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.

Improved amplitude evaluation of the frequency following response (FFR).

Three methods were combined to improve the evaluation of the amplitude of the frequency following response (FFR) in unfavorable signal-to-noise conditions. The first step was the application of a time-multiplex averager to reduce noise variance between amplitude values to be compared. The second step was the evaluation of amplitude in the frequency domain by means of a digital triangle filter. The third step was the correction of raw FFR amplitudes by an algorithm that takes into account several noise values.

Effects of crossmodal divided attention on late ERP components. I. Simple and choice reaction tasks.

We studied several effects of dividing attention between visual and acoustic inputs on different processing stages. Simple and choice responses were required to single letter stimuli. RTs and P300 latencies were delayed for divided attention (variable stimulus modality) as compared to focused attention (constant stimulus modality). In all but one condition, RT and P300 delays were similar. The exception was choice tasks to auditory stimuli, in which the RT delay was far larger than the P300 delay. Since the amplitude of the late ERP was larger in choice tasks than in simple tasks, the differences between the ERPs of choice and simple tasks were computed. They revealed that an additional late positive wave ("P-CR") occurred in all choice ERPs. In the divided attention condition the auditory (but not the visual) P-CR showed a longer delay compared to focused attention. We interpret the P-CR to be time-related to the response selection process. Our results suggest that the division of attention causes a slight impairment of stimulus evaluation (shown in P300 latency) and, after auditory stimuli only, a strong impairment of response selection (shown in P-CR latency). We therefore conclude that the observed RT effects are due to a bias of processing resources towards the visual modality, which mainly affects response selection. The results are in accordance with the theory of visual dominance.

Effects of crossmodal divided attention on late ERP components. II. Error processing in choice reaction tasks.

Reaction times and event-related potentials in correct and incorrect trials were studied in a bimanual choice reaction task. In a focused attention (FA) condition, the stimulus modality was constant (visual or auditory); in a divided attention (DA) condition, the modality was varied at random from trial to trial. Stimulus- and response-triggered averages were computed from the midline EEG leads. In error trials, the ERP amplitude was reduced in the P300 range (300-500 msec) and enhanced in the slow wave range (500-700 msec) compared to correct reaction trials. Difference plots between the ERPs (incorrect minus correct reaction trials) revealed a large fronto-central negativity ("NE") and a parieto-occipital "slow wave." These components appeared larger in the response-triggered averages. We believe that they reflect two different stages of error processing. After auditory stimuli the NE peaked much later for DA than for FA, which supports the idea of an asymmetrical allocation of processing resources to the disadvantage of the auditory modality in our DA condition.