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It has generally been thought that the primary visual cortex (V1) receives its driving input via the lateral geniculate nucleus and is modulated by input from the lateral pulvinar nucleus. Using several different techniques to manipulate lateral pulvinar activity, the authors report that the lateral pulvinar is able to gate information outflow from V1. The primary visual cortex (V1) receives its driving input from the eyes via the lateral geniculate nucleus (LGN) of the thalamus. The lateral pulvinar nucleus of the thalamus also projects to V1, but this input is not well understood. We manipulated lateral pulvinar neural activity in prosimian primates and assessed the effect on supra-granular layers of V1 that project to higher visual cortex. Reversibly inactivating lateral pulvinar prevented supra-granular V1 neurons from responding to visual stimulation. Reversible, focal excitation of lateral pulvinar receptive fields increased the visual responses in coincident V1 receptive fields fourfold and shifted partially overlapping V1 receptive fields toward the center of excitation. V1 responses to regions surrounding the excited lateral pulvinar receptive fields were suppressed. LGN responses were unaffected by these lateral pulvinar manipulations. Excitation of lateral pulvinar after LGN lesion activated supra-granular layer V1 neurons. Thus, lateral pulvinar is able to powerfully control and gate information outflow from V1.

In the middle temporal (MT) area of primates, many motion-sensitive neurons with a wide range of preferred directions respond to a stimulus moving in a single direction. These neurons are involved in direction perception, but it is not clear how perceptual decisions are related to the population response. We recorded the activities of MT neurons in rhesus monkeys while they discriminated closely related directions, and examined the relationship between the activities of neurons tuned to different directions and the monkeys' choices. Perceptual decisions were significantly correlated with the activities of the highest-precision neurons but not with those of the lowest-precision neurons. The combined performance of the high-precision neurons matched the monkeys' behavior, whereas the ability to predict behavior based on the entire active population was poor. These results suggest that fine discrimination decisions are crucially dependent on the activities of the most informative neurons.

Uncovering the relationship between sensory neural responses and perceptual decisions remains a fundamental problem in neuroscience. Decades of experimental and modeling work in the sensory cortex have demonstrated that a perceptual decision pool is usually composed of tens to hundreds of neurons, the responses of which are significantly correlated not only with each other, but also with the behavioral choices of an animal. Few studies, however, have measured neural activity in the sensory thalamus of awake, behaving animals. Therefore, it remains unclear how many thalamic neurons are recruited and how the information from these neurons is pooled at subsequent cortical stages to form a perceptual decision. In a previous study we measured neural activity in the macaque lateral geniculate nucleus (LGN) during a two alternative forced choice (2AFC) contrast detection task, and found that single LGN neurons were significantly correlated with the monkeys' behavioral choices, despite their relatively poor contrast sensitivity and a lack of overall interneuronal correlations. We have now computationally tested a number of specific hypotheses relating these measured LGN neural responses to the contrast detection behavior of the animals. We modeled the perceptual decisions with different numbers of neurons and using a variety of pooling/readout strategies, and found that the most successful model consisted of about 50-200 LGN neurons, with individual neurons weighted differentially according to their signal-to-noise ratios (quantified as d-primes). These results supported the hypothesis that in contrast detection the perceptual decision pool consists of multiple thalamic neurons, and that the response fluctuations in these neurons can influence contrast perception, with the more sensitive thalamic neurons likely to exert a greater influence.

The time it takes to transmit information along the human visual pathways introduces a substantial delay in the processing of images that fall on the retina. This visual latency might be expected to cause a moving object to be perceived at a position behind its actual one, disrupting the accuracy of visually guided motor actions such as catching or hitting, but this does not happen. It has been proposed that the perceived position of a moving object is extrapolated forwards in time to compensate for the delay in visual processing 1 , 2 , 3 .

In macaques, it was previously known that a majority of 4B neurons projecting to MT were of the spiny stellate type (that receives pure magnocellular input), but the exact connection patterns and types of pyramidal cells projecting to MT were not fully understood.\\n However, unless one can show that all the color-dependent modulation of metacontrast strength is due to correlated color-dependent modulation of parvocellular responses, Skottun's (2004) analysis does not rule out the role of the magnocellular system.

Speculation by Nijhawan that visual perceptual mechanisms compensate for neural delays has no basis in the physiological properties of neurons known to be involved in motion perception and visuomotor control. Behavioral and physiological evidence is consistent with delay compensation mediated primarily by motor systems.

In the target article Nijhawan speculates that visual perceptual mechanisms compensate for neural delays so that moving objects may be perceived closer to their physical locations. However, the vast majority of published psychophysical data are inconsistent with this speculation.