Jones D.M.The new C standard.An economic and cultural commentary.Sentence 0.2005

Visual information held in the visuo-spatial sketch pad decays very rapidly. Experiments have shown that people can recall four or five items immediately after they are presented with visual information, but that this recall rate drops very quickly after a few seconds. From the source code reading point of view, the visuo-spatial sketch pad is only operative for the source code currently being looked at.

While remembering digit sequences, readers may have noticed that the sounds used for them went around in their heads. Research has uncovered a system known as the phonological (or articulatory) loop. This kind of memory can be thought of as being like a loop of tape. Sounds can be recorded onto this tape, overwriting the previous contents, as it goes around and around. An example of the functioning of this loop can be found, by trying to remember lists of words that vary by the length of time it takes to say them.

Table 0.14 contains lists of words; those at the top of the table contain a single syllable, those at the bottom multiple syllables. Readers should have no problems remembering a sequence of five single-syllable words, a sequence of five multi-syllable words should prove more difficult. As before, read each word slowly out loud.

visuo-spatial memory

phonological loop

It has been found that fast talkers have better short-term memory. The connection is the phonological loop. Short-term memory is not limited by the number of items that can be held. The limit is the length of sound this loop can store, about two seconds.[26] Faster talkers can represent more information in that two seconds than those who do not talk as fast.

An analogy between phonological loop and a loop of tape in a tape recorder, suggests the possibility that it might only be possible to extract information as it goes past a read-out point. A study by Sternberg[423] looked at how information in the phonological loop could be accessed. Subjects were asked to hold a sequences of digits, for instance 4185, in memory. They were then asked if a particular digit was in the sequence being held. The time taken to respond yes/no was measured. Subjects were given sequences of different length to hold in memory. The results showed that the larger the number of digits subjects had to hold in memory, the longer it took them to reply (see Figure 0.23). The other result was that the time to respond was not affected by whether the answer was yes or no. It might be expected that a yes answer would enable the search to be terminated. This suggests that all digits were always being compared.

A study by Cavanagh[64] found that different kinds of information, held in memory, has different search response times (see Figure 0.24).

A good example of using the different components of working memory is mental arithmetic; for example, multiply 23 by 15 without looking at this page. The numbers to be multiplied can be held in the phonological loop, while information such as carries and which two digits to multiple next can be held within the central executive. Now perform another multiplication, but this time look at the two numbers being multiplied (see margin for values) while performing the multiplication. 26

While performing this calculation the visuo-spatial sketch pad can be used to hold some of the informa-12 tion, the values being multiplied. This frees up the phonological loop to hold temporary results, while the

memory span

Mean reaction time (msec)

Processing time (msec/item)

80

60

40

20

• nonsense syllables

random forms •

geometric shapes • • words

colors • • letters

• digits

memory

semantic memory

episodic declarative knowledge

developer visual manipulation

Figure 0.26: One of the two pairs are rotated copies of each other.

with working memory capacity and age making up the difference.

Source code constructs differ in their likelihood of forming semantically meaningful chunks. For instance, the ordering of a sequence of statements is often driven by the operations performed by those statements, while the ordering of parameters is often arbitrary.

Declarative memory is a long-term memory (information may be held until a person dies) that has a huge capacity (its bounds are not yet known) and holds information on facts and events (declarative knowledge is discussed elsewhere). Two components of declarative memory of interest to the discussion here are episodic and semantic memory. Episodic memory[24] is a past-oriented memory system concerned with remembering, while semantic memory is a present-oriented memory system concerned with knowing.

Having worked on a program, a developer may remember particular sections of source code through their interaction with it (e.g., deducing how it interacted with other source code, or inserting traces to print out values of objects referenced in the code). After working on the same program for an extended period of time, a developer is likely to be able to recall information about it without being able to remember exactly when they learned that information.[163]

16.2.2.1 Visual manipulation

How are visual images held in the brain? Are they stored directly in some way (like a bitmap), or are they held using an abstract representation (e.g., a list of objects tagged with their spatial positions). A study performed by Shepard[395] suggested the former. He showed subjects pairs of figures and asked them if they were the same. Some pairs were different, while others were the same but had been rotated relative to each other. The results showed a linear relationship between the angle of rotation (needed to verify that two objects were the same) and the time taken to make a matching comparison. Readers might like to try there mind at rotating the pairs of images in Figure 0.26 to find out if they are the same.

Kosslyn[229] performed a related experiment. Subjects were shown various pictures and asked questions about them. One picture was of a boat. Subjects were asked a question about the front of the boat and then asked a question about the rear of the boat. The response time, when the question shifted from the front to the rear of the boat, was longer than when the question shifted from one about portholes to one about the rear. It was as if subjects had to scan their image of the boat from one place to another to answer the questions.

A study by Presson and Montello[360] asked two groups of subjects to memorize the locations of objects in a room. Both groups of subjects were then blindfolded and asked to point to various objects. The results showed their performance to be reasonably fast and accurate. Subjects in the first group were then asked to imagine rotating themselves 90°, then they were asked to point to various objects. The results showed their performance to be much slower and less accurate. Subjects in the second group were asked to actually rotate 90°; while still blindfolded, they were then asked to point to various objects. The results showed that the performance of these subjects was as good as before they rotated. These results suggest that mentally keeping track of the locations of objects, a task that many cognitive psychologists would suspect as being

Figure 0.27: Proportion of errors (left) and time to recall (right) for recall of paired associate words. Based on Anderson.[10]

cognitive and divorced from the body, is in fact strongly affected by literal body movements (this result is more evidence for the embodied mind theory[467] of the human mind).

