Interaction between visual perception and mental representations of imagery and memory in the early visual areas

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http://urn.fi/URN:ISBN:978-951-51-0996-5
Title: Interaction between visual perception and mental representations of imagery and memory in the early visual areas
Author: Saad, Elyana
Contributor: University of Helsinki, Faculty of Behavioural Sciences, Institute of Behavioural Sciences
Thesis level: Doctoral dissertation (article-based)
Abstract: The relationship between mental representations based on external visual percepts (i.e., information held in short-term memory or via mental imagery) and the encoding of visual input remains unsettled. What stimulates this debate is the share of overlapping neural resources between visual short-term memory (VSTM), mental imagery and visual perception in the realm of the early visual cortex (EVC). This overlap raises a number of questions: how do the internal memory and imagery representations affect the perception of incoming visual information? What happens to imagery and VSTM abilities when cognitive resources need to be shared with the encoding of visual input? In short: how do visual memory/imagery and visual perception interact? This work addressed these questions by the use of behavioral paradigms coupled with transcranial magnetic stimulation (TMS) in situations where the encoding of the visual percept (measured via the tilt after effect (TAE) magnitude) happens either simultaneously or subsequently to holding information in VSTM/imagery. Therefore, when VSTM and the encoding of external input occurred concurrently, VSTM maintenance was found to inhibit visual encoding, reflected as a reduction of the TAE. Using TMS, it was shown that this inhibition takes place at the level of EVC. This reduction was found when the VSTM content matched the visual input, and when they were incongruent. However, when the encoding of external input occurred after VSTM maintenance phase had ended, VSTM maintenance was found to facilitate the former when the VSTM content matched the visual input. The subjective strength and the contrast of VSTM and mental imagery content (as reported by participants) affect visual detection of a briefly presented masked target. The reported visual contrast was positively associated with reporting target presence for both VSTM and mental imagery, in other words, inducing a more liberal bias. However, a differential effect was found for the subjective strength of the representations. Whereas the subjective VSTM strength was positively associated with the visual detection of the target, the opposite effect was observed for imagery. Finally, TMS applied at the EVC revealed a partial dissociation in the neural basis of VSTM and mental imagery by inducing delayed responses for the former selectively. Thus, while VSTM and mental imagery share neural resources, their neural mechanisms are partly dissociable at the level of early visual cortex.Many items constitute the visual environment. However, only those of relevance are maintained in mind. Once retained, the information will be kept active in the brain for a certain time. Therefore, the question that arises is, would this information, held in mind, have any simultaneous or subsequent effect on a real-time visual perception. This Work addressed this question by using simple tasks during which people where asked to memorize or imagine some shapes and subsequently or simultaneously perceive objects. Within the task, a non-invasive stimulation technique, known by transcranial magnetic stimulation (TMS), was applied. Therefore, participants received few magnetic pulses on their skull while performing the task. The results show that when information is held in mind while perceiving objects, the precision of the visual perception is reduced. This reduction is observed regardless of whether the percept is identical to that of the information held in mind. However, this is not the case when visual perception happens only after ceasing to hold information in mind. In this situation, a reduction of the precision of visual perception was found only when both information previously held in mind and the perceived to-be object were non-identical. The opposite pattern of results was observed when both were identical. Additionally, the processes of imaging and memorizing objects were found to have different effects on subsequent visual perception. Thus, whereas the subjective memory strength was positively associated with the visual detection of objects, the opposite effect was observed for imagery. Also TMS added to this discrepancy by affecting the speed of memory and imagery processes differently.
URI: URN:ISBN:978-951-51-0996-5
http://hdl.handle.net/10138/154167
Date: 2015-05-05
Subject: psychology
Rights: This publication is copyrighted. You may download, display and print it for Your own personal use. Commercial use is prohibited.


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