It is emphasised that for the present purposes it is not necessary that the subject actually continues with each problem

until a solution is achieved. It is necessary only that during the period in which the stimulus picture is presented, the subject is continuously processing information referent to the solution of the problem. The primary concern of the experiment is to ensure as far as possible that part of the subject's limited cognitive processing capacity is occupied during the time the stimulus picture is presented, so that only part of this capacity can be devoted to processing of the real visual sensory

information input.

In order to investigate and test these theoretical notions the following experiment was carried out.

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4.13 Method

(i) Apparatus

Sets of five similar black and white photographs were taken of 16 aspects of the local town and university environment

likely to be highly familiar to the experimental subjects to be used in the experiment. The five homogeneous

photographs forming each of the sixteen sets were obtained % either by photographing the same basic scene from five

slightly different angles, or the same scene from the one camera position but with dynamic features such as crowd and traffic patterns differing between each photograph. Prom the resulting pool of 80 transparencies, the best 10 sets of five were selected on the basis of photographic quality (e.g. clarity, contrast, brightness) and adequate perceptual discriminability between the five individual members of each setf

To present the visual stimuli, a Kodak Carousel S slide projector, equipped with a solenoid operated shutter device manually operated by a micro switch was used. The duration of exposure of the slide was controlled by a variable timer wired into the circuit. The slides were projected onto a white screen in front of the seated subject, the centre of the projected picture being approximately at the subject's eye level. The screen was at a distance of approximately 2.6 metres from the subject's eyes,.

A Perrograph Vortexion tape recorder was used to record subject's verbal descriptions.

The experimental room was well sound-attenuated, but in order to mask any possible distracting extraneous noises a white noise generator emitted very low intensity * Prints of these photographs are shown in Appendix III.

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noise through a small loudspeaker for the duration of the testing session.

(ii) Subjects

14 undergraduate and postgraduate students of the University of St. Andrews (7 male, 7 female) were employed in the study. All had normal corrected vision and no optical defects. None of the subjects had any knowledge in relation to the purpose of the experiment.

(iii) Procedure

At the commencement of the session, subjects were comfortably seated in the experimental room and the VVIQ was administered. (The complete l6-item questionnaire is shown in AppendixII). Subjects completed the VVIQ at their own pace, alone in the experimental room. The experimenter left the room after explaining the VVIQ procedure to minimise any distraction, time-pressure or embarrassment of the subjects, none of whom had previously encountered a questionnaire of this type. In the instruct-

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-ions to the VVIQ subjects were directed to complete "che whole 16 items, imaging with their eyes open, and secondly the entire 16 items imaging with their eyes closed. This procedure was adopted to minimise visual adaptation after effects resulting from alternating eyes-open, eyes-closed imagery.

On completion of the VVIQ, the experimenter returned to the room. A series of 20 stimulus slides, consisting of two homogeneous slides from each of the 10 heterogeneous stimulus categories was then shown to the

subject. The two homogeneous slides from each category were presented consecutively so that subjects could be made

aware of the sorts of differences they could expect between pictures within the homogeneous sets. The learning set of 20 slides was presented once for five seconds exposure per slide, then a second time, in the reverse order, again for five seconds exposure per slide, and, finally, a third time in the original order at an item exposure time of five

seconds. Thus, each picture was seen by the subject for a total of 15 seconds, and the subject had three occasions on which to compare the slides of each pair. For the reasons outlined earlier (p. 77 )» it was considered that, in view of the power of picture memory empirically demonstrated in experiments using large numbers of stimuli and very short exposure durations, 15 seconds presentation time would be more than sufficient to ensure over-learning of the stimulus pictures. Further, the consecutive display of the two homogeneous pictures within each category ensured that subjects were well aware of the extent to which these pictures varied. ^

As an additional control to show that all the pictures in the learning set had been memorised, subjects were asked in the absence of any visual stimuli to free- recall and describe to the experimenter in detail all the slides they had been asked to memorise. This verbal description was tape recorded for later analysis of recall order and amount and nature of detail.

The following instructions were read to all subjects prior to the presentation of the learning set of pictures:

"You will now be shown a series of 20 photographs of places and things that will be very familiar to you. I

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want you to commit these pictures to memory, as you will be required to recognise them among sets of similar pictures later on. Although there are 20 pictures in all in this first series, there are only 10 basic scenes, so that you will see two similar pictures of the same scene for each of

the 10 scenes. All the pictures are different - no two are f