Real Reading Behavior

The most obvious observable activities that accompany reading are the eye fixations on various parts of the text. Our laboratory has now developed the technology for automatically measuring and recording the sequence and duration of eye fixations that readers make in a fairly natural reading situation. This paper reports on research in progress to use our observations of this real reading behavior to construct computational models of the cognitive processes involved in natural reading. In the first part of this paper we consider some constraints placed on models of human language comprehension imposed by the eye fixation data. In the second part we propose a particular model whose processing time on each word of the text is proportional to human readers' fixation durations.t S o m e O b s e r v a t i o n s The reason that eye fixation data provide a rich base for a theoretical model of language processing is that readers' pauses on various words of a text are distinctly non-uniform. Some words are looked at very briefly, while others are gazed at for one or two seconds. The longer pauses are associated with a need for more computation [2]. The span of apprehension is relatively small, so that at a normal reading distance a reader cannot extract the meaning of words that are in peripheral vision [6]. This means that a person can read only what he looks at, and for scientific texts read normally by college students, this involves looking at almost every word. Furthermore, the longer pauses can occur immediately on the word that triggers the additional computation [4]. Thus it is possible to infer the degree of computational load at each point in the text. The starting point for the computer model was the analysis of the eye fixations of 14 Carnegie-Mellon undergraduates reading 15 passages (each about 140 words long) taken from the science and technology sections of Newsweek and Time magazines (see the Appendix for a sample passage). The mean fixation duration on each word (or on larger, clause-like sectors) of the text were analyzed in a multiple regression analysis in which the independent variables were the structural prcperties of the texts that were believed to affect the fixation durations. The results showed that fixation durations were influenced by several levels of processing, such as the word level (longer, less frequent 1This research was supported in part by grants from the Alfred P. Sloan Foundation. the National Institute of Education (G-79-0119) and the National institute of Mental Health (MH-29617) words take longer to encode and lexically access), and the text level (more important parts of the text, like topics or definitions take longer to process than less important parts). This analysis generated a verbal description of a model of the reading process that is consistent with the observed fixation durations. The details of the data, analysis, and model are reported elsewhere [5]. Some of the most intriguing aspects of the eye-fixation data concern trends that we have failed to find. Trends within noun phrases and verb phrases seem notable by their absence. Most approaches to sentence comprehension suggest that when the head noun of a noun phrase is reached, a great deal of processing is necessary to aggregate the meanings of the various modifiers. But this is not the case. While determiners and some prepositions are looked at more briefly, adjectives, noun-classifiers, and head nouns receive approximately the same gaze durations. (These results assume that word length effects on gaze duration have been covaried out). Verb phrases, with the exception of modals, show a similar flat distribution. It is also notable that verbs are not gazed at longer than nouns, as might be expected. Such results pose an interesting problem for a system which not only recognizes words, but also provides for their interpretation. Anotl"ler interesting result is the failure to find any associations with length of sentences (a rough measure of their complexity) or ordinal word position within sentences (a rough measure of amount of processing). That is to say, whether or not word function, character-length or syllables, etc., are controlled, there are no systematic trends associated with ordinal word position or sentence length. There is an added gaze duration associated with punctuation marks. Periods add about 73 milliseconds, and other punctuation (including commas, quotes, etc.) add about 43 milliseconds each above what can be accounted for by character-length or other covariates. T h e F r a m e w o r k The strategy for making sense of these and other similar observations is to develop a computational framework in which they can be understood. That framework must be capable of performing such diverse functions as word recognition, semantic and syntactic analysis, and text analysis. Furthermore, it must permit the ready interaction among processes implied by these functions. The framework we have implemented to accomplish these ambitious goals is a production system fashioned closely after Anderson's ACT system [1]. Such a production system is composed of three parts, a collection of productions comprising knowledge about how to carry out processes, a declarative knowledge base against which those processes are carried out, and an interpreter which provides for the actual behavior of the productions.