Delayed nonmatch-to-sample

Another developmental task that tests for explicit learning and memory is the delayed nonmatch to sample (DNMS) task. The DNMS task requires the subject to compare a presented sample object with a previously presented comparison object and encourages the selection of a novel object with an object’s second presentation. An edible reward is given to the subject to encourage the novel item selection. After learning the rule for avoiding the familiar object in favor of novelty, the delay period between object demonstrations is increased from 10 to 120 seconds and the number of displayed objects requiring recollection increases (Bachevalier & Beauregard, 1993). The DNMS task is likely both implicit and explicit in nature (Diamond, 1990). Its use of reward conditioning is likely implicit in nature (Squire, 1992). Its use of positive emotional conditioning helps to pair and associate item novelty with reward salience. This shapes the subject’s learning toward identifying novelty as the desired object quality for selection. The DNMS task itself is also likely explicit; it requires the infant or child to recognize an object’s previous exposure, to differentiate its qualities from that of a new object, to hold this information in mind during a delay period, and is characterized by rapid learning in developmentally capable older children and adults.

Surgical removal of either the hippocampal region’s perirhinal cortex or the ventromedial prefrontal cortex (vmPFC) in the nonhuman primate impairs the ability for acquiring the cognitive strategy needed for performing explicit aspects of the DNMS task (Bachevalier, Beauregard, Alvarado, 1999; Bachevalier & Mishkin, 1986; Malkova, Mishkin, & Bachevalier, 1995). This suggests that both brain regions are needed for successful completion of the DNMS task. Human research findings further demonstrate that human amnesics incurring damage limited to the hippocampus proper and damage extending into surrounding structures like the entorhinal cortex (Bayley, Hopkins, & Squire, 2003; Squire & Zola, 1997) are unable to both declare the inherent principle and perform as well as normal comparison controls on the DNMS task (Squire, Zola-Morgan, & Chen, 1988). According to these findings, amnesics also experience significant reductions in accuracy as DNMS task-dependent delays increase beyond 5 seconds.

Furthermore neuroimaging findings reference a significant role for the right hippocampus proper during DNMS training’s encoding phase and a trend for familiar-novelty comparisons and item recognition during the retrieval phase (Monk, Zhuang, Curtis, Ofenlock, Tottenham, Nelson, & Hu, 2002). A neuroimaging study by de Zubicaray and colleagues (2001) monitored and tracked brain activity with positron emission tomography (P.E.T.) regional cerebral blood flow (rCBF) during the encoding, retention, and retrieval phases of DNMS training (de Zubicaray, McMahon, Wilson, & Muthiah, 2001). It found significant lateral-medial activation shifts from more ventrolateral regions of the anterior prefrontal cortex or frontal pole region to the ventromedial prefrontal cortex and a posterior to anterior shift in the dorsolateral prefrontal cortex (dlPFC) from encoding to retention and retention to retrieval phases. Sensory cortical activations also shifted from posterior visual regions in the occipital lobe and lingual gyrus in the encoding phase, to the parietal lobe in the retention phase, and then to both the parietal and occipital lobules in the retrieval phase, suggesting successful engagement of visual areas during DNMS dependent short delays (Elliot & Dolan, 1999). Activation in the caudate tail during encoding and retention phases was probably associated with its receipt of visuomotor information from the occipital lobe and return relay to this structure (Brown, Desimone, & Mishkin, 1995) to facilitate successful subsequent task-related classification accuracy (Seger & Cincotta, 2005). Transient ventrolateral prefrontal cortical activity during the retention phase likely helped to facilitate increased cognitive demands during periods of increasing long delays (Elliott & Dolan, 1999) to avert tendencies for perseverative interference (Kowalska, Bachevalier, & Mishkin, 1991). Transient activations during the retrieval phase in the perirhinal cortex of the hippocampal region were likely evidence of the developing ability for cue-dependent retrieval that characterizes visual long-term memory recognition and familiarity (Buffalo, Ramus, Squire, & Zola, 2000; Hadfield, Baxter, & Murray, 2003; Holscher, Rolls, & Xiang, 2003; Malkova, Bachevalier, Mishkin, & Saunders, 2001). In total these findings suggest differential task-related activation sites for encoding, retention, and retrieval during DNMS learning. They also reflect that different brain regions are involved in task-related encoding, retention, and retrieval processes. They support that memory is not a single entity and is characterized by many different multiple systems which interact in a structurally time-dependent manner during the DNMS task. The nature “of representation stand(ing) between perception and production” (Meltzoff & Moore, 1997, p. 182) may be the developmental response of multiple memory systems underlying processes and abilities for storage, retention, and retrieval of representational memory and associated task-related behaviors.

The DNMS is developmentally more complex than deferred and elicited imitation tasks. Overman (1990) hypothesized that this “may be primarily due to slow learning of the (object’s) novelty- (external) reward rule and secondarily due to the inability to remember a particular item or rule” (p. 380). Because certain objects inherently possess their own rewarding qualities of color, texture, size, shape, etc. it is likely that a sample object’s intrinsic rewarding quality in the youngest of subject populations may distract and interfere in the establishment of the associational learning required for the novelty-initial presentation reward rule in infants. Infants older than four months can eventually acquire the DNMS strategy, with 4 month olds tolerating up to 10 second delays, 6 month olds tolerating 3 minute delays, and 12-9 month olds tolerating up to 10 minute delays between object demonstrations (Diamond, 1990). According to Overman (1990) and Overman and colleagues (1992) twelve to fifteen month old child can acquire the adult standard of the DNMS and reach performance criterion with up to 478 errors in 69-75 test days depending upon task-related age-dependent modifications (Overman, Bachevalier, Turner, & Peuster, 1992). Eighteen to twenty month olds can learn the DNMS strategy with up to 127 errors in 24-25 days over 10 weeks of testing. Twenty-two and thirty-two month olds can acquire it with up to 40 total errors in 11-14 days over four weeks of testing. The oldest children aged 45-81 months perform at levels similar to adults but with accuracy scores below those of adult. Finally adults can learn the DNMS strategy in two-three days with incurring an average of four errors.

Therefore increasing developmental age is associated with increased accuracy of DNMS performance and task related item complexity paired with increasing ability for managing the duration of delay periods between object presentations. These increasing capabilities are also likely supported by increasing developmental expression of memory systems supporting perception, retention, storage, and retrieval.

References

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