Perception of the intensity and duration of a stimulus within a unified framework: psychophysics and underlying neuronal processing

Every sensory experience is embedded in time, and is accompanied by the perception of the passage of time. The fact that perception of the content of a sensory event and the perception of the time occupied by that event are generated in parallel raises a number of questions: Do these percepts interact with each other? Do they emerge within separate neural populations? Which neuronal mechanism underlies this divergence? In the work of my thesis I explored how the perception of the intensity of a vibrotactile stimulus, interacts with the perception of its duration, in both humans and rats. I have carried out three main studies. Chapter I works out the details of the interaction between vibration amplitude and duration, revealing a symmetric confound: perceived duration depends on stimulus speed, and perceived intensity depends on stimulus duration. Quantification of this interaction allowed us formulate a testable computational model for the generation of both percepts, which posits that a single sensory drive provides input to two distinct downstream centers, which generate the two percepts in parallel. Chapter II addresses the effect of stimulus history. Systems neuroscience has given considerable attention in recent years to the effects of preceding stimuli on the perception of the current stimulus. We now ask whether the interaction found in Study I extends to an interaction in the memory trace of recent stimuli: are the perceptual priors mixed or separate? Through psychophysical testing, we were able to show that perception of the duration and the intensity of stimuli, are biased toward the perceived features of previously presented stimuli, and not their low-level physical properties, and that separate representations of prior perceived duration and prior perceived intensity exist in the brain. Chapter III begins to look for neuronal correlates of perceived duration, through extracellular recordings in behaving rats in Dorso-Lateral Striatum (DLS), a region which receives direct input from primary somatosensory cortex and has previously shown to be involved in time perception. The delayed comparison task, differently from many common behavioral paradigms, has the advantage of dissociating the first stimulus presented to the animal from any decisional and motor processes. This makes it particularly relevant for the search for the neural basis of stimulus duration perception. Moreover, the bias of stimulus intensity on perceived time found on Study I, posits the principle that the interaction between these two features should be present in the neural population that encodes the perception of stimulus duration in a behaviourally-relevant way. Ongoing recordings are showing that the unfolding of trial time can be decoded from the striatal neural activity, but the confound of stimulus speed is not encoded by the population. This findings points toward a role of striatum in representing temporal sequences of events, while questioning its involvement in encoding the perception of stimulus duration.