title: Effects of task difficulty on evoked gamma activity and ERPs in a visual discrimination task.
abstract: OBJECTIVE
  The present study examined oscillatory brain activity of the EEG gamma band and event-related potentials (ERPs) with relation to the difficulty of a visual discrimination task.
  
  
  METHODS
  Three tasks with identical stimulus material were performed by 9 healthy subjects. The tasks comprised a passive control task, and an easy and a hard visual discrimination task, requiring discrimination of the color of circles. EEG was recorded from 26 electrodes. A wavelet transform based on Morlet wavelets was employed for the analysis of gamma activity.
  
  
  RESULTS
  Evoked EEG gamma activity was enhanced by both discrimination tasks as compared to the passive control task. Within the two discrimination tasks, the latency of the evoked gamma peak was delayed for the harder task. Higher amplitudes of the ERP components N170 and P300 were found in both discrimination tasks as compared to the passive task. The N2b, which showed a maximum activation at about 260 ms, was increased in the hard discrimination task as compared to the easy discrimination task.
  
  
  CONCLUSIONS
  Our results indicate that early evoked gamma activity and N2b are related to the difficulty of visual discrimination processes. A delayed gamma activity in the hard task indicated a longer duration of stimulus processing, whereas the amplitude of the N2b directly indicates the level of task difficulty.
authors.affiliation: Max-Planck-Institute of Cognitive Neuroscience, Postfach 500 355, 04303 Leipzig, Germany.
authors.forename: Daniel
authors.initials: D
authors.lastname: Senkowski
authors.forename: Christoph S
authors.initials: CS
authors.lastname: Herrmann
mesh_terms: D000328:Adult; D004192:Discrimination, Psychological / Q000502:physiology*; D004569:Electroencephalography; D005071:Evoked Potentials / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D010775:Photic Stimulation; D011597:Psychomotor Performance / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D014796:Visual Perception / Q000502:physiology*