The thesis investigates the link between cortical excitation–inhibition (E–I) balance in the visual cortex, on the one hand, and psychophysical and neurophysiological measures, onthe other, using transcranial direct current stimulation (tDCS). The existing literature suggests that performance in the visual orientation discrimination task (ODT), peak gamma frequency, and the amplitudes of visual evoked potential (VEP) components (N1 and P2) can be indirect indicators of cortical E–I balance (Edden, Muthukumaraswamy, Freeman, & Singh, 2009; Siper et al., 2016), but some aspects of these relationships remain uncertain. As such, since tDCS has been suggested to modulate cortical E–I balance (Krause, Márquez-Ruiz, & Cohen Kadosh, 2013), it was used to attempt to manipulate these variables. The research presented in the thesis investigates, through a series of experiments, whether manipulating E–I balance of the visual cortex using a 10-min tDCS with an intensity of 2 mA could modulate the outcomes of the psychophysical (ODT performance) and neurophysiological (EEG) measures (peak gamma frequency, VEP amplitudes of N1 and P2). Additionally, it confirmed and extended the findings of previous studies investigating the relationship between ODT performance and peak gamma frequency by including the amplitudes of VEP components (N1 and P2) (Dickinson, Bruyns-Haylett, Smith, Jones, & Milne, 2016). The results provide no evidence of tDCS modulating the outcomes of the psychophysical and neurophysiological measures other than a noticeable placebo effect of tDCS on ODT performance. Furthermore, the result successfully replicated and extended previous studies’ findings of an association between performance in the oblique condition of ODT and peak gamma frequency as both high peak gamma frequency and/or lower VEP-N1 amplitude are associated with enhanced oblique ODT performance. The findings of the null effects of tDCS on the outcomes of the psychophysical and neurophysiological measures with a clear link to E–I balance add to the growing literature questioning the efficacy of tDCS on cognition (Medina & Cason, 2017). In addition, the finding of a strong relationship between ODT performance and both peak gamma frequency and the VEP-N1 amplitude in the same direction—as expected based on their association with gamma-aminobutyric acid (GABA) concentration and activity (as shown previously (Edden et al., 2009; Zemon, Kaplan, & Ratliff, 1980; Zeneroli, Penne, Parrinello, Cremonini, & Ventura, 1981)—supports the suggestion that these measures may be useful indirect indicators of E–I balance.
