Determinants of Short-term Forgetting : Decay , Retroactive Interference or Proactive Interference ?

In two experiments short-term forgetting was investigated in a short-term cued recall task designed to examine proactive interference effects. Mixed modality study lists were tested at varying retention intervals using verbal and non-verbal distractor activities. When an interfering foil was read aloud and a target item read silently, strong PI effects were observed for both types of distractor activity. When the target was read aloud and followed by a verbal distractor activity, weak PI effects emerged. However, when a target item was read aloud and non-verbal distractor activity filled the retention interval, performance was immune to the effects of PI for at least eight seconds. The results indicate that phonological representations of items read aloud still influence performance after 15 seconds of distractor activity. Short-term Forgetting 3 Determinants of Short-term Forgetting: Decay, Retroactive Interference or Proactive Interference? Most current models of short-term memory assert that to-be-remembered items are represented in terms of easily degraded phonological representations. However, there is disagreement on how the traces become degraded. Some propose that trace degradation is due to decay brought about by the prevention of rehearsal (Baddeley, 1986; Burgess & Hitch, 1992; 1996), or a switch in attention (Cowan, 1993); others attribute degradation to retroactive interference (RI) from other list items (Nairne, 1990; Tehan & Fallon; in press; Tehan & Humphreys, 1998). We want to add proactive interference (PI) to the possible causes of short-term forgetting, and by showing how PI effects change as a function of the type of distractor task employed during a filled retention interval, we hope to evaluate the causes of trace degradation. By manipulating the type of distractor activity in a brief retention interval it is possible to test some of the assumptions about decay versus interference explanations of short-term forgetting. The decay position is quite straightforward. If rehearsal is prevented, then the trace should decay; the type of distractor activity should be immaterial as long as rehearsal is prevented. From the interference perspective both the Feature Model (Nairne, 1990) and the Tehan and Humphreys (1995,1998) connectionist model predict that there should be occasions where very little forgetting occurs. In the Feature Model items are represented as sets of modality dependent and modality independent features. Forgetting occurs when adjacent list items have common features. Some of the shared features of the first item are overwritten by the latter item, thereby producing a trace that bears only partial resemblance to the Short-term Forgetting 4 original item. One occasion in which interference would be minimized is when an auditory list is followed by a non-auditory distractor task. The modality dependent features of the list items would not be overwritten or degraded by the distractor activity because the modality dependent features of the list and distractor items are different to each other. By the same logic, a visually presented list should not be affected by an auditory distractor task, since modality specific features are again different in each case. In the Tehan and Humphreys (1995) approach, presentation modality is related to the strength of phonological representations that support recall. They assume that auditory activity produces stronger representations than does visual activity. Thus this model also predicts that when a list is presented auditorially, it will not be much affected by subsequent non-auditory distractor activity. However, in the case of a visual list with auditory distraction, the assumption would be that interference would be maximised. The phonological codes for the list items would be relatively weak in the first instance and a strong source of auditory retroactive interference follows. This prediction is the opposite of that derived from the Feature Model. Since PI effects appear to be sensitive to retention interval effects (Tehan & Humphreys, 1995; Wickens, Moody & Dow, 1981), we have chosen to employ a PI task to explore these differential predictions. We have recently developed a short-term cued recall task in which PI can easily be manipulated (Tehan & Humphreys, 1995; 1996; 1998). In this task, participants study a series of trials in which items are presented in blocks of four items with each trial consisting of either one or two blocks. Each trial has a target item that is an instance of either a taxonomic or rhyme category, and the category label is presented at test as a retrieval cue. The two-block trials are the important trials Short-term Forgetting 5 because it is in these trials that PI is manipulated. In these trials the two blocks are presented under directed forgetting instructions. That is, once participants find out that it is a two-block trial they are to forget the first block and remember the second block because the second block contains the target item. On control trials, all nontarget items in both blocks are unrelated to the target. On interference trials, a foil that is related to the target is embedded among three other to-be-forgotten fillers in the first block and the target is embedded among three unrelated filler items in the second block. Following the presentation of the second block the category cue is presented and subjects are asked to recall the word from the second block that is an instance of that category. Using this task we have been able to show that when taxonomic categories are used on an immediate test (e.g., dog is the foil, cat is the target and ANIMAL is the cue), performance is immune to PI. However, when recall is tested after a 2-second filled retention interval, PI effects are observed; target recall is depressed and the foil is often recalled instead of the target. In explaining these results, Tehan and Humphreys (1995) assumed that items were represented in terms of sets of features. The representation of an item was seen to involve both semantic and phonological features, with the phonological features playing a dominant role in item recall. They assumed that the cue would elicit the representations of the two items in the list, and that while the semantic features of both target and foil would be available, only the target would have active phonological features. Thus on an immediate test, knowing that the target ended in -at would make the task of discriminating between cat and dog relatively easy. On a delayed test they assumed that all phonological features were inactive and the absence of phonological information would make discrimination more difficult. Short-term Forgetting 6 A corollary of the Tehan and Humphreys (1995) assumption is that if phonological codes could be provided for a non-rhyming foil, then discrimination should again be problematic. Presentation modality is one variable that appears to produce differences in strength of phonological codes with reading aloud producing stronger representations than reading silently. Tehan and Humphreys (Experiment 5) varied the modality of the two blocks such that participants either read the first block silently and then read the second block aloud or vice versa. In the silent aloud condition performance was immune to PI. The assumption was that the phonological representation of the target item in the second block was very strong with the result that there were no problems in discrimination. However, PI effects were present in the aloud-silent condition. The phonological representation of the read-aloud foil appeared to serve as a strong source of competition to the read-silently target item. All the above research has been based on the premise that phonological representations for visually presented items are weak and rapidly lose their ability to support recall. This assumption seems tenable given that phonological similarity effects and phonological intrusion effects in serial recall are attenuated rapidly with brief periods of distractor activity (Conrad, 1967; Estes, 1973; Tehan & Humphreys, 1995). The cued recall experiments that have used a filled retention interval have always employed silent visual presentation of the study list and required spoken shadowing of the distractor items. That is, the phonological representations of both target and foil are assumed to be quite weak and the shadowing task would provide a strong source of interference. These are likely to be the conditions that produce maximum levels of PI. The patterns of PI may change with mixed modality study lists and alternative forms of distractor activity. For example, given a strong phonological representation of the target, weak representations of the foil and a weak source of Short-term Forgetting 7 retroactive interference, it might be possible to observe immunity to PI on a delayed test. The following experiments explore the relationship between presentation modality, distractor modality and PI Experiment 1 The Tehan and Humphreys (1995) mixed modality experiment indicated that PI effects were sensitive to the modalities of the first and second block of items. In the current study we use mixed modality study lists but this time include a two-second retention interval, the same as that used by Tehan and Humphreys. However, the modality of the distractor activity was varied as well. Participants either had to respond aloud verbally or make a manual response that did not involve any verbal output. From the Tehan and Humphreys perspective the assumption made is that the verbal distractor activity will produce more disruption to the phonological representation of the target item than will a non-verbal distractor activity and the PI will be observed. However, it is quite possible that with silent-aloud presentation and a non-verbal distractor activity immunity to PI might be maintained across a twosecond retention interval. From the Nairne perspective, interfe