Many studies on memory formation and retrieval have shown the importance of mesial temporal structures for the encoding of incoming sensory information. Lesioned animals or humans demonstrate problems with recognizing objects they have previously encountered, even over short time scales like the famous patient HM. McTighe et al have now found a somewhat different memory impairment in rats with specific lesions of the perirhinal cortex (avoiding the hippocampus) (Science. 2010;330(6009):1408-1410). In an object recognition task, these lesioned animals seem to show a paradoxical false memory for novel objects, with exploration times similar to the times spent exploring previously encountered objects.
The authors used an object recognition task, in which the subject animal was first exposed to an initial test object for 3 minutes in a test chamber. The rat was then transferred to either a holding cage for 1 hour under the same ambient noise and light environment as the test chamber, or the rat was held in a visually restricted environment for 1 hour. The rats were then returned to the test chamber where they were either exposed to the same initial object again, or they were exposed to a novel object. The interesting finding of the study concerns the behavior of these perirhinal lesioned rats when exposed to the novel object during this second test.
The lesioned rats exposed to the novel object during the second half of the test demonstrated exploration times much quicker than controls (P < .001), and these times were statistically the same for the trials involving a repeated object implying that the rats were treating the novel object as if it were known. This effect could be completely eliminated if during the interval between the two test segments, the rats were kept in a reduced interference chamber which effectively removed any visual stimulation for the 1 hour waiting period. After 1 hour in the reduced interference chamber, the lesioned rats responded to the novel object in the second portion of the test as if it were novel, with exploration times statistically the same as nonlesioned animals.
In summary, McTighe et al have studied specifically bilaterally lesioned rats (perirhinal cortex) in an object recognition task. These rats behave toward novel objects as if they were familiar (“paradoxical false memory”), an effect that can be eliminated with a period of visual restriction before the presentation of the novel object. In these selectively lesioned animals, and perhaps in humans with similar lesions, the authors hypothesize that more simple feature level representations of the environment may be preserved more posteriorly in the temporal lobe. These simpler representations may then be more susceptible to interference effects (ie, experienced in the 1 hour holding cage), since many visual component features during the 1 hour interim experienced are likely contained in the novel object next presented. The lesioned animals no longer have the capacity to use the anterior temporal lobe system (more complex representations) to negate this association. This may have implications for our epilepsy surgery candidates and oncology patients with lesions in the temporal lobe. Although the clinical significance of a unilateral lesion might not be apparent, it may be possible to expand pre- and postoperative memory evaluations to account for similar or more subtle alterations due to sensory interference effects.