Detection theory trial#

In these models, each encoded item is internally represented by a continuous variable, usually with added Gaussian noise, that is positioned within a multidimensional perceptual space.īefore data collection began, participants were informed that their task was to match as closely as possible the relevant stimulus property of the probe stimulus with that of the stimulus at the cued location. Detection theory accounts, which suggest a continuous model of perceptual encoding, are more natural from a cortical viewpoint (Verghese, 2001). Second, it assumes that the internal state engendered in the absence of an item can never contribute to an “item seen” response (i.e., distracters can never produce enough misleading evidence to elicit a false positive answer). First, it posits that the relevant information used to determine whether a visual object has been seen previously is encoded as a discrete unit within the brain (i.e., the item is simply present or not, in the absence of internal noise). This type of high-threshold (HT) model has been shown to be based on a number of unattractive assumptions (Macmillan & Creelman, 1991), and its applicability has been questioned for other experimental paradigms, such as visual search (Eckstein, Thomas, Palmer, & Shimozaki, 2000 Palmer, 1995 Palmer, Verghese, & Pavel, 2000). This model classically envisages VSTM as a single high-level store within which items, often conceived as bundles of perceptual features, are stored.

Detection theory trial#

(1) where F is the probability on a given trial that an observer will incorrectly guess “change” when no change has occurred. Overall, the results suggest that observers encode stimuli independently and in parallel, and that performance is limited by internal noise, which is a function of set size. In Experiments 7–9, observers were asked to directly report the relevant feature attribute of a stimulus presented 1500 ms previously, from an array of varying set size. In addition to the HT model, two SDT models were tried: one assuming summation of signals prior to a decision, the other using a max rule. Three general models were tested against the receiver operating characteristics generated by the six experiments. In Experiments 4–6, set size was fixed while the number of stimuli that might change was varied ( T = 1, 2, 3, and 4). In Experiments 1–3, only one stimulus element difference could occur ( T = 1) while set size was varied. Observers were asked to report in a yes/no fashion whether there was any difference between the first and second arrays, and to rate their confidence in their response on a 1–4 scale. In Experiments 1–6, two arrays composed of multiple stimulus elements were presented for 100 ms with a 1500 ms ISI. Here we report findings from nine experiments in which VSTM for color, spatial frequency, and orientation was modeled using a signal detection theory (SDT) approach. However, the type of high-threshold (HT) model used to derive this estimate is based on a number of assumptions that have been criticized in other experimental paradigms (e.g., visual search). Previous studies have suggested that visual short-term memory (VSTM) has a storage limit of approximately four items.