Quantifying sensory and cognitive signals in visual and frontal cortex [electronic resource]
Keywords: Bioengineering.

The visual system has a limited capacity for capturing and processing the richness and intricate detail of the surrounding environment. Visual information that arrives in the retina is converted from relative light intensities to patterns of excitation and then transmitted to a hierarchy of visual areas, which process and combine increasingly complex features of the visual signal to form a visual percept. At each stage, the amount of task or stimulus-related information a neuron can encode depends on the separability of its responses to different conditions. Using electrophysiological recordings of extracellular spiking activity from single neurons in awake behaving monkeys, we explored ways to quantify information in neuronal firing rates in order to address specific questions about sensory and cognitive signals in visual cortex and frontal cortex during different behavioral contexts. In the first study, we addressed a question of latency differences in the visual pathways that process color using a passive fixation task. Color processing occurs generally along two separate chromatic pathways, and previous work has indicated that information from the two pathways arrive with a relative lag in primary visual cortex. However, to form a perception of color, these two pathways must converge at some stage of visual processing. We used information theory to examine the timecourse of chromatic information in neurons further up the visual hierarchy in area V4, which has been implicated as an area with an important role in color processing. We found that on average, information specific to each pathway arrived simultaneously in V4, suggesting that color signals from the different pathways converge at some point within or before V4 in the visual hierarchy. In order to select behaviorally relevant information from the large amount of visual information available, shifting the focus of gaze (via saccadic eye movements) and directing attention provide ways to allocate processing resources to selected locations in visual space. Studies have shown that perceptual enhancements at behaviorally selected spatial locations are accompanied by enhanced processing in visual cortex. The mechanisms by which neurons in the brain control the selection of sensory signals remain unclear. Previous works suggest that the control of attention and eye movements, as well as the modulation of sensory representations, originate in a distributed network that includes the frontal eye field (FEF) in frontal cortex. We studied responses of single neurons recorded separately in area V4 and the FEF of monkeys engaged in different visuospatial selection tasks. In addition to firing rate responses, we examined the trial-to-trial response variability, which has been suggested to reflect behavioral state. During natural vision, the eyes make frequent movements to selected targets. To better understand how these gaze shifts influence visual processing, we examined selective visual processing in area V4 during saccade preparation. We found that V4 neurons show transiently enhanced stimulus discrimination at the saccade target. This enhancement is due in part to changes in response magnitude, but may also be facilitated by reduced variability (increased reliability) of sensory representations. The similarity to effects of covert attention and experimental manipulations of FEF activity provides further evidence that the mechanisms driving visual modulation during saccade preparation and covert spatial attention rely on common neural resources. To explore signals that likely modulate visual responses through feedback connections, we examined the role of FEF neurons in the maintenance and selection of spatial information. In a task that required remembering and directing spatial attention to a cued location while withholding eye movements, neurons in the FEF exhibited spatially selective persistent activity, which continuously tracked the location of the cue. Moreover, this maintenance of spatial information correlated with successful deployment of attention. Despite robust visual and cognitive firing rate modulations that predicted behavioral performance on the task, declines in response variability appeared to be most effectively driven by visual stimulation, rather than spatial working memory or attention. This indicates that, at least in the FEF, behavioral engagement alone is not sufficient to drive changes in variability. Instead, changes in response variability may reflect shifts in the balance between feedforward and recurrent sources of excitatory drive.