An evaluation of the concurrent discrimination task as a measure of habit learning: performance of amnesic subjects.

Habit learning has been defined as an association between a stimulus and a response that develops slowly and automatically through repeated reinforcement. Concurrent discrimination (CD) learning, in which subjects learn to choose the rewarded objects in a series of pairs, is believed to be an example of habit learning in monkeys. Studies of human amnesic subjects, however, have produced equivocal results, revealing impaired or absent learning on the same CD tasks that monkeys with medial temporal-lobe (MTL) lesions learn normally. One possible explanation for impaired performance in human amnesic subjects is that, unlike monkeys, human subjects use explicit memory to solve CD problems. To test this hypothesis, we administered a 10-object pair CD learning task to two amnesic subjects, HM and PN, and normal control subjects (NCS). Both amnesic subjects have severe anterograde amnesia with little ability to form explicit memories. On the CD task, they demonstrated little or no learning and acquired no explicit knowledge of the task procedures or reward contingencies. In contrast, NCS learned the task quickly and easily using explicit memory strategies. These results suggest that CD tasks cannot be learned by habit in human subjects, and emphasize the discrepancies between the human and monkey literature on habit learning.

Does inhibition spread in a manner analogous to spreading activation?

Two experiments explored limited capacity inhibitory selective attention processes in working memory. Experiment 1 used a modified Sternberg-type 'short-term memory scanning' task, where both irrelevant and relevant memory-set words were included to see if an inhibitory fan effect operated on lexical associates of the should-be-ignored (irrelevant) words. Experiment 2 used a 'negative priming' task, where a target letter to be named was flanked by one, two, or three distractor letters to see if an inhibitory fan effect operated on the should-be-ignored letters. Results from both experiments supported the existence of a limited capacity spreading inhibition counterpart to spreading activation. The findings were discussed in terms of a model recently proposed by Neumann and DeSchepper (1991; 1992) in which two selective attention subprocesses (one excitatory and one inhibitory) in the brain each maximise opposed functions within their respective resource limitations in working memory.