Transsaccadic memory is the neural process that allows humans to perceive their surroundings as a seamless, unified image despite rapid changes in fixation points. Transsaccadic memory is a relatively new topic of interest in the field of psychology. Conflicting views and theories have spurred several types of experiments intended to explain transsaccadic memory and the neural mechanisms involved.
In many situations, human eyes move repeatedly in rapid, discontinuous steps, focusing on a single point for only a short period of time before moving abruptly to the next point. Rapid eye movements of this type are called saccades. If a video camera were to perform such high speed changes in focal points, the image on screen would be disorienting for a human viewer. In contrast, despite the rapidly changing sensory input to the visual system, the normal experience is of a stable visual world; this is an example of perceptual constancy. Transsaccadic memory is a system that helps maintain this stability despite rapid movement of the eyes.
Theories
Saccade target theory McConkie's and Currie's saccade target theoryCurrie C.B., McConkie G.W., Carlson-Radvansky L.A., Irwin D.E (2000), Perception & Psychophysics. 2000 May;62(4):673-83..The role of the saccade target object in the perception of a visually stable world. is similar to research by Schneider who came up with a similar "reference object theory".Deubel, H., Schneider, W. X., & Bridgeman, B. (2002). Transsaccadic Memory of Position and Form. Progress in Brain Research, 140, Chapter 11. Both theories hypothesize that each saccade is preceded by processes in the visual system that chose an object as the target for the next fixation point. The object is usually located in peripheral vision. The object's features are stored as a mental representation in transsaccadic memory for identification of future fixations. These target features are searched for by the visual system when the eye lands on its fixation point, and the physical features are compared to the mental representation of the target object. The theory assumes that visual stability is attained when these processes are successful (when the visual stimuli and the mental representation of the target object match). This process occurs before each saccade.McConkie, G. W., Carlson-Radvansky, L. A., & Irwin, D. E. (2000). The role of the saccade target object in the perception of a visually stable world . Perception and Psychophysics, 62(4), 673-683. Experiments performed by McConkie to support the role of a saccadic target in transsaccadic memory show two things: first, there is a limited peripheral area where a saccadic target exists, and second, attention is vital in recollection of items in the target area. The experiments involved recalling changes to an image that occurred in the peripheral area. Irwin performed similar experiments in which participants recalled letters that occurred near the target area. Due to confounding factors of the controlled environment in the studies, the involvement of saccade target objects is inferred and not established.
Spatiotopic fusion hypothesis Many radical views of transsaccadic memory exist. Some are favoured more than others because of the evidence that supports them. One less-accepted theory, Breitmeyer's spatiotopic fusion hypothesis, suggested that successive images are fused based on environmental coordinates and not retinal ones. In other words, our memory stores a sequence of images from our surroundings, and joins the images together to give us a full representation in our brain. The orientation of our retinas do not have an influence on this form of memory. This theoretical form of memory supposedly stores highly detailed representations of the world in a high capacity spatial buffer. Research and experiments by other psychologists provide empirical evidence against these claims.
Dennett
Daniel Dennett argued that the way we think we are seeing the world is, for the most part, an illusion. Part of Dennett's argument is the claim that each of us possess what he calls a "Cartesian theater", in which we believe there is a full representation of the visual world in our mind and that there is a place in the mind where it is observed.Blackmore, S.J., Brelstaff, G., Nelson, K. and Troscianko, T. 1995 Is the richness of our visual world an illusion? Transsaccadic memory for complex scenes. Perception, 24, 1075-1081c According to Dennett, none of this exists. Instead, the only place where a full and rich representation exists is directly on the fovea, and every time a saccade occurs, the information is overwritten. Therefore, there is no such thing as transsaccadic memory. Information previously lost only appears to be retained in visual memory because we can look again. In this way, the outside world acts as a visual memory. Since our eyes are constantly moving, we are not aware that visual inputs are constantly being refreshed to give the illusion of the completed picture we think we are seeing. Dennett makes a distinction between the presence of representation, and the representation of presence. The example he gives regarding this distinction is this: if you were to walk into a room covered in identical portraits of Marilyn Monroe, you would see that there are many of them, but you would not really be seeing them all at once. There would be no detailed representation of each individual portrait just the knowledge that they are present. Dennet's theory raises two relevant questions: 1) How does the visual system detect change in the environment? 2) How much information is retained in each saccade? The proposed answer to these questions lies in several mechanisms that support a high sensitivity to change in each visual fixation. These mechanisms are: retinal adaptation, "pop-out" systems, and motion detectors. The implication of this view is that little information is needed to be retained between each saccade.
