Source memory is memory for the time and place in which an event occurred. Without source memory for when she ran the dishwasher and when she put the dishes in, she can’t say which occurred more recently. And so the dishes may be dirty or they may be clean.
It's not just my mother. I remember going to an Indian restaurant. I remember the food, the décor of the place, and the waiter’s face. But I can't remember whether I went last week or the week before. Or what about the movie I watched about a year ago? I was pretty sure I'd seen it on Netflix. But, as my friend correctly reminded me, I really saw it on an airplane last summer. Even when we remember particular events, we can have trouble placing them in the time and place that they occurred.
There was an interesting study that asked how source memory is affected by age.
Subjects from 21 to 80 years old were shown 70 pictures, one at a time, on a screen. The chair to the right is an example of one of the pictures. The pictures appeared in one of four quadrants of the screen: top-left, bottom-left, top-right, or bottom-right. Subjects were later shown 100 pictures, which included the original 70. These 100 pictures were presented one at a time in the center of the screen, and subjects were asked (1) whether the picture had been shown previously (i.e. whether it was one of the earlier 70) and (2) which quadrant of the screen it had appeared in. The subjects could press one of 4 keys on a keyboard to indicate the quadrant.
The different colors of the dots in the graph below represent subjects of different ages. The color code is listed to the right of the graphs.
|From Cansino 2009, International Journal of Psychophysiology|
The vertical axis of the graphs shows subjects' % correct. So a bright green dot at the top of the graph would represent a subject in his 20s who scored near 100% correct.
The left graph shows % correct for recognition memory ('Did this picture appear during the previous presentation?'). The right graph shows % correct for source memory ('On which part of the screen did the picture appear last time?').
Older subjects, in general, did worse on both types of memory. But the age-related decline was much steeper for source memory.
But notice that even for people in their 70's, the individuals who performed well did as well or better than many of the youngsters in their 20's - even for source memory. Some people maintain very good memory as they get older.
This task seems pretty artificial, and not very much like trying to remember whether I saw a movie in an airplane or at home on Netflix. This is often the case in controlled psychology experiments, and it's the price you often pay in order to study a psychological phenomenon rigorously.
Source memory is strongly associated with function of the frontal lobes of the brain, and damage to the frontal lobes often results in disruption of source memory. (Activity within the hippocampus and other regions of the medial temporal lobes are associated with both source memory and memory for the sensory details of past remembered events -- see Electrical stimulation of memories).
You might imagine that different brain regions would be involved in remembering where an event occurred compared to remembering when it occurred. But both types of source memory appear to involve the same general region of the frontal lobes, within the prefrontal cortex.
|From Rajah et al 2011, Neuroimage|
2. The brain regions highlighted in blue represent those that were highly activated both when recalling where and when an event occurred.
3. Areas 44/45, 47, and 46 are all within regions of the prefrontal cortex. These prefrontal areas have previously been associated with source memory. L stands for left; R for right.
OK, Mom. You now know just about everything I do about source memory. There's nothing left that I know and you don't.
Here’s the little magnet I bought my mother to put on her dishwasher.