Transsaccadic memory
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 theory is similar to research by Schneider who came up with a similar "reference object theory". 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. This process occurs before each saccade. 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
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. 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'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 memory or a part of visual short-term memory. 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. 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. The information retained between saccades do not take into account positional information very well. Nevertheless, relational and identity information are well retained. Prime and colleagues 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 and information gathered after the saccade, and increased reaction times. 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 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 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
is responsible for maintaining a continuous, stable, visual world by reducing visual sensitivity to events occurring before, during and after a saccade. The more complex the background on which the stimulus is presented, the larger the suppression. The increase in saccadic suppression can lead to a decrease in detection of change in the visual field. Saccadic suppression can be linked to the phenomenon of change blindness, in which individuals lack the ability to detect small or large changes within an environment without the aid of directed attention. There are two types of saccadic suppression, with the first concerning the detection of light spot flashes during saccades. The lower the spatial frequency, meaning fewer objects in the visual field of the light flashes, the stronger the saccadic suppression. With fewer items in the visual field it is easier for saccadic suppression to occur. The higher the spatial frequency, which involves more complex objects within the visual field, the less likely there will be saccadic suppression. The second type concerns the detection of image displacement or a change in an image during eye movement. A displacement between saccades would not be detected because the movement of the eye eliminates the signal for the detection of the displacement. The location of the target forms an image feature that is used by the visual system to establish space constancy, which maintains the location of an object in space. Target blanking is used to study visual stability, space constancy and the content of information transferred across saccades. Blanking a target after a saccade eliminates an attempt to make a second saccade, resulting in the visual system failing to find the target information. Stability and constancy across the saccades is broken, as a result, image displacements become easier to detect when blanking occurs during or right after the saccade.