Transsaccadic memory is the neural process that allows humans to perceive their surroundings as a seamless, unified image despite rapid changes in fixation points.
Conflicting views and theories have spurred several types of experiments intended to explain transsaccadic memory and the neural mechanisms involved.
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.
[2] 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.
[2] The object's features are stored as a mental representation in transsaccadic memory for identification of future fixations.
Irwin performed similar experiments in which participants recalled letters that occurred near the target area.
[2] Due to confounding factors of the controlled environment in the studies, the involvement of saccade target objects is inferred and not established.
One less-accepted theory, Breitmeyer's spatiotopic fusion hypothesis, suggested that successive images are fused based on environmental coordinates and not retinal ones.
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.
The proposed answer to these questions lies in several mechanisms that support a high sensitivity to change in each visual fixation.
[5] Irwin's experiments showed that people cannot fuse pre-saccadic and post-saccadic images in successive fixations.
The lower the spatial frequency, meaning fewer objects in the visual field of the light flashes, the stronger the saccadic suppression.
The higher the spatial frequency, which involves more complex objects within the visual field, the less likely there will be saccadic suppression.
[2] Target blanking is used to study visual stability, space constancy and the content of information transferred across saccades.
[21] Studies have shown that there is a large amount of activation within the visual area V4 before the saccade even takes place.
[22] This dynamic change in receptive fields is thought to enhance the perception and recognition of objects in a visual scene.
[23] The lateral intraparietal cortex (LIP) is an area that is believed to be primarily responsible for keeping an image fluid and undistorted during a saccade (visual/spatial constancy).
The receptive fields of the LIP are quite large and therefore are not well suited to hold details about the perceived objects in the scene during a saccade.
The posterior parietal cortex (PPC) is a cortical area that is located in front of the parieto-occipital sulcus and is known to play a role in spatial awareness for eye and arm movements.
A study using transcranial magnetic stimulation (TMS) found that the PPC also plays an important role in the amount of information held across saccades.
[25] It is believed that transsaccadic memory has the ability to hold roughly three to four items across each saccade with a fully functional PPC.
Further research needs to be conducted in order to fully understand how the PPC is synthesized with object recognition within transsaccadic memory.
[25] Irwin's early experiments tested participants ability to identify random dot patterns as the same, or as different across saccades.
The control condition for this experiment presented dot patterns in the same spatial location without saccades, meaning participants had a single fixation point.
A no-overlap control condition presented dot patterns in different spatial locations, while participants maintained fixation on one point.
[5] The experimental condition showed dot patterns in the same spatial location but were viewed at separate fixations, forcing the use of transsaccadic memory.
[5] For the experimental condition, participants underwent a calibration phase, where they were shown five points in separate location to fixate on individually, for less than two seconds.
[3] Detection of change ended up being much higher when only the target object moved compared to when the entire image shifted.
It was found that transsaccadic memory was disrupted when TMS stimulation was delivered to the right posterior parietal cortex (rPPC) around the time of a saccade.
Prime et al. hypothesized that TMS interfered with the normal spatial remapping operations of the rPPC, in particular the parietal eye fields, that occur during a saccade.