Irwin Irwin's conclusion regarding transsaccadic memory is that there are no separate mechanisms, but that rather it is the same as short-term visual memory.Irwin, D. E. (1991). Information Integration across Saccadic Eye Movements . Cognitive Psychology, 23, 420-456. Irwin's experiments showed that people cannot fuse pre-saccadic and post-saccadic images in successive fixations. These results are evidence against spatiotopic fusion. According to Irwin, there is no transsaccadic buffer that holds visual information from one fixation point to the next. Also, transaccadic memory does not hold detailed spatial information, but a more abstract representation. Irwin describes transaccadic memory as an undetailed, limited capacity memory that is relatively long-lasting.
Features
Visual short-term memory
Many statements have been made concerning the relationship between transsaccadic memory and visual short-term memory. Researchers have noted several similar characteristics between the two systems, leading several to believe that transsaccadic memory is in fact visual short-term memoryIrwin, D. E., Yantis, S., & Jonides, J. (1983). Evidence against visual integration across saccadic eye movements. Perception & Psychophysics, 34, 49-57. or a part of visual short-term memory.Irwin, D. E. (1992). Memory for position and identity across eye movements. Journal of Experimental Psychology: Learning, Memory, and Cognition, 18, 307-317. Transsaccadic memory has a limited capacity of three to four items, a slow decay rate and maskable characteristics. Basically, transsaccadic memory can hold three to four items for each saccade, and the retention of items decays or disappears from consciousness slowly after the presentation of the stimulus. However, once a mask stimulus, such as a blank screen, is presented immediately after the stimulus, the items retained prior to the mask can be replaced and/or eliminated faster by the mask. All of these factors are typical characteristics of visual short-term memory. The content stored in transsaccadic memory are less-image like, more abstract and are sparse representations of the objects, which is found to be similar to the type of representations in visual short-term memory.Deubel, H., Schneider, W. X., & Bridgeman, B. (2002). Transsaccadic memory of position and form. Progress in Brain Research, 140, 165-180. Transsaccadic memory is different from visual short-term memory in that it takes into account the changes of the target's location due to the eye moving to new saccades.Prime, S. L., Tsotsos, L, Keith, G. P., & Crawford, J. D. (2007). Visual memory capacity in transsaccadic integration. Experimental Brain Research, 180, 609-628. The information retained between saccades do not take into account positional information very well.Irwin, D. E., & Gordon, R. D. (1998). Eye movements, attention, and trans-saccadic memory. Visual Cognition, 5, 127-155. Nevertheless, relationalCarlson-Radvansky, L. A. (1999). Memory for relational information across saccadic eye movements. Perception & Psychophysics, 61, 919-934. and identity information are well retained. Prime and colleagues (2006) hypothesize, transsaccadic memory utilizes egocentric mechanisms, like selective attention, to reduce the visual search of the target and allow for spatial information between saccades to be retained and updated by incorporation of information across saccades.
Attention
Transsaccadic memory is characterized by the integration of information gathered prior to the execution of a saccade (pre-saccadic) and information gathered after the saccade (post-saccadic stimuli. This involves attention, locating the targets necessary to perform a saccade and continuous updates of the environmental information. Attention occurs prior to the movement of the eyes, therefore it has a strong influence on the location of the saccade, what information is being encoded, remembered across saccades and stored into transsaccadic memory. In order to perform a successful and accurate saccade an individual is required to make an attentional shift to the saccadic target. This attentional shift causes the encoding of information to occur near the saccade target location. Attention to the target of the saccade results in better object identification and an increase in accuracy of identification. As well as, improvement in coding of locational information.Kowler, E., Anderson, E., Posher, B., & Blaser, E. (1995). The role of attention in the programming of saccades. Vision Research, 35(13), 1897–1916. Attention is restricted to one saccade target per area. It is impossible to keep attention on a target in one area, while performing another saccade simultaneously in another. This will cause prolonged saccadic latency periods (the time it takes to make a saccade from one area to another), and increased reaction times.Postsaccadic target blanking prevents saccadic suppression of image displacement. Vision Research, 36(7), 985-996. However, attention can be distributed around the saccade target, rather than the precise location that will fall on the fovea, allowing for the detection of one or two objects situated around the saccadic target. Kowler (1995) proposed two models to explain the relationship between attention, saccades and transsaccadic memory. The spatial model, states that attention is distributed among the perceptual site (visual field) and the saccadic target during the latency period, which allows for the identification of the saccadic target and object surrounding that area. The temporal model states that attention determines where the saccade will occur and releases a “go” signal to initiate a saccade within that area.
Space constancy