January 3, 2007
Which brain regions enable us to remember our past and anticipate our future?
SCR Feature, brain imaging, future thinking, memory, self-awareness — thomasr
In our minds, we often relive past events and contemplate possible future scenarios. This ability to mental time travel (MTT) into the past (episodic remembering) and future (pre-experiencing) is, arguably, an ability that is unique to humans. Non-human species clearly possess memory, which is necessary for the past-oriented component of MTT (episodic remembering). Some memory scientists, however, believe that there are different types of memory that each correspond to a different memory system inside the brain. Thus, it is possible that at least one type of memory system could be unique to humans.
The term episodic memory refers to our memory for past events (what happened, as well as when and where it happened) (Tulving, 1985), and it is this type of memory that is believed, by some people, to be unique to humans. The definition of episodic memory, however, does not center on the type of information that one stores about an event (what happened, when and where), instead it emphasizes the role that autonoetic consciousness (or autonoesis for short) plays when one re-experiences (or remembers) a past event.
Autonoetic Consciousness and Chronesthesia
Autonoesis is a type of consciousness that enables individuals to be aware of themselves as a continuous entity across time (Tulving, 1985; Wheeler, Stuss & Tulving, 1997). Chronesthesia is a form of consciousness that enables individuals to be aware of their personal past and future. It “confers the special phenomenal flavour to the remembering of past events, the flavour that distinguishes remembering from other kinds of awareness, such as those characterizing perceiving, thinking, imagining, or dreaming” (Tulving, 1985:3). Although the ability to MTT has been attributed to autonoesis, this ability has more recently been ascribed to chronesthesia, a neurocognitive capability that is required for various behavioural and cognitive activities that involve time (Tulving, 2002). Chronesthesia is a form of consciousness that enables individuals to be aware of their personal past and future.
Undoubtedly, autonoesis and chronesthesia are similar concepts, but they are not one and the same thing. Although autonoesis implies awareness of self in time, the focus on self, as opposed to time, is what differentiates it from chronesthesia. Perhaps the best way to convey the distinction between autonoesis and chronesthesia is to quote Tulving (2002: 315):
Thus it could be said that chronesthesia is the temporal component of autonoesis. The difference between these two types of consciousness may become clearer by considering the case of K.C. in relation to other brain-damaged patients.
The case of K.C. in relation to other brain-damaged patients
In contrast to other brain-damaged patients, who cannot reflect upon themselves or on information pertaining to them in a meaningful way (Eslinger & Damasio, 1985; Schacter, Glisky, & McGlynn, 1990; Stuss 1991), K.C. remains perfectly aware of himself in the present (Tulving, 1985). K.C.’s difficulties concern his ability to think about his personal past and future. On this basis, it seems that K.C.’s impairment is more related to impaired awareness of personal past and future (chronesthesia) rather than to a deficit of self awareness in subjective time (autonoesis) (Tulving, 2002).
An investigation of past and future thinking using positron emission tomography
Having branched off from Tulving’s work on episodic memory, chronesthesia has recently become a new topic of study. A handful of functional magnetic resonance imaging studies have already focused on past and future thinking and are now in the process of being published. One published study by Okuda, Fujii, Ohtake, Tsukiura, Kazuyo, Tanji, Suzuki, Kawashima, Fukuda, Itoh, and Yamadori (2003), however, has already investigated the topic of past and future thinking using positron emission tomography. These experimenters measured the regional cerebral blood flow of young, healthy subjects, while the subjects talked about the past and future with respect to two temporal windows: ‘far’ (several years) and ‘near’ (several days). The experimenters compared the brain activity associated with the past and future thinking tasks with that of a control task, where subjects explained the meaning of three nouns: car, surprise and money.
Okuda et al. (2003) found considerable overlap in brain activity for the past and future thinking about the future is closely related to retrospective memory tasks, compared to the control task, in various brain areas, including the left and right parahippocampal gyri and the right hippocampus, as well as frontopolar and medial occipitoparietal areas. All activations in the medial temporal lobes and most of the activity observed in the superior frontal gyri were common to at least one future task and one past task. Deactivations common to the past and future tasks were observed in the prefrontal cortices bilaterally, including the left inferior frontal gyrus (Broca’s area) and lateral parts of the bilateral anterior prefrontal cortices (BA 11).
The experimenters, however, also found differences in brain activity between the past and future thinking tasks. Specifically, the bilateral areas in the most anterior, but slightly medial, portion of the superior frontal gyrus (BA 10 in the left hemisphere and BA 11 in the right) showed greater activation during the future tasks than during the past tasks and greater activation during the ‘far’ tasks than during the ‘near’ tasks. The right hippocampus, as well as the left and right parahippocampal gyri, showed greater or equivalent levels of activation during the future tasks compared with the past tasks.
The results of this study by Okuda et al. suggest that thinking about the future is closely related to retrospective memory, but that specific areas in the frontal pole and the medial temporal lobes are more involved with thinking about the future than about the past.
Figure 1. Medial temporal and frontopolar areas showing significant effect for temporal direction. Red lines intersect at areas of activation. Activations for the future task are in yellow, and activations for past tasks are in green. Images were taken from Okuda et al. (2003) to construct this figure.
Study results in relation to chronesthesia
If one agrees that mentally traveling into the past (episodic remembering) and future (pre-experiencing) rely on chronesthesia, then it is not surprising that Okuda et al. found a good deal of overlap between the past and future thinking tasks. Nor is it surprising that Okuda et al. found some brain areas to be more activated for future thinking tasks compared to past thinking tasks, as there must be neural differences between these two types of thinking that are correlated with one’s conscious awareness of how they differ.
What will we learn from future studies on chronesthesia?
We have already started to learn about impaired chronesthesia in amnesics; K.C., for example, can operate mentally in the present but is helpless when he has to mentally travel into the past or into the future (Tulving, 1985). It seems likely that future functional neuroimaging studies will demonstrate differences in brain activity for tasks that are set in the present with those that are oriented in either the past or future. It will be interesting to see what we will learn from future studies that focus on this exciting topic.
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