Sven P. Heinrich

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.

Adaptation dynamics in pattern-reversal visual evoked potentials

Recording a VEP usually involves prolonged repetitions of the stimulus, but the influence of adaptation is rarely discussed in this context. Two experiments were performed. In Experiment 1 the time course of the response amplitude during steady-state stimulation was assessed. During the first seconds of stimulation we found an increase in amplitude, followed by a continuous exponential decline. This confirmed earlier results. There is considerable inter-subject variability concerning all aspects of the time course in our 19 subjects. Experiment 2 used two types of transient pattern reversal stimuli: one regular stimulus as used in standard clinical applications and one with a pause in between each reversal. N1 and P1 amplitudes did not show significant differential effects. N2 amplitude was reduced by 73% in the standard condition whereas P1 peak time increased slightly but significantly (3.2 ms).

Electrophysiological evidence for independent speed channels in human motion processing.

A variety of psychophysical studies suggests that motion perception in humans is mediated by at least two speed-tuned channels. To study the neurophysiological underpinnings of these channels in the human visual cortex, we recorded visual evoked potentials (VEPs) to motion onset. We applied an adaptation paradigm that allowed us (a) to isolate and extract direction-specific cortical responses and (b) to assess cross-adaptation in the speed domain. VEPs resulting from the onset of left- or rightward motion at either low or high speeds were recorded from three occipital recording sites in 11 subjects. For each of these test stimuli, responses were collected after adaptation to one of five different conditions: a static adaptation pattern (baseline), adaptation to low-speed motion (3.5 degrees/s) either in the same or in the opposite direction as the test, or adaptation to high-speed motion (32 degrees/s) either in the same or in the opposite direction as the test. We report considerable direction-specific adaptation for same adaptation and test speeds (by 28-37% of baseline response; p <.002), whereas there was no direction-specific adaptation across speeds. We supplement these electrophysiological data with corresponding psychophysical results. The lack of direction-specific cross-adaptation in the speed domain demonstrated with physiological and psychophysical techniques supports models of at least two speed-tuned channels in the human motion system.

Attention and visual texture segregation

Visual texture segregation is believed to be performed preattentively. Recent evidence, however, suggests that attention does play an important role. Using visual evoked potentials (VEPs), we investigated the effect of different tasks on texture segregation. Stimuli consisted of Gabor-filtered binary noise patterns. In segregated stimuli, local texture orientation contrasts defined global checkerboard patterns. VEP responses specific to texture segregation were obtained by computing the difference between VEPs to homogeneous and segregated stimuli. Four conditions were examined that required attending either the global pattern, the local structure, random numbers displayed on the screen, or a series of tones. Responses specific to texture segregation were dominated by two occipital negativities peaking around 110 and 230 ms. The earlier one was not affected by the task, whereas the later one was completely abolished when the subjects attended to either numbers or tones (p = .0005 and p = .006, respectively). The results suggest that early stages of texture segregation are not affected by attention, whereas task relevance is crucial for later processes. The timing is compatible with a recurrent processing pattern with initial bottom-up processing of basic stimulus characteristics and a subsequent top-down flow of higher level modulatory information. As attention effects occur across modalities, they cannot be simply explained by competition within the visual cortex.

Adaptation characteristics of steady-state motion visual evoked potentials

OBJECTIVE Motion visual evoked potentials (motion VEPs) are used in clinical diagnosis and basic research. Employing steady-state rather than the usual transient motion VEPs simplifies statistical evaluation and might drastically reduce examination durations. Protocols for recording transient motion-onset VEPs usually involve fairly long recovery intervals between trials to avoid neural adaptation. This is not feasible for steady-state VEPs. We investigated how adaptation affects the steady-state motion VEP. METHODS Oscillatory (13.3rev/s) and continuous uni-directional random-dot motion served as adaptation stimuli. Steady-state motion VEPs and, for comparison, transient motion VEPs were recorded. RESULTS In the first experiment, we investigated how adaptation affects the recordings. Contrary to our expectation, we did not find any sizable effect. However, there was a large inter-individual variability in steady-state amplitude and no correlation across subjects between transient and steady-state amplitude. In the second experiment, we confirmed that the steady-state VEP reflects veridical motion processing by assessing its susceptibility to uni-directional pre-adaptation. CONCLUSIONS Taken together, the results suggest that steady-state motion VEPs provide a fast method of recording motion responses without suffering from adaptation, but at the expense of inter-individual reproducibility.

Some thoughts on the interpretation of steady-state evoked potentials

Steady-state evoked potentials are popular due to their easy analysis in frequency space and the availability of methods for objective response detection. However, the interpretation of steady-state responses can be challenging due to their origin as a sequence of responses to single stimuli. In the present paper, issues of signal extinction and some characteristics of higher harmonics are illustrated based on simple model data for those readers who do not regularly hobnob with frequency-space representations of data. It is important to realize that the absence of a steady-state response does not prove the lack of neural activity. For the same underlying reasons, namely constructive and destructive superposition of individual responses, comparisons of amplitudes between experimental conditions are prone to inaccuracies. Thus, before inferring physiology from steady-state responses, one should consider an alternative explanation in terms of signal composition.

