Effect of selective attention on the latency of human frequency-following potentials.

While effects of attention on late and middle latency components of the evoked potential have been demonstrated, similar effects on brain stem evoked potentials--in particular on the human frequency-following potential (FFP)--are controversial. The FFP is a response to tone bursts in the frequency range of human language (optimum approximately 350 Hz). It has a latency of approximately 6.3 ms and is probably generated at a site peripheral to the inferior colliculus. We present data showing that the latency of the FFP can be shortened significantly (45 microseconds) if the subject is required to attend to the evoking auditory tone burst, while the amplitude of the FFP remains unaffected. This indicates an attention-controlled influence on signal processing in the earliest parts of the auditory pathway.

Action monitoring, error detection, and the basal ganglia: an ERP study.

The error negativity (Ne or ERN) is an event-related brain potential component, which is assumed to reflect error detection. Recently it has been hypothesized that the basal ganglia are assumed to play a crucial role in error detection. In the present study we ask whether the Ne is altered in patients with Parkinson's disease (PD), who have an impaired function of the basal ganglia. We recorded the Ne in patients and in matched controls, while they performed different tasks that require a relatively high cognitive control, which is supposed to pose particular problems on PD. The Ne was in fact smaller in the patients than in the controls in all tasks. Our results suggest an impairment of error detection in PD for different types of demanding tasks. This supports the hypothesis that the basal ganglia do play an important role for error detection in action monitoring.

Perceptual latencies are shorter for motion towards the fovea than for motion away.

Subjects had to align a brief test flash with a peripheral steady reference target during ocular pursuit, or with a moving reference target during steady eye fixation. The performance was determined by the direction of movement, of the reference target image on the retina--towards or away from fovea. The data are explained by the hypothesis that the passage of a retinal locus by a moving target image is experienced faster when the image moves foveopetally, regardless of whether the passage is caused by real target movement or by ocular pursuit.

Perception of visual motion with modulated velocity: effects of viewing distance and aperture size.

Subjects observed a random dot pattern that moved horizontally with modulated velocity within an invisible aperture. The velocity contrast, (V2-V1)/V1, was 2/3. Two different percepts occurred while observing this stimulus. At lower modulation frequencies, between 2 and 12 Hz, velocity changes were clearly seen; this percept is called "motion irregularity". At frequencies higher than 20 Hz velocity changes were no longer visible; the moving pattern appeared to be divided into stationary columns of different luminance. We call this percept "pattern irregularity". The critical frequency for detection of motion irregularity was independent of viewing distance; it was an inverted U-shaped function of the linear rather than the angular mean velocity of the pattern. At higher mean velocities the critical frequency increased with increasing aperture size; at lower mean velocities it was not affected by the size of the aperture. It is shown that detection performance is a function of the relative velocity of the pattern, i.e. of the ratio between the mean velocity in deg/sec and the aperture size in deg. Pattern irregularity could be detected at modulation frequencies even above 100 Hz. The critical frequency increased with increasing velocity and with decreasing viewing distance. It is suggested that detection of motion irregularity is determined by two distinct processes that are based on spatial analysis of motion at low relative velocities and temporal analysis at high relative velocities; both processes provide constancy of detection performance regardless of viewing distance. On the other hand, pattern irregularity seems to be detected on the basis of an analysis of the retinal luminance distribution at high modulation frequencies.

The time it takes to detect changes in speed and direction of visual motion.

We studied the ability of human observers to detect abrupt changes in velocity of motion of a random dot pattern. The pattern moved horizontally for 0.9 s at velocity V0, then changed to V1 either in speed, or in direction for a time T and returned to the initial motion. The threshold duration for detection of the change was measured for initial speeds of 2, 4, 8 and 16 deg/s. The time to detect a velocity reversal was equal to that for detection of an increase in speed by a factor of three. The time to detect an abrupt cessation of motion was equal to the time for detection of an increase in speed by a factor of two. The time to detect a direction change, the speed being constant, decreased gradually with increasing angle between V0 and V1 from 12 to 180 degrees and with increasing V0; the detection time was a function of (V1-V0) almost independent of the value of V0. This finding supports the hypothesis of Dzhafarov et al. (Percept Psychophys 1993;54:373-750), that the visual system effectively reduces the detection of velocity changes (from V0 to V1) to the presumably more simple detection of a motion onset, from 0 to (V1-V0). The characteristics of the detection process in the cases of uni- and two-dimensional velocity changes are discussed.

