ssVEP-Based Estimates of Contrast Sensitivity, Visual Acuity, and Orientation Sensitivity Do Not Correlate with Each Other or with Psychophysical Measures in Healthy Individuals

Traditionally, it is assumed that there is a common factor for spatial vision. However, recent psychophysical results have not found evidence for such a factor, suggesting that each test measures a rather distinct, independent component of visual perception. An alternative explanation is that psychophysical tests measure not only the underlying visual mechanisms, but also confounding non-visual aspects arising from behavioral responses. In this study, we tested 35 young adults with steady-state visually evoked potentials (ssVEP) to bypass behavioral responses and to “objectively” assess contrast sensitivity, visual acuity, and orientation sensitivity. The same participants also performed the corresponding psychophysical tests. ssVEP relies on brain responses elicited by stimuli flickering at specific frequencies and it is widely used in clinical settings, for instance to measure visual acuity in infants or patients with motor impairments. We hypothesized that EEG-based thresholds are less affected by non-visual confounding aspects than psychophysical tests, resulting in higher between-test correlation reflecting the underlying visual factor. Contrary to our expectations, although EEG-based thresholds demonstrated high test–retest reliability, between-test correlation were even lower than those between psychophysical tests. Furthermore, EEG-based thresholds did not correlate with the thresholds obtained from the corresponding psychophysical tests. These findings provide further evidence towards a multifactorial structure of the visual system, and suggest that ssVEP and psychophysical tests measure distinct aspects of visual processing, challenging their interchangeable use.