Iconic Memory

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Levy 1 Rebecca Levy Perception Lab Iconic Memory Full Revision 4/8/2014 Abstract How much can we see in just a single glance, and how much of that information stays in our minds even when we don’t process what we see? Iconic memory is the brief, high-capacity memory system which contributes to visual short term memory, involving the neural persistence of an image even after the stimulus has been taken away. By asking a group of Binghamton University undergraduates to report letters in a 3X4 grid shown to them for only fractions of a second, we tested for iconic memory by measuring their accuracy of recall under particular conditions. Past studies have confirmed the relationship between accuracy and reaction time, postulating that beyond a particular time threshold (at or beyond about 150 ms) visual information is no longer passively received, but instead begins to undergo neural processing. Here we examined these factors by imposing an audio delay of a tone which would indicate the line in the grid that was to be reported, and measuring the accuracy of recall in the

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Paper written by Rebecca Levy spring 2014 for an undergraduate Perception lab paper required for class. The phenomenon of Iconic Memory, as well as its implications and relation to function, is described and analyzed in this paper.

Transcript of Iconic Memory

Iconic Memory Full Revision.docx

Levy Rebecca LevyPerception LabIconic Memory Full Revision4/8/2014AbstractHow much can we see in just a single glance, and how much of that information stays in our minds even when we dont process what we see? Iconic memory is the brief, high-capacity memory system which contributes to visual short term memory, involving the neural persistence of an image even after the stimulus has been taken away. By asking a group of Binghamton University undergraduates to report letters in a 3X4 grid shown to them for only fractions of a second, we tested for iconic memory by measuring their accuracy of recall under particular conditions. Past studies have confirmed the relationship between accuracy and reaction time, postulating that beyond a particular time threshold (at or beyond about 150 ms) visual information is no longer passively received, but instead begins to undergo neural processing. Here we examined these factors by imposing an audio delay of a tone which would indicate the line in the grid that was to be reported, and measuring the accuracy of recall in the respective conditions. Further, we attempted to determine exactly how much or little can be recalled by imposing a partial condition, in which only a specific line of the grid was to be recalled, and a full condition in which participants recalled any and all letters seen in the grid without discrimination. Lastly, we tested for differences in recollection based on which line of the grid was selected for recall. We found that there was an unreliable difference in the percentage of accuracy between the full and partial conditions. We observed a significant effect in the location of the letters in question, as the recall of letters at the top of the grid tended to be more accurate than the recall of letters near the bottom. It was therefore concluded that iconic memory is most reliable in near-immediate instances and when the stimulus is closer to eye level. Lastly, we discovered a significant effect displayed in tone delay, in that larger delays caused a marked decrease in recall accuracy.

