Requests for reprints should be sent to Dr. George E. Stelmach, Motor Behavior ... .effectively by Adams, Goetz, and Marshall (1972) and. Stelmach (1973) to ...
Memory & Cognition 1975, Vol. 3 (1), 58-62
Memory trace strength and response biasing in short-term motor memory* GEORGE E. STELMACH and J. A. SCOTT KELSO University of Wisconsin, Madison, Wisconsin 53706
Two experiments, which attempted to create differential memory trace strengths in a response biasing paradigm, were performed. After the presentation of the criterion location, an interpolated target was presented which was either ± 40 deg from the criterion. The 8's task was to attend to both targets and recall each when instructed. The first experiment involved strengthening the criterion trace via repetition (0, 5, or 14 rep.) while the second involved providing additional feedback via visual, auditory, and heightened kinesthetic cues. In the initial experiment, a Repetition by Response Biasing interaction revealed that repetition systematically reduced error shifts at recall. The second experiment found that, m the combined feedback and visual conditions, response biasing was reduced. it seems feasible to suggest that both studies successfully manipulated memory trace strength which appears to be one determiner of error shifts at recall. I'
Directional error shifts in the' recall' of a positioning response following the presentation of an interpolated movement have been common in recent short-term motor memory (STMM) literature. Evidence has been presented which indicates that such error shifts are in the direction of the interpolated movement (Patrick, 1971; Pepper & Herman, 1970; Craft & Hinrichs, 1971; Laabs, 1973; Stelmach & Walsh, 1972, 1973). Thus, if the latter is of greater intensity or extent than the criterion, recall error is influenced in a positive direction. Similarly ,if the interpolated movement is of lesser intensity or extent, constant error at recall is shifted in a negative manner. At present, two views have been expressed to account for this phenomenon, both of which rely to some extent on the concept of assimilation (Helson, 1964). Pepper and Herman (1970) have postulated a theory of trace interaction in which the memory traces of the criterion and interpolated movements interact to yield a memory trace that is a combination of both. The recall response of the criterion is, therefore, made with reference to the altered trace representation and the directional error at recall is seen as an assimilation effect. Laabs (1973), on the other hand, views the reproduction of the criterion as being made in reference to an "average" or "central" movement, which he terms adaptation level (Helson, 1964), and to the criterion memory trace. Changes in adaptation level as a result of interpolated motor activity are thus seen as responsible for shifts in recall errors. As yet, empirical data which differentiates between these two positions has been tThis research was supported by Research Grant MH 22081-01 from the National Institute of Mental Health and by Grant NE-G-oO-3-0009 from the National Institute of Education awarded to the first author. The research was conducted in the University of Wisconsin-Madison Biotron, a controlled environment research facility, supported by the National Science Founcation and the University of Wisconsin. Requests for reprints should be sent to Dr. George E. Stelmach, Motor Behavior Laboratory, University of Wisconsin, 2000 Observatory Drive, Madison, Wisconsin 53706.
