Reply to Farrell & Lewandowsky: Changes in the shape of the lag-CRP predicted by TCM due to recency

In the temporal context model (TCM), the current state of context is used as a cue for episodic recall. Farrell & Lewandowsky (in press) argue that the lag-CRP should be examined over a much wider range of lags than have previously been considered. Farrell and Lewandowsky (in press) show that TCM predicts a characteristic change in the shape of the conditional response probability as a function of lag (lag-CRP). This change manifests as a non-monotonicity at extreme lags as well as a skew favoring forward recalls. We show that TCM predicts the distortion to the extent that endof-list context persists as a retrieval cue for subsequent recall attempts and to the extent that end-of-list context generates a recency effect. Empirically, the degree of skew and non-monotonicity in the lag-CRP seem to be more prominent in immediate than delayed free recall and more prominent in continuous-distractor than delayed free recall. There even appear to be skew and non-monotonicity across lists in a final free recall experiment that exhibited a strong recency effect across lists. TCM predicted the existence of these relatively subtle distortions of the lag-CRP and their correlation with the recency effect. Rather than a reason to suspend work on TCM, these effects provide strong support for an associative engine based on retrieved temporal context (e.g., Sederberg, Howard & Kahana, in press). REPLY TO FARRELL & LEWANDOWSKY 2 In free recall, participants are presented with a list of words and then instructed to recall them in the order they come to mind. Because the order of recall is unconstrained by the experimenter, regularities in the transition probabilities presumably reflect properties of the organization of memory. Perhaps the most important of these regularities in constraining models of episodic memory retrieval is the conditional response probability as a function of lag, or lag-CRP (Kahana, 1996). Given that a participant has just recalled the item from serial position i, the lag-CRP estimates the probability that the next item recalled will be i + lag, attempting to control for the availability of potential recalls in a number of ways. In delayed recall studies, the lag-CRP has a canonical shape, exhibiting a strong contiguity effect favoring adjacent transitions over more remote transitions and an asymmetry favoring forward transitions over remote transitions (see Kahana, Howard, & Polyn, 2008, for a review). Farrell and Lewandowsky (in press) have identified a novel set of predictions of the temporal context model (TCM, Howard & Kahana, 2002). Previous work has explored the ability of TCM to account for the effects of hippocampal lesion on temporally-defined associations (Howard, Fotedar, Datey, & Hasselmo, 2005), the effect of aging on the shape of lag-CRP curves (Howard, Kahana, & Wingfield, 2006) and the dynamics of immediate and continuous-distractor free recall (Sederberg, Howard, & Kahana, in press). Nonetheless, this subtle prediction about the shape of the lag-CRP curve has not previously been reported, nor have tests of this prediction been undertaken. Farrell and Lewandowsky (in press) point out that TCM predicts that the persistence of the recency effect across multiple retrieval attempts should lead to a distortion of the lagCRP. That is, to the extent there is a recency effect, transitions to nearby items should be supplemented with transitions to items at the end of the list. This recency effect leads to a bias towards forward transitions that can manifest as a non-monotonicity in the lag-CRP. If we follow the forward lag-CRP outward from zero, eventually the tendency to make recalls to the end of the list overcomes the advantage from being nearby the just-recalled word. This results in an increase in the lag-CRP at extreme values of lag. The non-monotonicity in the lag-CRP reflects an excess of transitions from extreme serial positions to other extreme serial positions. A persistent primacy effect would manifest as an increase in the lag-CRP in the backward direction—extreme negative lags—whereas a persistent recency effect would manifest as an increase at extreme positive lags. The persistence of the primacy effect in recall transitions has been known for some time. Laming (unpublished manuscript) observed that there was an excess of transitions to the first serial position—we noted this persistent primacy effect early on in describing the lag-CRP (p. 939, Howard & Kahana, 1999). A moment’s reflection reveals that the existence of the primacy effect in the serial position curve obtained in immediate free recall, coupled with the tendency to initiate free recall from the recency portion of the list necessitates an excess of remote transitions to the early part of the list across subsequent retrieval attempts. That is, to the extent that the primacy effect in the serial position curve is not The authors acknowledge support from National Institutes of Health research grants MH069938 to MWH, MH072138 and MH080526 to PBS and MH055587 MJK. Correspondence concerning this article may be addressed to Marc Howard ([email protected]). REPLY TO FARRELL & LEWANDOWSKY 3 solely attributable to the tendency to initiate recall with the first item in the list, then there must have been an excess of remote transitions to the first item. Farrell & Lewandowsky’s primary empirical contribution is to suggest that there is a non-monotonicity in the forward direction consistent with what would be expected from a persistent recency effect. Although the statistics reported by Farrell & Lewandowsky (in press) are not appropriate to determine whether a particular experiment demonstrates a non-monotonicity (see Appendix 1), there are three additional sources of evidence that convince us that lag-CRPs exhibit non-monotonicity at extreme positive values of lag. These sources of evidence are Farrell and Lewandowsky’s (in press) meta-analyses (their Figure 2), their observation that the version of TCM they refer to as TCMevo provides a better fit than the model they refer to as TCMpub across a wide variety of experiments, and our own secondary analyses (reported here). In this reply we explore the variables that affect the change of shape of the lag-CRP by examining lag-CRPs from a set of experiments with largely similar methods but differing delay schedules. Qualitative modeling assesses the degree to which this pattern of results is consistent with the predictions of TCM. We start by describing in more detail the source of the distortions in extreme values of the lag-CRP predicted by TCM. Distortions in the lag-CRP predicted by TCM TCM proposes that the cue for episodic recall is the current state of a graduallychanging temporal context vector. Potential recalls are cued by a state of context to the extent that it overlaps with the context that obtained when they were studied. The current state of temporal context is driven by presented items, which can also recover their study context. This enables the model to account for contiguity effects—when a studied item is recalled, the input it causes to the temporal context vector resembles the encoding context of neighboring list items, resulting in an increased tendency to recall neighbors of the recalled item. These basic ideas are common to all of the studies that have applied TCM to a variety of topics, although these treatments have varied in a number of details (see Howard & Kahana, 2002; Howard et al., 2005, 2006; Rao & Howard, 2008; Sederberg et al., in press). TCM predicts a distortion in the shape of the lag-CRP evident in extreme lags to the extent that end-of-list context persists as part of the retrieval cue and to the extent that this end-of-list context supports a recency effect.1 In TCM, the degree of contextual drift at any given time step is a function of the amount of information that is provided as input to temporal context. This leads to the interesting property that when no input is provided, there is no change in the state of temporal context, predicting that recency can remain intact in response to an unfilled delay (Baddeley & Hitch, 1977; Murdock, 1963). It is perfectly reasonable to suppose that the amount of information provided as While the primacy effect has been identified with rehearsal (e.g., Brodie & Murdock, 1977; Rundus, 1971; Tan & Ward, 2000), there is, in addition, a one-position primacy effect that is observed in the PFR and remote recall transitions in many data sets (see e.g., Figure 2). While we have occasionally included a descriptive model of primacy in treatments of TCM (e.g., Howard, et al., 2006; Sederberg, et al., in press), this rehearsal-resistant primacy effect is not an integral part of TCM, at least as currently formulated. It is simple enough to add a descriptive account of primacy to TCM to account for primacy in the PFR and persistent backward non-monotonicities in the lag-CRP, such that the existence of primacy does not place a strong constraint on the model. We will not consider primacy further here. REPLY TO FARRELL & LEWANDOWSKY 4