Electroencephalography and clinicalNeurophysiology , 88 (1993) 143-150 143 © 1993 Elsevier Scientific Publishers Ireland, Ltd. 0168-5597/93/$06.00 EVOPOT 92611 Spatial frequency response functions obtained from cat visual evoked potentials Kenji Arakawa *, Neal S. Peachey and Gastone G. Celesia Department of Neurology, Stritch School of Medicine, Loyola University of Chicago, Maywood, IL (USA), and Hines VA Hospital, Hines, IL (USA) (Accepted for publication: 19 October 1992) Summary Visual evoked potentials (VEPs) were obtained from the surface of the cat visual cortex in response to contrast reversing sinusoidal gratings. Gratings of different spatial frequencywere presented either separately, using signal averagingto increase the signal-to-noise ratio, or as a spatial frequencysweep, in which spatial frequencywas sequentially increased every 5 sec during a 40 sec trial (3.99 Hz) or every 3 sec during a 24 sec trial (6.65 Hz). The second harmonic amplitude- and phase-spatial frequencyfunctions derived from averagingor from sweep trials were similar, indicating that the swept stimulus method can be used to provide a rapid and reliable measure of the VEP-spatial frequency function. Intravenous administration of physostigmine, an acetylcholinesterase inhibitor, evoked a spatial frequency-dependent change in VEP amplitude. At 3.99 Hz, responses to low spatial frequencies were enhanced to a greater extent than were responses to high spatial frequency stimuli. At 6.65 Hz, responses to mid-range spatial frequencies were enhanced to a greater extent than were responses to low and high spatial frequency stimuli. VEP phase at both 3.99 and 6.65 Hz was advanced to a greater degree at the higher spatial frequencies. These results indicate that the swept spatial frequency method may be useful in studyingspatial frequency-dependentpharmacologicaleffects on the VEP and support the possibility that pharmacologicaldisruption of the cholinergicvisual system can produce such changes. Keywords: Visual evoked potential (VEP); Spatial frequency; Physostigmine;Swept spatial frequencymethod; (Cat) A number of studies have examined how the re- sponse characteristics of the visual evoked potential (VEP) are modified by various pharmacological agents (e.g., Zemon et al. 1980; Onofrj and Bodis-Wollner 1982; Harding et al. 1983; Adachi-Usami et al. 1990; Arakawa et al. 1992). Typically, these studies record averaged VEPs repetitively to a single stimulus before, during and after drug administration. However, spatial vision is thought to be mediated by several distinct channels, each tuned for a limited range of spatial frequencies (reviewed by De Valois and De Valois 1990). Since it cannot be determined whether these channels are affected to different degrees by applied agents if only a single spatial frequency stimulus is used, it would be of value to record VEPs to stimuli of different spatial frequency within a single session. One approach is to obtain averaged VEP responses to a Correspondence to: Dr. Neal S. Peachey, Department of Neurol- ogy, Loyola University Medical Center, 2160 S. First Avenue, May- wood, IL 60153 (USA). * Present address: Department of Neurology,Neurological Institute, Faculty of Medicine, Kyushu University 60, Fukuoka 812, Japan. range of stimuli of different spatial frequency (Maffei and Fiorentini 1973). However, this approach is time- consuming and may not be suitable for studies in which the effects of pharmacological agents are transient. The swept spatial frequency method (Regan 1977; Tyler et al. 1978, 1979) appears to satisfy the demand of rapidly obtaining VEPs across a range of stimulus spatial frequency. In this approach, VEPs are analyzed in real time while the spatial frequency of the stimulus is systematically varied. As a result, an entire ampli- tude-spatial frequency function can be developed in the course of a single short duration trial. However, before applying this approach to the pharmacologic study of the VEP, it is important to verify that the spatial frequency function obtained from the swept stimulus method is similar to that obtained using con- ventional averaging techniques. In the present study, we first compared VEP ampli- tude- and phase-spatial frequency functions obtained from anesthetized cats using swept stimuli and averag- ing techniques at two temporal frequencies. Upon find- ing that the functions obtained were similar, the swept spatial frequency method was used to examine the effect of physostigmine on the VEP. Physostigmine blocks the acetylcholinesterase responsible for acetyl-