Temporal thresholds and reaction time to changes in velocity of visual motion.

A random dot pattern moved at a velocity V1. The velocity then increased or decreased abruptly to another value V2 for some time and again returned to V1. The temporal threshold, i.e. the duration of V2 that was necessary to detect the change was measured. Thresholds for the detection of the same velocity increment, V2 = 2 x V1, were shorter when the baseline velocity V1 increased from 1 to 8 deg/sec (Expt 1). The temporal threshold decreased as the velocity contrast (V2 - V1)/(V1 + V2) increased from 0.33 to 0.77. The thresholds for the detection of velocity decrements were in general longer than those for the detection of increments (Expt 3). In Expts 2 and 4 the random-dot pattern moved with velocity V1, which abruptly increased or decreased to V2, without returning to V1. The reaction time to the change was measured for the same velocity pairs as those used in the temporal threshold measurements. There was a good correspondence between changes in the reaction times and changes in the thresholds under the various conditions. The data are interpreted on the basis of two hypotheses: higher velocities are detected by mechanisms that respond more rapidly; and integration of velocities occurs when temporally-adjacent motions are presented.

Multi-aperture viewing: perception of figures through very small apertures.

A new phenomenon of viewing objects through multiple apertures is reported. When a solid figure is hidden behind a sieve of very small apertures (pinholes), only its general shape may be perceived. Outline figures may be entirely invisible behind the sieve. Motion of the figure markedly improves the visibility of the outline figures and of the small details and edges of the solid figures. Two experiments are presented. The first demonstrates that the size of a dark stimulus moving behind a rear-illuminated row of holes is perceived with an accuracy that may be considerably better (i.e. the error is smaller) than the interhole distance. The results of the second experiment show that the visibility of an outline figure, a ring, is significantly better when the figure smoothly moves than when presented at random positions behind a two-dimensional sieve. Changing the velocity within the range of 5.18-10.36 deg/sec does not affect the visibility of the figure while it moves; however, a lower rate of discrete presentation leads to significantly better visibility. An explanation of the phenomenon is presented in terms of integration of information about the relative positions of the covered/uncovered holes, with a possible involvement of motion analyzing mechanisms.

Mechanisms of simple and choice reaction to changes in direction of visual motion.

Experiments are presented in which a random dot pattern moved vertically upwards (velocity vector V(1)) and then abruptly changed its direction of motion by the angle alpha (velocity vector V(2)), either to the left or to the right, without changing the speed. Subjects performed simple reactions to the direction change, disregarding its sign. In another experiment choice reactions to the same stimuli were performed: the subjects pushed a left button when the direction change was to the left and a right button when the change was to the right. The simple reaction time decreased monotonically with alpha increasing from 11 degrees to 169 degrees, whereas, within the same range of angles, a U-shaped curve described the function of the choice reaction time versus alpha. Both types of reaction time increased with decreasing the base speed. Difficulties are outlined which occur when the angle of change alpha is considered as 'intensity' of the stimulus. Instead, the parameter mid R:V(2)-V(1)mid R:, the absolute value of the difference between the velocity vectors before and after the change, is shown to be a meaningful 'intensity' parameter for the simple reaction task. The parameter V(2N), the speed of the velocity component normal to the initial velocity vector V(1), is suggested as an 'intensity' parameter for the choice reaction task. It is shown that the simple and choice reactions to changes in direction of visual motion are performed by two distinct mechanisms which seem to work in parallel and may be nearly equally fast for small angles of change, when mid R:V(2)-V(1)mid R: approximately V(2N).

