Abstract
Purpose:
To investigate the spatial selectivity of the chromatic visual evoked cortical potential (VECP) using pseudo random stimuli.
Methods:
We evaluated 14 normal trichromats and 5 dichromats with normal or corrected to 20/20. All subjects had color vision evaluated by HMC anomaloscope and Cambridge Colour Test. For the electrophysiological tests, the stimuli were driven by VERIS 6.09. Sinusoidal red-green gratings at 8 spatial frequencies (SF) ranged between 0.2-10 cpd. The stimulus was temporally modulated by a binary m-sequence in a pattern reversal mode presentation. Recordings were obtained by one channel electrodes (active - Oz, reference - Fp, ground - Fpz). The VERIS system was used for extract first and second slice of the second order kernel. The baseline-to-peak amplitude of the components was measured. We fit the VECP amplitude versus spatial frequency function with difference of Gaussians functions. From the model, we extracted the optimal frequency, spatial frequency tuning bandwidth. We also estimate the chromatic visual acuity by fitting a log function at the data from the peak amplitude to the amplitude at the highest tested spatial frequency.
Results:
Trichromats. The waveforms of the second order kernel first slice had a negative component (N1) followed by a positive component (P1). 8 out of 14 subjects had band-pass tuning for the VECP N1 amplitude versus SF function, and the mean optimal SF was 2.l cpd ± 5.1. The mean visual acuity estimated from N1 amplitude data was 14.6 cpd ± 5.1. 3 out of 14 subjects had band-pass tuning for the VECP P1 amplitude versus SF function, and the mean optimal SF was 1.5 cpd ± 1.5. Mean visual acuity estimated from P1 amplitude data was 16.4 cpd ± 5.2. The second order kernel second slice was dominated by a negative component (N2). 5 out of 14 subjects had band-pass tuning for the VECP N2 amplitude x SF function, and the mean optimal SF was 2.8 cpd ± 1.3. The mean visual acuity estimated from N2 amplitude data was 16.4 cpd ± 6. Dichromats. The first and second slices of the second order kernel had few or none signals at all spatial frequencies.
Conclusions:
Negative components had prevalent band-pass tuning and positive components had low pass tuning, but they had equivalent optimal SF and visual acuity. It suggests that different low SF mechanisms may contribute to the VECP along the SF range of the human visual system.
Keywords: 471 color vision •
507 electrophysiology: clinical •
640 pattern vision