The influence of stimulus chromaticity on the isoluminant motion-onset VEP

Motion-onset visual evoked potentials (VEPs) were elicited by low spatial frequency chromatic isoluminant gratings presented in a central 7 degrees circular field. The chromatic composition of the stimuli was varied so as to modulate along different axes in colour space. For slow speeds (<5 degrees/s) changing the chromatic axis induced large response differences between the S- and L/M-cone VEPs. At faster speeds (5-12 degrees/s) the effects were not as marked. A dichotomy between the slow and fast responses was also shown to exist in terms of their contrast dependencies, the former exhibiting a stronger dependency on contrast than the latter. These findings suggest that neural substrates with chromatic sensitivity are involved in the generation of S- and L/M-cone mediated motion-onset VEPs at low velocities. At higher velocities, responses are generated by different mechanisms that possess little or no chromatic sensitivity.

Spatial property of motion visual evoked potentials

The cortical evoked response to drifting patterns (motion visual evoked potentials) was investigated. When the direction of motion of the stimulus pattern was reversed upward or downward at intervals, the cortical evoked response was triggered at the moment when the pattern changed direction. The polarity reversal of the main negative component occurred between upper and lower visual field stimulations as seen in pattern reversal visual evoked potentials. Our study indicates these potentials have a compound property reflecting the visual field.

Motion adaptation in chromatic motion-onset visual evoked potentials

The aim of this study was to investigate the influence of motion adaptation on visual evoked potentials (VEPs) elicited by the onset of isoluminant chromatic motion. VEPs were recorded from the occipital cortex of human subjects using a sinusoidal grating stimulus of one cycle per degree which moved at either a velocity of 2 or 10 degrees/s and subtended a field of 7 degrees with a mean luminance of 30 cdm(-2). In the first experiment the effects of adaptation were investigated via the manipulation of the stimulus duty cycle which was varied between 11-90%. The results showed a significant (p < 0.001) reduction in the N2-P2 amplitude of the chromatic response. In contrast, P1-N2 amplitude was not significantly affected by motion adaptation. Subsequent experiments demonstrated that the chromatic motion onset VEP was attenuated not only following adaptation to isoluminant chromatic motion, but also to luminance motion as well. These results indicate that the chromatic motion onset VEP is just as susceptible as its luminance counterpart to motion after effects (MAEs) and as a result it is highly likely that it is a motion specific response. Furthermore, the fact that the VEP shows that there are cross-adaptation effects between motion defined by change in colour and by change in luminance, suggests that the two types of motion stimuli have inputs into a common motion mechanism.