Is visual cortex involved in memory?
Essay type – Option 1 [REVIEW OPTION]
Lopez-Aranda et al. 2009. Role of Layer 6 of V2 Visual Cortex in Object-Recognition Memory, Science 325, 87
Cattaneo et al., 2009. Contrasting early visual cortical activation states casually involved in visual imagery and short term memory
Though a lot of information enters the brain, retention does not occur for all of it, and it is considered to be a selective process. One of these retentions is short term memory, also known as working memory. In vision, working memory is interpreted as the maintenance of a whole object, instead of the components of the object’s image: orientation, texture, etc. (Super, 2003).
Memory retention and formation is typically associated with increased activity in mammalian prefrontal and parietal cortex, with little evidence for activity in sensory areas, beyond of the initial sensory stimulation (Pasternak and Greenlee, 2005). The Multiple Memory Systems is a widely accepted view that sustains that the brain is divided into sections in respect of their own specific function. In this interpretation, the Medial Temporal Lobe (MTL) has a role in memory, particularly in explicit memory function, and includes structures such as the hippocampus, entorhinal cortex, parahippocampal cortex, and perirhinal cortex, in addition to the prefrontal cortex (Bussey and Saksida, 2007).
However, recent findings have suggested a role of sensory cortex in memory processing: increased brain activity in visual cortex has been found during the short-term retention of visual information after stimulus presentation (Kaldy and Sigala, 2004). It therefore has been more common to say that visual cortex role goes beyond encoding sensory information and also participates in memory consolidation.
This essay reviews two papers in which evidence of the role of visual cortex in memory consolidation is presented by the use of different techniques: 1) Cellular techniques as protein overexpression and immunocytochemistry (Lopez-Aranda et al., 2009) and 2) Transcranial magnetic stimulation (Cattaneo et al., 2009)
Role of Layer 6 of V2 Visual Cortex in Object-Recognition Memory
Using rats as a model, Lopez-Aranda et al. (2009) tried to elucidate the specific role of layer 6 in V2, in regards to memory processing and retention. To do this, they utilised two methodologies in a paired-sample experiment (same group tested on two different occasions). One addressed the problem by analysing the overexpression of a certain G-protein regulator (RGS-14) in layer 6 of V2, that acted as a protease, and which permitted rats do better in Object recognition memory (ORM) tests. ORM tests consisted on evaluating the exploration time after an object was presented for 3 minutes, and presented again after a delay period of 30 minutes, 45 minutes or 60 minutes. Rats could recognise the object after 30 or 45 minutes had passed, but failed to do it after the 60 min delay. A group of these rats was then injected with a lentivirus coupled with the RGS-14 gene into layer 6 of V2, at 2/3 of V2 (dorsal to layer 6 of V2), at CA1 and at the dentate gyrus of the hippocampus (both ventral to layer 6 of V2). This permitted the overexpression of RGS-14 at those sites. Rats were ORM tested again 3 weeks after the injection was done. What was found was that rats improved in their ORM tests when they were injected exactly at layer 6 of V2, whereas the rest of the rat groups (injected at other sites), did not show any difference in performance, and their activity was similar to that of the initial non-injected rats. The layer 6 injected rats were ORM tested again, to see how much retention they could support, showing up to 6 object retention (in comparison to the 2 object retention non-injected rats showed), and increasing its retention time to about 24 weeks. They then proceeded to make immunocytochemistry analysis to localize protein expression, which showed that RGS-14 was primarily being expressed at layer 6 of V2.
The other methodology used focused on presenting the result of layer 6 of V2 destruction, by the injection of Ox7-SAP into this layer in non-injected rats and RGS-14 injected rats, and later doing the ORM test to both groups. Non-injected rats showed an increased reduction in retention time, not being able to perform equally as they did when layer 6 of V2 was not ablated. RGS-14 lentivirus injected rats also showed a reduction in their ORM test performance. A group of rats, either injected or non-injected, were tested again, only that before having layer 6 of V2 ablated by Ox7-SAP an object was presented for three minutes. Rat performance was not reduced when ORM test was done with object that was previously presented, but did showed reduction when the ORM was done with a new object, presented after layer 6 removal.
The results showed an involvement of layer 6 of V2 in memory trace, though not storage. It is not explicitly said if the RGS-14 G protein regulator is naturally expressed in layer 6 of V2. As far as the obtained results, it is possible to say that RGS-14 could act as treatment option for short memory disorders or impairments, though more trials are possibly needed.
Contrasting early visual cortical activation states casually involved in visual imagery and short term memory
With the use of Transcranial Magnetic Stimulation (TMS), Cattaneo et al., (2009) evaluated the role of early visual areas in memory and visual imagery. They essentially established two similar experiments involving two tasks, the imagery task and the memory task, in subjects who were either undergoing occipital TMS (over V1/V2), Vertex TMS (as a control) or No TMS.
