Moving Ripple Stimuli
In order to examine how similarity relationships among stimuli affect subjects' responses, we used moving ripple sounds, stimuli that could be continuously varied, and whose inter-item similarities could be measured. Moving ripple sounds are broadband sounds that vary sinusoidally in both time (with a period w cycles per second) and in frequency content (with a period Ω cycles per octave). In the horizontal axis represents time, and the vertical axis represents the frequency content of two sample stimuli. These stimuli were generated by superimposing sounds at many frequencies whose loudness at any time (t), and for any frequency (f) is defined by
Figure 2 Spectro-temporal plots of ripple sounds. The horizontal axis shows time in seconds, while the vertical axis shows frequency content in Hertz. Darker colors represent sounds of greater amplitude. Modulations over time are referred to as the ripple's velocity, (more ...)
) and f0
is the lowest allowed frequency. ψ
is the phase of the ripple, and D
is modulation depth. D0
is the base loudness, which was set to 1.0. In order to simplify the stimulus space, only one parameter (w
) was varied among the stimuli. Other parameters took fixed values: Ω=1, ψ
=0.9, and f0
=200 Hz. Frequencies ranged over 3 octaves above f0
, that is, from 200 to 1600 Hz. Each stimulus contained 20 logarithmically-spaced frequencies per octave. Each stimulus has a spectral profile that drifts in time, so that different frequencies are at their peaks at different times. For each stimulus, duration was set to one second. Example stimuli can be found at http://people.brandeis.edu/~sekuler/rippleSoundFiles/movingRippleSounds.html
. The advantages of this particular kind of stimulus for studying memory were described by Visscher et al. (2007)
, who also showed that short term memory for these stimuli exhibits strong parallels to short term memory for visual stimuli such as oriented sinusoidal gratings. An additional benefit for studying ripple sounds is that they share similarities to speech sounds (see Shamma, 2001
). For example, their frequency bands modulate in time. Thus, findings using these ripple sounds are likely to generalize to speech sounds. The ripple sounds are difficult to verbalize, however, allowing examination of memory for language-like sounds independent of verbal labels.
Subjects were between the ages of 18 and 30, and came from the student population of Brandeis University. At the outset, each potential subject underwent audiometric screening. Using a MAICO MA39 audiometer, thresholds were measured at 250, 500, 750, 1000 Subjects were between the ages of 18 and 30, and came from the student population of Brandeis University. At the outset, each potential subject underwent audiometric screening. Using a MAICO MA39 audiometer, thresholds were measured at 250, 500, 750, 2000, 3000, 4000 and 6000 Hz. Each subject had normal or above-normal hearing, that is, thresholds at or below 20 dBHL at each frequency.
Twelve subjects participated in eight sessions each, following an initial session in which just noticeable difference thresholds (JND) for the w parameter (cycles per second) were measured (see below), and 200 practice trials were performed. Experimental sessions, lasting about one hour each, comprised 586 trials. At the beginning of every session, each subject completed at least 30 practice trials that were excluded from data analysis. Successive sessions were separated by at least six hours, and for any subject, all sessions were completed within three weeks. Subjects participated for payment of $72, plus a performance-based bonus of up to $16.
The methods used in the study were approved by the Institutional Review Board of Brandeis University.
Apparatus and sound levels
Subjects listened to stimuli through Sennheiser Pro HD 280 headphones. Stimuli were generated by an Apple iMac computer, and Matlab, including its PsychToolbox add-on (Brainard, 1997
). In order to characterize the stimulus intensity at the subject's eardrum, sound levels for this system were verified using a Knowles electronic mannequin for acoustic research. All stimuli were 79 dBSPL
, well above our subjects' hearing thresholds.
On each trial, either one or two study items were presented, followed by a probe. The analyses presented here focus on the two-item lists. One-item trials were included in order to quantify pairwise perceived similarity, a parameter needed for the NEMo model fits presented in Visscher et al. (2007)
. Study lists were restricted to no more than two items in order to provide control of the variables required for the questions of experimental interest. The subject's task was to judge whether the probe (p
) matched any of the study items (s1
). The response was indicated by a button press. During the presentation of study items, subjects fixated on a ‘+’ in the center of a computer screen. Trials with one study item were intermixed among trials with two.
