The human motion onset VEP as a function of stimulation area for foveal and peripheral vision

An investigation into the relationship between stimulus property, neural response and its manifestation in the visual evoked potential involving retinal resolution

The visual evoked potential (VEP) has been shown to reflect the size of the neural population activated by a processing mechanism selective to the temporal - and spatial luminance contrast property of a stimulus. We set out to better understand how the factors determining the neural response associated with these mechanisms. To do so we recorded the VEP from 14 healthy volunteers viewing two series of pattern reversing stimuli with identical temporal-and spatial luminance contrast properties. In one series the size of the elements increased towards the edge of the image, in the other it decreased. In the former element size was congruent with receptive field size across eccentricity, in the later it was incongruent. P100 amplitude to the incongruent series exceeded that obtained to the congruent series. Using electric dipoles due the excitatory neural response we accounted for this using dipole cancellation of electric dipoles of opposite polarity originating in supra- and infragranular layers of V1. The phasic neural response in granular lamina of V1 exhibited magnocellular characteristics, the neural response outside of the granular lamina exhibited parvocellular characteristics and was modulated by re-entrant projections. Using electric current density, we identified areas of the dorsal followed by areas of the ventral stream as the source of the re-entrant signal modulating infragranular activity. Our work demonstrates that the VEP does not signal reflect the overall level of a neural response but is the result of an interaction between electric dipoles originating from neural responses in different lamina of V1.

A brain computer interface based on motion-onset VEPs

In this article, a novel brain-computer interface (BCI) based on motion-onset visual evoked potentials (mVEP) is proposed. Examination on the spatio-temporal pattern of motion-onset VEPs showed that the amplitude of N2 and P2 components of mVEP, evoked by attended target, was significantly higher than that by unattended ones. The area of N2 and P2 component was used as features for classifying the offline data of a five-class BCI, achieving an average accuracy of 98.33% in five subjects.

Cortical processing of visual motion in young infants

High-density EEG was used to investigate the cortical processing of a rotating visual pattern in 2-, 3-, and 5-month-old infants and in adults. Motion induced ERP in the parietal and the temporal-occipital border regions (OT) was elicited at all ages. The ERP was discernable in the 2-months-olds, significant and unilateral in the 3-month-olds and significantly bilateral in the 5-month-olds and adults. The motion induced ERP in the primary visual area was absent in the 2-month-olds and later than in the OT area for the 3-month-olds indicating that information to OT may be supplied by the V1 bypass at these ages. The results are in agreement with behavioural and psychophysical data in infants.

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.

Motion-onset VEPs: Characteristics, methods, and diagnostic use

This review article summarises the research on the motion-onset visual evoked potentials (VEPs) and important motion stimulus parameters which have been clarified. For activation of the visual motion processing system and evocation of the motion-onset specific N2 peak (with latency of 160-200ms) from the extra-striate temporo-occipital and/or parietal cortex, the following stimulus parameters can be recently recommended: low luminance (<ca. 20cd/m(2)) and low contrast (<ca. 10%-sinusoidally modulated) of a moving structure with low velocity and temporal frequency (<ca. 6Hz). A short (up to 200ms) duration of motion and a long (at least 1s) inter-stimulus interval reduce adaptation to motion and predominance of a pattern-related P1 peak. Radial motion (with increasing velocity and decreasing spatial frequency towards the periphery) produces larger reactions as compared to a unidirectional translation. In view of the slow maturation (up to the age of 18 years) and early ageing of the visual motion processing system, the use of age-dependent latency norms may be necessary. Since early or selective involvement of the motion processing system is suspected in some CNS disorders, we suggest an evaluation of the utility of motion-onset VEPs as part of the electrophysiological CNS examination since this method may recognise motion processing involvement better than other methods. Motion-onset VEPs might increase the sensitivity of this examination for diagnosing CNS diseases including Multiple Sclerosis, Neuroborreliosis, Glaucoma, Dyslexia and Encephalopathies.

Motion-onset VEPs to translating, radial, rotating and spiral stimuli

Motion-onset related visual evoked potentials (M-VEPs) were recorded as a result of the three basic (translating, radial and rotating) and one complex (spiral) motion stimulations in five subjects. Low contrast, retinotopically scaled patterns evoked potentials with major motion-onset specific negativity N160 with maximum in the parieto-temporal region. All multidirectional motion stimuli elicited the motion-onset response of significantly higher amplitude and shorter latency compared to the translating (unidirectional) motion. The rotation-onset evoked potentials had significantly shorter latencies than the rest of explored stimuli. The most stable responses with the largest N160 amplitude were recorded to the radial motion. After masking of the central 20 degrees of the visual field these motion-onset VEPs were acquired without statistically significant amplitude drop. The efficiency and usefulness of the radial stimulus is presented in two clinical cases.

