Gamma oscillations in gerbil auditory cortex during a target-discrimination task reflect matches with short-term memory

Tightly stimulus-locked "evoked" and not tightly stimulus-locked "induced" gamma-band oscillations have been implicated in a variety of cognitive functions as well as more basic stimulus-related aspects of neuronal activity. The present study aimed at dissociating both aspects using a preparation in rodents trained to perform in a target-discrimination task while recording the intracerebral electrocorticogram in parallel from left and right auditory cortex. While stimulus-related aspects of gamma-band activity were already evident in the naïve subjects, aspects related to successful target-discrimination only emerged with learning. Frequency-modulated tones were employed as stimuli and were varied with respect to two stimulus dimensions, viz. "spectral content" and "modulation direction". A target stimulus had to be discriminated in a GO/(NO-GO) paradigm from three non-target stimuli matching the target in only one or none of these dimensions. Analysis of the event-related potentials indicated that the physical stimulus parameters explained most of the variance in amplitude of most event-related potential components. For the evoked gamma-band activity no learning related modulations were found. The induced gamma-band activity showed a relationship with the similarity of a stimulus with the target that was most prominent in the right hemisphere. The correspondence between task-specific behavior and the amplitude of induced gamma-band activity, both developing with learning, supports a functional relevance of this form of oscillation beyond merely processing physical stimulus attributes.