Frequency-domain analysis of fast oddball responses to visual stimuli: a feasibility study.

Event-related potential responses to oddball stimuli, including the P300 component, have been proposed as a diagnostic tool for discerning psychiatric or higher-level neural disorders from malingering, for instance in cases of unexplained visual loss. For clinical use, short recording durations and easy statistical assessment are highly desirable. With this aim, we investigated the feasibility of recording oddball responses in a fast steady-state regime. We used gratings with two possible orientations in a rapid oddball paradigm with an inter-stimulus interval of 214 ms. Six consecutive presentations of one stimulus type (frequent) were followed by a single presentation of the other (infrequent) stimulus type. Subjects were attending to the rare stimulus type. The electroencephalographic recordings were analyzed in the frequency domain. All subjects produced significant harmonic responses related to the processing of the rare stimulus, demonstrating the feasibility of the technique, with the potential of reducing recording times substantially compared to conventional slow stimulation. We furthermore found that the regularity of the occurrence of infrequent stimuli, which is necessary for frequency-domain analysis, does not per se reduce the P300 responses, as would have been expected in the framework of some hypotheses regarding the role of the P300.

Signal and noise in P300 recordings to visual stimuli

The P300 of the event-related potential is typically obtained for infrequent target stimuli that are embedded in a sequence of frequent irrelevant stimuli. The P300 has been suggested as a marker of high-level cognitive processing and might be useful in ophthalmology to confirm the diagnosis of a functional disorder. However, typical P300 measurements require relatively lengthy recording sessions. It would therefore be desirable to minimize the required time and to maximize the signal-to-noise ratio by finding the optimal balance between parameters such as stimulus probability and the number of target trials or the recording time. This is different from previous studies, which assessed the amplitude only. We recorded event-related potentials to visual stimuli using standard oddball paradigms with various target frequencies ranging from 2:1 (target majority) to 1:16 (massive non-target majority). We compared the signal-to-noise ratios for a fixed number of target trials as well as for a fixed total recording time and assessed effects of the immediate stimulus history. As expected, P300 amplitudes depend strongly on target infrequency. This did not reach saturation within the range tested. For a given number of target trials, the signal-to-noise ratio also increases with target infrequency. For a given recording duration, the signal-to-noise ratio is optimal around 1:8. In the 1:4 condition, the signal-to-noise ratio can be improved by excluding trials that were preceded by a target trial.

Treatment of optic neuritis with erythropoietin (TONE): a randomised, double-blind, placebo-controlled trial—study protocol

Introduction Optic neuritis leads to degeneration of retinal ganglion cells whose axons form the optic nerve. The standard treatment is a methylprednisolone pulse therapy. This treatment slightly shortens the time of recovery but does not prevent neurodegeneration and persistent visual impairment. In a phase II trial performed in preparation of this study, we have shown that erythropoietin protects global retinal nerve fibre layer thickness (RNFLT-G) in acute optic neuritis; however, the preparatory trial was not powered to show effects on visual function. Methods and analysis Treatment of Optic Neuritis with Erythropoietin (TONE) is a national, randomised, double-blind, placebo-controlled, multicentre trial with two parallel arms. The primary objective is to determine the efficacy of erythropoietin compared to placebo given add-on to methylprednisolone as assessed by measurements of RNFLT-G and low-contrast visual acuity in the affected eye 6 months after randomisation. Inclusion criteria are a first episode of optic neuritis with decreased visual acuity to ≤0.5 (decimal system) and an onset of symptoms within 10 days prior to inclusion. The most important exclusion criteria are history of optic neuritis or multiple sclerosis or any ocular disease (affected or non-affected eye), significant hyperopia, myopia or astigmatism, elevated blood pressure, thrombotic events or malignancy. After randomisation, patients either receive 33 000 international units human recombinant erythropoietin intravenously for 3 consecutive days or placebo (0.9% saline) administered intravenously. With an estimated power of 80%, the calculated sample size is 100 patients. The trial started in September 2014 with a planned recruitment period of 30 months. Ethics and dissemination TONE has been approved by the Central Ethics Commission in Freiburg (194/14) and the German Federal Institute for Drugs and Medical Devices (61-3910-4039831). It complies with the Declaration of Helsinki, local laws and ICH-GCP. Trial registration number NCT01962571.

Oblique effects beyond low-level visual processing.

A number of studies have demonstrated a reduction in neural activity for oblique gratings as compared to horizontal or vertical gratings. This has been associated with the psychophysical oblique effect. Using event-related potentials, we now assessed the neural activity associated with the processing of higher-order stimuli of different orientations. We applied a novel stimulus paradigm that is particularly suited to investigate mid- and high-level vision by obviating low-level responses. It consisted of a line grid that emerged perspicuously from a continuous movement of stimulus elements without any temporal discontinuances in stimulus presentation. This Gestalt could be oriented along the cardinal axes or rotated by 45 degrees . We obtained distinct event-related potentials with a moderate task-dependence. They showed a correlate of Gestalt processing that did not depend on the orientation, followed by a P300-like component that was 50% larger for the 45 degrees Gestalt. Surprisingly, this oblique effect is opposite to previous studies using gratings. We propose that it originated from a bias in neural processing, induced by the long-term environmental experience of the subjects.