Effect of stimulus localisation on motion-onset VEP

Reliable motion-onset visual evoked potentials (result of the dorsal stream activation) were recorded to motion stimuli with the temporal frequency of five cycles per seconds in 20 different locations with eccentricity up to 42 degrees to periphery of the visual field. Amplitudes and latencies of the positive-negative-positive (P1-N1-P2; 84-144-208 ms) complex were evaluated in occipital (OZ and two derivations 5 cm to the left and right from OZ) and central region (CZ) in 10 subjects. We observed: (1) Shortening of the N1 latency toward periphery of the visual field. (2) The N1 amplitude maximum and latency minimum moved from occipital into central region (CZ derivation) as stimulus moved from centre toward periphery of visual field. (3) The P1 and N1 peaks displayed significantly greater amplitudes and shorter latencies when the lower part of the visual field was stimulated. (4) The N1 peak changed lateralisation of its maximum amplitude in dependence on the eccentricity. Up to 17 degrees, it corresponds to striate projection of the "optic radiation" whilst more in periphery, there was paradoxical lateralisation of higher amplitude and shorter latency. The retinotopic dependence shows that the motion response includes position information and that the motion-onset VEPs are not generated solely in the higher extrastriate areas (MT or MST).

Automatic detection of motion direction changes in the human brain

The possibility that the visual system is able to register unattended changes is still debated in the literature. However, it is difficult to understand how a sensory system becomes aware of unexpected salient changes in the environment if attention is required for detecting them. The ability to automatically detect unusual changes in the sensory environment is an adaptive function which has been confirmed in other sensory modalities (i.e. audition). This deviance detector mechanism has proven to be based on a preattentive nonrefractory memory-comparison process. To investigate whether such automatic change detection mechanism exists in the human visual system, we recorded event-related potentials to sudden changes in a biologically important feature, motion direction. Unattended sinusoidal gratings varying in motion direction in the peripheral field were presented while subjects performed a central task with two levels of difficulty. We found a larger negative displacement in the electrophysiological response elicited by less frequent stimuli (deviant) at posterior scalp locations. Within the latency range of the visual evoked component N2, this differential response was elicited independently of the direction of motion and processing load. Moreover, the results showed that the negativity elicited by deviants was not related to a differential refractory state between the electrophysiological responses to frequent and infrequent directions of motion, and that it was restricted to scalp locations related to motion processing areas. The present results suggest that a change-detection mechanism sensitive to unattended changes in motion direction may exist in the human visual system.

Perception of apparent motion in depth: a high-density electrical mapping study in humans

We evaluated brain activity using 64-channel visual evoked potentials (VEPs) while subjects perceived apparent motion in depth. Checkerboard patterns (CBPs) within small and large circles were presented in turn as experimental conditions. Motion in depth was perceived when the CBP in the large circle was coarser than in the small circle; when coarseness did not change, no motion in depth was perceived. As control conditions only fine or coarse CBPs were presented. We used ANOVA to compare VEPs associated with experimental vs. control conditions and with coarse vs. fine CBPs. Negative potentials at a latency near 190 ms showed statistically significant interactions between these comparisons in the right lateral occipital and posterior parietal areas when apparent motion in depth was perceived. This suggests that higher tiers of the dorsal stream mediate this motion perception.

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.

Visual evoked potentials elicited by chromatic motion onset

Visually Evoked Potentials (VEPs) were recorded in response to the onset of chromatic and luminance motion gratings of 1 cpd and luminance 40 cd m(-2) subtending a 7 degrees field. At slow speeds (< or =2 cycles s(-1)) the motion onset response exhibits a clear amplitude minimum at isoluminance. Over the Michelson contrast range tested (0.05-0.75) the chromatic response at 2 cycles s(-1) possesses a linear response function compared to the saturating function of the luminance response and the contrast dependency of the former is a factor of 5-6 times greater than for the latter. These differences are suggestive of different neural substrates for the chromatic and luminance motion VEPs at slow speeds. At 10 cycles s(-1) the chromatic motion onset VEP exhibits no amplitude minimum at isoluminance and becomes more like its luminance counterpart in terms of its saturating contrast response function. Furthermore, the contrast dependency of the chromatic and luminance responses differs by only a factor of 1.6 at this faster rate. These findings are consistent with the idea of separate motion mechanisms that operate at fast and slow speeds, the latter having separate channels for colour and luminance motion.