16.2.2.2 Longer term memories

People can store large amounts of information for long periods of time in their long-term memory. Landauer[239] attempts to estimate the total amount of learned information in LTM. Information written to LTM may not be held there for very long (storage), or it may be difficult to find (retrieval). This section discusses storage and retrieval of information in LTM.

One of the earliest memory research results was that practicing an item, after it had been learned, improves performance of recall at a later time (first published by Ebbinghaus in 1885, and reprinted several times since[102]). The relationship between practice, P , and time, T , to recall has been found to follow a power law T = aP b (where a and b are constants). This relationship has become known as the power law of learning. A similar relationship has been found for error rates— more practice, fewer errors.

How is information stored in LTM? The brain contains neurons and synapses; information can only be represented as some kind of change in their state. The term memory trace is used to describe this changed state, representing the stored information. Accessing an information item in LTM is thought to increase the strength of its associated memory trace (which could mean that a stronger signal is returned by subsequent attempts at recall, or that the access path to that information is smoothed; nobody knows yet).

Practice is not the only way of improving recall. How an item has been studied, and its related associations, can affect how well it is recalled later. The meaning of information to the person doing the learning, so-called depth of processing, can affect their recall performance. Learning information that has a meaning is thought to create more access methods to its storage location(s) in LTM.

The generation effect refers to the process whereby people are involved in the generation of the information they need to remember. A study by Slamecka and Graf[411] asked subjects to generate a synonym, or rhyme, of a target word that began with a specified letter. For instance, generate a synonym for sea starting with the letter o (e.g., ocean). The subjects who had to generate the associated word showed a 15% improvement in recall, compared to subjects who had simply been asked to read the word pair (e.g., sea–ocean ).

An example of the effect of additional, meaningful information was provided by a study by Bradshaw and Anderson.[50] Subjects were given information on famous people to remember. For instance, one group of subjects was told:

memory

information elaboration

memory serial lists

primacy effect memory

recency effect memory

while other groups were given two additional facts to learn. These facts either elaborated on the first sentence or were unrelated to it:

Newton became emotionally unstable and insecure as a child

Newton’s father died when he was born

Newton’s mother remarried and left him with his grandfather

After a delay of one week, subjects were tested on their ability to recall the target sentence. The results showed that subjects percentage recall was higher when they had been given two additional sentences, that elaborated on the first one (the performance of subjects given related sentences being better than those given unrelated ones). There was no difference between subjects, when they were presented with the original sentence and asked if they recognized it.

The preceding studies involved using information that had a verbal basis. A study by Standing, Conezio, and Haber[421] involved asking subjects to remember visual information. The subjects were shown 2,560 photographs for 10 seconds each (640 per day over a 4-day period). On each day, one hour after being shown the pictures, subjects were shown a random sample of 70 pairs of pictures (one of which was in the set of 640 seen earlier). They had to identify which of the pair they had seen before. Correct identification exceeded 90%. This and other studies have confirmed people’s very good memory for pictures.

16.2.2.3 Serial order

The order in which items or events occur is often important when comprehending source code. For instance, the ordering of a function’s parameters needs to be recalled when passing arguments, and the order of statements within the source code of a function specifies an order of events during program execution. Two effects are commonly seem in human memory recall performance:

1.The primacy effect refers to the better recall performance for items at the start of a list.

2.The recency effect refers to the better recall performance for items at the end of a list.

A number of models have been proposed to explain people’s performance in the serial list recall task. Henson[162] describes the start–end model .

developers organized knowledge

identifier memorability

The general conclusion from the, many, study results is that interference occurs in both learning and recall when there are multiple associations for the same item. The improvement in performance of subjects in the second category, of proactive inhibition, is thought to occur because of a practice effect.

16.2.2.5 Organized knowledge

Information is not stored in people’s LTM in an unorganized form (for a detailed discussion, see[12]). This section provides a brief discussion of the issues. More detailed discussions are provided elsewhere in the specific cases that apply to reading and writing source code.

Whenever possible, the coding guidelines given in this book aim to take account of the abilities and limitations that developers have. An example of how it is possible to use an ability of the mind (organizing information in memory) to overcome a limitation (information held in LTM becoming inaccessible) is provided by the following demonstration.

Readers might like to try remembering words presented in an organized form and as a simple list. Read the words in Figure 0.29 out loud, slowly and steadily. Then try to recall as many as possible. Then repeat the process using the words given below. It is likely that a greater number of words will be recalled from the organized structure. The words in the second list could be placed into the same structure as the first list, instead they appear in a random order.

identifier cognitive resource usage

identifier syntax selection statement syntax

identifiers containing a greater number of syllables consume more resources in the phonological loop.

There has been some research on the interaction between human memory and software development. For instance, Altmann[7] built a computational process model based on SOAR, and fitted it to 10.5 minutes of programmer activity (debugging within an emacs window). The simulation was used to study the memories, called near-term memory by Altmann, built up while trying to solve a problem. However, the majority of studies discussed in this book are not directly related to reading and writing source code (your author has not been able to locate many). They can, at best, be used to provide indicators. The specific applications of these results occur throughout this book. They include reducing interference between information chunks and reducing the complexity of reasoning tasks.

memory aging

visuo-spatial 0 memory

16.2.2.8 Memory and aging

A study by Swanson[432] investigated how various measures of working memory varied with the age of the subject. The results from diverse working memory tasks were reasonably intercorrelated. The following are the general conclusions:

•Age-related differences are better predicted by performance on tasks that place high demands on accessing information or maintaining old information in working memory than on measures of processing efficiency.

•Age-related changes in working memory appear to be caused by changes in a general capacity system.

•Age-related performance for both verbal and visuo-spatial working memory tasks showed similar patterns of continuous growth that peak at approximately age 45.