Stimulus Set Meaningfulness and Neurophysiological Differentiation: A Functional Magnetic Resonance Imaging Study

A meaningful sequence of stimuli, like movie frames, triggers varied experiences, while meaningless stimuli, like TV noise, produce a uniform experience despite physical differences. The differentiation of cortical responses, measured by Lempel-Ziv complexity of functional MRI images, reflects the overall meaningfulness of a stimulus set rather than differences among stimuli. Brain activity patterns showed highest differentiation during a movie, intermediate for a temporally scrambled movie, and minimal for spatially scrambled TV noise, even though overall cortical activation was strong and widespread in all conditions. Meaningfulness also correlated with higher information integration among cortical regions. This approach can assess stimulus meaningfulness without identifying relevant features or neural locations.

Study at a glance

Abstract

A meaningful set of stimuli, such as a sequence of frames from a movie, triggers a set of different experiences. By contrast, a meaningless set of stimuli, such as a sequence of 'TV noise' frames, triggers always the same experience--of seeing 'TV noise'--even though the stimuli themselves are as different from each other as the movie frames. We reasoned that the differentiation of cortical responses underlying the subject's experiences, as measured by Lempel-Ziv complexity (incompressibility) of functional MRI images, should reflect the overall meaningfulness of a set of stimuli for the subject, rather than differences among the stimuli. We tested this hypothesis by quantifying the differentiation of brain activity patterns in response to a movie sequence, to the same movie scrambled in time, and to 'TV noise', where the pixels from each movie frame were scrambled in space. While overall cortical activation was strong and widespread in all conditions, the differentiation (Lempel-Ziv complexity) of brain activation patterns was correlated with the meaningfulness of the stimulus set, being highest in the movie condition, intermediate in the scrambled movie condition, and minimal for 'TV noise'. Stimulus set meaningfulness was also associated with higher information integration among cortical regions. These results suggest that the differentiation of neural responses can be used to assess the meaningfulness of a given set of stimuli for a given subject, without the need to identify the features and categories that are relevant to the subject, nor the precise location of selective neural responses.