Long range coupling between V4 and PF in theta band during visual short-term memory

S. Liebe, G. Hoerzer, N. Logothetis, W. Maass, and G. Rainer

Abstract:

Both extrastriate area V4 and the lateral prefrontal cortex (PF) are thought to be part of a neural network contributing to sensory and mnemonic processing of visual information. However, it is not well understood how V4 and PF might interact during visual memory. Here, we addressed this question by recording Local Field Potentials (LFP) simultaneously in both brain regions while two rhesus monkeys performed a delayed matching-to sample task. In the task, a sample stimulus (250ms) was presented followed by a probe stimulus (600ms) after a delay period (1500ms). A lever press was required if the sample stimulus matched the probe. We assessed coupling between LFP sites within and between the different brain regions by both measuring pair-wise phase-synchrony (phase locking value, PLV) using a wavelet based method and employing a coupling measure that relies on the concept of Granger causality (partial-directed coherence; PDC) using multivariate autoregressive (MVAR) modeling. In both monkeys we consistently found increases in theta-band phase synchrony (3.5-7 Hz) between V4 and PF LFP site pairs during the delay period of the task. Specifically, a significant proportion of pairs (26.1%, 62/231 for monkey 1 and 25%, 40/160 for monkey 2, p<0.001) showed increased coherence during the delay phase compared to the pre-stimulus baseline period. In contrast, only a small proportion of sites showed significant coupling in gamma (42-97 Hz, 5.9%/13% for monkeys 1/2, respectively) or beta (16-36Hz, 6.9%/16%) frequencies. In addition, we obtained comparable results using PDC, which also assesses the directionality of information flow between the brain areas. Our preliminary results indicate that the interaction between V4 and PF during short-term memory might be primarily mediated through neuronal coherence in the theta band. Furthermore, our analyses using MVAR modeling suggest that this interaction can be characterized by a bidirectional information flow between these areas. These findings support the idea that long-range interactions play an important role in short-term maintenance of short-term memory.

Reference: S. Liebe, G. Hoerzer, N. Logothetis, W. Maass, and G. Rainer. Long range coupling between V4 and PF in theta band during visual short-term memory. 39th Annual Conference of the Society for Neuroscience, Program 652.20, Poster Y31, 2009.