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Abstract:
The class of ambiguous stimuli is a powerful tool to study the neuronal correlates of consciousness; under the constant physical stimulation, the conscious experience of the stimulus spontaneously flips back and forth over time. Previous electrophysiological studies with concurrent behavioral measurements during ambiguous percepts concentrated on the trial-by-trial relationship between the activity of isolated single neurons and monkeys’ reports. In these studies, either the stimulus or the recorded neuron was carefully selected so that the two alternative conscious experiences maximally differentiate the spike counts of the recorded neurons. Focusing on single neurons ignores a potentially information-rich signal: the temporal correlation in the spikes of neighboring neurons. We were interested to learn how and when the firing of many neurons began to reflect the conscious perception of an ambiguous stimulus over time. Specifically, we wanted to learn the extent to which we could track the development over time of a neural correlate of consciousness. To address this issue, we trained two monkeys to report their percepts while they were seeing an ambiguous structure-from-motion stimulus. We recorded neuronal activity from the motion sensitive area MT, with 8-10 microelectrodes. We used a decoding approach to quantify how monkeys’ reports are correlated with the activity of the simultaneously recorded multiple neurons over time. The time resolved decoding performance was compared between ambiguous and unambiguous conditions. The ambiguity was manipulated via binocular disparity. The decoding performance attained with many neurons in both conditions was very accurate, although a significant difference emerged between the ambiguous and unambiguous conditions over time. For the unambiguous condition the decoding performance was very accurate from shortly after the stimulus onset and remained high throughout the stimulus presentation. In a stark contrast, the decoding performance for the ambiguous condition built up gradually (almost linearly) over time, and reached at the peak at around.4-.8 sec after the stimulus onset. Our results show that the initial neuronal activity evoked by the onset of a stimulus reflects the physical properties of the input and thus is less correlated with conscious percept, while the later activity is increasingly reflective of the conscious percept of the animal.
