No abstract

The distinction between episodic and semantic memory was proposed in 1972 by Endel Tulving and is still of central importance in Cognitive Neuroscience today. Data obtained in the last 30 years or so, however, support the idea that the frontiers between perception and knowledge and between episodic and semantic memory are not as clear cut as previously thought, prompting a rethinking of the episodic-semantic distinction. Here, we review recent research on episodic and semantic memory, highlighting similarities between the two systems. Taken together, current behavioral, neuropsychological and neuroimaging data are compatible with the idea that episodic and semantic memory are inextricably intertwined, yet retain a measure of distinctiveness, despite the fact that their neural correlates demonstrate considerable overlap.

Declarative memory is thought to consist of two independent systems: episodic and semantic.Episodic memory represents personal and contextually unique events, while semantic memory represents culturally-shared, acontextual factual knowledge. Personal semantics refers to aspects of declarative memory that appear to fall somewhere in between the extremes of episodic and semantic. Examples include autobiographical knowledge and memories of repeated personal events. These two aspects of personal semantics have been studied little and rarely compared to both semantic and episodic memory. We recorded the event-related potentials (ERPs) of 27 healthy participants while they verified the veracity of sentences probing four types of questions: general (i.e., semantic) facts, autobiographical facts, repeated events, and unique (i.e., episodic) 2 events. Behavioral results showed equivalent reaction times in all 4 conditions. True sentences were verified faster than false sentences, except for unique events for which no significant difference was observed. Electrophysiological results showed that the N400 (which is classically associated with retrieval from semantic memory) was maximal for general facts and the LPC (which is classically associated with retrieval from episodic memory) was maximal for unique events. For both ERP components, the two personal semantic conditions (i.e., autobiographical facts and repeated events) systematically differed from semantic memory. In addition, N400 amplitudes also differentiated autobiographical facts from unique events. Autobiographical facts and repeated events did not differ significantly from each other but their corresponding scalp distributions differed from those associated with general facts. Our results suggest that the neural correlates of personal semantics can be distinguished from those of semantic and episodic memory, and may provide clues as to how unique events are transformed to semantic memory.

Anticipation of predictable events is crucial for organizing motor performance. Using instructed delay tasks, it has been shown that even when delay duration is kept constant, reaction time fluctuates from trial to trial. As time estimation is at the core of anticipatory behavior, it is reasonable to speculate whether neuronal delay activity correlates with the subjective estimate of time. As a consequence of the scalar property of time estimation processes, the variability in time estimation increases continuously as time passes during the delay. This scalar property may then be reflected in the increasing variability in neuronal delay activity. We thus studied the influence of temporal prior information on neuronal delay activity in monkey motor cortex in two conceptually different tasks in which two equally probable delay durations were randomly presented. We hypothesized that if one considers the animal's subjective time as the time which elapses between the first (instruction) signal and movement onset, then, by suppressing this temporal variability, across-trial variability in neuronal discharge should decrease. We thus defined a new time scale in each trial such that, after rescaling, the time between the instruction signal and movement onset was identical in all trials. Each spike was then displaced in time accordingly. As expected, the variability in the timing of neuronal peak discharges no longer increased during the trial. This suggests a direct link between the temporal profile of spiking activity and time estimation. The timing of motor cortical activity reflected the 'elasticity' of the animal's subjective time.