Involvement of the hippocampus in implicit learning of supra-span sequences: the case of SJ

Learning of supra-span sequences was assessed in a densely amnesic individual (SJ) who suffers from a substantial circumscribed bilateral lesion to the hippocampus. SJ's ability to lay down information originating from repetitive memory recall episodes was assessed using Hebb's supra-span procedure. After assessment of short-term memory span, 25 sequences of span +1 items were presented to SJ for immediate serial recall (ISR), one sequence being presented repeatedly eight times. Learning was deduced by the comparison of ISR scores on the repeated versus nonrepeated sequences of span +1 items. SJ's learning capacity was examined using four different types of stimuli: digits, spatial locations (Corsi block tapping test), words, and pseudowords. Implicit learning of sensorimotor sequences was also assessed in SJ using a serial reaction time (SRT) paradigm. Findings with the supra-span ISR task revealed evidence of learning in SJ with all four types of stimuli. The learning magnitude, as well as learning rate, observed in SJ were comparable to those observed in matched control participants. SJ showed evidence of implicit learning on the SRT paradigm. We conclude that the hippocampus is not required to learn certain types of recurrent information, and that the supra-span ISR task can be considered as an implicit-based learning paradigm. These findings have significant implications for our conceptualisation of implicit learning, and for understanding of the role of the hippocampus in learning.