title: Spatial luminance contrast sensitivity measured with transient VEP: comparison with psychophysics and evidence of multiple mechanisms.
abstract: OBJECTIVE
  To compare the spatial luminance contrast sensitivity function (CSF) obtained with transient visual evoked potentials (VEPs) with that obtained with psychophysical measurements.
  
  
  METHODS
  The stimuli consisted of horizontal luminance gratings. In the VEP experiments, 0.4, 0.8, 2, 4, 8, and 10 cpd of spatial frequency were used, at 1 Hz square-wave contrast-reversal mode. Eight to 10 Michelson contrasts were used at each spatial frequency. Contrast thresholds were estimated from extrapolation of contrast response functions. Psychophysical sensitivities were obtained with spatial gratings of 0.4, 0.8, 1, 2, 4, 6, 8, and 10 cpd and presented at 1 Hz square-wave contrast-reversal or stationary mode (dynamic and static presentation, respectively). CSF tuning was estimated by calculating the ratio between peak sensitivity and the sensitivity at 0.4 cpd.
  
  
  RESULTS
  In all subjects tested (n = 6), VEP contrast-response functions showed nonlinearities-namely, amplitude saturation and double-slope amplitude functions that occurred at low and medium-to-high spatial frequencies, respectively. Mean electrophysiological and psychophysical CSFs peaked at 2 cpd. CSF tuning for electrophysiology and dynamic and static psychophysics were, respectively, 1.08, 1.11, and 1.31. Correlation coefficients (r(2)) between electrophysiological CSF and dynamic or static psychophysical CSF were, respectively, 0.81 and 0.45.
  
  
  CONCLUSIONS
  Electrophysiological and psychophysical CSFs correlated more positively when temporal presentation was similar. Spatial frequencies higher than 2 cpd showed that at least two visual pathways sum their activities at high contrasts. At low contrast levels, the results suggest that the transient VEP is dominated by the magnocellular (M) pathway.
authors.affiliation: Departamento de Fisiologia, Universidade Federal do Pará, Brazil.
authors.forename: Givago S
authors.initials: GS
authors.lastname: Souza
authors.forename: Bruno D
authors.initials: BD
authors.lastname: Gomes
authors.forename: Cézar A
authors.initials: CA
authors.lastname: Saito
authors.forename: Manoel
authors.initials: M
authors.lastname: da Silva Filho
authors.forename: Luiz Carlos L
authors.initials: LC
authors.lastname: Silveira
mesh_terms: D000328:Adult; D015350:Contrast Sensitivity / Q000502:physiology*; D004594:Electrophysiology / Q000379:methods*; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005260:Female; D006801:Humans; D008027:Light; D008297:Male; D010775:Photic Stimulation; D011601:Psychophysics / Q000379:methods*; D012684:Sensory Thresholds