In the imagery task of the first experiment, subjects had to create a mental image of something. It consisted on presenting a black dot in the middle of a white screen, followed by a series of digits (that represented an hour, e.g. 10.10, 6.50, etc.), for about 1000 ms. Then this digits disappeared and a black circle showed up. Subjects were then asked to imagine the clock hands in the position that would describe the digits they had just seen. After a 2 second period passed, a single pulse of TMS was applied, depending on the condition previously defined for them. Next, a black dot was shown (inside the black circle) and subjects were asked to tell if this dot had appeared inside or outside the area the clock hands were supposed to be, by either pressing 1 or 2 on a keyboard for either inside or outside the area.
In the memory task in the same experiment, subjects also had to fix their eyes at a black dot in the white screen. Then, the clock hands (describing an hour) inside a circle appeared for about 1000 ms. When this period had passed, the hands disappeared but the circle remained, and subjects were asked to continue on thinking on the clock hands for about 2 s. TMS was applied at the end of this 2 s (retention) period, in the same mode as in the imagery task. A block then appeared inside the circle and subjects were asked to describe whether the dot was inside or outside the area the clock hands formed.
By doing ANOVA, they found no relevant differences between the mean detection accuracies between TMS conditions: Occipital TMS, Vertex TMS and No TMS, in both imagery and memory tasks. However, the mean reaction times did show relevant differences between those conditions, in both tasks. A Post hoc comparison showed that performance was better in the Occipital TMS than when condition were Vertex TMS or no TMS. There was also no significant variation when the analysis was done between Vertex TMS and no TMS.
Experiment two was fairly similar to the one described above. It also involved a memory and imagery task, with the only difference being when was TMS applied: at the beginning of the 2 s period after subjects had seen the digits and were asked to imagine the clock hands inside the circle, for the imagery task, and at the beginning of the 2 s period when they were asked to continue on thinking on the clock hands, for the memory task. By performing ANOVA they found no significant difference between conditions for the mean detection accuracies and reaction time, in the imagery task. Conversely, in the memory task, ANOVA showed a relevant effect in mean detection accuracy and mean time, as well as the Post hoc analysis showed occipital TMS had an effect in comparison to the other conditions, both of which was impairment in performance.
MTL structures have been presented as the major components in perception and working memory, and it is seen as a domain where ORM is thought to be processed (Kaldy and Sigala, 2004).
Lopez-Aranda et al, (2009) results of the role of layer 6 neurons in the formation of both normal (short-term) and long-term ORM highlight the importance of V2, an area placed outside of MTL. Not much is known about the protein overexpressed at V2, RGS-14. It is integrated by a Regulators of G protein Signaling domain, as well as by a motif that permit its binding to inactive GDP; and by a tandem Rap1/2–binding domain. Acting as a GTPase activating protein, the protein increases the rate of conversion of the GTP to GDP. This allows the G alpha subunits to bind subunit heterodimers, and eventually ending a signal (NCBI, 2013). It would be interesting to know what made the authors determine to test this protein in that specific layer of V2, as it is not fully stated in the article, and because RGS14 was found to be expressed naturally/primarily in CA2 hippocampal neurons and to show memory “obstruction” when expressed in mice (Lee et al., 2010). Perhaps differences between species (as both studies were done with model animals: rats and mice) are more relevant than thought, and should be taken in account before making any definite conclusion or investigate of how the signaling process occurs and affects a cognitive behavior, such as memory.
However, findings involving TMS analysis in humans by decrease of activity, as the one presented by Cattaneo et al. (2009), in which there was a noted decrease in subject performance in the memory task when TMS was applied in the beginning of the “retention period” at V1/V2, indicate that memory of visual information involves activity in early visual cortex that goes further than the periods of sensory perception. In early visual cortex, memory of visual content is topographically organized. These results are possibly due to less vulnerability to interference after the retention period, and a possible interaction with higher order areas activity with visual cortex activity (van de Ven and Sack, 2013).
The previous results can be paired with Harrison and Tong (2009) results, were they used functional magnetic resonance imaging (fMRI), in conjunction to Blood Oxygen Level Dependent (BOLD) analysis, to monitor cortical activity while participants did a delayed orientation discrimination task, where 2 gratings were shown to the subjects, followed by a cue that indicated which grating to remember (first or second) and an 11 s period (delay period). Then the grating was showed again and subjects had to say if the image was rotated in a sense or antisense (clockwise) matter. They examined the role of visual areas in working memory through different experiments; fMRI decoding was specifically used to evaluate the patterns in brain activity, in areas corresponding to V1 to V4 (to the 120 most responsive voxels) to try to predict its representation in working memory. The accuracy of predicted orientation that was held in memory reached 83%, which is considered to be very high, one of the experiments where subjects had to fix its eye to a letter, and not the grating, showed high prediction to those gratings in areas V1, V2 and V3. Ultimately, their findings suggest that memory related information may be encoded in these structures (showing increased activity in areas V1/V2) and that early visual areas can hold up information, not only displaying sensory processing functions.
Different approaches can be taken to evaluate visual cortex relation with memory, as the ones reviewed in this essay: TMS, protein overexpression, fMRI among others. Evidence that sensory cortical areas are an active element of the circuitry that underlies short term retention of sensory signals is emerging and improving our understanding of memory. It can be concluded that not only the MTL is important for visual memory processing, but also early visual cortex and evidence of what is happening at the cellular level needs to be improved in order to eventually delimit its potential in cognitive treatments.
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