IntroductionThe concept of iconic memory stems from the question of how much an individual can perceive in just a single glance. What has been found time and time again is that iconic memory seems to be stronger than one might suppose, yet has common limits in terms of how many items can be accurately recalled. Keeping all other factors constant, its been shown that an individual can accurately remember 4-5 items after only glancing at them for a moment. Iconic memory is this neural persistence of an image after viewing said image. It involves maintaining and processing information about the stimulus seen, and can last a few hundredths of a second (known as visible persistence) or create a slightly longer memory that can be retrieved (known as informational persistence). The type of short-term memory per stimulus can be affected by a few different things, some of the factors including the amount of time the image was seen (Coltheart, 1980), the amount of time between viewing and retrieving (DiLollo, 1980), as well as familiarity with the object, orientation and color of the object (Blaser, 2010). The visual working memory, which lasts a few full seconds, is more limited in capacity, while shorter-lived iconic memory is much stronger. This has been tested in many ways since studies on this concept were first performed in 1960 by George Sperling. These experiments, which have since been replicated many times and are being replicated in this study as well, typically involve the subject being exposed to a grid of letters which are shown to them for differing lengths of time, usually under 500 ms. The subject then gives a report, which is simply a record of either a specific given part or the entire stimulus, depending on a partial or full report request.Iconic memory has been known to be an extremely brief, yet high capacity memory system which contributed to visual short term memory by indicating a surprisingly accurate representation of what is perceived for less than a full second. The role of this memory, visual persistence, has gotten much attention from researchers in the last century, due to its proposed role as an unprocessed representation of visual information in short term memory. George Sperling first began to run experiments in order to confirm the existence, capacity and duration of visual memory, particularly involving immediate recall of images after exposing subjects to various durations of viewing. He conceded that testing this would be most effective when requiring that the subject only report part of what they have seen, because if the number of letters in the stimulus exceeds the participants memory, it would be impossible for them to give an account of all that they saw. Sperlings first two experiments confirmed that immediate-memory for stimuli is separate from the amount of exposure time, while his third experiment showed how the available information on the stimulus immediately after the offset of the stimulus is dependent on the means of sampling, which would be a partial report (Sperling, 1960). His studies utilized grids of letters which were presented to subjects using a tachistoscope, which is simply a device which displays an image to the viewer for a specific amount of time. After being exposed to the grid-like images for about 50ms each, subjects were asked to report as many of the letters as they could recall in a procedure known as a whole report. However, Sperling found that increases in exposure duration (ranging from 15ms to 500ms) didnt affect a subject's full report.Additionally, Sperling investigated the proponency to focus on certain sections of the grid by having participants provide a partial report in which they were only asked to identify one line for a particular trial. The line in question was indicated to subjects via either a high, medium or low pitched sound, heard while simultaneously viewing the grid. Sperling concluded that a portion of the visual presentation remained briefly accessible after presentation because the partial reports showed that while subjects can recall attended information (something that is to be assumed in an individual with average concentration and attention), some portion of unattended information gets processed as well. That said, by using grids made up of both letters and symbols, it was found that even symbols in the grid which were unfamiliar had a higher rate of accurate recall than more familiar stimuli that went unattended.Two separate additions to visual memory began to emerge after Sperlings original publication: informational and visual persistence. Sperling tended to test mainly the information of a stimulus, where other researchers tested for visual persistence. Max Coltheart, for example, found differing results and even rejected some of the claims made by Sperling, particularly those that supported that neural activity may be recalled by a stimulus remaining in the visual system after viewing, that the stimulus may be visible or retrievable after offset, and that information about visual properties prevails after offset at all. He instead found, in his 1980 study of iconic memory and visible persistence, that the two relevant after-effects were the inverse effect (in which the more time one is exposed to a stimulus, the less accurate the memory), and the inverse intensity effect (where the more intense the stimulus is, the more brief the persistence) (Coltheart, 1980). There are exceptions to this, which he explains are due to excessive exposure, in which these images are encoded into a longer-term memory. His explanation for this, as well as his theories on inverse effects, is as follows:There are neural persistences in the visual system from the photoreceptor level on up. Both rods and cones continue to signal for some time after stimulus offset. Such photoreceptor persistence may be one of the neural bases for visible persistence. If one accepts claims that cones inhibit rods, it is even possible to explain why, at least at mesopic light levels, there is an inverse relationship between stimulus luminance and duration of visible persistence. Sustained cells in the visual pathway also exhibit poststimulus persistence. If visible persistence ceases when this persistence is cut off by inhibition via the off-response, or alternatively if the off-response itself directly signals stimulus offset, then one can explain the inverse relationship between stimulus duration and persistence duration. The off-response has a minimum latency as measured from the onset of the stimulus; therefore, if visible persistence is measured from stimulus offset, its duration will be inversely related to stimulus duration, up to a duration equal to the minimum off-response latency: beyond this point, persistence duration will be independent of stimulus duration. (Coltheart, 1980)

Colthearts idea that visible persistence, a very short-term memory, is inversely related to how long the stimulus is visually available - and further, that if the stimulus is visually available for the same amount of time that it takes for the subject to respond - would mean that the more immediately a subject responds to a grid of letters seen for less than a second, the more accurate it would be as opposed to a subject that might take their time in thinking about and trying to remember what theyve seen. This speaks to the immediacy of a subliminal memory, as well as the power that even unprocessed visual events can have.In agreement with Colthearts notions of inverse effect and inverse intensity effect, Vincent DiLollo proposed his two-state model of visual memory (1978). The first state is activated by the onset of a stimulus whose features are extracted and stored in a rudimentary, unprocessed format. The second state begins about 100ms or 150ms after the initial onset, and this is the point where items are identified and organized in a processed from. This is the informational persistence talked about in Colthearts study, and is believed to be the basis and key contributor to the pre-categorized, unprocessed sensory information. This model, while having since been disputed (Irwin, 1986), is consistent with Colthearts perception of visible and informational persistence and has contributed to our present-day understanding of iconic memory, distinguishing between the visual and informational persistence which are, in essence, different in their core properties. Irwins 1986 study tested two alternate ideas to Colthearts informational persistence. The first was that, at stimulus offset, visual information persists for about 150-300ms in a visual memory, regardless of the length of exposure. Another idea was that informational persistence comes from neural nonvisual memory that contains spatial arrangements for displayed items. He ultimately found that regardless of the exposure duration, spatially-specific memory persisted after stimulus offset, favoring the visual component of informational persistence proposed by Coltheart. However, increased accuracy and decreased errors in item intrusion were the result of increased stimulus duration.DiLollo found that when testing subjects with grids of dots and asking them to report the position of the dot that was missing, a marked impairment was observed when the duration of the display was more than 100ms, implying that during longer durations the display was seen in two successive portions, perhaps due to saccadic eye movement and even change blindness. This meant that images seen for only 10ms were, astonishingly enough, reported back more accurately than other, longer ones (DiLollo, 1977).Our current study sought to test iconic memory much like Sperling (1960), Coltheart (1980) and DiLollo (1977) did, measuring coherence of iconic memory with accuracy in reporting under a multitude of conditions. Analyses were done using a three-way ANOVA within subjects. The independent variables were the report conditions (either full or partial), the appropriate line within the partial condition (indicated by either a high, medium or low pitched tone) and finally the tone delay (either 0ms, 150ms or 500ms). The dependent variable was the accuracy with which participants recalled the letters in question. We hypothesized that in the full condition, more letters would be accurately described overall than in any of the partial conditions because of the lack of potential pressure put on the participant to hone into and study a particular row in a limited amount of time. Also, we hypothesized that, in the partial condition, trials in which subjects were instructed via a low pitched tone would, on average, have less accuracy due to our natural proponency to reading from top to bottom. Lastly, we hypothesized that as the time between offset and the tone playing increased, the accuracy with which the subjects respond would decrease, as it would be reasonable to assume that unprocessed memories are more fragile and, therefore, will fade quicker than a processed memory would.