Selective directional sensitivity in visual motion perception.

We present two experiments demonstrating that: (i) the latency of perception of the position of a small visual target moving towards the fovea is shorter than that of the same target moving away from the fovea; (ii) the reaction time (RT) to onset of motion of the same type of target is also shorter when it moves towards the fovea; and (iii) the RT to onset of motion away from the fovea may be shorter when larger, textured stimuli are employed. The relation of the findings to the existence of two systems for visual motion information processing and to recent neurophysiological findings is discussed.

The discrimination of abrupt changes in speed and direction of visual motion.

A random dot pattern that moved within an invisible aperture was used to present two motions contiguously in time. The motions differed slightly either in speed (Experiments 1 and 3) or in direction (Experiments 2 and 4) and the subject had to discriminate the sign of the change (e.g. increment or decrement). The same discrimination task was performed when the two motions were temporally separated by 1 s. In Experiments 1 and 2 discrimination thresholds were measured with motion durations of 0.125, 0.25, 0.5 and 1.0 s and mean speeds of 2, 4, 8, and 16 degrees/s. In Experiments 3 and 4 thresholds were measured with aperture widths of 5 and 20 cm. The discrimination of contiguous motions progressively deteriorated with decreasing duration and mean speed of motion. For the lowest value of duration the Weber fraction for contiguous speeds was more than three times as the Weber fractions for separate speeds. For the same low value of duration the thresholds for discrimination of direction of contiguous motions were only about 50% higher than the thresholds for separate motions. The Weber fraction for contiguous speeds was ca. three times higher with the smaller aperture than with the larger one, provided the ratio 'aperture width mean speed' (i.e. the lifetime of the moving dots) was less than 0.3 s. Aperture width did not affect the discrimination of direction of contiguous motions. The discrimination of contiguous motions is discussed together with the known data for detection of changes in speed and direction. It is suggested that both, detection of changes in speed and discrimination of the sign of speed changes, may be performed by a common visual mechanism.

Late visual and auditory ERP components and choice reaction time.

Some relations between different late positive ERP components and choice reaction time (RT) were studied. In order to identify the different components we used visual and auditory stimuli, as well as simple and choice reaction tasks, since one of the components is thought to be modality dependent and the other one task dependent. In the paradigm the stimulus modalities were mixed, which was expected to lead to a maximum dissociation of the components after auditory stimuli (Hohnsbein et al. (1991). Electroencephalography and Clinical Neurophysiology, 78, 438-446). The results demonstrated the overlap of two positive waves in choice reaction tasks: a central one (P-SR), and a parietal one (P-CR). The latency of the P-SR varied greatly across modalities, but did not vary with RT, whereas the latency of the P-CR varied strongly with RT. The different overlap of these components on fast and slow trials caused amplitude and latency variations of the "P300" and the positive slow wave. Our results suggest a relation of the P-SR with stimulus evaluation (identification), and of the P-CR with response selection (stimulus-response mapping).

Effects of attention and time-pressure on P300 subcomponents and implications for mental workload research.

Our approach to objective measures of mental workload is establishing relationships between components of the event-related brain potential (ERP) and information processing stages. These relationships can be used to infer the influence of specific workload conditions on specific processing stages. We recently showed that the ERP component P300 in choice tasks is composed of two subcomponents, P-SR and P-CR, which are time-related to stimulus-evaluation and response-selection. With these relations we could specify which processing stages were affected when certain workload conditions are varied. When attention was divided between the visual and auditory modalities compared to (unimodal) focused attention, the choice reaction time (RT) was prolonged, primarily in the auditory modality. This delay was mainly reflected in the P-CR latency, which shows that the division of attention mainly impairs the response-selection process in the auditory modality due to a bias of attention towards the visual modality. When the time-pressure was increased, the latency of the P-CR (and not of the P-SR) was shortened, but less than the choice RT. This suggests a (limited) acceleration of response-selection but not of stimulus evaluation. Since the response-selection process was accelerated less than the overt choice RT, an increase of the error rate was consequently observed. In summary we showed that increases of mental workload can induce accelerations or decelerations of specific processing stages which can be monitored by observing latency changes of the affiliated ERP components.

