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When a near-threshold stimulus is presented, a sensory percept may or may not be produced. The unpredictable outcome of such perceptual judgment is believed to be determined by the activity of neurons in early sensory cortex. We analyzed the responses of neurons in primary somatosensory cortex, recorded while monkeys judged the presence or absence of threshold stimuli. We found that these responses did not covary with the monkeys' perceptual reports. In contrast, the activity of frontal lobe neurons did covary with trial-by-trial judgments. Further control and microstimulation experiments indicated that frontal lobe neurons are closely related to the monkeys' subjective experiences during sensory detection.

Imagine sitting next to a window in a restaurant atop a high building. You see people moving in the plaza below, and you instinctively direct your gaze in the direc- tion that most people are moving. This effortless action—identifying the net direc- tion of motion of a more-or-less randomly moving group of objects—involves cortical neurons from the middle temporal area (MT) of the brain. These neurons activate in response to visual objects moving in a par- ticular direction and inform the brain that an object is in motion 1 ,b ut understanding how this neuronal representation is used to causal relationships between neuronal fir- ing patterns and perception. Because MT neurons are very selective about the physi- cal attributes of the stimuli that activate them, the researchers first determined the appropriate combination of size, location, velocity and direction of the moving dots that optimally activated the neurons in the vicinity of the recording electrode. Once the best activating stimulus was found, the monkeys' task was designed such that the dots moved in the direction preferred by the neurons near the recording electrode in half of the trials, and in the opposite direc- tion in the other half. For example, if the recording electrode was near a group of neurons with upward direction preference, the stimulus set was designed such that monkeys had to decide whether the net motion of the dots was upward or down- ward. Thanks to this design, the authors could artificially activate neurons while monkeys performed the direction discrimi- nation task and add motion information that was either congruent or opposite to the direction of the moving dots. They could then evaluate how the addition of either type of information affected the monkeys' choice behavior. How this artificial addition of information affected the monkeys' behavior depended on the direction of the dots being presented with respect to the microstimulated neuron popu- lations. If the dots were moving in the direc- tion preferred by the microstimulated neurons, the authors found that the monkeys were substantially more likely to choose to make an eye movement in that direction. Additionally, the stimulus-viewing intervals (reaction times) were faster. This suggested that microstimulation was adding informa- tion to the stimulus, biasing the monkeys' behavioral responses in the direction pre- ferred by the stimulated neurons and causing the monkeys to reach faster decisions, pre- sumably due to a quicker accumulation of sensory evidence. These results elegantly confirmed the causal relationship between the activity of MT neurons and the percep- tion of motion direction 4 .