MethodsParticipantsThe participants of this study were undergraduate psychology students at Binghamton University, in a Lab of Perception class. Class credit was awarded to those who participated. Subjects were informed of exactly what the experiment was testing for, and all gave their consent to participate prior to the experiment. It was assumed that all participants had normal vision and motor skills to complete this task, which involved typing on a desktop computer.

ApparatusRequired for this experiment were computers compatible with Psychmate software, working keyboards, and a quiet place in which to administer the test. Additionally, participants will need speakers or headphones for the audio stimuli.

DesignStimuli were 3X4 grids of letters and, in some conditions, accompanying tones. All letters of the alphabet were used over the course of the experiment, and the twelve letters in each trial were chosen and placed randomly. The grids were presented for 100ms each, and the accuracy with which a subject reported back the letters in the grid was emphasized, as reaction time was not a relevant variable. The audio stimuli were consisted of three types of low, medium and high tones which were used to signify which line of the grid to report. There were also three timing conditions for the tone delay: a tone at 0ms played at the same time the grid was in view. The 150ms delay tone played 150ms after viewing the grid, and the 500ms delay tone was heard 500ms after the grid was in view. The tone delays were also presented in blocks.The two report conditions were full, meaning that the participant was to relay all of the letters in the grid to the best of their ability, and partial, meaning that the participant was to relay just one line of the letters to the best of their ability. The different conditions were blocked, meaning that there would be a certain amount of full trials all in a row, followed by a certain number of partial trials all in a row, and so on. The report conditions were a within-subjects variable.In the partial report, the subjects were instructed to recall the letters of the line signified by a tone. A high pitched tone signified the top line, a medium pitched tone signified the middle, and the lowest tone signified the bottom line. Tone was not blocked the way the partial and full groups were. Instead, tone varied at random within a partial block, and each tone was heard 10 times per 30 block trial. Pitch, delay and full/partial conditions were all within-subjects variables.ProcedureThe subjects were sent to rooms with computers. Using the Psychmate software on these PCs, they logged in with their unique and premade accounts and were given instructions on how to effectively do the assigned task. The instructions on the screen indicated that they would do 120 trials of practice in order for the participant to successfully make a more automatic association between tone pitch and the line of letters they were ideally meant to recall. They then performed a series of four blocks of 30 trials each. The four blocks included a full report, a partial report with 0ms delay, partial report with 150 ms delay, and partial report with 500 ms delay. The order of each of the blocks was counterbalanced among participants.A single trial would be performed as follows, for example. The participant presses the spacebar in order to indicate preparedness for an individual trial, thus initiating the stimulus which is a grid of letters. In the full condition, there would be no audio, but in the partial condition the participant hears either a low, medium or high pitched tone, corresponding to the bottom, middle and top rows, respectively. This tone is either 0ms, 150ms or 500ms after the offset of the visual stimulus. In both the full and partial scenarios, the screen that followed was one in which participants could type in the letters that they saw. In the full condition, as they attempt to remember all twelve letters, participants enter in 12 different letters. In the partial condition, they type in only the four letters they thought they saw.

