Understanding Human Cognition through Experimental and Computational Methods
Jay McClellandSymbolic Systems 100
Spring, 2011
Early History of the Study of Human Mental Processes
• Introspectionism (Wundt, Titchener)– Thought as conscious content, but two problems:
• Suggestibility• Gaps
• Freud suggests that mental processes are not all conscious
• Behaviorists (Watson, Skinner) eschew talk of mental processes altogether
Can Experiments Teach Us About the Contents of the Mind?
• Conrad: Verbal coding in short-term memory
• Sachs: Representation of meaning in long-term memory
Conrad’s Experiment
• You will see a series of letters.
• Try to remember them so that, when you see the word recall, you can write them down in the correct order.
• There will be six letters, followed by a brief delay, then the word ‘Recall’ will appear.
• After you see the word recall, write down the letters in order, starting with the first letter and then proceeding through the list.
.
B
M
S
F
X
T
V
N
.
.
.
Recall
B M S F X T V N
Sachs’ Experiment
• Participants heard a story containing a sentence such as:– He sent Galileo, the great Italian Scientist, a letter about it.
• Either immediately, or after reading a few more sentences, the participants were asked which of the following sentences they had heard:– He sent Galileo, the great Italian Scientist, a letter about it.– He sent a letter about it to Galileo, the great Italian Scientist.– Galileo, the great Italian Scientist, sent him a letter about it.
• When tested immediately, nearly all participants chose the correct sentence.
• After a delay, many participants chose the second sentence, but no one chose the third.
A Question:
• What sort of a mechanism should we use to capture the processes that underlie human thought?– A mechanism like the brain?– Or a mechanism like a computer?
The McCulloch-Pitts Neuron
Neuron i
Output fromneuron j
wij
Input
ThresholdOut
put
McCulloch-Pitts neurons can be used to compute logical functions,such as A-AND-B, A-OR-B, A-AND-NOT-B, etc
0
1
The Perceptron
1 if ; else 0i ii
w
; i iw
Problems for the Perceptron
• Depends crucially on the φi
• Some functions require an exponential number of φi
• No one figured out how to train the weights coming in to the φi
– all the possible φi that might ever be needed had to be provided in advance
The Rise of Symbolic Computation
• Mathematics and logic grew up around the use of symbols:– Marks on paper that stand for things.
• Computer programs that do math and logic make use of symbols too.
• Rules of mathematics and logic can be expressed in terms of statements about symbols.– ‘If p then q’ and ‘p’ implies ‘q’
• So symbolic models seemed like they might be effective ways of using computers to model human reasoning.
But AI Didn’t Live Up to It’s Promise Either
• Computers could do math and logic, but they couldn’t:– Recognize objects– Recognize speech– Understand sentences– Retrieve relevant information from memory
• Was there something wrong with the specific models or languages people were using or was there something wrong with the whole approach?
Ubiquity of the Constraint SatisfactionProblem
• In sentence processing– I saw the grand canyon flying to New York– I saw the sheep grazing in the field
• In comprehension– Margie was sitting on the front steps when she heard the
familiar jingle of the “Good Humor” truck. She remembered her birthday money and ran into the house.
• In reaching, grasping, typing…
Graded and variable nature of neuronal responses
Lateral Inhibition in Eye of Limulus
(Horseshoe Crab)
Neural Network Models of Cognition: The Interactive
Activation Model
Newer Directions
• Cognitive Neuroscience:– Using measurements of human brain activity to learn more
about mental processing
• Reasoning with uncertain information:– Probabilistic models of cognition
• Cognition as an embodied process, tied to experience and action.