Memory

Psy391h - Classes 15-16

(This page last updated 4 April 2006.)
 

 Separate Memory Systems 

• Sensory memory
  • Iconic (visual)
  • Echoic (auditory)
• Working memory (short term)
• Long term memory

Sensory memory demonstrated by partial report task.

• Whole report task
  • Display shown for 100 msec or less.
  • Number of letters varies from trial to trial.
  • Report as many letters as you can.
  • Can never report more than 4 or 5 letters correctly.
  • From this, we might conclude that only 5 letters are ever held in memory, but…
• Partial report task
  • After display disappears, one of 3 tones is heard (high, medium,or low pitch.
  • Tone indicates which row to report.
  • Don't know which row to report until stimulus is gone.
  • Performance much better than with Whole Report.
  • Near perfect recall.
  • Entire display of letters is held in memory, but only 4.5 can be reported.
• Iconic memory decays within seconds, down to the level at which subjects are again reporting just 4.5 letters.
  • Can hold a large amount of unidentified visual information.

Working memory vs. long term memory

• Distinction suggested by William James.
• Working memory holds info that is currently active.
• Very limited in capacity.
• Long term memory has very large capacity.
• Although psychologists have been trying to distinguish between short-term and long-term memory for over a century, the strongest evidence that they are separate systems comes from studies of anterograde amnesia.

Working memory will be covered in the section on Executive Functions. Here we will focus on long term memory.

Retrograde amnesia: Cannot remember events prior to brain damage.
Anterograde amesia: Cannot record memories of events that occur after brain damage.

Bilateral hippocampal damage produces strong anterograde amnesia, which is often accompanied by weaker retrograde amnesia.

Patient H.M.

• Epilepsy
• Treated in 1953 by removing hippocampus bilaterally and parts of temporal lobe surrounding it.
• After surgery, severe anterograde amnesia.
• Also weak retrograde amnesia.
• Amnesia persists to this day.
• Forms no new explicit long term memories.
  • Little or no memory of any events that have happened since his operation.
  • Retains memories for events before operation.
• Working memory unaffected.
  • Can carry on a normal conversation.
• Evidence for separation of short-term and long-term memory systems.

What is the function of the hippocampus?

It is NOT where all old memories are stored.

It does seem to be crucial for recording new explicit memories. The information is actually stored elsewhere in the brain, however.

Clive Wearing

• British musician
• Viral encephalitis
• Damaged hippocampus bilaterally
• Also frontal lobe damage
• Often feels like he has just become conscious.
• Remembers his wife, but can no longer recall many important aspects of his life.
• Can form no new episodic memories, and has little ability to acquire new semantic memories.
• Retains his musical ability.

H.M. and Clive Wearing are not the only patients with this type of amnesia.

• Anoxia
  • Lack of oxygen in the blood
• Ischemia
  • Lack of blood to the brain
  • Patient R.B.
    • during heart bypass surgery
• Neurons in the hippocampus are particularly sensitive to lack of oxygen.
• Korsakoff's Syndrome
  • Thiamine deficiency due to alcoholism
  • Does not damage the hippocampus, but mammilary bodies and dorsomedial nucleus of the thalamus.
• Patient N.A.
  • Stab wound to the medial thalamus

Amnesic patients have normal implicit memory.

• Normal procedural learning
• Mirror reading
• Mirror tracing
• Serial Reaction Time Task
• Note Clive Wearing's piano playing
• Serial reaction time task
  • See sequence in lights and recreate it with button presses.
  • Portion of pattern is repeated.
  • Faster at reproducing repeated portion.
  • Subjects not aware of repetition.
• Normal priming
  • Lexical decision
  • Word stem completion
• Artificial grammar learning
  • Set of rules (artificial grammar) used to generate strings of letters.
  • Subject does not know rules, but must judge which new strings are formed by same rules as old strings.
  • Performance above chance, even though subjects cannot describe rules.
  • Seemed to have learned the rules implicitly.
  • Amnesic patients can also learn the rules implicitly.

Patient M.S.: Damage to priming without affecting explicit memory.

• Most of areas 18 and 19 on right removed.
• Blind in most of left field.
• Impairment of priming in task of reading masked words.
• No impairment in explicit recognition.

Retrograde Amnesia without Anterograde Amnesia.

• Damage to anterior temporal lobe near temporal pole.
• Can extend back many decades.
• Some patients can still form new long term memories.
• Memories are probably distributed across cortex. Anterior temporal lobe may not hold the information, but may help in retrieving it.

Memory Consolidation

• The hippocampus is important for encoding new explicit long term memories.
• Also important for consolidating those encoded memories.
  • Consolidation takes up to a few years.
• Evidence
  • Some retrograde amnesia in hippocampal patients.
  • Studies with hippocampal lesions in monkeys.

Animal studies also show that amygdala is not required for encoding regular memories, but that cortical areas near hippocampus do play a role.

• Entorhinal cortex
• Perirhinal cortex
• Parahippocampal cortex

Brain Imaging: Wagner et. al. (1998)

• Does memory failure arise from encoding problems or retrieval problems?
• Study list of words, one appearing every 2 seconds.
• Recognition test.
• Compare brain activation for words remembered and words forgotten.
• Less Activation when forgotten words were encoded.
  • Parahippocampal/Fusiform gyri
  • Posterior left inferior frontal gyrus
  • Frontal operculum
• Encoding is responsible for at least part of the memory failure in this task.

Other brain imaging studies also show frontal activity in memory.

• Surprisingly, more activity on left during episodic encoding, and more on right during episodic retrieval.
• More evidence on left during semantic retrieval.

Brain Activity in Priming

• PET scan during word stem completion.
• Some words represented after being presented earlier.
• For old words, less occipital activity (area 19).

For a summary of different long-term memory systems, take a careful look at Figure 8.40 in the textbook.

Cellular Mechanisms Underlying Long-Term Memory: Long Term Potentiation (LTP)

• Stimulate neuron with burst of pulses
  • At least 5 pulses/sec
• Afterward, EPSP is cell is stronger.
• A slow rate of pulses can produce long-term depression (LTD).
• Found in hippocampus, and also in other brain regions.

Relevant Website

• Newspaper story about Clive and Deborah Wearing

next: language

Psych 391h: Cognitive Neuroscience Kyle Cave Psychology Dept. U. Mass.