Attentional shifts between surfaces: effects on detection and early brain potentials

Two consecutive events transforming the same illusory surface in transparent motion (brief changes in direction) can be discriminated with ease, but a prolonged interference ( approximately 500 ms) on the discrimination of the second event arises when different surfaces are concerned [Valdes-Sosa, M., Cobo, A., & Pinilla, T. (2000). Attention to object files defined by transparent motion. Journal of Experimental Psychology: Human Perception and Performance, 26(2), 488-505]. Here we further characterise this phenomenon and compare it to the attentional blink AB [Shapiro, K.L., Raymond, J.E., & Arnell, K.M. (1994). Attention to visual pattern information produces the attentional blink in RSVP. Journal of Experimental Psychology: Human Perception and Performance, 20, 357-371]. Similar to the AB, reduced sensitivity (d') was found in the two-surface condition. However, the two-surface cost was associated with a reduced N1 brain response in contrast to reports for AB [Vogel, E.K., Luck, S.J., & Shapiro, K. (1998). Electrophysiological evidence for a postperceptual locus of suppression during the attentional blink. Journal of Experimental Psychology: Human Perception and Performance, 24(6), 1656-1674]. The results from this study indicate that the two-surface cost corresponds to competitive effects in early vision. Reasons for the discrepancy with the AB study are considered.

The development of hemispheric asymmetry in human motion VEPs

In six healthy adults we examined the sources underlying P1 and N2 of the motion VEP. For this purpose was acquired, in addition to the VEP, MRI images and patterns of regional cerebral blood flow with SPECT for three of the subjects. With the same motion stimulus we also examined the spatial distribution of N2 in children. In both adults and children left and right half-field responses were compared. It was found that N2 is generated by extrastriate activity and that motion stimuli are not equivalently processed in the two cerebral hemispheres. In adults, N2 dominates in one hemisphere irrespective of the visual half-field used for stimulation whereas children show an ipsilateral maximum for N2 upon half-field presentation.

Visual evoked potentials related to motion-onset are modulated by attention

The effects of attention on visual evoked potentials triggered by motion-onset were examined in four experiments. A set of randomly oriented bars was used as stimuli. The first experiment showed that responses to motion-onset following pattern-onset by less than 300 ms were suppressed. In the other three experiments, the amplitude of N170 was reduced when attention was drawn away from the moving elements and towards spatially interspersed bars that remained static. The superposition of the two sets made spatial selection unlikely. These results support the existence of an attentional 'motion filter' (separating stationary from moving elements) that can operate at early stages of visual processing.

Visually evoked potentials evoked by moving unidimensional noise stimuli: effects of contrast, spatial frequency, active electrode location, reference electrode location, and stimulus type

We determined the relative importance of electrode derivation, stimulus type, spatial frequency and contrast in determining the relative size of the late negative and early positive responses of motion elicited VEPs. Seven subjects aged 22-48 years with normal vision were tested binocularly. Motion onset and motion reversal were employed as modes of stimulus presentation. For both, pseudo-random one-dimensional noise patterns whose peak power was at 5.2, 2.6, 1.3, 0.325 and 0.1625 cycles per degree (cpd) were stimuli. Contrasts were 70% and 5%. Active electrodes were placed at Oz, 5 cm to the left of Oz, 5 cm to the right of Oz and a frontal midline position (Fpz) and referenced to linked mastoids. Transient motion reversal elicited a prominent positive response present in all subjects and at low contrasts. Motion onset VEPs have a complex waveform which may be either predominantly positive or negative. The most important variables in determining whether a prominent positivity or negativity is present in the motion onset VEP are the contrast and the spatial frequencies. Data such as these are first efforts in developing recommendations for the motion VEP.

Time course of motion adaptation: motion-onset visual evoked potentials and subjective estimates

The aim of this study was to quantitatively describe the dynamics of adaptation to visual motion with electrophysiological and psychophysical methods in man. We recorded visual evoked potentials (VEPs) to motion onset of random dot patterns from occipital and occipito-temporal electrodes during a succession of adaptation-recovery sequences. In these sequences the test stimulus was used to set the adaptation level: seven trials with 70% motion duty cycle (adaptation) followed by seven trials of 7% motion duty cycle (recovery). In a similar paradigm we determined the length of the perceptual motion after-effect to obtain a psychophysical measure of the time course of motion adaptation. Our results show a highly significant reduction of the N2 amplitude in the maximally compared to the minimally adapted condition (P < 0.001). Electrophysiological and psychophysical results both indicate that adaptation to visual motion is faster than recovery: The data were fit with an exponential model yielding adaptation and recovery time constants, respectively, of 2.5 and 10.2 s for the N2 amplitude (occipito temporal derivation) and of 7.7 and 16.7 s for the perceptual motion after-effect. Implications for the design of motion stimuli are discussed, e.g. a motion stimulus moving 10% of the time may lead to about 30% motion adaptation.

