Studies in selective listening

Cherry (1955) discovered that if a subject listens to a passage of prose presented to one ear and repeats it aloud, he remains ignorant of the content, though not of the general characteristics, of another, different prose passage presented simultaneously to his other ear. The present thesis examines the nature of this selective blocking of content in selective dichotic listening, suggests a model for the mechanisms involved, discusses the relations between this and other "kinds" of attention, and speculates on the neurophysiological mechanisms involved.
Using Cherry's original technique it is shown that even if the rejected passage is a list of seven words repeated 55 times it leaves no detectable trace (Chapter XI). Certain stimuli can break down this block, however, and these seem to be "important" stimuli such as the subject's own name. An attempt to make a word "important" by pairing it with electric shock la described, with another attempt to change the affective value of stimuli by giving the subject instructions to alter his "set*. The first of these attempts was successful (chapter IV). The block appears to be extraordinarily effective, since the subject's attention does not switch even when there is a considerable disparity in loudness between the two messages (Chapter III).
A review is given of some of the recent work on selective listening, and on the basis of this a model is suggested for the mechanisms which underlie the observed phenomena.
The model supposes that the input from either ear is treated in two ways. One pathway from the ear to perceptual mechanisms treats words simply as sounds, and this pathway is not blocked in selective dichptic listening. But there is a second pathway, which leads to the higher analytical mechanisms which treat sounds as words, and in selective listening this may be blocked below the level of conscious perception. The signals passing into the central nervous system are sampled by a localisation device, which compares the two inputs and takes a decision as to whether one or more messages are present and where they are localised. The subject's voluntary decision to listen to one ear and not the other "biasses" the system towards accepting signals along one input pathway rather than the other, unless the localisation device decides there is only one message present in a position which renders the voluntary selection impossible. The brain receives the early parts of the message, and on the basis of past experience of the transitional probabilities between the signals of a language, it predicts what signals are likely to appear later in the message. Whichever input channel receives a message which most nearly matches these predictions will tend to be accepted (Chapter III). Modifications of this basic model are suggested to deal with the role of "important" signals, and situations involving more than two messages (Cbapter IV).
Various experiments to test the model are described, such as one in which the transitional probabilities between signals are varied by using statistical approximations to English. Further, if the same message is sent to the two ears, but with a large time difference between the arrival of a given word at the ears, the subject does not realise that they are the same message if he listens to one of them. If he listens to, and repeats, the one which leads in time he realises that they are the same (his attention breaks down) when the two messages are as much as six or more seconds apart. If he listens to the one which lags in time this only happens wben the messages are less than 1½ seconds apart. The difference is shown not to be an artefact of the time taken to repeat what is heard, which takes only about 1 second. The difference would be predicted by the model, and is related to it in discussion. Other data from the review are also related to the model.
Two phenomena which bear on the general problem of selective listening are discussed - the time taken to switch attention, and the apparent diversity of the "kinds" of attention.
The failure to break down the subject's attention by "setting" him with instructions raised the question of switching time, for one of the conditions used in that experiment was such that the estimate of switching time of attention made by Broadbent (1954), and similar work by the writer, should have allowed the subject ample time to switch to that message for which he was set. An experiment is described which produces results which differ from Broadbent's as regards switching time, and which cast doubt on his "Filter Theory" model. He has suggested that there is a very short term memory store peripheral to a filter which can select one of the possible channels through which signals enter the nervous system, but the data here presented show that the effect of the rate of presentation upon recall of stimuli in an immediate memory experiment is not such as would support Broad- bent's model. A critique of the rationale of experiments on switching time is given, pointing out that the present experimental designs are inadequate to ensure that the subject is indeed switching (Chapter V).
The way in which the concept of attention has been used in modern experimental psychology is reviewed, and four apparently distinct uses are described. Firstly there is the "mentalistic" use of the early experimental psychologists, such as Wundt and Titchener. Secondly, there is the use in vigilance experiments, which seem best described as a prolonged task of signal/noise discrimination. Thirdly, there is the use such as is found in the present thesis, where attention refers to the selection of one from a number of possible channels along which signals may arrive. Lastly, there is the use in the sense of "mental concent- ration". Titchener's list of attention-catching stimuli is compared with the results found in selective dichotic listening. It is suggested that it would be both useful and possible to enquire whether separate mechanisms underlie those different uses of the concept of attention, and a start is made in this direction by showing that there is no significant correlation between scores on a test of concentration and scores on a listening-and-repeating situation (Chapter VI).
In considering the neurophysiology of these phenomena the evidence about four parts of the mechanism is assessed. Firstly evidence is brought from ablation studies in animals and dysphasia in humans to show that the meaning-analysis mechanisms of the brain can be affected without affecting those responsible for comparatively crude discriminations. For example in word deafness aphasia there is little or no lose in auditory sensitivity as measured with an audiometer, but the meaning of words and tunes is utterly lost. Secondly, the position of the auditory localisation mechanisms is considered, and that for comparing the early and later parts of messages received. It is tentatively concluded that the auditory localisation mechanisms are located in the olivary nuclei of the trapezoid body; and that a pathway from the parieto-temporal area of the cortex which is brought into relation with the afferent inflow by way of a circuit involving the reticular formation may be the mechanism of the prediction devices. Thirdly, the level of the block for meaning is discussed. On the basis of ablation studies in animals, GSR conditioning experiments in the awake human, and EEG studies of the ability of the sleeping human to discriminate between signals, it is suggested that the block lies between the auditory cortex and the reticular formation. Lastly, the question of the way in which "switching" of the attention is brought about is discussed. It is tentatively suggested that the initial "biassing" of the system is through a fronto-reticular pathway, and that the reticular formation is probably the centre of the system, where the several factors responsible for producing the particular response observed on any given occasion interact to produce the final result (Chapter VII).
In a final chapter a recapitulation of the model is given together with a brief discussion of the writer's attitude to models in general. The present thesis is considered as a preliminary progress report on work in a field which seems to be as yet untouched except by the writer and Taylor (1960). Points of particular interest are noted and a programme of research using both behavioural and neurophysiological techniques is indicated.