Abstract
Purpose :
ON- and OFF-pathways may be differently affected by vision-loss in glaucoma. Therefore, a measure that can describe ON and OFF pathway functional integrity is desirable. Here we develop a Steady-State Visually Evoked Potential (SSVEP) method that simultaneously measures ON and OFF pathway responses from the upper and lower visual fields.
Methods :
We measured ON and OFF biased responses in the upper and lower visual fields across a range of luminance contrasts in 17 observers with normal vision using a 128 channel EEG system. Upper and lower visual field luminance-modulated probes were flickered at 3.0 and 3.75 Hz with a sawtooth profile, the polarity of which was set to bias responses towards the ON or OFF pathways. Spectral analysis separates these simultaneously recorded responses. To analyze these data, each observer’s 128 channel dataset was algorithmically reduced to a small number of stimulus-tracking components, separately for the upper and lower fields. Signal amplitudes were fit with a transducer function at the group-level, the parameters of which describe the sensitivity of the underlying neuronal populations. Parameters for the ON and OFF pathways in the Upper and Lower visual field were compared via bootstrapping.
Results :
The contrast response functions of the ON and OFF pathways were reliably measured in the upper and lower visual fields, although they differed in form. OFF-favoring stimuli generated larger responses overall. Response functions in the upper visual field were shallower and continued to increase over a wider range of contrast levels. This difference, described by the semi-saturation parameter of the fits, was significant (p < .05) when comparing the ON pathway in the upper visual field to the OFF pathway in the lower visual field (m.d. = 62%, 95CI = 1.4 – 106%), and the OFF upper pathway to the OFF lower pathway (m.d. = 21%, 95CI = 4.3 – 73%). Qualitatively, ON and OFF responses in the lower visual field were more alike at high contrasts, where responses in the upper visual field were readily discriminable.
Conclusions :
Our two-frequency sawtooth SSVEP protocol simultaneously measures ON and OFF pathway responses in the upper and lower visual field. Probes of intermediate contrast, where pathway responses functions are most different, may be better suited to assess pathway integrity.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.