Temporal modulation of responses to drifting grating stimulation is observed in visual neurons in different brain structures. The common measure of intensity of such modulation is the modulation index (MI; Movshon et al. 1978), defi ned as the ratio of the amplitude of the response component at the stimulus temporal frequency (f1 ) and the net response of the cell. However MI works correctly in a limited range of net responses. If stimulation causes only a weak change in mean fi ring rate, MI can take any value independently of actual modulation in the response. Here we present a new, simple method of determining the strength of modulation, based on the detection of a peak in an amplitude spectrum at the frequency of stimulation. We defi ne modulation strength (MS) as the ratio of f1 value above the mean value of amplitude spectrum and standard deviation (SD) of amplitude values along all frequencies in the spectrum. We assume the response to be modulated if f1 value exceeds the mean amplitude by at least one SD (MS>1). We test the method on data recorded from a number of structures of the extrageniculate visual pathway and compare it to the classical MI and the modulation depth (measure used in the amplitude modulation radio transmission – here the change of the spectrum component at the stimulation frequency between the spontaneous activity and the response). Supported by MNiSW grants: N303 070234, COST/127/2007, N303 046 31/1483, OTKA/NKTH Hungary grant 68594, OTKA grant PD75156.
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