Switching attention without shifting the spotlight object-based attentional modulation of brain potentials

Although psychophysical evidence for object-based attention has been reported, corresponding studies with event-related potentials (ERPs) are scarce. Here subjects were presented with perceptual fields containing two superimposed objects (transparent surfaces generated by two sets of dots in rigid rotation around fixation, each set of a different color and direction of motion) or only one object (the same dots but either at rest or all rotating in the same direction). Brief (150-msec) rectilinear displacements affected either of the sets at random ISIs of 350 to 550 msec. Attention was directed to one set of dots, guided by color, in order to discriminate the direction of their displacement. Motion-onset ERPs elicited by these displacements were compared for attended and unattended dots. When the perceptual field consisted of two objects, strong suppression of P1 and N1 was obtained in the ERPs associated with the unattended object. No suppression was found with the field containing a single object, although an enhanced selection negativity was found in ERPs associated with attended dots (selected by color). Since the two objects occupied the same region of visual space, the suppression of P1/N1 cannot be explained by the space-based mechanisms but is consistent with object-based attentional selection at early stages of vision. The results highlight the role of perceptual organizations in enabling alternative attentional mechanisms.

Contrast dependency of motion-onset and pattern-reversal VEPs: interaction of stimulus type, recording site and response component

We compared the contrast dependency (from 0.4 to 98%) of the visual evoked potential (VEP) to motion onset and to pattern reversal at an occipital and lateral recording site using sinewave grating stimuli of 0.9 c/deg, drifting at 4.9 deg/sec. Two differing VEP components were identified: a positive component, peaking at around 130 msec, dominating the occipital derivation, enhanced in pattern-reversal stimulation, a high-threshold, late-saturating contrast response characteristic with a half-amplitude contrast above 7%; and a negative component at around 180 msec, dominating the lateral derivation, enhanced in motion-onset stimulation, exhibiting a low-threshold, saturating contrast characteristic with a half-amplitude contrast below 4%. The results suggest: (1) The negative component (N180) represents motion mechanisms, located more laterally, while the positive component (P100-P130) represents form-processing mechanisms, located near the V1/V2 areas. (2) A pattern-reversal stimulus triggers both form-processing and motion mechanisms that can be discriminated by latency. In an occipital derivation, the clinical reversal VEP P100 will be little contaminated by motion responses.

Visual evoked potentials elicited by a moving unidimensional noise pattern

(1) Motion onset and offset visual evoked potentials (VEPs) were recorded in normal human subjects using a unidimensional noise pattern moving at 1, 8 and 64 degrees/s. The maximum N1-P1 amplitude of the motion onset response was obtained when using a fine noise pattern (maximum energy at 5.2 cpd) moving at 8 degrees/s. (2) At a velocity of 8 degrees/s, the motion onset response (fine pattern, 0.70 contrast) showed a morphology similar to the pattern disappearance response. Both at a lower (1 degrees/s) and a higher velocity (64 degrees/s) the N1-P1 amplitude of the motion onset complex was significantly reduced. The latency of the motion onset response (8 degrees/s) and the pattern disappearance complex were significantly different. (3) The effect of lowering the spatial content of the noise pattern on the amplitude of the motion onset response was different for the 3 velocities tested: the largest effect was at the lower velocity of 1 degrees/s; there was no similar effect on the pattern disappearance response. (4) With decreasing contrast, the N1-P1 amplitude of the motion onset response at 8 degrees/s decreased, but this reduction in amplitude was much less than that of the disappearance response. The contrast dependency of the motion onset complex was identical for binocular and monocular recordings. (5) Increasing the motion duration or the duration of the interstimulus interval did not alter the general morphology of the motion response.

Isolation and characteristics of a steady-state visually-evoked potential in humans related to the motion of a stimulus

We have examined the visual potential evoked by two motion stimuli. In the first stimulus (termed coherent motion) a random-dot pattern oscillated between phases of coherent and incoherent ("snowstorm") motion, and in the second a random-dot pattern alternated in direction of motion (termed direction change). We found that the response to the coherent motion stimulus is low-pass with respect to speed, has low contrast sensitivity and increases steadily with the contrast of the stimuli. The direction change visually-evoked potential (VEP) is band-pass with respect to speed, has high contrast sensitivity but then saturates and even reduces as the stimulus contrast is raised above 0.1. The behaviour of the direction change VEP is similar in nature to results from psychophysical experiments of motion perception and to the known properties of directionally selective cells of the cortex. On the other hand the behaviour of the coherent motion VEP suggests this may not be mediated by a mechanism specific to motion.