ResultsOut of the 66 total subjects, 8 individuals were excluded for displaying a flat learning curve or a negative slope during the practice trials, leaving 58 individuals whose data we could reliably use. The dependant variables were the numbers of correct letters, while the independent variables were the pitch of the tone, the delay of the tone and whether or not it was a partial or full report condition. Analyses were calculated with an ANOVA in order to measure a full effect between all three independent variables. The mean accuracy of the full report condition was .41 while the mean for the partial was .45 (F 1,112 = 2.67, p = .11). If we combine across all 3 partial conditions, participants accuracy was unreliably higher in the partial condition than in the full condition.There was a main effect of tone delay condition (F 2, 168 = 13.50, p < 0.001). The mean of the 0ms = .515, 150ms = .451 and 500ms = 380. The more delayed the presentation of the cue was, the lower the accuracy for iconic memory.There was a main effect of row condition (F 2, 168 = 9.46, p < 0.001). The mean of the top row = .50, middle row = .47, and bottom row = .37. When required to report the top and middle rows, participants gave more accurate responses.

DiscussionWe originally theorized that in the full condition, more letters would be accurately described overall than in any of the partial conditions. Because the means for the partial and full conditions were so close, there was a negligible difference between the two, indicating that there wasnt a significant difference in the quality of recall. But while the full and partial conditions had a negligible effect on recall, perhaps the length of the exposure of those conditions would have an effect. This study could be altered in the future to reflect a variety of exposure times in the reports.Also, we hypothesized that, in the partial condition, trials in which subjects were instructed via a low pitched tone would, on average, have less accuracy due to our natural proponency to orient our reading from top to bottom. This did have a significant effect, as the means for the top and middle rows were about even with one another, while the mean for the bottom row was significantly less. This could also be because the higher lines may have been closer to the subjects eye level, so they are seen first, or even sometimes exclusively if the exposure time was too small to allow the subjects eyes to travel to the middle and bottom rows. Lastly, we hypothesized that as the time between offset and the tone playing increased, the accuracy with which the subjects respond would decrease. It seems that the tendency for decline in accuracy in relation to a lengthened tone delay was supported. While seeing the stimulus and hearing the tone simultaneously, it seemed to be easier to immediately associate what was being heard with what was being seen. By 500ms after seeing the grid, hearing the tone was significantly less effective in triggering correct responses, probably because the need to quickly scan a random grid and then try to remember it half a second later was more disjointed than a simultaneous influx of stimuli. This speaks to not only the astonishingly fast and accurate immediate iconic memory of unprocessed items, but also the tendency to forget those unprocessed items in a relatively small amount of time simply because they are vague and unprocessed. This is consistent with the findings of Coltheart and DiLollo, as they also found an inverse relationship between delay and accuracy. Some factors that we could not account for were potential fatigue within the sample, which may have affected the possibility of there being a significant difference in the accuracy reports between the full and partial conditions. The implications of this study are that quick glances can transmit quite a lot of information to the brain, sometimes even more than a slightly longer look. This neural phenomenon could have implications in advertising, for example, where images can be rapidly intertwined in a commercial to provide a subliminal effect. It may not do much in the way of transmitting any hard information, but a 500ms image of a hamburger flashed in the middle of a restaurant ad might be enough to awaken the hunger in a viewer. It also implies that the storage of iconic memory decays very rapidly, thereby reinforcing the subliminal effect that might prevail from its use in advertising without the consumer ever being aware.

References

Blaser, E. & Kaldy, Z. (2010). Infants get five stars on iconic memory tests: A partial report test of 6-month-old infants' iconic memory capacity. Psychological Science, 21, 1643-1645.Bradley, C., & Pearson, J. (2012). The sensory components of high-capacity iconic memory and visual working memory. Frontiers in Psychology, 3.Coltheart, M. (1980). Iconic memory and visible persistence.Perception & Psychophysics, 27(3), 183-228.Dick, A. (1974). Iconic memory and its relation to perceptual processing and other memory mechanisms. Perception & Psychophysics, 16(3), 575-596.DiLollo, V. (1997). Temporal characteristics of iconic memory. Nature, 267, 241-243.Di Lollo, V. (1980) Temporal integration in visual memory. Journal of Experimental Psychology: General, 109(1), 75-97.Entwisle, D., & Huggins, W. (1973). Iconic memory in children. Child Development, 44, 392-394.Irwin, David; James Yeomans (1986). "Sensory Registration and Informational Persistence". Journal of Experimental Psychology: Human Perception and Performance 12(3), 343360Sperling, G. (1960). The information available in brief visual presentations. Psychological Monographs: General and Applied, 74(11), 1-29.