Spectral decomposition of local field potentials uncovers frequency-tuned gain modulation of working memory in primate visual system

Working memory has been shown to modulate visual processing in a variety of ways, including changes in visual response gain, oscillatory power, spike timing, and phase coding of information ^1–3 . Here we probe working memory’s influence on various oscillatory components within visual areas, using the Maximal Overlap Discrete Wavelet Transform (MODWT) technique to decompose the local field potential(LFP) ⁴ ; this method allows single-trial quantification of the properties of various oscillatory components. We examine the impact of spatial working memory on visual processing within the extrastriate middle temporal (MT) visual area of rhesus macaque monkeys, comparing the responses in MT when remembering a location inside or outside the receptive fields of the neurons being recorded. Using traditional bandpass filtering, we replicate previous reports that working memory enhances visual responses, low-frequency oscillatory power, spike-phase locking, and phase coding of visual information. Applying the MODWT method, we find that working memory modulates both oscillatory power and the precise frequency of oscillations within a general frequency band. The precise frequency of several lower frequency components (alpha, theta, and beta frequencies, but not gamma or high gamma) correlates with visually-evoked firing rates of MT neurons on a trial-by-trial basis. This relationship between firing rate and oscillatory component frequency is maintained across memory conditions, indicating a close link between the neural mechanisms driving oscillatory frequency and firing rates.