SAGE Publications

Bartlett viewed thinking as a high level skill exhibiting ballistic properties that he called its “point of no return”. This paper explores one aspect of cognition through the use of a simple model task in which human subjects are asked to commit attention to a position in visual space other than fixation. This instruction is executed by orienting a covert (attentional) mechanism that seems sufficiently time locked to external events that its trajectory can be traced across the visual field in terms of momentary changes in the efficiency of detecting stimuli. A comparison of results obtained with alert monkeys, brain injured and normal human subjects shows the relationship of this covert system to saccadic eye movements and to various brain systems controlling perception and motion. In accordance with Bartlett's insight, the possibility is explored that similar principles apply to orienting of attention toward sensory input and orienting to the semantic structures used in thinking.

The selective impairment of semantic memory is described in three patients with diffuse cerebrallesions. These patients, selected on the basis of a failure to recognize or identify common objects (agnosia for objects), were investigated in detail. In particular, their perceptual, language and memory functions were assessed, and the limits and properties of their recognition difficulties explored.

It was found that knowledge of pictorial representations of objects, and of words, was impaired or impoverished, and in both instances knowledge of subordinate categories was more vulnerable than superordinate categories. Evidence is presented that this impairment of semantic memory cannot be accounted for by intellectual impairment, sensory or perceptual deficits, or expressive language disorder. The implications of damage to the semantic memory system for the operation of other cognitive systems, in particular short and long-term memory functions, are considered. Some tentative evidence for the structural basis for a hierarchically organized modality-specific semantic memory system is discussed.

Two experiments were carried out to investigate the difficulty of making the contra-positive inference from conditional sentences of the form, “if P then Q.” This inference, that not-P follows from not-Q, requires the transformation of the information presented in the conditional sentence. It is suggested that the difficulty is due to a mental set for expecting a relation of truth, correspondence, or match to hold between sentences and states of affairs. The elicitation of the inference was not facilitated by attempting to induce two kinds of therapy designed to break this set. It is argued that the subjects did not give evidence of having acquired the characteristics of Piaget's “formal operational thought.”

In shadowing one of two simultaneous messages presented dichotically, subjects are unable to report any of the content of the rejected message. Even if the rejected message consists of a short list of simple words repeated many times, a recognition test fails to reveal any trace of the list. If numbers are interpolated in prose passages presented for dichotic shadowing, no more are recalled from the rejected messages if the instructions are specifically to remember numbers than if the instructions are general: a specific set for numbers will not break through the attentional barrier set up in this task. The only stimulus so far found that will break through this barrier is the subject's own name. It is probably only material “important” to the subject that will break through the barrier.

Two messages were presented dichotically and subjects were asked to “shadow” whatever they heard on one ear. Somewhere in the middle the two passages were switched to the opposite ears. Subjects occasionally repeated one or two words, at the break, from the wrong ear, but never transferred to it for longer than this. The higher the transition probabilities in the passage the more likely they were to do this. One explanation might be that the “selective filter” (Broadbent, 1958) acts by selectively raising thresholds for signals from the rejected sources rather than acting as an all-or-none barrier.

Experiment I studied short-term memory (STM) for auditorily presented five word sequences as a function of acoustic and semantic similarity. There was a large adverse effect of acoustic similarity on STM (72·5 per cent.) which was significantly greater (p < 0·001) than the small (6·3 per cent.) but reliable effect (p < 0·05) of semantic similarity.

Experiment II compared STM for sequences of words which had a similar letter structure (formal similarity) but were pronounced differently, with acoustically similar but formally dissimilar words and with control sequences. There was a significant effect of acoustic but not of formal similarity.

Experiment III replicated the acoustic similarity effect found in Experiment I using visual instead of auditory presentation. Again a large and significant effect of acoustic similarity was shown.

Right, mixed and left handers are found in binomial proportions in seven samples of varied subjects whose lateral preferences were ascertained by several methods. These proportions have been obtained in previous studies of humans and animals when the performance of several actions has been recorded in complete samples and when consistent right and left subjects have been separated from those of mixed usage.

Does our limited capacity in selective listening tasks arise primarily in perception or in response organization? To examine this, subjects were given two dichotic messages, one primary and one secondary, and had to make two different responses: the primary response was to “shadow” the primary message; the secondary response was to tap on hearing certain target words in either message. Since the secondary response was identical for the two messages, any difference in its efficiency with the two messages must be due to a failure in perception of the secondary message. Any interference between the primary and secondary responses (repeating and tapping) to target words in the primary message must be due to a limit in performing simultaneous responses, since if either was correctly performed the target word must have been perceived. The results clearly showed that the main limit is perceptual.

Various target words were used to investigate the nature of the perceptual and response limits. Factors investigated were (1) the information content of the target words, (2) their range of meanings, (3) their grammatical class, and (4) the compatibility between stimuli and responses. A relative lack of response competition was found, which might be due to successive organization of the two responses at different stages in the perceptual sequence. The results were interpreted in terms of signal detection theory and the effects of reduced signal-to-noise ratio produced by inattention were compared with those produced by an external masking noise.

The systems underlying word recognition were investigated in a single case study of a patient (K.F.) with an acquired dyslexia. His reading performance was related to parts of speech, word frequency and word concreteness, and his reading errors were analysed. There was a very striking difference between his ability to read concrete and abstract words. Furthermore visual errors, which could not be attributed to a deficit at a peripheral level, predominated; phonemic errors did not occur. It is argued that these findings support a dual encoding model of word recognition, the present case illustrating the impairment of the phonemic route, a direct graphemic-semantic route being relatively spared. These findings and interpretation are for the most part consistent with Marshall and Newcombe's (1973) studies of acquired dyslexia. The present findings are discussed in terms of more general theories of word recognition.