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Diego Mendoza-Halliday, Santiago Torres, Julio Martinez-Trujillo; Sharp emergence of working memories along the primate dorsal visual pathway. Journal of Vision 2014;14(10):168. doi: 10.1167/14.10.168.
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© ARVO (1962-2015); The Authors (2016-present)
The temporary storage of visual information in the absence of retinal inputs is known as visual working memory. It is long established that in primates, visual working memory representations are encoded in the sustained spiking activity of neurons in high-order cortical areas far downstream along the visual processing pathways, such as the lateral prefrontal cortex (LPFC). Several studies have recently argued that these representations are also encoded in the spiking activity of neurons in early visual cortical areas. Where along the visual stream working memory representations emerge remains highly controversial. Here we show that in macaque monkeys, working memories of visual motion direction are not encoded in the spiking activity of direction-selective neurons in early visual area middle temporal (MT). Surprisingly, these memories robustly emerge immediately downstream, in multimodal association area medial superior temporal (MST). Working memories in MST were as strong as (mean auROC, P = 0.13, t-test) and lasted longer than (% significant bins, P = 0.03, t-test), those found in LPFC. On the other hand, activity during working memory maintenance was more predictive of task performance in lPFC than in MST (mean choice probability, cp = 0.61 in LPFC; cp = 0.55 in MST; P = 0.02, t-test). Our findings reveal a sharp functional boundary between early visual areas, mainly encoding sensory inputs, and downstream association areas, additionally encoding the contents of working memory. Moreover, we found that local field potential oscillations in MT encoded the memorized directions and, in the low frequencies, were phase-coherent with spikes from LPFC neurons in 12.5% (14 of 112) of the recorded LPFC-MT pairs. This suggests that LPFC modulates synaptic activity in MT, a putative top-down mechanism by which working memory signals influence sensory processing in early visual cortex.
Meeting abstract presented at VSS 2014
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