Mathew E. Diamond: Unified framework for perception of stimulus intensity and perception of stimulus duration in humans and rats.

In natural environments, most events are characterized by stochastic, “noisy” sensory signals. Noisy stimuli can be reliably identified only by accumulating evidence over time. We present studies designed to elucidate how an uncertain tactile stimulus is integrated over time to produce a percept and how that percept is transformed into a decision. Subjects discriminate between two vibrations (v1 and v2) delivered either to their whiskers, in rats, or fingertips, in humans. Vibrations are normally distributed velocity noise. Both rats and humans show progressive improvement as vibration duration increases, provided v1 and v2 are of equal duration. In contrast, unequal duration of v1 and v2 leads to a perceptual bias corresponding to an overestimate of the intensity of the longer stimulus.

 

We formulate a model that is able to replicate both (i) the subjects’ improvement in performance as stimulus duration is increased (ii) the bias by which subjects judge longer stimuli as being more intense. Because the model specifies a relationship between stimulus duration and perceived stimulus intensity, we posit that a reformulation of the same model will specify a relationship between stimulus intensity and its perceived duration. As a test, we present the same stimulus pairs but ask subjects to compare duration. Resulting psychometric curves in humans reveal that intensity affects perceived duration as predicted from the model. Training of rats in duration comparison is in progress. Neurons recorded from various regions of neocortex in behaving rats express the computations inherent to the model.

In sum, temporal integration is a component of stimulus perception just as stimulus features are a component of time perception.