AbstractsBiology & Animal Science

In order to use visual information for behavior, humans and other primates must preferentially process behaviorally-relevant information among abundant irrelevant information, a function known as selective attention. When relevant information becomes unavailable to the eyes, the brain must temporarily maintain and manipulate it internally, a function known as working memory (WM). This thesis examines the relationship between WM and the activity of neurons in different processing stages along the dorsal visual pathway of macaque monkeys. We also investigate the interaction between the neural substrates of WM and attention, and their effects on visual perception. WM may play a crucial role during attention: it is thought that a representation of to-be-attended visual information, held in WM, guides the top-down modulation of neural activity in visual cortical areas, selectively enhancing perception. We show in Chapter 2 that in humans, the perception of a visual feature – motion direction – is modulated not only by attention to that feature, but also by the maintenance of that feature in WM, and by a cumulative interaction of both processes when acting simultaneously.Current theories propose that WM representations of visual features are encoded in the sustained spiking activity of neurons not only in high-order areas such as the lateral prefrontal cortex (LPFC), but also in feature-selective areas of visual cortex. In Chapter 3, we show that sustained activity encoding memorized motion directions is absent in direction-selective neurons in early visual area middle temporal (MT), but sharply emerges immediately downstream, in multimodal association area medial superior temporal (MST), and is also present in LPFC. Furthermore, local field potentials in MT encode the memorized directions and are synchronized with LPFC spikes, suggesting that LPFC modulates synaptic activity in early visual cortex. In Chapter 4, we investigate whether the same or different populations of LPFC neurons encode visual features during attention and WM. We show that while a fraction of the neurons exclusively encoded attended or memorized motion directions, other neurons encoded both, suggesting that these two functions are partially segregated. Moreover, activity correlated with behavioral performance during both WM and attention.We propose a putative mechanism that links WM, attention, sensory processing and perception. Early visual areas (e.g., MT) exclusively encode the features of current sensory stimuli, whereas multimodal association areas further downstream (e.g., MST) additionally store WM representations of these features. These representations are "read out" by a population of prefrontal neurons, while a partially-segregated population encodes representations of attended stimulus features. Both memorized and attended representations in LPFC may serve to guide behavior, as well as to modulate sensory processing in early visual cortex, ultimately influencing perception. Afin d'utiliser l'information visuelle utile pour le…