Gamma oscillations as a neural signature of shifting reference time in language
Verbs and other temporal expressions allow speakers to specify the location of events in time, as well as to move back and forth in time, shifting in a narrative between past, present and future. The referential flexibility of temporal expressions is well understood in linguistics but its neurocognitive bases remain unknown. Here we aimed to obtain a neural signature of time shifting in natural language. We analyzed event-related potentials (ERPs) and oscillatory responses to occurrences of the word ‘now’ and of verbs in Punctual (‘An hour ago the boy stole a candy and now he peeled the fruit’) and Iterative (‘The entire afternoon the boy stole candy and now he peeled the fruit’) contexts. ‘An hour ago’ introduces a time frame that lies entirely in the past, ‘now’ shifts the narrative to the present, and ‘peeled’ shifts it back to the past. These two time shifts in Punctual contexts are expected to leave very similar traces on electrophysiological responses. ‘The entire afternoon’ may encompass past and present: both ‘now’ and ‘peeled’ are consistent with that time frame, therefore no time shift is required. We found no difference in ERPs between Punctual and Iterative contexts either at ‘now’ or at the verb. However, time shifts modulated brain oscillations. ‘Now’ and ‘peeled’ in Punctual contexts resulted in nearly identical signals: an increase in gamma power with a left-anterior distribution. Gamma bursts were absent in Iterative contexts. We propose that gamma oscillations here reflect operations that bind temporal variables to the values allowed by the constraints introduced by temporal expressions in discourse.