Discussion

The third experiment showed that the fears expressed concerning the efficacy of the pre-masks was groundless. Letters presented at 35 Hz, a rate at which they are

normally nonrecognisable (as shown when subjects tried to read them when presented in the form of a grid in

experiments 1 and 2) were shown to be, as in the first second experiments readily visible if presented without a premask. In this condition, the subjects were able to recognise 90% of the monochromatic letters and 27% of the heterochromatic ones. However, the accuracy with which subjects identified tachistoscopically presented letters reduced to chance or near chance if they were preceded by a premask. Two masks, static and counterphase, were used in this experiment. An important finding was that there was no difference between the effects of static compared to

counterphase masks.

In the introduction to this experiment, the fears were expressed that the do not know response option of the first two experiments could have effected their results. Indeed it was shown that there were significantly more do not know responses for experiment 2 (which used premasks) than in experiment 1 (which did not use premasks). Thus the

difference between the two experiments might have not been due to the existence of premasks blocking the hypothesised buffer but effected how subjects responded to letters they

were not sure they could identity. For instance, the presence of premasks before tachistoscopically presented letters might have biased subjects to make more do not know responses to letters they otherwise would have identified correctly thus artificially inflating the error scores to letters preceded by a mask. An alternative possibility is that subjects responded with a do not know response because they genuinely could not identify the letters to which they had been exposed. The do not know response was not available

in the third experiment thus the two possibilities could be distinguished. Subjects responded in the third experiment much as they did in the first second experiments suggesting that the second possibility is most likely to be correct: that they responded with a do not know response because they genuinely could not identify they letter — if forced to guess they would not have guessed it correctly. Thus the presence of a do not know response is unlikely to affected the results of the first two experiments.

4.6. Conclusions

The introduction suggested that counterphase stimuli might be recognisable either through temporal resolution of their counterphase or by processes that could side-step it.

The first experiment showed that the identification of letters was affected by their rate of counterphase. It had been predicted that very high rates of counterphase 35 Hz

would be difficult to see compared to lower ones. Moreover that monochromatic stimuli would be visible at higher rates compared to heterochromatic ones. This in general was

confirmed both by the grid and the tachistoscopic presentation of letters.

However, the first experiment also showed that counterphase images can be perceived through mechanisms

which by-pass the temporal resolution of their counterphase. Tachistoscopic presentation of single letters suggested that this perception occurred at the onset image of the

counterphase.

The second experiment shows that the perceptibility of the initial image of counterphase could to some degree be minimised by the use of a premask. Presumably this works by occupying a buffer which otherwise could be used to store the initial image of the counterphase.

The second experiment also showed that the problem identified in the first experiment concerning the

perception, without temporally resolution, of counterphase letter can be ameliorated by using a pre-mask. The existence of this problem and the existence of a means of minimising suggests all work using counterphase images should where appropriate proceed them with suitable premasks.

The third experiment ruled out an explanation of the effectiveness of the premask in terms of a difference between the experiments in the use of the nonguessing

option. When this option was absent the premasks were still as effective, as when present, in inhibiting the visibility of tachistoscopically seen letters.

The third experiment also showed that the nature of the premask — counterphase or static — did not matter as each was equally effective as the other. This removes doubts raised earlier in this chapter whether the premasks used in the later experiments were not as effective as static ones.

N o t e s .

1. The presentation of letters in the form of a grid was settled upon after preliminary explorations. One of the problems of presenting letter stimuli in rapid flicker are artifact dark ghosts that form around the letter outlines

(this was discussed in the previous chapter). It was found that a black line box tightly fitting around a letter

minimised the perception of otherwise detectable visual g h o s t s . Organising black line boxes in horizontal and

vertical rows and columns produces the appearance of letters contained in grids. Letters presented tachistoscopically were also in black line boxes.

2. Unfortunately, electronic alternatives which would have increased accuracy were either not feasible or not

available. The reason was limited video buffer memory accessible in the programming language. In computer terminology, there were only two video pages of memory available. One was used for each of the two frame-images: the consequence of this was that there was not a third which could act as a dummy page to hid the two containing the two frame-images. Ironically, the Super Seven vga card allows up to eight video p a g e s . Unfortunately, they are not accessible accept through machine level programming. In the experiments reported in this thesis, a different programming technique was used for which this problem does not arise. However, the technique was limited to images of around 2 cm by 2 cm and

not applicable to the presentation of a large array of letters. The hand timing follows from the lack of an

electronic start. Further, there would have been problems in electronically determining the end point of a sequence of 2 0 named letters. While this could have been overcome, the

limited accuracy of the means used in this experiment were adequate since the aims of it were more qualitative then quantitative (quantitative timing was carried out in part B) .

3. A further problem was that subjects varied in the pacing of their letter reading particularly whether they paused at the beginning of the second line.

4. There were two further factors: first, writing errors and manually recording time are difficult concurrent tasks. Second, this was compounded by the experimenter's limited short term digit span. In retrospect, use of a tape recorder should have been made.

5. The reasons for this were (a) to ensure a steady rate of subject responding and (b) to minimise subject

discomfort. Preferably, subjects should have been forced to guess since the findings of subliminal perception suggest subjects often claim not to be able to see things, which they do if forced to guess. However, preliminary experience with the use of flickering stimuli suggested subjects found them uncomfortable. It was therefore decided to minimise

this as far as possible. Forced guessing is not pleasant and it was felt that its removal while not compromising the central aims of the experiment made the experiment easier on the subject.