Visual memory


Visual memory describes the relationship between perceptual processing and the encoding, storage and retrieval of the resulting neural representations. Visual memory occurs over a broad time range spanning from eye movements to years in order to visually navigate to a previously visited location. Visual memory is a form of memory which preserves some characteristics of our senses pertaining to visual experience. We are able to place in memory visual information which resembles objects, places, animals or people in a mental image. The experience of visual memory is also referred to as the mind's eye through which we can retrieve from our memory a mental image of original objects, places, animals or people. Visual memory is one of several cognitive systems, which are all interconnected parts that combine to form the human memory. Types of palinopsia, the persistence or recurrence of a visual image after the stimulus has been removed, is a dysfunction of visual memory.

Neuroanatomy

In humans, areas specialized for visual object recognition in the ventral stream have a more inferior location in the temporal cortex, whereas areas specialized for the visual-spatial location of objects in the dorsal stream have a more superior location in the parietal cortex. However, these two streams hypothesis, although useful, are a simplification of the visual system because the two streams maintain intercommunication along their entire rostral course.

Posterior parietal cortex

The posterior parietal cortex is a portion of the parietal lobe, which manipulates mental images, and integrates sensory and motor portions of the brain.
A majority of experiments highlights a role of human posterior parietal cortex in visual working memory and attention. We therefore have to establish a clear separation of visual memory and attention from processes related to the planning of goal-directed motor behaviors.
We can only hold in mind a minute fraction of the visual scene. These mental representations are stored in visual short-term memory. Activity in the posterior parietal cortex is tightly correlated with the limited amount of scene information that can be stored in visual short-term memory. These results suggest that the posterior parietal cortex is a key neural locus of our impoverished mental representation of the visual world.
The posterior cortex might act as a capacity-limited store for the representation of the visual scene, the frontal/prefrontal cortex might be necessary for the consolidation and/or maintenance of this store, especially during extended retention intervals.

Visual cortex

There is a visual cortex in each hemisphere of the brain, much of which is located in the Occipital lobe. The left hemisphere visual cortex receives signals mainly from the right visual field and the right visual cortex mainly from the left visual field, although each cortex receives a considerable amount of information from the ipsilateral visual field as well. The visual cortex also receives information from subcortical regions, such as the lateral geniculate body, located in the thalamus. However, ample evidence indicates that object identity and location are preferentially processed in ventral and dorsal cortical visual streams, respectively.
Comparison of rCBF during performance of the two tasks again revealed differences between the ventral and dorsal pathways.

Dorsal stream pathway

The dorsal stream pathway is mainly involved in the visual-spatial location of objects in the external world, and it is also known colloquially as the 'where' pathway. The dorsal stream pathway is also involved in the guidance of movements, and is therefore implicated in the analysis of the movement of objects in addition to their spatial locations.
The dorsal stream pathway begins with purely visual information in the occipital lobe, and then this information is transferred to the parietal lobe for spatial awareness functions. Specifically, the posterior parietal cortex is essential for "the perception and interpretation of spatial relationships, accurate body image, and the learning of tasks involving coordination of the body in space."

Ventral stream pathway

The ventral stream pathway is mainly involved in object recognition, and is known colloquially as the 'what' pathway. It has connections to the medial temporal lobe, the limbic system, and the dorsal stream pathway. Therefore, the ventral stream pathway not only deals with the recognition of objects in the external world, but also the emotional judgement and analysis of these objects.
The ventral stream pathway begins with purely visual information in the primary visual cortex, and then this information is transferred to the temporal lobe.

Occipital lobes

Located at the back of the brain, the occipital lobes receive and process visual information. The occipital lobes also process colors and shapes. Whereas the right occipital lobe interprets images from the left visual space, the left occipital lobe interprets images from the right visual space. Damage to the occipital lobes can permanently damage visual perception

Occipital lobe injury complications

Damage to the occipital lobe is characterized by loss of visual capability and the inability to identify colors both important processes in visual memory.

Short term visual memory

Visual short term memory is the capacity for holding a small amount of visual information in mind in an active, readily available state for a short period of time. Although visual short term memory is essential for the execution of a wide array of perceptual and cognitive functions, and is supported by an extensive network of brain regions, its storage capacity is severely limited.
Visual short-term memory storage is mediated by distinctive posterior brain mechanisms, such that capacity is determined both by a fixed number of objects and by object complexity.

Long term visual memory

Recall of the patterns from long term visual memory is associated with rCBF increases in different areas of the prefrontal cortex and the anterior cingulate cortex.
The retrieval of long term visual memories is associated with activation of both anterior and posterior temporal cortices. Posterior temporal cortical regions are more associated with retrieval of category-specific aspects of visual memory, whereas anterior regions of the temporal cortex are more associated with category-independent visual memory.

Methods of study

Benton visual retention test

The Benton Visual Retention Test is an assessment of visual perception, and visual memory abilities. More than 50 years of proven clinical use is the staple of the Benton Visual Retention Test. This test has proven its sensitivity to reading disabilities, nonverbal learning disabilities, traumatic brain injury, attention-deficit disorder, alzheimer's, and other forms of dementia. During testing participants are presented with 10 cards for 10 seconds with unique designs on each. After the time has passed participants are asked to immediately reproduce the designs from each card using their visual memory. In the second stage participants are asked to copy each of the 10 card designs while the cards are in view. The participants results from each task are then assessed and placed into six categories; omissions, distortions, preservations, rotations, misplacements, and sizing errors. The further the participant's scores varies from the averages provided in the Benton Visual Retention Test manual the worse the participant is assessed to be on visual memory ability. The Benton Visual Retention Test has proved to be a generalizable test with the ability to be accurately administered to participants aged 8-adult, and no gender effect. Some studies have suggested a significant gender and education interaction indicating that an age-associated decline in visual memory performance may be more prominent for those individuals with a lower education level.

Neuroimaging tests

studies focus on the neural networks involved in visual memory using methods designed to activate brain areas involved in encoding, storage, and recall. These studies involve the use of one or multiple types of brain imaging techniques designed to measure timing or activation within the brain. The data collected from neuroimaging studies gives researchers the ability to visualize which brain regions are activated in specific cognitive visual memory tasks. With the use of brain imaging devices researchers able to further investigate memory performance above and beyond standard tests based on exact response times, and activation.

Control condition

The subject's resting brain activation level is first determined in order to form a control or 'baseline' to measure from. Subjects are blindfolded and instructed to lay motionless while simultaneously eliminating any visual imagery present in their mind's eye. These instructions are intended to minimize the activation of brain regions involved in visual memory to form a true resting brain state. After the scan is complete a control has been formed which can be compared with activated regions of the brain while performing visual memory tasks.

Activation condition

During encoding, participants are typically exposed to 1–10 visual patterns while connected to a brain imaging device. As the subject encodes the visual patterns researchers are able to directly view the activation of areas involved in visual memory encoding. During recall subjects again need to have all visual stimuli removed by means of a dark room or blindfolding to avoid interfering activation of other visual areas in the brain. Subjects are asked to recall each image clearly in their mind's eye. While recalling the images researchers are able view the areas activated by the visual memory task. Comparing the control 'baseline' state to the activated areas during the visual memory task allows researchers to view which areas are used during visual memory.