difficult to obtain. Recent research has focused rather on characteristics of the interpolated response which influence recall error shifts in an effort to shed some light on the foregoing theories. For example, Pepper and Herman (1970), Herman and Bailey (1970) and Craft and Hinrichs (1971) have all demonstrated reproduction errors proportional to the magnitude of the interpolated movement. Stelmach and Walsh (1972, 1973) have examined, in turn, the duration of time spent at an interpolated location and the temporal placement of the interpolated movement within the retention interval, and found both variables to have potent effects on shifts in recall error. In the first of these studies it was assumed that, since S remained at the interpolated location during the retention interval, the interpolated memory trace remained stable, while the memory trace associated with the criterion movement decayed over time. Similarly in the second study, it was argued that the later the interpolated movement was presented within a retention interval (i.e., the closer to recall), the stronger its trace would be relative to the criterion trace when S made his recall responses. Response shifts at recall in both studies were therefore interpreted in terms of .the relative strengths of the two memory traces. The present study reports two experiments which further examined the relative trace strength interpretation. The first experiment attempted to strengthen the memory trace of the criterion movement by means of repetition. The Ss made either 0, 5, or 14 repetitions to a criterion location prior to the presentation of an interpolated location (Adams & Dijkstra, 1966). The second experiment manipulated trace strength by increasing the information feedback to the S via additional auditory, visual, and heightened kinesthetic cues. This latter technique has been used .effectively by Adams, Goetz, and Marshall (1972) and Stelmach (1973) to reduce forgetting in STMM. A
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reduction in error shifts as a function of the strength of movement either 5 or 14 times. On each occasion S remained at the criterion memory trace was predicted in the present the target location for 2 sec and then returned to the 0 deg starting position for an identical period of time. Following the experiments on the basis of the relative trace strength designated number of repetitions, S was presented the BT (which hypothesis. was called MOVE TWO by E) and returned to the starting EXPERIMENT I Method Subjects. Thirty Ss (15 males and 15 females) between the ages of 18 and 26 years were obtained through the Financial Aids Office at the University of Wisconsin. Each was randomly assigned to one of three experimental conditions with the restriction that an equal number of both sexes appeared in each condition. All Ss were paid $1.50 for participation. Apparatus. The appara tus consisted of a manual lever identical to one described previously by Stelmach and Walsh (1972). The S was instructed to move the free-moving, near frictionless lever at a steady rate to variable target locations which were defined by stops inserted by E. Positioning and reproduction responses were made in a right to left manner and errors were recorded to the nearest 0.5 deg (1 deg =4 mm of displacement). Design. A 3 by 2 by 2 by 5 design using three independent groups (repetition conditions) with repea ted measures on the last three factors (retention interval, response biasing, and targets) was utilized to examine accuracy of kinesthetic recall. Five criterion movements were combined with a positive and negative biasing target which in turn was paired with a 5-sec and 20-sec retention interval, giving a total of 20 trials which were randomized for all Ss. Procedure. The S assumed a seated position facing the lever apparatus. Standardized instructions were read to each S who was then given one familiarization trial. From his position behind the lever apparatus, E administered testing procedures and recorded S's recall estimates. Each S was informed that it was necessary to remember two movements in each trial. For the zero repetition condition (0 REP), S was simply presented the criterion target (CT = 50, 70, 110, 115, and 130 deg) followed by the biasing target (BT) which was systematically varied from the criterion by plus 40 deg (positive) or minus 40 deg (negative). Either immediately or after a 20-sec retention interval which commenced when S returned from the CT, S recalled both targets (CT and BT) in order of presentation. The 5 repetitions (5 REP) and 14 repetitions (14 REP) conditions were identical to the aforementioned except that S, after receiving the criterion presentation, repeated that
position for the duration of the retention interval, after which he recalled the CT and BT in order of presenta tion. During all trials S maintained a firm grip on the lever handle, but was allowed to release and rest during the 20-sec intertrial interval. The E administered all verbal commands on the basis of light signals which were provided by a programmed Lafayette eight-channel timer.
Results Mean constant and absolute errors for criterion recall with accompanying standard errors for the experimental variables of repetitions, retention intervals and response biasing are presented in Table I. Analysis of constant error revealed that only the main effects of response biasing and target locations were significant F(1 ,27) =68.48, P < .01 and F(4,108) = 31.48, P < .01, respectively. In addition, only the Repetitions by Response Biasing interaction was found significant F(2,27) =8.39, p < .01. Inspection of this interaction indicates that response biasing (positive and negative) reduces as a function of repetition. To further examine the magnitude of this reduction, difference scores were calculated by subtracting the negative biasing error from the positive biasing error for each of the three repetition conditions. The means in degrees for the 0, 5, and 14 REP conditions were 7.40, 3.40, and 2.37, respectively, and clearly illustrate a reduction in error shifts as a function of repetition. While absolute error is not as interesting as constant error in response biasing studies, due to the fact that these scores do not reflect directional shifts in error, they are of value in assessing the behavioral effects of repetition, retention interval and targets, and are therefore included in this analysis (see Table 1). The main effects of repetitions and retention intervals were significant F(2,27) = 12.39, p
