Perceptual decision-making and working memory in rats and humans: Interaction of stimulus intensity and duration

Summary

Date: 
September 21, 2016 - 1:00pm - 2:00pm
Location: 
Northwest 243
About the Speaker
Name: 
Arash Zakeri
Speaker Affiliation: 
SISSA

We carried out studies to elucidate how an uncertain tactile stimulus is integrated over time to produce a percept and how that percept is kept in memory and transformed into a decision. Our earlier work showed that rats could perform a tactile delayed comparison task with level of performance overlapping that of human subjects. The current study aimed to understand whether uncertain sensory information is integrated over time to produce increasingly accurate estimates about the external world and if so, how. Humans and rats compared the magnitudes of two sequential vibrations, Stimulus 1 and Stimulus 2, defined by mean speeds sp1 and sp2, respectively. Both in humans and in rats, performance improved as vibration duration increased, provided the two vibrations were of equal duration. Unequal duration of Stimulus 1 and Stimulus 2 led to a perceptual bias corresponding to an overestimate of the intensity of the longer stimulus: “longer feels stronger”. Perception of stimulus intensity is thus confounded by stimulus duration. We propose a behavioral model where perceived intensity is proportional to the weighted integral of instantaneous speed, which explains both improvement and bias due to the stimulus duration manipulation.

Sensory integration in behavior implies the existence of at least two levels of neuronal processing – one level at which the instantaneous quantity of the stimulus is encoded and a further level at which information accumulates over time. However, the neuronal circuitry by which sensory information is accumulated remains to be identified. We searched for the neuronal basis of time-dependent changes in perceived stimulus magnitude and found that the activity of neurons in rat somatosensory cortex varied according to the stimulus speed but not duration. In contrast, activity in premotor cortex varied according to both speed and stimulus duration, matching the bias observed in behavior.

Observation of the confound in perception and in the premotor cortex motivated us to design another experiment in which both humans and rats reported the relative duration of stimuli. If “longer feels stronger” does “stronger feel longer”? Manipulation of stimulus intensity had a symmetric effect on perceived duration. In conclusion, tactile information in rats and humans is accumulated, not averaged, an operation that leads to confusion between duration and magnitude; the confound may originate in a neuronal population of frontal cortex that multiplexes intensity and duration.