Each stimulus was one second in duration. When two study items were presented, they were separated by 0.25 seconds. The probe was presented 0.75 seconds after the final study item, and was accompanied by the presentation of a ‘?’ on the computer screen. Subjects responded with a button press, indicating whether the probe matched a study item (“Yes”) or not (“No”). Immediately after the subject's response, a distinctive tone provided feedback about response correctness. To increase motivation, after each trial subjects were shown their percent correct thus far in the session, and the difference between that value and their goal of at least 70% correct. Subjects were rewarded at the end of a session with a candy bar if their percent correct exceeded 70%. For every percentage point above that value, subjects received a $0.25 increment to their base payment.
Adjustment for discrimination threshold
Stimuli were adjusted to each subject's auditory discrimination threshold, thereby eliminating one source of potential individual differences, and making the memory task comparably difficult for all subjects (Zhou, Kahana, & Sekuler, 2004
). In addition, the similarity among stimuli made it difficult for subjects to use naming or categorizing strategies in a consistent, reliable fashion. In a subject's first experimental session, pairs of stimuli were presented in succession on each trial, and the subject identifed which stimulus had the faster rate of modulation. Watson and Pelli (1983)
's QUEST algorithm found the difference in modulation rate (
) that just permitted correct identification of the more rapidly modulated stimulus on 70% of trials. This value was taken as the just noticeable difference (JND).
This JND value was then used to generate the stimuli that would be used in subsequent sessions to test that subject's recognition memory. The lowest value of w
= 7 Hz and successive values were given by w0
(1 + JND
, where n varies from 0 to 9. This generates stimuli that increment in one JND steps. In order to reduce the possibility that subjects could memorize the set of stimuli and assign verbal labels to them, we increased the number of stimuli to which subjects would be exposed. A second set of ten stimuli was created whose values lay midway between successive stimuli in the first set; taking on values
, where n
again varies from 0 to 9. Trials whose test items were drawn from the first series were randomly intermixed with trials whose test items came from the second series. Thus the complete collection of possible stimuli comprised twenty sounds. Items in the stimulus pool were tightly packed along the dimension w
, with separations of just 0.5 JNDs. This tight packing was meant to make absolute identification of individual stimuli difficult. On a particular trial, stimuli were drawn from only one series or the other, meaning that a trial's stimuli [s1
] were always an integer number of JNDs from each other.
Trials were self-paced, each initiated by the press of a key on a computer keyboard. On equal numbers of trials, the probe matched one of the study stimuli, or did not match either of the study stimuli. We designated matching trials as Target trials, and non-matching trials as Lure trials. Target and Lure trials were randomly intermixed during memory testing.
In order to assess the carry-over of item information and relational information from trial to trial, we manipulated the stimulus materials that were presented on successive trials. For each trial pair, the first trial (Trial A) comprised the set up trial, intended either to establish some particular item information or to produce some particular relational information (operationally defined by inter-item homogeneity). Following each set up trial, the response on the next, test trial (Trial B) provided an index of the influence that had been established on the preceding trial. The details of the various conditions represented in the design are given below and in .
Table 1 Trial types in experimental design. Columns refer to the effect examined for each group of trials, the condition, the number of types of lure trials (combinations of two list items and a probe), number of types of target trials, and the number of repetitions (more ...)
To minimize subjects' awareness of the complex regularities in the stimulus presentation schedule, trial pairs were randomly interleaved with trials of other types (a total of 320 carefully controlled pairs of trials within the 4680 trials presented to each subject). Trials listed as “Model testing” in were analyzed in addressing a separate issue (Visscher et al., 2007
). Trials on which just one study item was followed by a probe were randomly interleaved among all trials, and were used to gauge stimulus similarity.
summarizes the effects that were targeted by each condition in our experiment. Note that the column headed “Condition” signifies the relationship among presented stimuli [s1, s2, p] rather than specific choices of stimuli, which varied from trial to trial. Many sets of stimuli consistent with the rules defining each condition were generated; examples were generated for both Target trials (on which the probe replicated a study item) and Lure trials (on which the probe did not replicate a study item). For example, the last row in the table refers to trials on which there was just one study item (s1) and it did not match the probe (p). As each trial's stimuli were chosen from a set of 10 stimuli, there are 90 possible pairings of s1 and p. For the conditions represented in the bottom two rows of the table, all possible pairings were used; other conditions used only a random subset of all possible pairings.
Carry-over of item information
In order to gauge carry-over of item information from one trial to the next, pairs of successive trials were constructed so that the stimuli from the first trial in the sequence (Trial A), A
], were similar to the probe, pB
, on the second trial (Trial B). This condition, represented in , is labeled hi
Sim after the relatively high similarity of the probe from Trial B to the stimuli from Trial A. On these trials,
were all within 3 JNDs of each other, as seen in ). If item information were carried over from trial to trial, memory of the study items on Trial A might influence recognition, inducing subjects to judge erroneously that pB
matched a study item on Trial B (
). In other words, carry-over of item information from Trial A to Trial B would be characterized by the proportion of false positive recognitions.
