Behavioural Brain Research 131 (2002) 105 – 114 Research report Modulation of flash stimulation intensity and frequency: effects on visual evoked potentials and oscillatory potentials recorded in awake, freely moving mice Luisa Lopez a,b , Adriana Brusa a , Antonello Fadda a , Stefano Loizzo a , Alfredo Martinangeli a , Walter G. Sannita c,d , Alberto Loizzo a, * a Istituto Superiore di Sanita ` , Viale Regina Elena 299, 00161 Rome, Italy b ‘Eugenio Litta’, Rehabilitation Center for Deelopmental Disabilities, Via Anagnina Nuoa 13, 00046, Grottaferrata, Italy c Center for Neuropsychoactie Drugs, Uniersity of Genoa, Viale Benedetto XI, 12, 16132, Genoa, Italy d Department of Psychiatry, SUNY, Stony Brook, NY, USA Received 17 January 2001; received in revised form 13 August 2001; accepted 13 August 2001 Abstract Visual evoked potentials (VEP) responses to flash stimulation at nine intensities, from 0.611 to 945.6 cd/m 2 *s, and two frequencies (0.2 and 1 Hz) were recorded and oscillatory potentials (OPs) extracted after digital 50-Hz high pass filtering in unanaesthetized unrestrained mice. Both VEP and OPs morphology were replicable for all conditions and were similar to values reported in the literature. In particular OPs spectral analysis showed that the main frequency component remained stable at 66 – 77 Hz, for both stimulation frequencies, although it displayed an increase in amplitude, as a function of stimulus intensity. OPs amplitude at 1 Hz versus 0.2 Hz stimulus frequency was higher after taking into account the different noise contributions in the two conditions. Root mean square values calculated at selected time windows, revealed that, at 1 Hz, the main contribution to OPs occurs at the onset of the response (14–27 ms) while, at 0.2 Hz, the higher RMS was recorded later (42–56 ms). This difference accounts for the longer duration of the oscillatory event in the 0.2-Hz condition and suggests that oscillatory activity, modulated and carried along the visual pathway, is recorded at the cortical electrode after further elaboration at the cortical/subcortical level, depending on stimulus properties. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Oscillatory potentials; Mouse; Chronic electrode implant; Feature binding www.elsevier.com/locate/bbr 1. Introduction Visual evoked potentials (VEPs) are a well estab- lished means of investigation of the optic pathway in humans and in animals. In particular, flash stimulation has been used both in clinical and experimental re- search, because of its feasibility in non-cooperative, non-sedated subjects. Scalp recorded oscillatory potentials (OPs) in re- sponse to luminance changes have been investigated by many authors in the last decade. Since their first ap- pearance [4], their origin has been controversial. A volume conduction from the previously observed retinal oscillatory potentials was postulated. Nevertheless, lu- minance and contrast sensitivity [28], clinical findings [20], intracortical [24] and magnetoencephalographic recordings [16] have suggested a cortical/subcortical origin of the scalp recorded OPs. In agreement with findings in the cat visual cortex [10], a dependence on stimulus features has been observed for OPs in man, Abbreiations: ERG, electroretinogram; ERG-OPs, electroretino- gram oscillatory potentials; FFT, Fourier analysis; HP, high pass; LP, low pass; OPs, cortically recorded oscillatory potentials; phy, photoisomerizations per rod per second expressed in log units; RMS, root mean square; VEPs, visual evoked potentials. * Corresponding author. Tel.: +39-06-4990-2882; fax: +39-06- 4938-7104. E-mail address: a – loizzo@sun.iss.it (A. Loizzo). 0166-4328/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0166-4328(01)00351-5