Human Memory and the Medial Temporal Lobe

Human Memory and the Medial Temporal Lobe Yael Shrager Squire Lab December 1, 2005

Patient HM

Patient HM • What events led to his brain lesion? • Knocked down by bicycle at age 9 • Was unconscious for 5 minutes and sustained a head injury • Seizures • Minor seizures since the age of 10 • Major, incapacitating seizures since the age of 16 • Unresponsive to medication • Surgically removed the epileptogenic brain areas

Patient HM • What brain regions were removed? • The medial temporal lobe (MTL) • The specific structures that were removed: amygdala, uncus, anterior two thirds of the hippocampus, anterior portion of hippocampal gyrus

HM’s Lesion

Patient HM • After surgery, it was noticed that something wasn’t quite normal about HM’s behavior… • Could no longer recognize hospital staff or find his way around hospital • Recalled nothing of daily events, such as what he ate for lunch • Read same magazines and did the same puzzles day after day • Did not remember events in the recent past, such as the death of an uncle three years before

Patient HM • But HM’s remote memories (i.e., memories from his childhood) were intact

Patient HM • Formal testing of HM’s cognitive abilities: • Tests of intelligence • Tests of perception • Tests of memory

You test a patient with a lesion of the MTL on a certain task. They do as well as normal people. What can you say? You test the same patient on another task. They don’t do as well as normal people. What can you say?

Tests of Intelligence and Perception

RESULT: No impairment of personality, perception, or general intelligence following bilateral removal of the medial temporal lobe

Tests of Memory

Tests of Memory Does impairment on this task imply a memory impairment? What else would you like to see as a control?

Rey-Osterrieth Complex Figure Hippocampal Lesion MTL Lesion (HM) Control

HM was unable to form new memories. This ability to form long-term memories is dependent on the MTL.

A Short Number Sequence Task: remember the number “584”. A few minutes later, when asked for the number, HM said “584”. How did HM remember the number sequence? HM: “It’s easy. You just remember 8. You see 5, 8, and 4 add to 17. You remember 8, subtract it from 17, and it leaves 9. Divide 9 in half and you get 5 and 4 and there you are: 584. Easy.” A few minutes later, after he had been engaged in another task, HM had no recollection of the number or of his train of thought about it.

Therefore, there is another (immediate) memory system that is intact in HM and is independent of the MTL.

Mirror Tracing Task

Day 1 Day 2 Day 3 Number of Errors How would you expect a normal person to perform on mirror tracing across a number of practice sessions? So, this task involves learning? How do you expect HM to do on mirror tracing?

HM’s Performance on Mirror Tracing • Before each session, HM would claim that he had never done the task. • Yet, once he performed the task… Milner 1965

HM was capable of learning skills even though he had no recollection of doing so. This sort of memory (skill learning) is independent of the MTL.

Remote Memory in Another Amnesic (EP) 100 90 80 70 60 Percent Correct 50 40 30 20 10 0 Familiar Navigation Novel Navigation Alternate Routes

EP’s Knowledge of His Current Neighborhood 100 90 80 70 60 Percent Correct 50 40 30 20 10 0 Current Neighborhood

Summary • The ability to acquire new memories is a distinct function (separate from functions such as perception or reasoning), and it is localized to the medial temporal lobes. • The medial temporal lobes are not required for immediate memory. • The medial temporal lobe is not necessary for acquiring some kind of knowledge. • The medial temporal lobe cannot be the site of final storage for long-term memory of knowledge that was acquired before lesion. • There are multiple memory systems in the brain.

Human Memory and the Medial Temporal Lobe