Temporal Processing in the Visual System

Abstract

Encoding time is one of the most important features of the mammalian brain. The visual system, comprising almost half of the brain is of no exception. Time processing enables us to make goal-directed behavior in the optimum “time window” and launch a ballistic eye movement, reach/grasp an object or direct our processing resources (attention) from one point of interest to another. In addition, encoding time is critical for higher cognitive functions, enabling us to make causal inferences. The limitations of temporal individuation in the visual stream seem to vary across the visual field: the resolution gradually drops as objects become farther away from the center of gaze, where little differences were found in terms of resolution for objects in the upper versus lower visual field. This resolution of temporal individuation is vastly different from the resolution ascribed to spatial individuation. If individuation is mediated through attention, as some researchers have proposed, the general term ”attention” seems to possess different properties, at least regarding temporal and spatial processing. Next we looked at another aspect of encoding time: Temporal Order Judgments (TOJ), where animals had to judge the relative timing onset of two visual events. After training two monkeys on the task, we recorded from neurons in the lateral intraparietal area (LIP), while the animals reported the perceived order of two visual stimuli. We found that LIP neurons show differential activity based on the animal’s perceptual choice: when the animal reports the stimulus inside the receptive field of the neuron as first, the cells show an increased level of activity compared to when the animal reports he same stimulus as second. This differential activity was most reliable in the tonic period of the response \((\sim100 ms\) after stimulus onset). However, no difference in visual response latencies was observed between the different perceptual choices. The parietal cortex has previously been implicated in temporal processing based on patient studies as well as neuroimaging investigations. Physiological studies have also suggested the involvement of parietal area in encoding elapsed time. However, our study is the first to demonstrate parietal neurons encoding relative timing.