By Thomas Meindertsma and Tobias Donner.
Even the most elementary perceptual decisions result from complex cascades of neural computations in the brain. Such elementary decisions entail responding to simple patterns of light on the retina with arbitrarily mapped motor act. Previous research has shown that such elementary decisions are associated with rapid state changes in visual cortex, which are indexed by a modulation of neural activity in the beta-frequency range. These decisions are also followed by other events that might affect the state of visual cortex, including the motor command associated with the behavioral report of the decision, an increase in pupil-linked arousal, fixational eye movements, and fluctuations in bottom-up sensory processing. Previous studies that identified the beta-frequency modulation did not disentangle any of these processes from the decision.
Here, we uncoupled the perceptual decision from behavioral report, while volunteers performed a simple detection task during which we measured their magnetoencephalography (MEG). We asked our participants to report the onsets and offsets of a salient visual target either by direct button press or by silently counting the number of offsets and reporting the total number at the end of the run (Figure 1A). As in previous studies, we found suppression of beta-band power after target offsets, and an increase of beta-power after target onsets (Figure 1B). Importantly, we found this modulation also in the counting condition, indicating a non-motor origin.
Figure 1
Furthermore, we studied the relation between visual cortex and boosts in phasic arousal, as indexed by transient pupil dilations. Our results show that pupil-linked arousal indeed affects the state of visual cortex, but it does not drive the beta-frequency modulation around perceptual decisions (Figure 1C). Other potential factors that might affect visual cortex, including fixational eye movements and fluctuations in bottom-up stimulus encoding, were also ruled out as drivers of the beta-band modulation.
In sum, we have here identified a highly specific neurophysiological correlate of the perceptual decision, in the earliest stage of cortical visual processing. We propose that the beta-band modulation reflects a decision-related state change, which is induced by the conversion of graded perceptual signals into a categorical format underlying behavior; the resulting decision signal is fed back to reinforce the decision state already in early visual cortex. This hypothesis can now be tested by us or other labs interested in the large-scale interplay between brain regions involved in perceptual decision-making. Our results also shed new light on the function of beta-band oscillations in cortex, wich are commonly studied in motor cortex as a correlate of motor movement.
Meindertsma T, Kloosterman NA, Nolte G, Engel AK, Donner TH (2017) Multiple transient signals in human visual cortex associated with an elementary decision. Journal of Neuroscience. DOI: 10.1523/jneurosci.3835-16.2017