Sentence Analyzing Machinery (SAM) is a phrase coined by psycholinguists used to describe the way the brain makes sense of sentences. SAM relies on general assumptions about what sort of sentence it is about to receive. The following is a short description of the way it behaves based on experimental evidence.
SAM starts with the basic assumption the sentence will be in the form: doer, the act, and then the done-to. This works well because most sentences are in the active form (“John hits Fred”). However, if SAM does encounter a passive sentence (“Fred is hit by John”) it changes its initial assumption upon reaching the “by”. SAM then goes back and re-reads the sentence recognizing the first noun as the done-to and the second as the doer. Studies of reaction time have shown evidence for this bias towards active sentences.
Embedded sentences are approached by recursively interpreting the smaller sentences in the context of the wider sentence. When you read, “The man who hit Fred hit John”, SAM is expecting to hear a verb after “man” but is interrupted. This causes SAM to interpret the sentence embedded, then start again using it in terms of the wider sentence “The man … hit John”. SAM reads, “who hit Fred” and understands it to be clarifying exactly which man is being referenced. It can then go back and continue with what the man did. Interestingly, studies have shown these sentences take longer than sentences with strings of propositions such as “The man hit Fred, and the man hit John”.
One of the clues SAM relies on in deducing the meaning of a sentence is function words. Function words are words that operate on the structure of the sentences. Examples are words such as “who” or “that”. As we have seen they are used by SAM to deduce whether a sentence is in the active or passive form, among other things. Although often omitted from everyday speech they speed up sentence recognition and in a few isolated cases they are necessary to understand what is being said.
Garden Path Sentences
Garden path sentences are sentences that lead the SAM down the wrong path and trick it. An example of this is “the horse raced past the barn fell”. When you first read this sentence most people’s reaction is that the sentence is perfectly normal except for the last word, “fell”. This sentence is perfectly well formed and grammatically correct (althought some function words have been omitted). Consider these two tricks to realizing the true meaning of the sentence. Firstly, read it with the omitted function words added, “the horse that was raced past the barn fell”. If you're still having trouble, as you read it remember that it is the past tense version of raced. “Which horse fell?” “The horse raced past the barn fell.”
Why does this sentence fail? What most likely happens is that when SAM approaches "raced" and then "past the barn" it seems like a model doer, act, done-to sentence. The thought of "raced" being in the sense of a human racing the horse past the barn does not seem to apply in this case and so is disregarded (or never considered) by SAM. Perhaps the fact that this sentence is presented in isolation inclines SAM to believe that it is the horse doing the racing (as opposed to being raced) because there is no mention of anyone else that could be "racing" or have "raced" the horse. Consider "the horse raced past the barn by Henry fell" - no confusion.
Garden path sentences show how SAM rigidly sticks to its assumption of doer, act, done-to. They also show that sometimes SAM forgets that the initial assumptions were assumptions at all, and continues on believing the “fell” to be a mistake rather than re-evaluating the sentence. One needs to remember, however, that a case like the one in point is a purposely constructed, artificial example.
Other examples show how SAM can begin on the wrong path, realize and recover. Consider:
The detective examined…
SAM quickly picks up on this as being an active voice sentence in which the subject, the detective, is examining something. SAM lies in wait, expecting another noun that the detective is examining. However, when the rest of the sentence reads:
…by the reporter revealed the truth about the CIA.
SAM realizes upon reaching “by” that this is a passive sentence and so re-reads it with detective as the done-to. Experiments on people reading sentences such as these show their eyes jutting back re-reading the sentence. However if it had begun:
The document examined…
There is no confusion - documents don’t examine, they are examined. Thus SAM is paying attention to semantics just as you would imagine it would. It can begin interpreting a sentence as passive as long as the first noun verb combination can only be a done-to.
This is merely a brief overview of the powerful SAM inside all of us. For further information I recommend:
Psychology by Henry Gleitman (ISBN: 0-393-97364-6)