Figure 3 Upper panel: Schematic diagram of design examining maintenance of item information across trials. Trial A immediately precedes Trial B. In the hiSim condition, the probe for Trial B (with value pB) is very similar in perceptual space to the stimuli from (more ...)
Sim condition was contrasted with the lo
Sim condition, in which pairs of trials were arranged so that pB
had a low similarity to Trial A's stimuli. Trials A and B in the lo
Sim condition were the same as in the hi
Sim condition, except that trials were paired such that Trial A's stimuli (
) differed from pB
by at least 5 JNDs. Such low similarity between stimuli on subsequent trials should give rise to very little proactive interference of item information, and few false positive recognitions.
In Trial A of both hi
Sim and lo
Sim conditions, the probe (pA
) and both study items (
) were all very similar to each other (that is, within 3 JNDs of each other). On this subset of trials, the probe and both study items all took values among the three highest allowed stimulus values, or the three lowest allowed stimulus values. The following trial, Trial B, always contained study items (
) that were only 1 or 2 JNDs from each other. The probe (pB
) differed from the closest study item by 5 JNDs. In all conditions, only the similarity among s1
was constrained; their ordering in stimulus space along the w
axis was not. Thus, s1
was equally likely to take a value greater than or less than s2
. For simplicity, illustrates only the case where s1
. In addition, the probe's value was equally likely to be greater than or less than the study items. On Trial A (but not Trial B), the probe could even fall at a stimulus value between two study items, or hold an identical stimulus value to one of the items.
Previously, related procedures, using a “recent negative probe” condition have been shown to provide a sensitive assay of the degree of carry-over of item information (Monsell, 1978
; D'Esposito et al., 1999
). If information in memory did not carry over between trials, recognition performance on instances of Trial B in hi
Sim should be no different from instances of Trial B in the lo
Sim condition. One difference between the “recent negative probe” design and our own is that our design controlled the similarity of the probe from Trial B (pB
) to the stimuli in A
. Thus in the hi
Sim condition pB
could either exactly match a stimulus from Trial A (pB A
) or be highly similar, though not identical to a stimulus from Trial A (pB A
). In the lo
Sim condition, pB
was highly dissimilar from stimuli on Trial A (lo
Sim condition). The similarity among stimuli could be quantified, allowing evaluation of the specificity of item information maintained from previous trials. Note that Trial B consistently has a low value of homogeneity. Trial B is the same in both hi
Sim and lo
Sim conditions and is likely, based on previous experiments (Kahana & Sekuler, 2002
; Kahana et al., in press
; Nosofsky & Kantner, 2006
; Visscher et al., 2007
; Yotsumoto et al., in press
), to give rise to a relatively low false alarm rate on those trials.
Carry-over of relational information
The relationships among study stimuli on a trial robustly affect subjects' responses on that trial (Kahana & Sekuler, 2002
; Kahana et al., in press
; Nosofsky & Kantner, 2006
; Visscher et al., 2007
). To evaluate the possibility that such information was maintained from one trial to the next (Gorea & Sagi, 2000
), we used a design parallel to that described above. Again, pairs of successive trials were generated, with the set up trial (first in the pair) varying in study item homogeneity. Guided by NEMo, we generated two kinds of set up trials, which we label as hi
Hom and lo
Hom. On hi
Hom trials, s1
were highly homogeneous, differing from one another by just one JND. These trials were expected to promote a high, stricter criterion and fewer false alarms on that trial. On lo
Hom trials, s1
differed from one another by at least 4 JND units, and thus had relatively low homogeneity. These trials were expected to promote a lower, more liberal criterion and more false alarms.
Presentation of either a hi
Hom trial or a lo
Hom trial was followed by the presentation of a neutral, test trial. These neutral, test trials were drawn from a pool of four different Lure
stimulus sets (four sets of values for
, and pB
). Study stimuli and probes were chosen randomly for each set in the pool of neutral, test trials, and each of these random trials followed hi
Hom and lo
Hom trials with equal frequency, 20 times each. Any systematic difference in performance on neutral, test trials after lo
Hom trials vs. hi
Hom trials would indicate that relational information had been maintained and carried over to the neutral, test trial. For each subject, 40 hi
Hom and 40 lo
Hom trial pairs were randomly intermixed among all trials. Note that these condition labels refer to the characteristics of the first trial in a pair.