The Effect of Presentation Rate on Categorization

Homogeneous experimental (acoustically and semantically related) and control lists were presented to different groups of subjects at either 1 s or 6 s per word. The difference score between these two lists was used as a measure of categorization. Only words in the middle serial positions (representing retrieval from secondary memory) were scored. Clustering was also measured. Slow presentation rate significantly increased categorization of the semantically similar words. This was not true of acoustically similar words. A possible explanation of this is that semantic categorization is time-consuming and acoustic categorization is not. Alternatively, the acoustic similarity effect may be a retrieval effect. The absence of acoustic clustering would seem to fit in with this latter interpretation.

EEG Correlates of Cumulative Expectancy and Subjective Estimates of Alertness in a Vigilance-type Task

Occipital EEG was monitored during a slow presentation rate vigilance task. EEG samples were taken for each of 400 task events. The EEG is correlated with increases and decreases in “expectancy” built into the task. Post-trial subjective estimates of alertness parallel the EEG changes. Reaction time to “wanted signals” does not correlate with measures of pre-signal EEG.

Stimulus Set and Response Set: The Alternation of Attention

Recently Gray and Wedderburn showed that if a meaningful phrase of three words was presented together with three digits, in such a way that each ear received some items of each of the two types of material, it was no harder to recall the items grouped by type than it was to recall them grouped by ear. This finding is repeated and confirmed in several forms, culminating in the use of three letters of the alphabet and three digits as the six items presented. It is also shown, however, that even when all material is presented to one ear, it is harder to recall a list made up of alternate items of two classes than it is to recall the same items arranged as two successive sub-lists. Thus Gray and Wedderburn's result does not appear to reveal a situation in which alternation of attention between the ears is especially easy, but rather one in which continued attention to one ear is especially difficult because it requires alternation between classes of item. A further series of experiments showed that a reduction in the presentation rate of stimuli produced a much greater improvement in performance when the items were of two alternated classes than when the classes were left separate. Equally, a slow presentation rate is more helpful when alternation between ears is required than when each ear is to be dealt with separately. These results support the idea that attention takes time to shift; but require a separate kind of attention, to a class of item rather than to a source of stimulation.