Effects of cooling prefrontal cortex on cell firing in the nucleus medialis dorsalis

The spike activity of single units of the thalamus was explored during hypothermia of dorsolateral prefrontal cortex in monkeys trained to perform a delayed response (DR) task. Lowering prefrontal temperature to 15–20 °C — measured subdurally — induced a profound and reversible deficit of DR performance and a concomitant alteration of firing in cells of the parvocellular portion of the nucleus medialis dorsalis (MDpc). Spontaneous firing frequency was diminished in most of the units affected, while a tendency to grouped, rhythmic discharge was generally accentuated. The normal firing patterns related to DR performance were modified by prefrontal cooling in 63% of the units examined in MDpc. Prefrontal cooling induced a disruption of the sustained elevations of discharge shown by certain types of units during the delay period of DR trials. Sustained activation was attenuated, abbreviated or converted into a paradoxical inhibition as a result of cooling. The effects of prefrontal cooling on behavior and on unit activity in medialis dorsalis are discussed in the context of what is known about input and output connections of the prefrontal cortex. It is apparent that cryogenic depression of this cortical region interferes with the normal neural processes that constitute the substrate of short-term memory function.