Retinocortical gain in the foveal pathway: The effect of spatial frequencyand stimulus size

The amplitude and the phase of the simultaneously recorded steady-state pat tern electroretinogram (PERG) and visual evoked potential (VEP) were evalua ted in humans as a function of the vertical diameter (D) of unidimensional Gabor stimuli. In the other dimension, parallel to the horizontal gratings, the patterns all had constant diameter (see Methods and Materials). Spatia l frequencies (SFs) of 1 cycle per degree (cpd) and 5.3 cpd were counterpha se modulated at a rate of 6.8 Hz. After off-line artifact rejection, the re sponse was subject to Fast Fourier Transformation (FFT), Amplitude and phas e of the first and second harmonics of both ERG and VEP were displayed for each SF and stimulus diameter. Both ERG and VEP amplitude were found to increase as a function of D. Using trend analysis we found that ERG amplitude increased linearly as a functio n of D. VEP amplitude was found to be both linear and cubic, as a function of D. We calculated the ratio of VEP amplitude and ERG amplitude at each D and termed it retinocortical gain (G), G normalized to stimulus area was hi gh for small D and decreased with D independently of SF. Unity gain occurre d at stimulus sizes of 6 degrees to 7 degrees. ERG phase was found to be more negative at 5.3 cpd than at 1 cpd, Although no significant difference was found between VEP phases at the two SFs teste d, neither ERG nor VEP phase changed as a function of D. The results suggest that retinocortical gain is highest for the foveally ce ntered low spatial frequency small patch Gabors. The results provide suppor t for the notion of the "foveal window" in human vision.