ERP components on reaction errors and their functional significance: a tutorial.

Some years ago we described a negative (Ne) and a later positive (Pe) deflection in the event-related brain potentials (ERPs) of incorrect choice reactions [Falkenstein, M., Hohnsbein, J., Hoormann, J., Blanke, L., 1990. In: Brunia, C.H.M., Gaillard, A.W.K., Kok, A. (Eds.), Psychophysiological Brain Research. Tilburg Univesity Press, Tilburg, pp. 192-195. Falkenstein, M., Hohnsbein, J., Hoormann, J., 1991. Electroencephalography and Clinical Neurophysiology, 78, 447-455]. Originally we assumed the Ne to represent a correlate of error detection in the sense of a mismatch signal when representations of the actual response and the required response are compared. This hypothesis was supported by the results of a variety of experiments from our own laboratory and that of Coles [Gehring, W. J., Goss, B., Coles, M.G.H., Meyer, D.E., Donchin, E., 1993. Psychological Science 4, 385-390. Bernstein, P.S., Scheffers, M.K., Coles, M.G.H., 1995. Journal of Experimental Psychology: Human Perception and Performance 21, 1312-1322. Scheffers, M.K., Coles, M. G.H., Bernstein, P., Gehring, W.J., Donchin, E., 1996. Psychophysiology 33, 42-54]. However, new data from our laboratory and that of Vidal et al. [Vidal, F., Hasbroucq, T., Bonnet, M., 1999. Biological Psychology, 2000] revealed a small negativity similar to the Ne also after correct responses. Since the above mentioned comparison process is also required after correct responses it is conceivable that the Ne reflects this comparison process itself rather than its outcome. As to the Pe, our results suggest that this is a further error-specific component, which is independent of the Ne, and hence associated with a later aspect of error processing or post-error processing. Our new results with different age groups argue against the hypotheses that the Pe reflects conscious error processing or the post-error adjustment of response strategies. Further research is necessary to specify the functional significance of the Pe.

Different error types and error processing in spatial stimulus-response-compatibility tasks: behavioural and electrophysiological data.

We tested the hypothesis that in spatial stimulus-response-compatibility (SRC) tasks two different error types occur: A noise-induced 'general error' independent of SRC and reaction time and a 'position driven error' in incompatible trials with short RT being driven by the irrelevant stimulus position. A second issue was whether error detection is different for these two types of errors, which should be reflected by differences in the error negativity (Ne), since the Ne is seen as a neural correlate of error detection. To study these issues, we used a Simon- and a spatial Stroop-task. In incompatible (vs. compatible) trials we found more errors and a below chance accuracy in fast responses. Neither the amplitude nor the latency of the Ne were significantly affected by the experimental factors. This pattern of behavioural results supports the above hypothesis of two error types in such tasks. The Ne results indicate that error detection is similar for both types of errors.

The human frequency-following response (FFR): normal variability and relation to the click-evoked brainstem response.

The frequency-following response (FFR) was recorded from twenty human subjects (11 female and 9 male) over a frequency range of 128-832 Hz in order to study the normal variability of this evoked potential and its dependence on age and sex. Moreover the relation of the FFR to the click-evoked brain stem response (BER) was analyzed in order to contribute to the FFR source discussion. The FFR had a maximum amplitude of about 400 nV and a latency of about 6.4 ms for stimulus frequencies around 350 Hz; the inter-individual variance of the best frequency and of the shape of the frequency function was considerable. Large second harmonics were seen in the FFR to stimuli below about 200 Hz. The FFR amplitude tended to be larger in younger subjects, whereas no such effect was found for the BER. No significant sex effect was found for the FFR amplitude, whereas the BER waves IV and VI were larger for females than for males. There were no correlations between FFR and BER latencies. Significant correlations were found between the amplitudes of the FFR and BER components II, III and IV, but not of waves V and VI. The results support the notion that the FFR and the BER reflect different mechanisms. Moreover the results do not favor the common hypothesis that the inferior colliculus is the major source of the scalp-recorded human FFR, but rather point to lower brainstem levels.

