Dependence of rhythmic activity and oddball effects in the rat cortex on the depth of sedation during dissociative anesthesia.

The reactions to novelty manifesting in mismatch negativity in the rat brain were studied. During dissociative anesthesia, mismatch negativity-like waves were recorded from the somatosensory cortex using an epidural 32-electrode array. Experimental animals: 7 wild-type Wistar rats and 3 transgenic rats. During high-dose anesthesia, deviant 1,500 Hz tones were presented randomly among many standard 1,000 Hz tones in the oddball paradigm. "Deviant minus standard_before_deviant" difference waves were calculated using both the classical method of Naatanen and method of cross-correlation of sub-averages. Both methods gave consistent results: an early phasic component of the N40 and later N100 to 200 (mismatch negativity itself) tonic component. The gamma and delta rhythms power and the frequency of down-states (suppressed activity periods) were assessed. In all rats, the amplitude of tonic component grew with increasing sedation depth. At the same time, a decrease in gamma power with a simultaneous increase in delta power and the frequency of down-states. The earlier phasic frontocentral component is associated with deviance detection, while the later tonic one over the auditory cortex reflects the orienting reaction. Under anesthesia, this slow mismatch negativity-like wave most likely reflects the tendency of the system to respond to any influences with delta waves, K-complexes and down-states, or produce them spontaneously.

Distracters impair and create working memory-related neuronal activity in the prefrontal cortex.

The prefrontal cortex (PFC) has a central role in working memory (WM). Resistance to distraction is considered a fundamental feature of WM and PFC neuronal activity. However, although unexpected stimuli often disrupt our work, little is known about the underlying neuronal mechanisms involved. In the present study, we investigated whether irregularly presented distracters disrupt WM task performance and underlying neuronal activity. We recorded single neuron activity in the PFC of 2 monkeys performing WM tasks and investigated effects of auditory and visual distracters on WM performance and neuronal activity. Distracters impaired memory task performance and affected PFC neuronal activity. Distraction that was of the same sensory modality as the memorandum was more likely to impair WM performance and interfere with memory-related neuronal activity than information that was of a different sensory modality. The study also shows that neurons not involved in memory processing in less demanding conditions may become engaged in WM processing in more demanding conditions. The study demonstrates that WM performance and underlying neuronal activity are vulnerable to irregular distracters and suggests that the PFC has mechanisms that help to compensate for disruptive effects of external distracters.

Effects of an NMDA-receptor antagonist MK-801 on an MMN-like response recorded in anesthetized rats.

In the human brain, auditory sensory memory has been extensively studied using a well-defined component of event-related potential named the mismatch negativity (MMN). The MMN is generated in the auditory and frontal cortices in response to deviant stimuli. In monkeys, cortical N-methyl-d-aspartate (NMDA) receptors have a central role in the generation of the MMN. MMN-like responses have also been recorded in other animals, including rats. The present study aimed at determining whether the MMN-like response in rats depends on an intact NMDA-receptor system. We recorded auditory evoked responses during an oddball paradigm epidurally in anesthetized rats that had received intraperitoneal injections of saline or an NMDA-receptor antagonist MK-801. An MMN-like response was recorded in the oddball paradigm in saline-treated rats. Further, this response was dose-dependently blocked by MK-801. These results suggest that the MMN-like response in rats depends on an intact NMDA-receptor system.

Processing of auditory and visual location information in the monkey prefrontal cortex.

Visuospatial working memory mechanisms have been studied extensively at single cell level in the dorsolateral prefrontal cortex (PFCd) in nonhuman primates. Despite the importance of short-term memory of sound location for behavioral orientation, there are only a few studies on auditory spatial working memory. The purpose of this study was to investigate neuronal mechanisms underlying working memory processing of auditory and visual location information at single cell level in the PFCd. Neuronal activity was recorded in monkeys performing a delayed matching-to-sample task (DMTS). The location of a visual or auditory stimulus was used as a memorandum. The majority of the neurons that were activated during presentation of the cue memorandum were selective either for visual or auditory spatial information. A small group of cue related bimodal neurons were sensitive to the location of the cue regardless of whether the stimulus was visual or auditory, suggesting modality independent processing of spatial information at cellular level in the PFCd. Most neurons that were activated during the delay period were modality specific, responding either during visual or auditory trials. All bimodal delay related neurons that responded during both visual and auditory trials were spatially nonselective. The results of the present study suggest that in addition to the modality specific parallel mechanism, working memory of auditory and visual space also involves modality independent processing at cellular level in the PFCd.