What comes first? Electrophysiological differences in the temporal course of memory and future thinking.

It is now widely accepted that remembering the past and imagining the future rely on a number of shared processes and recruit a similar set of brain regions. However, memory and future thinking place different demands on a range of processes. For instance, although remembering should lead to early associative retrieval of event details, event construction may be slower for future events, for which details from different memories are combined. In order to shed light on the question of how the brain distinguishes between memories and future thoughts, we investigated the differences in the electrophysiological correlates of the vivid elaboration of future and past events. In the slow cortical potentials of 24 healthy human participants, differences during early elaboration were observed at temporo-parietal and parieto-occipital electrode sites, presumably reflecting differential recruitment of sensory and semantic detail retrieval. Additional differences emerged over the right pre-frontal cortex during later elaboration, which could be linked to differential retrieval demands. In conclusion, the time course differences, which presumably reflect the varying recruitment of sub-processes engaged during mental time travel, will help to understand the mechanisms with which the brain separates memories from future thoughts.

Foreseeing the future: occurrence probability of imagined future events modulates hippocampal activation.

Episodic memory and episodic future thinking are known to share a set of brain regions. Potential differences in activation patterns associated with the two conditions are as yet inconclusive, in particular with respect to hippocampal involvement. Hippocampal activation is modulated by a range of phenomenal qualities during the imagination of both past and future events (Addis et al. (2004) Hippocampus 14:752-762; Addis and Schacter (2008) Hippocampus 18:227-237). A relevant variable in this regard is the occurrence probability of an episode, which varies for future but not past events and thus cannot be equated across conditions. Using parametric modulation analysis, we investigated the effect of occurrence probability of imagined future events on brain activation patterns, while effects of temporal distance, amount of details, and emotionality were controlled for. Activation of right anterior hippocampus increased with decreasing occurrence probability, presumably reflecting higher processing demands during binding of more disparate details for unlikely events. This finding may contribute to the understanding of previously reported inconsistent results concerning hippocampal involvement during the imagination of past and future events.

Impairment of probabilistic reward-based learning in schizophrenia.

Recent models assume that some symptoms of schizophrenia originate from defective reward processing mechanisms. Understanding the precise nature of reward-based learning impairments might thus make an important contribution to the understanding of schizophrenia and the development of treatment strategies. The present study investigated several features of probabilistic reward-based stimulus association learning, namely the acquisition of initial contingencies, reversal learning, generalization abilities, and the effects of reward magnitude. Compared to healthy controls, individuals with schizophrenia exhibited attenuated overall performance during acquisition, whereas learning rates across blocks were similar to the rates of controls. On the group level, persons with schizophrenia were, however, unable to learn the reversal of the initial reward contingencies. Exploratory analysis of only the subgroup of individuals with schizophrenia who showed significant learning during acquisition yielded deficits in reversal learning with low reward magnitudes only. There was further evidence of a mild generalization impairment of the persons with schizophrenia in an acquired equivalence task. In summary, although there was evidence of intact basic processing of reward magnitudes, individuals with schizophrenia were impaired at using this feedback for the adaptive guidance of behavior.

Aging affects acquisition and reversal of reward-based associative learning.

Reward-based associative learning is mediated by a distributed network of brain regions that are dependent on the dopaminergic system. Age-related changes in key regions of this system, the striatum and the prefrontal cortex, may adversely affect the ability to use reward information for the guidance of behavior. The present study investigated the effects of healthy aging on different components of reward learning, such as acquisition, reversal, effects of reward magnitude, and transfer of learning. A group of 30 young (mean age = 24.2 yr) and a group of 30 older subjects (mean age = 64.1 yr) completed two probabilistic reward-based stimulus association learning tasks. Older subjects showed poorer overall acquisition and impaired reversal learning, as well as deficits in transfer learning. When only those subjects who showed evidence of significant learning were considered, younger subjects showed equivalently fast learning irrespective of reward magnitude, while learning curves in older subjects were steeper for high compared to low reward magnitudes. Acquired equivalence learning, which requires generalization across stimuli and transfer of learned contingencies to new stimuli, was mildly impaired in older subjects.

Illusory contrast-induced shifts in binocular visual direction bias saccadic eye movements toward the perceived target position.

The perceived binocular visual direction of a fused disparity stimulus with an interocular contrast difference is biased toward the direction signaled by the eye presented with the higher contrast image (J. S. Mansfield & G. E. Legge, 1996). Does the amplitude of binocular saccadic eye movements have a similar bias? We examined saccades to fused disparate Gabor patches with interocular contrast differences. The effect of these contrast differences on saccadic amplitudes was compared to the perceptual biases in binocular direction obtained in a vernier acuity task. Saccades to unequal contrast targets landed between the end points for equal contrast and monocular targets. For three of our eight subjects, the saccadic bias equaled the perceptual effect. For the other subjects, however, saccades were affected to a lesser extent. Three models for binocular combination were used to evaluate these responses: A maximum-likelihood model failed to predict our results, whereas a model with contrast-dependent weighting of direction estimates by two monocular channels and a gain control model of binocular contrast summation gave a better approximation to our data. Both models showed that for the perceptual system, the influence of the eye that was presented with the higher contrast image was more dominant in the binocular combination than expected from the stimulus contrast ratio. The oculomotor system, however, was close to following linear summation.

When the future becomes the past: Differences in brain activation patterns for episodic memory and episodic future thinking.

Episodic memory and episodic future thinking activate a network of overlapping brain regions, but little is known about the mechanism with which the brain separates the two processes. It was recently suggested that differential activity for memory and future thinking may be linked to differences in the phenomenal properties (e.g., richness of detail). Using functional magnetic resonance imaging in healthy subjects and a novel experimental design, we investigated the networks involved in the imagery of future and the recall of past events for the same target occasion, i.e. the Christmas and New Year's holidays, thereby keeping temporal distance and content similar across conditions. Although ratings of phenomenal characteristics were comparable for future thoughts and memories, differential activation patterns emerged. The right posterior hippocampus exhibited stronger memory-related activity during early event recall, and stronger future thought-related activity during late event imagination. Other regions, e.g., the precuneus and lateral prefrontal cortex, showed the reverse activation pattern with early future-associated and late past-associated activation. Memories compared to future thoughts were further related to stronger activation in several visual processing regions, which accords with a reactivation of the original perceptual experience. In conclusion, the results showed for the first time unique neural signatures for both memory and future thinking even in the absence of differences in phenomenal properties and suggested different time courses of brain activation for episodic memory and future thinking.

[Mental time travel - the neurocognitive basis of future thinking]. / Mentales Zeitreisen - Neurokognitive Grundlagen des Zukunftsdenkens.

The ability to travel in time mentally, i. e. the re-experiencing of personal past events as well as the ability to mentally simulate potential future events, forms part of the "episodic memory" concept. Evidence for the notion that episodic memory and episodic future thinking share a common neural basis stems from different lines of research, namely functional neuroimaging, assessment of clinical groups, behavioral investigations of the phenomenological characteristics of mental time travel, and developmental research. The present article summarises the evidence from these lines of research which indicate a common neural network underlying episodic memory and episodic future thinking, consisting of medial prefrontal, medial temporal, medial parietal, lateral parieto-occipital, as well as lateral temporal regions. Both abilities, episodic memory and future thinking, seem to develop around the age of four years, feature similar phenomenological characteristics, and are impaired to a similar extent by brain lesions and brain dysfunction. These findings yielded different hypotheses concerning the function and evolutional significance of the mental time travel network, which will also be addressed.