ERP components in Go/Nogo tasks and their relation to inhibition.

In visual Go/Nogo tasks the ERP usually shows a frontal negativity after Nogo stimuli ("Nogo-N2"), which possibly reflects an inhibition process. However, the Nogo-N2 appears to be very small after auditory stimuli, which is evidence against the inhibition hypothesis. In the present study we tested this hypothesis by evaluating performance differences between subjects. Assuming that for Ss with a high false alarm rate the inhibition process is weakened and/or delayed, they should reveal a smaller and/or later Nogo-N2 than Ss with a low false alarm rate. This prediction was confirmed, which supports the inhibition hypothesis. However, the Nogo-N2 was again much smaller and had a different topography after auditory than after visual stimuli despite similar performance in both modalities. This modality asymmetry was explained by assuming that the inhibitory mechanism reflected in the Nogo-N2 is located at a pre-motor rather than at the motor level. In the second part of the study we compared the Nogo-N2 with a similar phenomenon, the error negativity (Ne), which occurs in trials with commission errors (false alarms). Earlier work suggests that the Ne is a correlate of error detection or inhibition. This raises the possibility that the Ne is a delayed Nogo-N2, i.e., the Ne may reflect a late and hence unsuccessful attempt to inhibit the response after a nontarget. However, the Ne amplitude showed no difference between performance groups and stimulus modalities, as found for the Nogo-N2. Moreover, Ne and Nogo-N2 had different scalp topographies. This suggests that different mechanisms and generators underlie the Ne and the Nogo-N2.

Error processing in patients with Parkinson's disease: the influence of medication state.

One of the hallmarks of Parkinson's disease (PD) is a depletion of dopamine. Error processing, as reflected in a component of the event-related potential, the so-called error (related) negativity (Ne or ERN) is likely dependent on the midbrain dopaminergic system. In case of an unfavourable event such as an error, this system is assumed to send an error signal to the mediofrontal cortex, which elicits the Ne. Hence, the Ne should be altered in patients with PD. In fact, we earlier found a reduction of the Ne in medicated patients with PD in different tasks while another group found no such reduction in "off-medication" patients in a flanker task. In the present study, we reinvestigated this issue by measuring the Ne in a large group of treated PD patients in the "on"- and "off"-parkinsonian medication state and in matched control subjects in a flanker task. The Ne was found to be the same in the "on-medication" and "off-medication" state, while the motor score in the Unified Parkinson's Disease Rating Scale was different. In both medication states the Ne was smaller in the patients than in the controls. The results show that the Ne reduction found earlier is unaffected by short-term differences in parkinsonian medication. The question remains open whether the long-term medication could have contributed to the Ne reduction.

Effects of stimulus-response compatibility in Parkinson's disease: a psychophysiological analysis.

The present study investigated the mechanisms underlying stimulus-response compatibility effects in Parkinson's disease patients and matched controls. Since basal ganglia are involved in the selection and inhibition of competing responses we examined whether basal ganglia dysfunction in Parkinson's disease leads to greater interference effects compared to the control subjects. Reaction times and lateralized movement-related cortical potentials (lateralized readiness potential: LRP) were recorded in two modified Eriksen flanker tasks. Both groups were influenced by compatibility conditions; interference was seen as enhanced reaction time and error rate, as well as incorrect early LRP and delayed late LRP in incongruent trials. Altogether, behavioral and electrophysiological measures showed the interference to be rather smaller for the patients than for the controls. In contrast, facilitation did not differ among groups. Hence the claim that Parkinson's disease patients are more influenced than controls by interfering directional stimuli appears not always valid.